Cisco CCNA 1

Essential

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Table Of Contents 1. INTRODUCTION TO NETWORKS.............................................................................................................. 5 1.1. CONNECTION TO A NETWORK........................................................................................................................... 5 1.1.1. Material ..................................................................................................................................................... 5 1.2. NUMERICAL SYSTEMS ...................................................................................................................................... 6 1.2.1. Data representation................................................................................................................................... 6 1.2.2. Numerical systems ..................................................................................................................................... 7 1.2.3. Conversions ............................................................................................................................................... 8 1.3. NETWORK TERMINOLOGY ................................................................................................................................ 9 1.4. MEASUREMENT UNITS ................................................................................................................................... 10 2. THE OSI AND TCP/IP MODELS ................................................................................................................ 11 2.1. OSI MODEL OVERVIEW .................................................................................................................................. 11 2.2. TCP/IP MODEL .............................................................................................................................................. 13 2.3. COMPARISON BETWEEN TCP/IP AND OSI MODEL.......................................................................................... 14 3. LAYER 1: MEDIA AND NETWORK DEVICES ....................................................................................... 15 3.1. SIGNALS AND CODING .................................................................................................................................... 15 3.1.1. Comparing analog and digital signals .................................................................................................... 15 3.1.2. Representing a bit in a physical medium ................................................................................................. 16 3.1.3. Factors that can affect a bit..................................................................................................................... 16 3.2. COPPER MEDIA ............................................................................................................................................... 18 3.2.1. Unshielded Twisted Pair ......................................................................................................................... 18 3.2.2. Shielded twisted pair cable...................................................................................................................... 19 3.2.3. Coaxial Cable.......................................................................................................................................... 20 3.2.4. RJ45 connectors ...................................................................................................................................... 21 3.3. OPTIC MEDIA.................................................................................................................................................. 21 3.3.1. Physical phenomena................................................................................................................................ 21 3.3.2. Optical components ................................................................................................................................. 24 3.4. WIRELESS TECHNOLOGY ................................................................................................................................ 26 3.4.1. Wireless network presentation................................................................................................................. 26 3.4.2. Authentication and security ..................................................................................................................... 27 3.4.3. Wireless network implementation............................................................................................................ 28 3.5. LAYER 1 COMPONENTS AND DEVICES ............................................................................................................ 29 3.5.1. Repeaters ................................................................................................................................................. 29 3.5.2. Hubs ........................................................................................................................................................ 29 3.5.3. Transceiver / Receiver ............................................................................................................................. 29 3.6. BASIC TOPOLOGIES USED IN NETWORKING .................................................................................................... 30 3.6.1. Bus topology ............................................................................................................................................ 30 3.6.2. Ring topology........................................................................................................................................... 30 3.6.3. Star topology ........................................................................................................................................... 31 3.6.4. Extended star topology ............................................................................................................................ 31 3.6.5. Tree topology........................................................................................................................................... 32 3.6.6. Full mesh topology .................................................................................................................................. 32 4. LAYER 2: ETHERNET TECHNOLOGIES................................................................................................ 33 4.1. INTRODUCTION TO LAN TECHNOLOGIES ....................................................................................................... 33 4.2. INTRODUCTION TO ETHERNET........................................................................................................................ 33 4.2.1. Ethernet and OSI model .......................................................................................................................... 33 4.2.2. Norms and Specifications ........................................................................................................................ 33 4.2.3. Ethernet IEEE 802.3 Frames................................................................................................................... 34 4.3. ETHERNET OPERATION ................................................................................................................................... 34 4.3.1. Relation to the OSI model........................................................................................................................ 34 4.3.2. Possible errors......................................................................................................................................... 35

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5. LAYER 2: ETHERNET SWITCHES ........................................................................................................... 37 5.1. COLLISION DOMAIN ....................................................................................................................................... 37 5.2. SEGMENTATION ............................................................................................................................................. 37 5.2.1. Segmentation with bridges....................................................................................................................... 37 5.2.2. Segmentation with switches ..................................................................................................................... 37 5.2.3. Spanning Tree.......................................................................................................................................... 38 6. LAYER 3: IP PROTOCOL............................................................................................................................ 39 6.1. ROUTED PROTOCOLS ..................................................................................................................................... 39 6.1.1. Connection and Connectionless oriented protocols ................................................................................ 39 6.1.2. Routed and routable ................................................................................................................................ 39 6.2. INTERNET PROTOCOL (IP).............................................................................................................................. 40 6.2.1. IP packet.................................................................................................................................................. 40 6.2.2. IP addresses ............................................................................................................................................ 40 6.2.3. IP address classes.................................................................................................................................... 41 6.2.4. IPv4 and IPv6 .......................................................................................................................................... 42 6.3. ATTRIBUTION OF IP ADDRESSES .................................................................................................................... 42 6.3.1. Acquiring Methods .................................................................................................................................. 42 6.3.2. Address resolution ................................................................................................................................... 43 6.3.3. Internet Control Message Protocol (ICMP)............................................................................................ 44 7. LAYER 3: SUBNETTING ............................................................................................................................. 45 7.1. SUBNETTING UTILITY ..................................................................................................................................... 45 7.2. CALCULATION METHOD ................................................................................................................................. 45 7.2.1. Classic method......................................................................................................................................... 45 7.2.2. Magic number method ............................................................................................................................. 46 8. LAYER 3: INTRODUCTION TO ROUTING............................................................................................. 48 8.1. FUNDAMENTAL PRINCIPLES ........................................................................................................................... 48 8.2. BROADCAST DOMAIN .................................................................................................................................... 48 8.3. NETWORK LAYER DEVICES: ROUTERS ........................................................................................................... 48 8.4. PATH DETERMINATION ................................................................................................................................... 49 8.5. AUTONOMOUS SYSTEMS, IGP AND EGP ........................................................................................................ 49 9. LAYER 4: TRANSPORT LAYER................................................................................................................ 51 9.1. INTRODUCTION .............................................................................................................................................. 51 9.2. TCP AND UDP............................................................................................................................................... 51 9.2.1. Port numbers ........................................................................................................................................... 52 9.2.2. TCP Segment structure............................................................................................................................ 52 9.2.3. UDP datagram structure ......................................................................................................................... 53 9.3. TCP CONNECTION METHOD ........................................................................................................................... 53 9.3.1. Three step connection initialization sequence ......................................................................................... 53 9.3.2. Positive Acknowledgement Retransmission............................................................................................. 54 9.3.3. Windowing............................................................................................................................................... 54 10. LAYER 5: SESSION LAYER...................................................................................................................... 55 10.1. DIALOG CONTROL ........................................................................................................................................ 56 10.2. DIALOGUE SYNCHRONIZATION .................................................................................................................... 56 10.3. DIALOG DIVISION ......................................................................................................................................... 56 11. LAYER 6: PRESENTATION LAYER ....................................................................................................... 58 11.1. FUNCTIONS AND NORMS .............................................................................................................................. 58 11.2. DATA ENCRYPTION ...................................................................................................................................... 59 11.3. DATA COMPRESSION .................................................................................................................................... 59

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12. LAYER 7: APPLICATION LAYER........................................................................................................... 60 12.1. INTRODUCTION ............................................................................................................................................ 60 12.2. DNS............................................................................................................................................................. 60 12.2.1. Presentation of the protocol .................................................................................................................. 60 12.2.2. Host name, « domain name system »..................................................................................................... 60 12.2.3. Internet domain codes ........................................................................................................................... 61 12.3. FTP AND TFTP ............................................................................................................................................ 62 12.3.1. FTP........................................................................................................................................................ 62 12.3.2. TFTP...................................................................................................................................................... 62 12.4. HTTP........................................................................................................................................................... 62 12.5. SMTP .......................................................................................................................................................... 63 12.6. SNMP.......................................................................................................................................................... 63 12.7. TELNET PROTOCOL ...................................................................................................................................... 63 12.7.1. Presentation of the Telnet protocol ....................................................................................................... 63 12.7.2. Notion of virtual terminal ...................................................................................................................... 64

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1. Introduction to networks Networks are systems of connected beings or objects. Nowadays when we say network, we mean an enterprise network that connects machines which allows them to communicate. Let it be the file sharing or the message exchange, most enterprises have a network today, so that they are more efficient when transferring information. One would agree that it is simpler to transfer a file by Internet than posting it on a CD. During this course we will study how the information (file, data, etc.) circulates on networks of small size (PAN, LAN) or large size (MAN, WAN), as well as the network devices.

1.1. Connection to a network 1.1.1. Material A computer is composed of various elements. Before connecting your computer on a network, it is necessary to know what composes it, so in case of a breakdown one would know how to better identify the problem. Besides it allows one to be familiar with his machine, which can help in daily work. Here is the list of different PC components, and their respective descriptions: Components Motherboard

Descriptions The main electronic card in a computer. The motherboard contains logical buses, a microprocessor, the integrated circuits used to control external devices as a keyboard, a graphic display, serial and parallel ports, or USB ports and Firewire. Central Processor Unit A silicon microchip in charge of all the arithmetic and logical calculations in a computer. It manages information fluxes as well. RAM (Random Random-access memory stocks on hold instructions, as well as temporary data. Access Memory) Once the computer shuts down, this memory empties itself, contrary to the hard disk. Hard Disk Physical disk for data storage. It is on the hard disk where one saves his data. Contrary to the RAM, the hard disk keeps the data even while the computer is off. Bus Internal communication channel of a computer by which the data transits between the different components. Power supply Component that provides the power supply to the computer. ROM (Read only Read only memory, written to only once. This kind of component serves to Memory) stock the information that must not be erased. CD-ROM player Device that reads compact discs.

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Backplane components allow one to add extensions on the motherboard. Components Graphics Card Sound Card

Descriptions Device that displays visual information on a monitor Extension card that serves to manipulate and to produce sounds via audio speakers. Extension card that connects physically a computer to a network. A port standard for connecting plug and play devices to a computer. Concurrent standard of USB that provides higher speed transfers.

Network Interface Card (NIC) USB (Universal Serial Bus) Firewire

1.2. Numerical systems When the computers were created, they were very expensive because of the number of components they required, in addition to their complexity. The reduction of the cost was achieved by their miniaturization. Computers use the binary numbering system. A computer could be resumed to a set of electric switches capable of being in two states: • On (the current passes) • Off (the current doesn't pass)

1.2.1. Data representation Because the humans work with the decimal system, the computer must be able to do a “translation” in order to be able to treat the user information. The binary numbers are expressed in bits, which constitute the smallest unit of information. A group of 8 bits corresponds to one byte, which represents a character of data. For a computer, a byte also represents an addressable memory site. For example, the binary representation of the characters of the keyboard and the characters of control is given in the picture of the ASCII codes (American Standard Code heart Information Interchange): Decimal 0 1 2 3 4 7

Hexadecimal 0 1 2 3 4 7

Octal 000 001 002 003 004 007

Binary 00000000 00000001 00000010 00000011 00000100 00000111

Char NUL SOH STX ETX EOT BEL

This figure shows the equivalences between different systems of numbering that we are going to study. If we look at the binary column, we can see that all characters are expressed thanks to an 8 bit combination.

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Because of the size of the information contained in computers, different units of measure have been created: Unit Bit Byte (b)

Definition Byte binary digit 1 bi 1 or 0

Bits

Examples

1 bit

+5 volts or 0 volts

8 bits

8 bits

01001100 corresponds to L in ASCII

1 octet

1 kilobyte 8192 Kilobyte (Kb) =1024 byte 1024 byte bits 1 megabyte =1024 kilobytes 1 gigabyte Gigabyte =1024 (Gb) megabytes 1 terabyte Terabyte (Tb) =1024 gigabytes Megabyte (Mb)

E-mail : 2kb first PC : 64kb of RAM

8 388 Floppy disk = 1,44 Mo 1 048 576 608 CD-ROM = 650 Mo octets bits 1 048 576 8 billion Hard disk = 4 Go kilobyte bits 1 048 576 8 trillion Theoretical bandwidth of megabytes bits the optic fiber

1.2.2. Numerical systems People are accustomed to use a system of numeration to represent numerical values since the early age. This system includes 10 symbols: 0 1 2 3 4 5 6 7 8 9 and is called a decimal system. This system constitutes the basis of the calculation for the men, mainly because we have 10 fingers. We will use this system as the reference in the course. However, there are other numerical systems capable of representing values. Thus, a value is an abstract notion that can be expressed differently according to its system: A computer uses a simple system that has only two states, 0 (off) or 1 (on), easily reproducible in an electric circuit. The hexadecimal system includes 16 symbols 0 1 2 3 4 5 6 7 8 9 TO B C D E F. The 6 letters correspond in decimal to 10 11 12 13 14 15. This system is used to simplify the too large decimal values. That is one of the reasons that make several numerical systems useful.

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1.2.3. Conversions Between these bases methods of conversions exist: • • • • • •

Decimal Î Binary Decimal Î Hexadecimal Binary Î Decimal HexadecimalÎ Decimal Binary Î Hexadecimal Hexadecimal Î Binary

To convert the decimal toward another basis, one uses this formula:

No Decimal number superior to (base-1)

We divide by base

We keep the result

Result inferior to (base-1)?

Yes

It’s the last remainder

We keep the remainder

One divides a number by the base to which one wants to convert it and continues until the number does not become lower than the base. It suffices to take the different remains and of the concatenate them from the last to the first (from right to the left). The conversion to a decimal base is done by a decomposition of the number in digits. And then one multiplies every digit by the power of the base while starting on the right with the one with a power zero (if the number is hexadecimal then one would multiply the digits by 160, 161, 162…). It is the set of values of the different digits this way multiplied that forms the decimal number, as shown in the formula.

Finally, to convert the binary to the hexadecimal, one takes a group of 4 bits and converts them in hexadecimal via the powers of 2. For the other way round, it is sufficient to make the same process while using the first formula as if one converted to base 2, while using the groups of 4 bits here as well.

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Hexadecimal Binary Hexadecimal Binary 0

0000

8

1000

1

0001

9

1001

2

0010

A

1010

3

0011

B

1011

4

0100

C

1100

5

0101

D

1101

6

0110

E

1110

7

0111

F

1111

Figure of binary/hexadecimal conversion

1.3. Network terminology A network is a system of objects or people that communicate between each other. Networks have appeared because of a need of a way to simplify the communication between computers and to avoid the duplication of units such as printers. A data network is a communicating computing unit ensemble. The first classification that can be established is based on thee distance separating the units: •

LAN Networks: o Operate within a limited geographic area o Allow multi-access to high-bandwidth media o Provide full-time connectivity to local services (Internet, mails, etc.) o Connect physically adjacent devices ƒ Example: A classroom

•

WAN networks: o Operate over large geographical area o Allow access over serial interfaces operating at lower speeds o Provide full-time and part-time connectivity o Connect devices separated over wide, even global areas. ƒ Example : Internet

These types of networks are the most current; nevertheless others exist, like the MAN (Metropolitan Area Network), which connects one or several LANs in the same geographical region. This type of network has emerged because of the development of the Wireless networks. They are often found in a city, around the public zones. Another type of network is the SAN (Storage Area Network) that is a zone of storage and transfer of data.

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The SANs: • Use a different network than that, used by hosts, in order to not to overwhelm the traffic. • Permit a distinctly higher rate of transfer between servers, in order to permit a safer replication of the data. • Permit to duplicate some data between servers until a distance of 10 km. • Use various technologies that permit not to take account of the system used. A VPN (Virtual Private Network) is a private network that is constructed on public network infrastructure, such as the Internet. On the Internet, a secure tunnel can be placed between a user PC and a VPN router, being at the enterprise head office. This way, one can log into his enterprise network at home.

1.4. Measurement units The bandwidth of a network represents its capacity, which means the quantity of data transferred in a given period of time. It is measured in bits per second. According to the different network capacities, the following conventions are used: Bandwidth unit Bits per second Kilobits per second Megabits per second Gigabits per second

Abbreviation bits/s kbits/s Mbits/s Gbits/s

Equivalence 1 bit/s = fundamental unit 1kbit/s = 1000 bits/s 1Mbit/s = 1 000 000 bits/s 1Gbit/s = 1 000 000 000 bits/s

The throughput is the real bandwidth, measured at a precise time of the day. The output is often inferior to the bandwidth, because the bandwidth represents the maximal output which is reduced by: • • • • • •

The client (PC) The server Other users on your LAN Routing within the “Cloud” The topology of all networks involved Type of data being transferred

The formula to calculate the time needed to transfer a file is: Best Download T = S / BW Typical Download T = S / P BW: Maximum theoretical bandwidth of the slowest link between the source and the destination (bps) P: Actual throughput at the moment of transfer T: Time for file transfer to occur S: File size in bits

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2. The OSI and TCP/IP models 2.1. OSI model overview The first evolution of the networks was most anarchic, each manufacturer providing its own technology, in most of the cases not compatible with the other ones. The result of this evolution was an impossibility to interconnect networks together. To address the problem of networks being incompatible and unable to communicate with each other, the International Organization for Standardization (ISO) researched network schemes like DECNET, SNA, and TCP/IP in order to find a set of rules. As a result of this research, the ISO created a network model that would help vendors create networks that would be compatible with, and operate with, other networks. The OSI reference model (not to be confused with ISO) released in 1984 was the descriptive scheme they created. It provided vendors with a set of standards that ensured greater compatibility and interoperability between the various types of network technologies that were produced by the many companies around the world. This model is a conceptual model. The aim of this model is to analyze the communication by separating the different steps in 7 layers, each having a specific task: • What kind of information passes through? • Under what form does it circulate? • What paths does it borrow? • What are the rules that govern the information flow? The seven layers of the OSI model are: • Layer 1 Î Physical layer: This layer defines medium specifications (wiring, connector, voltage, bandwidth …) • Layer 2 Î Data link layer: The data link layer takes care of the sending of the data across the media. This layer is divided in 2 sub-layers: o The MAC sub-layer (Media Access Control) is in charge of the media access. It’s at this layer that we find data link addresses (MA, DLCI). o The LLC sub-layer (Layer Link Control) is in charge of the communication management between stations and interact with the network layer. • Layer 3 Î Network layer: This layer manages the level 3 addressing, path selection and packet delivery across the network. • Layer 4 Î Transport layer: The transport layer carries out quality transmission allowing retransmission of segments in case of transmission errors. This layer also takes care of sending data flow control. • Layer 5 Î Session layer: The session layer, establishes manage and close sessions in communication between applications. • Layer 6 Î Presentation layer: The presentation layer specify the data format for applications (encoding, MIME, compression, encryption). • Layer 7 Î Application layer: This layer makes the interface with the applications, it the layer nearest to the user.

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Layer number

Name

Description

7

Application

Communication at software level

6

Presentation

Data representation

5

Session

Interhost communication

4

Transport

End-to-end connections

3

Network

Path selection

2

Data link

Access to Media

1

Physical

Binary Transmission

Figure 1- The seven layers of the OSI model

The advantages of this model are: • A division of the communication network in elements smaller and simpler for a better understanding. • The standardization of the elements in order to permit the network device production by different constructors. • The possibility to modify an aspect of the communication network without modifying the rest (Example: a new media) • In order to communicate between the layers and between the hosts of a network, OSI has found the principle of encapsulation. Application

Data

Application

Presentation

Data

Presentation

Data

Session

Session

Encapsulation

Transport

Segment

Network

Packet

Header

Data

Transport

Header

Segment

Network

Data link

Frame

Physical

010010110011010110011001100110001100

Hearder

Packet

Trailer

Desencapsulation

Data link Physical

Encapsulation: process of treating the data that consists in adding a certain protocol header before the data is transmitted to the lower layer: When 2 hosts communicate, it means that the n layer of the source communicates with the recipient's n layer.

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When a source layer receives some data, it encapsulates it with its information and then passes it to the lower layer. The inverse mechanism takes place on the recipient's side, where a layer receives the data of the lower layer, removes the information that concerns it, and transmits the remaining information to the superior layer. The data in transit of the n layer at the source side is therefore the same as the data at the destination side, in the n layer. To identify the data at the time of its passage through a layer, the PDU appellation (Protocol Data Unit) is used.

Layer

PDU

7

Data

6

Data

5

Data

4

Segments

3

Packets

2

Packets

1

Bits

2.2. TCP/IP Model The U.S. Department of Defense (DOD) created the TCP/IP reference model because it wanted a network that could survive any conditions. To illustrate further, imagine a world at war, crisscrossed by different kinds of connections including wires, microwaves, optical fibers, and satellite links. Then imagine that you need information/data (in the form of packets) to flow, regardless of the condition of any particular node or. The DOD wanted their packets to get through any time, under any conditions, from any one point to any other point. It was this very difficult design problem that brought about the creation of the TCP/IP model, and which has since become the standard on which the Internet has grown. The TCP/IP model has 4 layers:

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2.3. Comparison between TCP/IP and OSI model Those two protocols are very similar because of the layer division concept and the use of encapsulation. However, two differences are to be noted: • TCP/IP groups some layers of the OSI model in more general layers • TCP/IP is more than a theoretical conception model, it is the foundation of the Internet OSI Model Layer Application Presentation Session Transport Network Data Link Physical

TCP/IP Model

Designation

Layer

Application Layers

Application

Data Flow Layers

Designation Protocols

Transport Internet Network Access

OSI and TCP/IP models

Network

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3. Layer 1: Media and network devices 3.1. Signals and coding 3.1.1. Comparing analog and digital signals During transmission of data on the network, it transit using physical links, it is useful to observe how those links are represented.

I Analog and digital signal representation

Signal: refers to information that can be transmitted by varying, in some way, a measurable quantity, such as electrical voltage, light, or radio waves. All of these can carry networking data. One type of signal is analog. An analog signal has the following characteristics: • Is wavy • Has a continuously varying voltage versus time graph • Is typical of things in nature • Has been widely used in telecommunications for over 100 years The two important characteristics of a sine wave are its amplitude (A), or its height and depth, and its period (T), or the length of time to complete one cycle. The frequency (f) of the wave can be calculated with the formula: F = 1/T Another type of signal is digital .A digital signal has the following characteristics: • Has discrete, or jumpy, voltage versus time graphs • Is typical of technology, rather than nature Digital signals have fixed amplitude but their pulse width and frequency can be changed. Although this is an approximation, it is a reasonable one, and will be used in all future diagrams.

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3.1.2. Representing a bit in a physical medium The basic building block of information is one binary digit, known as the bit or pulse. One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. This may be as simple as 0 (zero) volts for binary 0, and +5 volts for binary 1, or a more complex encoding. Signal reference ground is an important concept relating to all networking media that use voltages to carry messages. In order to function correctly, a signal reference ground must be close to the digital circuits inside a computer. Engineers have accomplished this by designing ground planes into circuit boards. The computer cabinets are used as the common point of connection for the circuit board ground planes to establish the signal reference ground. Signal reference ground establishes the 0 (zero) volts line in the signal graphics. Examples: • light (1) or darkness (0) with optical signals • Short waves (0) and a longer burst of waves (1) with wireless signals

3.1.3. Factors that can affect a bit Eight effects concerning a bit carried on the media may be observed: Propagation: Time required for data to travel over a network, from its source to its ultimate destination. It should be consistent through the whole network.

Attenuation: causes signals propagating through the medium (cable, optical fiber) to reduce in strength.

Reflection: Energy echo caused by the crossing of impulses across the media. If the echo is too high, it can affect the following impulsions.

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Noise: Undesirable communications channel signals caused by electromagnetic interferences produced by external power supplies, thermal variations Far-end Crosstalk (FEXT): noise induced by an external electromagnetic field, such as another cable. Near-End Crosstalk (NEXT): crosstalk induced internally in the cable by adjacent pairs. Delay - Distortion (Dispersion): Happens when the signal broadens in length and takes more time. If the dispersion is too pronounced, one bit can interfere with the next bit, hence merging the signal. This causes an information loss. Jig: All digital systems are clocked, meaning it is the clock pulses that cause everything to happen. Clock pulses cause the CPU to calculate, the data to be stored in memory, and the NIC to send bits. If the clock on the source host is not synchronized with the destination, which is quite likely, timing jig will occur. Latency: A phenomenon where transmitted information experiences delay.

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Collisions: Occurs when two bits from two different communicating computers are on a shared medium at the same time. In the case of copper media, the voltages of the two binary signals are added, and cause a third voltage level. This voltage variation is not allowed in a binary system, which only understands two voltage levels. The bits are corrupted. As soon as a bit reaches the media, it is vulnerable to all these parameters. These phenomena can disrupt the transmission and therefore should be taken seriously. A communication between two equipments, A and B, may be: • Simple (unidirectional): A is always the transmitter and B the receiver. • Half-Duplex (bi-directional to alternate it): The roles of A and B alternate, the communication changes sense in turns (the principle of walkie-talkies). • Full-Duplex (bi-directional simultaneous): A and B can send out and can receive in the information at the same time (ex. Telephone conversation).

3.2. Copper media 3.2.1. Unshielded Twisted Pair UTP is a four-pair wire medium composed of pairs of wires. Each of the eight individual copper wires in the UTP cable is covered by insulating material. Each pair of wires are also twisted around each other. This type of cable relies solely on the cancellation effect, produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. UTP has an external diameter of approximately 0.43 cm, so its small size can be advantageous during cable installation. It was once considered slower at transmitting data than other types of cable. UTP is now considered the fastest copper-based media. Advantages: • Easy to install • Cheaper than other types of networking media. • Small diameter (important when cabling thought wiring ducts) Disadvantages: • Prone to electrical noise and interference than other types of networking media • Its maximum length (100m)

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UTP Cable

3.2.2. Shielded twisted pair cable Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires. Each pair of wires is wrapped in metallic foil. The four pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150 Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise, both within the cable (pair to pair coupling, or cross talk) and from outside the cable, electromagnetic interference (EMI), and radio frequency interference (RFI). Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). STP affords greater protection from all types of external interference but is more expensive and difficult to install than UTP. A new hybrid of UTP with traditional STP is Screened UTP (ScTP), also known as Foil Twisted Pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or "screen". It is usually 100 or 120 Ohm cable. The metallic shielding materials in STP and ScTP need to be grounded at both ends. If improperly grounded, STP and ScTP become susceptible to major noise problems. Any discontinuities in the entire length of the shielding material will allow the shield to act like an antenna receiving unwanted signals. However, this effect works both ways. Not only does the foil (shield, screen) prevent incoming electromagnetic waves from causing noise on our data wires, but it minimizes the outgoing radiated electromagnetic waves, which could cause noise in other devices. STP and ScTP cable cannot be run as far as other networking media (coaxial cable, optical fiber) without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. The shielding materials also make terminations more difficult and susceptible to poor workmanship. STP and ScTP still have their role, especially in Europe.

STP Cable

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3.2.3. Coaxial Cable Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. A copper conductor located in the center of the cable. Surrounding it is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit, and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket. For LANs, coaxial cable offers several advantages. It can run for longer distances between network nodes than either STP or UTP cable. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known. It has been used for many years for all types of data communication. Other types of communication also utilize coaxial cable. As the thickness or diameter of the cable increases, it becomes more difficult to work with. It is important to remember that cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable. This was because it had historically a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable can be too rigid to install easily in certain situations. The general rule is that the more difficult the network media is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted pair cable. Thicknet cable now used primarily for special purpose installations. Coaxial cable with an outside diameter of only .35 cm (sometimes referred to as thinnet) was used in Ethernet networks in the past. It was especially useful for cable installations that required the cable to make many twists and turns. Since it was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet .The outer copper or metallic braid in coaxial cable comprises half the electrical circuit, so special care must be taken to ensure that it is properly grounded. This is done by ensuring that there is a solid electrical connection at both ends of the cable. Installers frequently fail to do this. As a result, poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmittal on the networking media. It is for this reason that, despite its small diameter, thinnet is no longer commonly used in Ethernet networks.

Thinnet cable

.

Thicknet cable

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It is important to put an emphasis on the grounding. One must assure a solid electric connection to the two extremities of the cable. Otherwise, the staff that works with the cables may be in danger of an electric shock. Furthermore, the electric parasites may cause interferences on the signal.

3.2.4. RJ45 connectors The connector for 10BaseT standard is called RJ-45 (RJ for regular jack). It is conceived in a way so that it can reduce the electrical parasites’ impact, the reflection and the mechanical stability problems. It looks like a telephone connector, except that it has eight wires instead of four. The RJ-45 connectors are inserted in the RJ-45 sockets.

RJ-45 socket and connector

Here is a table that sums up the different types of cables and their respective bandwidths: Technology

Cable Type

Theoretical Bandwidth

Maximal Length

Connector

Cost

10 Base 2 (Thinnet) 10 Base 5 (Thicknet) 10 Base T 100 Base TX 10 Base FL 100 Base FX

Coaxial Coaxial UTP cat 5 UTP cat 5 Optical fiber Optical fiber

10 Mbits/s 100 Mbits/s 10 Mbits/s 100 Mbits/s 10 Mbits/s 100 Mbits/s

200 m 500 m 100 m 100 m 2000 m 400 m

BNC BNC RJ45 RJ45 SC SC

Not expensive Not expensive Cheap Cheap Expensive Expensive

3.3. Optic media 3.3.1. Physical phenomena Electromagnetic spectrum: The radio waves, the infrared light, the visible luminous rays, as well as the gamma and X rays are all types of the electromagnetic energy. This energy is created when a source changes repeatedly in intensity. The broadcasts amplified and decreased create waves, vibrations that move like water waves created by a stone thrown in a pool.

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wavelength

time t

amplitude Electromagnetic waves’ propagation

The distance between the waves is called wavelength and is designated by λ. It depends on the frequency of charge alterations. The bigger is the frequency, the smaller is the distance between the waves' peaks. The electromagnetic waves share similar properties. For example, they are all propagated at the light speed c (299 792 458 m /s) in vacuum. When they traverse other environment, like air or water, their speed v is attenuated. When the electromagnetic waves are grouped with those that have the smallest wavelength to the waves with the longest wavelength, the electromagnetic spectrum is obtained. The wavelengths between 400 nm and 700 nm constitute the visible light. The light having longer wavelength is called the infrared light. The lengths frequently used for the information transportation in the optic fiber are in particular the lengths of the infrared light: 850 nm, 1310 nm and 1550 nm.

normal

Reflection: A ray passing the environment 1 that strikes on the surface of another environment 2 is incident ray. Once it has stroke the surface, the ray reflects. According to the reflection law, the incident ray θ1 equals θ2. Ray reflection : θ1 = θ2

Refraction: Let's suppose that an incidental ray crosses a transparent environment, for example air, and arrives on the surface of another environment, also transparent, let it be water. Instead of reflecting, it is possible that the incident ray crosses the surface that separates the two surroundings, and hence penetrates the water. When the ray crosses the surface, its angle diminishes towards the normal (the vertical line passing through the center). You can observe this case on the figure below, where the angle θ1 is superior to θ3. This phenomenon is called the refraction, and the ray that has traversed is called the refracted ray.

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23 / 64 In order that a ray is reflected without being refracted, it is necessary that its incidence angle is greater than the critical angle.

Inc ide nt

Environment 1

Ra y

ay

Re

Environment 2

c fle e R

R te d

c fle ted Ra y

Refraction of a ray

It is important to know the factor which determines the magnitude of deviation of the refracted ray. This coefficient, named the index of refraction, is the ratio between the speed of light in the vacuum and the speed of the ray that passes through an environment: n = c / v.

It is necessary to remember that the index of refraction depends on the wavelength λ. It means that two rays having two different wavelengths do not behave the same way in a given environment E; that is, one ray may travel faster than the other in a particular environment. It is for this reason mostly that we chose the infrared light and not some other as the bearer signal in the optic fiber. Total internal reflection: In an optic fiber, data is transmitted the similar way as the transmission over a copper media: if there is light, the information is translated as bit 1, otherwise it is bit 0. The objective is evidently that the ray arrives from the source to the destination without being attenuated. Therefore, the ray must be guided in the fiber without refraction; it must be propagated via total internal reflection. Two principal conditions needed to achieve the total internal reflection are: • The index of refraction n0 of the fiber core has to be superior to the index of refraction of the cladding n1, • The incident ray has to enter the fiber within the cone of acceptance. cone of acceptance

Internal total reflection

Refraction

Total internal refraction

On the scheme above notice that the arriving ray is outside of the acceptance cone, with an angle superior to θ0. The first zoomed part, on the right, shows that the ray is refracted. Recall that in this case, the incident angle α is inferior to the critical angle.

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The second ray traverses the cone and its incidence angle β is superior to the critical angle. Hence it is propagated by the total internal reflection all along the fiber. It is a guided ray.

3.3.2. Optical components An optic fiber transmits data in one direction only. Therefore, to obtain a bidirectional communication known as full duplex, an optic cable must contain at least two optic fibers: one for transmission and the other for reception. A cable contains usually from 2 to 48 fibers. The fibers put together in a cable are not sensible to noise and they do not induce any electromagnetic interference, because they do not transmit any electric energy. For that reason there is no need to protect them by a shield, the way the copper wires are protected.

Full duplex with 2 optic fibers

A fiber optic cable is fixed with reinforcement fibers in Kevlar (plastic matter). This makes it more resistant.

Optic cable

Light is guided in the center of the fiber, known as the core. The core is made of silicon dioxide (silica), enriched with other elements. It is surrounded by the optic cladding. The cladding is also made of silica, but its index of refraction is inferior to the core’s index. This is exactly what permits the ray to travel by reflection. The optic cladding is protected by an envelope, frequently made of plastic.

Two optic fibers: monomode and multimode, respectively

The path which a ray traverses is called mode. When an optic fiber guides only one ray, it is called fiber monomode. The fiber that transmits several rays is called multimode fiber. When it transmits several rays, with different paths, the core of the multimode fiber is bigger than the one of the monomode fiber.

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Cladding Core

Ray propagation in a multimode fiber

The light sources that distribute light in the fiber are not the same for the monomode fiber and the multimode fiber. Indeed, a multimode fiber uses the LED (Light Emitting Diode), whereas the monomode fiber uses the laser, which is in general more expensive. The laser emits rays that have longer wavelength than LED. As a result, the maximal length of the multimode fiber is 2000 m. while the maximal length of the monomode fiber is 3000 m. The monomode fibers are more expensive and their use is frequently destined for the WAN links, between different buildings. The multimode fibers are less expensive and more frequently used in the enterprise.

The diameters of the fibers have different sizes. On the diagram underneath one can see the different types of multimode and monomode with different diameters.

The most of LAN devices transmit their data in the electric form. In order to integrate the optic fiber in such a network, the electric signals must be transformed in light impulses. That is why we have created the transmitters that are capable of transforming the electric current into signals of light. As already stated, there are two types of light source: • •

LED – (Light Emitting Diode) produces infrared light having wavelength of 850 nm, and 1310 nm. LASER – (Light amplification by stimulated emission radiation) Infrared light has a

grand intensity and has wavelengths 1310 nm and 1550 nm long. The receiver is connected on the other end of the fiber. It transforms the light impulses back to the electric signals. The fiber ends attached to the connectors are plugged into transceiver and receiver sockets. The monomode fibers use the SC type connectors (Subscriber Connecter) and the multimode fibers use the ST type connectors (Straight Tip). The figure below shows the ST connectors and SC, respectively.

Two (simplex) fiber optic connectors: ST and SC

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A pair of attached connectors in a common box is called a duplex connector. A connector simplex is a simple connector, attached to one fiber. When the optic cables are supposed to connect devices separated by a distance that is greater than the fiber’s maximal length, the fibers are extended by repeaters, light amplification devices. Signals and noise in optic fibers: In spite of the fact that the optic fiber is currently the best transmission media, it is not perfect and the signals it transmits can be attenuated by different factors. The most important factor is the signal attenuation caused by scattering. It can happen when the fiber cable is bent too much or fixed too tightly. Then, the incidence angle becomes inferior to the critical angle causing the ray to refract. The absorption is another shape of attenuation. It happens when a ray encounters impurities on its path. To counter the attenuation problems, we test the optic fiber links with tools that measure the loss of energy and the time taken by a signal to arrive to its destination.

3.4. Wireless technology 3.4.1. Wireless network presentation The wireless LANs have succeeded to unite all the advantages of a traditional wired network, such as Ethernet or Token Ring, without the cabling limitations. The possibility to plan out, establish and change a LAN with an ultimate mobility is one of the main assets looked for by the enterprises of today. Obviously, a WLAN communicates with signals and uses a sort of media to transmit them, like any other network. However, instead of electric pulses and cables it uses radio frequencies 2.4 GHz and 5 GHz. When one speaks of wireless local area network, it could be misleading because these networks are not totally exempt of wires. In fact, these networks are often integrated in the traditional LANs and are considered to be an extension. Thanks to standardization organizations IEEE and Wi-Fi Alliance, the wireless devices are produced under common norms and are hence compatible. This facilitates and reduces the cost of the wireless devices’ production. Low production cost usually means faster evolution and deployment, and we should remind that the first wireless networks offered only a 1 Mbps bandwidth. Many domains have rapidly adopted the wireless technology and so WLANs have spread in different enterprises, production facilities, hospitals, and education institutions. Again, this rapid development and employment was the primary reason to standardize the technology. An alliance of constructors Wireless Ethernet Compatibly Alliance, or WECA, was created in 1991. Their name has changed afterwards, and we know them today as Wireless Fidelity (Wi-Fi) Alliance. IEEE published 802.11 standards for wireless local area networks in June 1997. The wireless networks function basically in two frequencies, depending on the technology used. The 802.11b and 802.11g work on about 2.4 GHz and the 802.11a works on frequencies around GHz

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The most used frequency for the moment is the Industrial Scientific and Medical (ISM) frequency and it corresponds to a bandwidth from 2.4 GHz to 2.4835 GHz. Frequency Maximum bandwidth

802.11b 2,4 GHz 11 Mbps

802.11a 5 GHz 54 Mbps

802.11g 2,4 GHz 54 Mbps

Summary table of the frequencies and their maximum throughput

The radio waves laws: • Bigger bandwidth = Smaller coverage • Greater broadcast power = Bigger coverage, but lesser battery duration • The higher radio frequency = Better transmission bandwidth, but smaller coverage For a WLAN communication at least two devices are needed: the access point (AP) and a wireless network card. Here are the different components one could find in a WLAN: Client interfaces: • PCI: Internal NIC for desktop computers • PCMCIA: Used for laptop/notebook computer, with an integrated antenna. • LM: Identical to PCMCIA, with the same bus, but without an antenna • Mini PCI: Used for laptop/notebook computers as an internal devices; it needs a supplementary antenna. Access Points (AP): The Cisco Aironet models 1100 and 1200 are the most utilized for the client access. Wireless Bridges (BR): Principally used to link two wire based networks. Antennas : • Directional • Omni directional Native wireless devices: • PDA • Notebooks • IP telephones • Printers

3.4.2. Authentication and security With the arrival of the wireless networks, the security issue arose. The wave propagation poses an obvious problem. Clearly, one cannot control the propagation of a wave like the electric signals in a cable. At the beginning the waves were spread in all directions and it was easy for everyone to capture them. As a solution, the directional antennas were invented. Furthermore, the filters came about so that windows could be isolated. However these safety measures were still too costly for many enterprises. So, several software solutions have been conceived since. One software solution is based on the use of a Service Set to Identify (SSID). One can connect to a wireless network using this identification means. Nevertheless this solution is not very secure, since any capturer of frames can take hold of the frame carrying the SSID.

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Another similar software security solution is based on the MAC address as the identification. Still, it is prone to the same security issues as SSID and since the MAC addresses are static the solution is not convenient for large environment enterprises. The third solution consists of using an encryption key to encode transfers. This key is necessary to connect to an AP and maintain that connection. It is called the Wired Equivalent Privacy (WEP) key. The encoding is done with 64 or 128 bits. The norm that elaborates the dynamic key system is Wi-Fi Protected Access (WPA). This resolution has a higher security level. It is clear that the combination of these different solutions can increase the security of a network. However the security remains lower compared to the electric cable networks. Right now, the 802.11i security specifications, or the WPA 2, are being developed. They should eventually raise the security to an acceptable level for large scale networks.

3.4.3. Wireless network implementation Let’s consider two computers equipped with wireless network cards. We have two possibilities two connect them: • •

Either by connecting them directly one to another (exactly like one would do with a crossover RJ45 cable) Or by using an access point (like with a hub and a pair of straight through cables).

In the case of Wi-Fi connection PC to PC, instead of a different wire layout, we have to configure the NIC itself. Indeed, a Wi-Fi card is not configured the same way when it serves to establish an Ad-Hoc connection (direct station interconnection) or an Infrastructure connection (with an AP). Ad-Hoc mode has an advantage of contributing the mobility. For example, two stations may be put together in a common space, for a reunion, where each machine can be reached by another. So the two machines can be easily connected and the both stay mobile. Infrastructure mode, allows an interconnection to a wired network, the Internet for example. Note: Contrary to the Ethernet, it is possible to connect more station between each other in the AdHoc mode. However, it is not rare that in these topologies one looses every so often the carrier signal, which makes this service inherently instable. For the quality reasons of connection, it is discouraged to connect more than four machines in Ad-Hoc mode. Ad-Hoc mode: Hosts directly connected via their wireless interface (the equivalent to a crossover wire)

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Infrastructure: Connection made via an AP (the equivalent to an Ethernet hub)

3.5. Layer 1 components and devices 3.5.1. Repeaters Repeaters regenerate and resynchronize signals and so they enable cables to extend farther to reach longer distances. They only deal with packets at the bit level.

Repeater Symbol

3.5.2. Hubs Hubs or Multiport repeaters combine connectivity with the amplifying and retiming properties of repeaters. It is typical to see four, eight, 12, and up to 24, ports on multiport repeaters. This allows many devices to be cheaply and easily interconnected. Multiport repeaters are often called hubs, instead of repeaters, when referring to the devices that serve as the center of a star topology network. Hubs are very common internetworking devices. Since the typical unmanaged hub only requires power and plugged in RJ-45 jacks, they are great for setting up a network quickly.

Hub Symbol

Like the repeaters on which they are based, they only deal with bits, and are Layer 1 devices.

3.5.3. Transceiver / Receiver A transceiver is a combination of transmitter and receiver. In networking applications, this means that they convert one form of signal to another form.

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3.6. Basic Topologies used in Networking Topology described the way in which the network equipment is connected between them. We will separate physical topologies, describing the way in which the equipment is connected by media, and logical topologies, describing the way in which the equipment communicates.

3.6.1. Bus topology Physical perspective: All the nodes are directly connected to a link Logical perspective: All the hosts see all the signals from all others devices

Bus topology

3.6.2. Ring topology Physical perspective: Nodes are chained in a closed ring. Logical perspective: Each host must pass the information to its directly connected stations

Ring topology

An alternative of this topology is the double ring where each host is connected to 2 rings. These two rings do not communicate between them. The second ring is used as redundant link in the event of breakdown in the first.

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3.6.3. Star topology Physical perspective: This topology has a central node wit all links radiating from Logical topology: The flow of all information would go through one device

Star topology

3.6.4. Extended star topology This topology repeats a star topology, except that links to the center node is also the center of another star.

Extended star topology

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3.6.5. Tree topology Physical perspective: The trunk is a wire that has several layers of branches Logical perspective: The flow of information is hierarchical

Tree topology

3.6.6. Full mesh topology Physical perspective: Each node is connected to every other nodes Logical perspective: Depends greatly on the devices used

Complete topology

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4. Layer 2: Ethernet Technologies 4.1. Introduction to LAN technologies The local area networks (LAN) are limited in size (a few thousand meters maximum). Their characteristic is a high bandwidth and a feeble error percentage caused by attenuation. They connect different components, terminals and computers.

4.2. Introduction to Ethernet Ethernet is the basic LAN technology and is currently the most used. The principle is that all machines are connected to the same line of communication. The IEEE institute has adapted the standard Ethernet into IEEE 802.3 norm. These two technologies are very similar (the only difference is one field in the frame).

4.2.1. Ethernet and OSI model The Ethernet technology functions in the physical and the data link (only the MAC part) layers. While two stations communicate through a shared media, data goes through one collision domain. Every station can access another station. They speak to each other via this media. Collisions are then created, because the media is used in a concurrent manner.

4.2.2. Norms and Specifications Each Ethernet technology has its specific name. This is due to IEEE nomenclature rules and makes it easy to identify the technology’s characteristics and the way it is utilized. The following pattern explains how an Ethernet technology is named: Speed in Mbps – signal type – cable type (ex: 100 base TX) • • •

There are two types of signal: Baseband (digital transmission) and Broadband (analog carrier). Cable type: Simply designates the cable type, for example Unshielded Twisted Pair (UTP), or the optic fiber. To indicate that the technology supports Full Duplex, an X is put after the cable type letter (except the 10 BASE T which does support Full Duplex).

L’IEEE has defined norms for different Ethernet technologies: Norm 802.3 802.3u 802.3z 802.3ab 802.3ae

Designation Ethernet Fast Ethernet Gigabit Ethernet Gigabit Ethernet 10 Gigabit Ethernet

Bandwidth 10 Mbps 100 Mbps 1000 Mbps 1000 Mbps 10 000 Mbps

Media used Coaxial / UTP / Optic fiber UTP / Optic Fiber Optic Fiber UTP Optic Fiber

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4.2.3. Ethernet IEEE 802.3 Frames

Ethernet Frame

IEEE 802.3 Frame

• • • • • •

•

•

Preamble: composed of alternating 1s and 0s; it announces whether the frame is of type Ethernet or 802.3. Start frame delimiter: IEEE 802.3: the separator bye is finished with two bits set to 1. It serves to synchronize the frame reception of all stations Destination address: can be in Unicast, multicast or broadcast. Source address: always in Unicast. Type (Ethernet): specifies the protocol type of the superior layer. Length (802.3): specifies the number of bytes in the Data field. o It’s on this part that 802.3 and Ethernet frame differs: The value of this field allow specifying the frame type: 802.3 or either. o The frame is from 802.3 type if the hexadecimal value of the field is strictly inferior of 0x600; The frame is from Ethernet type if the hexadecimal value is equal to 0x600 Data: Superior layers information. o Ethernet: once layer 1 and layer 2 processing has been done, data are transmitted at the upper layer protocol as indicated in the “type” field. We can use bit padding if there is not enough data to fill the first 64 bytes of the frame. o IEEE 802.3: once layer 1 and layer 2 processing has bee done, data are transmitted at the upper layer protocol indicated in the data field of the frame. We can also use bit padding. Frame Check Sequence (FCS): Contains a Cyclic Redundancy Code (CRC) that allows the receiving machine to verify the frame integrity.

4.3. Ethernet operation 4.3.1. Relation to the OSI model Ethernet uses two sub layers of the data link layer: MAC and LLC. The lower data link layer is called MAC for Medium Access Control (to not to be confound with the MAC address). It serves for “bridging” between the physical layer and the superior layers. In other terms it exists so that the Ethernet protocol does not have to worry about the physical properties of the underlying network.

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The Logical Link Control, or LLC, serves to communicate the information to the layer 3 and is independent of the physical equipment. The Data Link layer uses a principle of non deterministic access to media: CSMA/CD (Carrier Sense Multiple Access with Collision Detection). All the hosts share the same media. If one of them wants to transmit data, it verifies whether the media is not already spreading some other host’s data. If the media is free, the host starts emitting its information. If it happens that two hosts “talk” at the same time, a collision produces. The first station that detects that collision starts sending JAM data. That way all the hosts will know that there is a collision. All the bits are destroyed then and every host calculates a random value to which it counts before it is ready to (re)transmit data.

4.3.2. Possible errors During a data transmission, many factors may generate a data corruption. The objective is to correctly detect the errors and determine which frames have to be retransmitted. Collisions: In a shared environment, the most common corruption is the one provoked by a collision. When two, or more, hosts emit a signal at the same moment, the current augments in tension (voltage). That electric signal does not mean anything in terms of information, but is recognized as a sign of collision. These collisions are produced in a Half-Duplex environment only. In Full-Duplex environment every twisted pair relies just two hosts and leads information in one direction only. There are three different types of collisions: • Local collision • Remote collision • Late collision The local collision happens, as said earlier, when two signals overlap. Either they cancel out, or they double the tension. The over-voltage is sensed by the hosts. The remote collision manifests usually in damaged frames that have lost some data and have irregular incorrect FCS. This phenomenon is usually caused by a local collision that happens on the other side of a distant bridge. The bridge does not let through the over-voltage signal, but a part of a traveling frame. When a NIC transmits more than 64 bytes of a frame, and the frame collides, the NIC does not perceive this collision automatically. This is called the late collision. The higher level protocols would recognize that a problem would have occurred and instruct the NIC to retransmit the frame in question. Long frames: The maximal length of the data field is 1500 bytes. If a frame arrives with the data field longer than the maximal length, than it is an illegal frame. Short frames: Same as for the long frames, the short frames are about an illegal data field length. If a frame has data field smaller than 64 bytes, then it is a short frame. However, its FCS has a good checksum.

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Other types of errors: Other errors can occur because of the bad media quality of the media, or because of external interferences. • Incorrect FCS: the Frame Check Sequence carries bad checksum. • The length field does not accord with the real length of the Data field. • Incorrect field length: for example, the preamble field is shorter than 7 bytes. Once a mistake of this type is detected, the superior layer (on the side of the receiving station) asks for a retransmission of the damaged frame.

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5. Layer 2: Ethernet Switches 5.1. Collision domain A collision domain is a network part which is in a shared environment. In this domain hosts have concurrent access to a resource. In this zone, collisions are made. All layer 1 devices extend the collision domain.

5.2. Segmentation The collision domains pose problems proportional to their size. Indeed, the bigger a collision domain the smaller gets the hosts’ bandwidth and the number of errors increases. To diminish these negative effects, one collision domain should be segmented in smaller collision domains. The segmentation lays on the principle of sending the data to a smaller part of the network, hence sparing the other parts of the network. A frame sent over the wire has a lesser probability to create a collision, and if it does, it spares all other hosts outside its domain. To create a collision domain, we need layer 2 equipment. At this level, the equipment takes decisions based on the layer 2 MAC addresses.

5.2.1. Segmentation with bridges Bridges segment a network in two collision domains by sending information only to one of its two sides. A bridge “knows” where to send information, because it keeps a MAC address table where every host is associated to one of its ports. When it receives a frame it looks up its destination MAC address and filters sends it out of the port that is associated with destination machine’s MAC address. If it does not have the MAC address in its table, it sends out the frame on the both collision domains.

5.2.2. Segmentation with switches The switches function absolutely like bridges. Switches learn MAC addresses and put them into MAC tables, they divide a collision domain into more collision domains, all the same. The difference is that switches have more interfaces (for this reason switches are also called multiport bridges), they are faster and are more intelligent. One notion should to be retained: it is the microsegmentation. A switch uses this technique to make virtual circuits. So every time a host sends data to another host, the switch make a unique circuit for this communication, by internally (and temporarily) connecting the input and the output port.

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5.2.3. Spanning Tree

In a network that uses numerous switches, the redundant links are often created so that a reliable connectivity exists. The objective of a redundant link is that if a switch brakes down, it does not bring the whole network down. The problem that arises without the spanning tree protocol is that loops would be created. A frame could loop forever, provoking congestion. Spanning Tree Protocol (STP) prevents frames from looping indefinitely by blocking some switch ports. It designates a root switch that has a role of a chief with all ports open. All other switches must accept, transfer, or block data. The election of the root switch is elaborated by HELLO messages, where each message carries the switches priority. A switch delegates its ports to be: Root ports : ports that receive frames with the lowest cost (the best path to the root switch), Designated ports : ports that forward root switch frames to one collision domain, Blocking ports: ports that are closed for transfers, but that remain operation in case of a topology change. Switch messages are transferred in Bridge Protocol Data Units (BPDUs). A switch port may be in one of the four following states: • Blocking • Listening • Learning • Forwarding Listening and Learning states are transitory and last 15 seconds each. When in Listening state, switch awaits for BPDUs that eventually bring a message coming from a new root switch. Learning state serves to renew the MAC address table. The STP permits one to create a network that is breakdown tolerant and loop free.

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6. Layer 3: IP protocol 6.1. Routed Protocols Protocol: Formal collection of rules and conventions that govern the information exchange between network entities. In the level of the network layer, information transmitted passes by the use of protocols so that the data is routed toward its destination in a correct manner, and as quickly as possible. This set of rules and conventions permits to maintain a certain organization on a network.

6.1.1. Connection and Connectionless oriented protocols A connection oriented protocol establishes the connection first and then sends a packet. It is like a telephone communication. The telephone rings first and once the other party has answered, the connection is established. Connectionless protocol is also known as the best effort protocol. IP belongs to this category. It means that an IP packet is sent without connecting the destination host first, or making sure how the packet will travel to its destination. Instead, the packet is released with the destination address and a possibility to arrive to its destination via many different paths. The communication where packets are forwarded over logical circuits is also called packet commutation.

6.1.2. Routed and routable These two terms are similar, but they have slightly different meanings. • Routed protocol: Defines the packets’ format and furnishes the addressing information. o Routable protocol: The protocol distinguishes the network from the host part. o Non Routable protocol: The inverse of the routable protocol. Here is a non exhaustive list of some routed protocols: Name IP IPX Appletalk CLNP NetBEUI SNA

Is it routable? Yes Yes Yes Yes No No

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6.2. Internet Protocol (IP) 6.2.1. IP packet A segment, also referred to as the Layer 4 Protocol Data Unit (PDU) is encapsulated with a Layer 3 header. This new PDU of Layer 3 is known as a packet. We can examine on a figure below an IP packet header:

IP packet example

Fields Version Length Time to Live (TTL) Checksum Source address Destination address Data Padding

Descriptions Indicates the IP version used (4 bits). Specifies the length of the packet A counter that decreases gradually. Once it attains zero, the packet is deleted. It prevents packets from circulating around the network infinitely Assures the integrity of the IP header (16 bits). The IP address of the sender (32 bits). The IP address of the receiver(32 bits) PDU of the fourth layer Zeros are added to the header so that is a multiple of 32 bits

6.2.2. IP addresses An IP address is a 32 bits address written in the form of 4 decimal numbers separated by dots. There are two parts of an IP address: • A prefix part that designates the network. It is also called the network number. • The suffix part on the right designates the host’s number.

An IP address example

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6.2.3. IP address classes The organization in charge of the IP address attribution is InterNIC (Internet Network Information center). The IP addresses are distributed in different classes, depending on their first byte value: Class A B C D E

Fixed bits in the 1st byte 0 10 110 1110 1111

1st byte Range 1 to 126 128 to 191 192 to 223 224 to 239 240 to 255

Mask 255.255.255.0 255.255.0.0 255.0.0.0 None None

The two parts of an IP address, network and host, are delimited with use of a binary mask. The mask is binary added to an IP address. The bits set to 1 in a mask represent the network prefix in the IP address. For example: IP address = 172.16.0.15 IP mask = 255.255.0.0

in binary: in binary:

From here, we obtain the network number : 0000 Or in the decimal form : 172.16.0.0

1011 0000. 0001 0000. 0000 0000. 0000 1111 1111 1111. 1111 1111. 0000 0000. 0000 0000 1011 0000. 0001 0000. 0000. 0000. 0000

So as we have seen, the suffix part is reserved for hosts that belong to a particular network. So we use the bits in the suffix part to obtain a range of hosts’ IP addresses. However, there are two special suffix bit combinations that are reserved and must not be assigned as host IP addresses. The IP suffix that is empty (all bits are set to zeros) is reserved as the network address. Here are some examples of network addresses, where the prefix part is colored: 10.0.0.0, 172.16.0.0, 192.168.1.0. When the suffix has all bits set to 1s, it represents the broadcast address. 10.255.255.255 - in binary: 0000 1010. 1111 1111. 1111 1111. 1111 1111 When an IP packet has the broadcast address used as the destination address, every host on that network receives the packet. Every device connected to the Internet has to have an IP address. To obtain an IP or a range of IP addresses one should address INTERNIC. Nevertheless, it is frequent that Enterprises have their own private networks which computers do not have a need for a public Internet address. This is why INTERNIC have reserved a range of private addresses in each IP class, so that local networks can communicate internally, without running the risk of creating address conflicts on the global network: • • •

10.0.0.1 to 10.255.255.254 172.16.0.1 to 172.31.255.254 192.168.0.1 to 192.168.255.254

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6.2.4. IPv4 and IPv6 IPv4 protocol is the actual standard used today. Its creators presumed that the number of all IP address would be enough for the IPv4 networks. Nevertheless during the Internet expansion and the number of devices using the Internet addresses had grown and network engineers realized that the limit of available addresses was going to be reached and exceeded. To counter this problem, the Internet Engineering Task Force (IETF) decided in 1992 to “modernize” the IP. Different solutions have been invented so that the IP address lack is reduced to a certain level. The ultimate solution however is a new version of the IP addressing. IPv6 employs 128 bits rather than 32 bits actually utilized by IPv4. The IPv6 addresses are written in hexadecimal form, whereas the IPv4 addresses are often represented in four byte dotted decimal form. IPv6 provides 2128, or 3.4*1038 IP addresses. Clearly, the new version should largely cover the future needs in terms of IP addresses, for quite some time. IP v4 address example: Value: 34.208.123.12 Number of bytes used: 4 IP v6 address example: Value: 21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A Simplified form: 21DA:D3::2F3B:2AA:FF:FE28:9C5A Number of bytes used: 16 You can notice that the new addresses are more complicated to be worked with, or to be memorized. That is why a simplification method has been invented: Any bloc beginning with zeros is replaced by “::”.

6.3. Attribution of IP Addresses 6.3.1. Acquiring Methods Hosts may be attributed IP addresses either dynamically or statically: • Statically: Each device is configured manually with a unique IP address. • Dynamic: IP addresses are assigned by a special network protocol: o RARP: This protocol associates MAC addresses to IP addresses. This how terminals without hard disks can obtain their IP addresses. o BOOTP: This protocol allows a machine to recuperate its IP address during the initialization phase. The sender broadcasts a query (destination is everyone: 255.255.255.255). The server receives the query and sends back to the host its IP address. o DHCP: It replaced BOOTP. The principal objective is basically the same, but the mechanism is more sophisticated. With DHCP it is possible to obtain a complete IP configuration (IP address with a mask, and a gateway IP address).

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The host broadcasts a DISCOVER query expecting to find a Server.

When the server receives the query, it looks up into its IP address range. If it has a free address, it sends back an OFFER message.

The client knows now that there is a DHCP server on the network ready to assign it an address. So, it demands with REQUEST query an IP address and negotiates the lease time.

Finally, the server concludes the dialog with an ACK message containing the host’s IP address, and the lease time accorded.

6.3.2. Address resolution ARP (Address Resolution Protocol) has an important place in the TCP/IP protocol stack. It allows a host to find out the MAC address of another host on the same network. MAC is the physical unique address made of 48 bits, and is attributed to every network device. In order to find out a MAC of a host, we need to know its IP address. Here is an example of an ARP query: host A needs the MAC address of host B. It broadcasts out an ARP query to the network asking “I, host A, would like to know what is the MAC address of the host B”. All the hosts receive this query and only B responds to A giving it out its IP address. It is as simple as that. One should know however that names “host A” and “host B” are replaced by their respective IP addresses in the ARP query. Once a MAC address is discovered, it is matched with its IP pair in the ARP table. RARP (Reverse Address Resolution Protocol) as its name indicates does a reverse address resolution. It matches a known MAC address to an IP address. This is useful for hosts that do not have their IP addresses set manually.

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6.3.3. Internet Control Message Protocol (ICMP) As already stated, the IP packets are sent in the “best effort” manner. That means that the routing protocols do not establish connections for the packet transfers. Furthermore, none of the layer 3 IP routing protocols provide help that could describe an exact route taken by a packet, or an error message that would show why a packet never reached its destination. ICMP is a complementary protocol to IP. Hence all TCP/IP networks must have it implemented. Its principal and only objective is to inform. On one hand it serves as a means of error notification among routers. For example, a router may inform other network nodes that congestion occurred. On the other hand it is a helpful tool for administrators. The most common example of ICMP use would be the Ping command: one host sends an echo reply request to another host. If the other host replies, the administrator knows that the connection in the network layer is good.

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7. Layer 3: Subnetting 7.1. Subnetting utility Not long after the Internet started to expand rapidly, everyone understood that the IP networks were experiencing an exponential decrease of the free IP addresses. That is why many solutions have been created to counter this problem. One of the first was subnetting, also known as the sub-addressing. Subnetting divides an existing network into more, smaller networks. Not only that this technique allows us to use address spaces more wisely, but the division into more broadcast domains results in a bandwidth augmentation and a congestion reduction.

7.2. Calculation method In order to divide a subnet, one borrows bits from the suffix (host) part of the IP address and uses them to create subnets. This means that a network can have a certain number of subnets, each of which has its own hosts.

7.2.1. Classic method It is done in six steps: • Borrow the sufficient number of bits • Calculate the new subnet mask • Identify the different IP address ranges • Identify the IP address ranges that cannot be used • Identify the broadcast address • Determine the IP address ranges that can be assigned to hosts Borrow the sufficient number of bits: First, we should find out the number of bits that should be borrowed from the host part. We either determine the maximal number of hosts, or the maximal number of subnets need. One should keep in mind that when we create an IP address range for hosts, we must not forget to reserve the first and the last IP address. These two addresses, all zeros and all ones in the suffix part, use as the network IP number and network broadcast address, respectively. Examples: • An administrator needs 3 subnets. If we count the first and the last reserved address, we need to add 2 more addresses. So, we obtain 5 addresses that have to fit in the required range. The first address is all zeros. We are left with the number 4 which is written: 100 in binary. Therefore, we borrow 3 bits from the host part. •

An administrator wants 43 hosts per subnetwork. In binary 43 is: 10 1011. The sub-network address 00 0000, and the broadcast address 11 1111 fit in the range. So we need 6 bits only for 43 hosts per subnet.

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Calculate the new subnet mask: Now that we know how many bits we are going to borrow, we calculate the new mask for all sub-networks. To do this, we use the old network mask; we convert it in its binary form and add the borrowed bits to it. Example : If this were our old mask 255.0.0.0, this would be its binary form 1111 1111.0000 0000.0000 0000.0000 0000. Suppose that we needed to borrow 5 bits for subnet creation: Simply set those five bits in the host part: 1111 1111.1111 1000.0000 0000.0000 0000. Identify the different IP address ranges: With the subnet mask we calculate different IP ranges. It suffices to write down every possible binary combination for the subnet part. Identify the IP address ranges that cannot be used Example: 192.168.1.0 with mask 255.255.255.0 Subnet mask 255.255.255.192 (3 bits borrowed) Possible subnets: 192.168.1.0 192.168.1.64 192.168.1.128 192.168.1.192 The first subnet address 192.168.1.0 is reserved because it is equal to the network address. The subnet 192.168.1.192 is reserved as well, because both of the bits are set to ones: 192.168.1. 1100 0000, Which it is needed for global network broadcasts: 192.168.1. 1111 1111. Identify the broadcast addresses: A broadcast address is the inversed subnet mask that we add in binary to the subnet address. Example: Network: 10.0.0.0, Mask 255.0.0.0 Subnet Mask: 255.240.0.0 (1111 1111.1111 0000.0000 0000.0000 0000) Broadcast address for subnet 10.16.0.0 is obtained: Broadcast mask 0000 0000. 0000 1111. 1111 1111. 1111 1111 Subnet in binary + 0000 1010. 0001 0000. 0000 0000. 0000 0000 Subnet broadcast 0000 1010. 0001 1111. 1111 1111.1111 11111 Î 10.31.255.255.255 Determine the IP address ranges that can be assigned to hosts: Finally all we have to do is to assign the IP addresses from the valid subnets to every host.

7.2.2. Magic number method This method can be done with two formulas:

256 = Subnet Mask + Network Range 256 = Subnet Range * Number of Subnets

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These two formulas allow us to calculate rapidly: • A Subnet mask • Number of hosts per subnet • Number of subnets If you wanted to calculate the number of hosts in five subnets with the following mask : 255.255.255.0. The nearest power of 2 (2n) superior to 5 is 8. We apply the formula 256 = Network range * Number of Subnets Æ 256 = Network range * 8 Network range = 256/8 = 32 We subtract the two reserved address (the original network address and the broadcast address): 32 – 2 = 30 So there are 30 usable IP addresses per subnetwork. This number is simply added to every last subnet’s IP address. This gives us the following mask: 255.255.255.224 (256-32 = 224) Thus we obtain the next configuration: First address of the subnet: 192.168.0.1 Last address of the subnet: 192.68.0.31 First address of the subnet: 192.168.0.31 Last address of the subnet: 192.68.0.63 First address of the subnet: 192.168.0.64 Last address of the subnet: 192.68.0.95 And so on.

Using the magic number formulas one can subnet a network easily and it takes less time than the first method. However, one should understand the binary math method, before using the magic number formulas.

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8. Layer 3: Introduction to routing 8.1. Fundamental principles First of all, we define the difference between the routing (network layer packet switching) and data link layer switching. The two terms designate two completely different mechanisms. Switching decisions are based on data link information and routing decisions on network layer information. A switch can be seen as a link between all collision domains in a logical network or a subnetwork. A switch can broadcast frames in one broadcast domain, but not farther. To send information outside of the broadcast domain, one needs a layer three device. In order to make a commutation decision, a router examines the destination address of the network layer packet. First, routers maintain their routing tables. That means that if a network topology changes, this change is communicated to all the routers. So every router would learn the good topology, and would update its routing table. Second, a router has for objective to switch arriving packets to the good interface. It keeps its layer 3 commutation information in a routing table, just like a switch keeps that has a MAC address table. The difference is that a switch associates MAC address to its interfaces, whereas a router associates interfaces to the IP addresses. It calculates the best route using its internal algorithm and one or more metrics associated to the known paths.

8.2. Broadcast Domain Simply put, a broadcast domain is a logical domain of a network, where any host can attain another host without having to use a network layer device. Two hosts that are in the same broadcast domain do not need a router to communicate.

8.3. Network Layer devices: Routers Router connects two or more networks relying on layer 3 addresses.

Router Symbol

Cisco Router type 2600

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A router possesses network interface cards (NIC) which connect it to different network. Every router’s NIC has a network layer address. We can see this concept in the scheme below, where every the router connects the two networks.

Suppose we wanted to send data from A network to B network: • The router receives a 2nd layer frame and deletes its header and trailer. • It examines the 3rd layer address. • It does a logical AND between the IP address destination and the logical mask in order to find the destination network. • It consults its routing table to determine the interface on which it will send out the packet. This is why every interface has to be on a different network, or else, the router cannot determine which interface to use for the packet forward.

8.4. Path determination The path selection methods allow Layer 3 devices, routers, to establish routes for different networks. Routing services utilize the network topology information in order to evaluate the routes. This process makes use of different parameters, known as routes’ metrics, for example: • Number of routers between the source and the destination host. • Load • Delay • Bandwidth • Reliability • Link speed

8.5. Autonomous systems, IGP and EGP An autonomous system is a network, or a collection of networks that are under the same administration control. There are two routing protocol families: Interior Gateway Protocol) (IGP) and Exterior Gateway Protocols (EGP). The IGP protocols route data inside an autonomous system: • RIP and RIPv2 • IGRP • EIGRP • OSPF • IS-IS

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EGP routes packets between the autonomous systems. An example of an Exterior Gateway Protocol is Border Gateway Protocol (BGP). There are different routing protocols that belong to different families and each has its own characteristics. Here is a summary table with descriptions of a few important routing protocols: Protocol

Type (IGP or EGP)

Algorithms

Metrics

Updates

RIP

IGP

Distance Vector

15 hops maximum

30 sec

RIP v2

IGP

Distance Vector

15 hops maximum

30 sec

IGRP

IGP

Distance Vector

Delay ,load, bandwidth, reliability

90 seconds

EIGRP

IGP

Hybrid

Delay ,load, bandwidth, reliability

Triggered Updates

OSPF

IGP

Link State

IS-IS

IGP

Link State

BGP

EGP

Cost

Triggered Updates Triggered Updates

Remarks 15 hops maximum Includes masks in routing information exchanges Chooses the best route according to its metrics. Cisco proprietary Faster than IGRP, Cisco proprietary Used for large scale networks Supports Multiple routed protocols. Used by grand companies and Internet Service Providers

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9. Layer 4: Transport layer 9.1. Introduction The Transport layer has for the objective to maintain a conversation between two endpoints on a network. The main functions are connection-oriented or connectionless data transfers, error recovery, reliability, flow control and segmentation of higher level layers data

9.2. TCP and UDP La pile de protocoles TCP/IP comprend 2 protocoles de couche 4 : TCP et UDP TCP est un protocole orienté connexion, c'est-à-dire qu’il associe au transport des informations la notion de qualité en offrant les services suivants : • Fiabilité • Division des messages sortants en segments • Ré assemblage des messages au niveau du destinataire • Ré envoi de toute donnée non reçue Segments : PDU de couche 4 UDP est lui un protocole non orienté connexion, c'est-à-dire qu’il n’offre pas de fonction de contrôle du bon acheminement : • Aucune vérification logicielle de la livraison des messages • Pas de réassemblage des messages entrants • Pas d‘accusé de réception • Aucun contrôle de flux Cependant, UDP offre l’avantage de nécessiter moins de bande passante que TCP. Il peut donc être intéressant d’utiliser ce protocole pour l’envoi de messages ne nécessitant pas de contrôle de qualité.

Application

TCP

Transport UDP Internet Network Access

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9.2.1. Port numbers As we saw already, every application that uses Transport layer should have its logical port assigned. If it does not, it gets one randomly. Here are some port numbers associated their applications, services, protocols: Protocol

nº de port

Description

FTP data FTP SSH Telnet SMTP Domain HTTP POP3 NNTP IMAP2 NEWS HTTPS

20 21 22 23 25 53 80 110 119 143 144 443

File Transfer (transfer) File Transfer (control) Secure SHell Telnet Simple Mail Transfer Domain Name Server World Wide Web HTTP Post Office Protocol - Version 3 Network News Transfer Protocol Interactive Mail Access Protocol v2 News Secure HTTP (by SSL) Ports number

Here is the port number division: Port range 0 - 1023 1023 - 65535

Utilization Reserved for public applications Assigned to companies for commercial application and use by the operating system for attribution of source ports.

9.2.2. TCP Segment structure The datagram that contains data from higher layers, encapsulated by TCP, is called a TCP segment. You can see on the figure below a TCP header organization:

TCP datagram structure

Fields Source port Destination port Sequence number Number of acknowledgement Control checksum Data

Descriptions Number of the source port Number of the destination port Number used to ensure correct sequencing of incoming data Next TCP byte awaited Control checksum calculated from header and data field Data of the upper layer protocol

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9.2.3. UDP datagram structure Since UDP is a connectionless oriented protocol, it has a header of shorter size than TCP header:

UDP datagram structure

The UDP protocol is conceived for applications that don’t have to gather segments sequence number. Friability is leaved in charge of application layer protocols.

9.3. TCP connection method The connection establishment consists of three steps: • A unique path is determined between the endpoints • Data is transmitted in a sequential order and when received they are sorted chronologically. • The connection is closed once the communication is done.

9.3.1. Three step connection initialization sequence

The sender dispatches a segment with the initial sequence number set to x. A bit in the segment header is set, indicating the connection demand.

The other host receives the segment, and responds with the acknowledgment x + 1, plus it includes its own sequence y.

The sender receives x+1 (so it knows the connection is ok) and sends y+1 to confirm that everything has functioned

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9.3.2. Positive Acknowledgement Retransmission Positive Acknowledgement Retransmission (PAR) consists in sending a packet, starting a counter and waiting for a confirmation that the next packet may be sent. If the counter reaches to its expiration before the confirmation arrives, the data is retransmitted and the counter is reset. This simplistic technique uses up too much bandwidth, and is not efficient enough. That is why windowing has been created.

9.3.3. Windowing The windowing mechanism lays in the principle of the amount of data sent at a time. The windowing field in a segment header specifies the number of unacknowledged bytes allowed to be sent. At first the machines let the number of transfers grow. Once the errors occur, the endpoint hosts reduce the window size. This procedure assures the optimal amount of segments transferred before the acknowledgment is transmitted. Computer sender

Computer receiver

Send 1 Send 2 Send 3 Receive 1 Receive 3 Send AR 2

Receive AR2 Send 2 Send 3 Send 4 Receive 2 Receive 3 Receive 4 Send AR 5

Receive AR5

Transmission without packet loss

Transmission with packet loss: here the packet 2 will be resent (the third too, even if it has been received).

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10. Layer 5: Session layer As we saw previously, a session is a whole of transactions between two network unit or more. An analogy to understand the session layer is a communication between several individuals. If you want the conversation to unfold correctly, it is imperative to make rules in order that interlocutors don’t interrupt themselves. This notion of “dialogue control” is an essential point of the session layer. The role of the layer session is to open, run and close the session between the applications. This means that this is it which takes into account: • The throwing of the sessions • The resynchronization of the dialogue • The interruption of the sessions So it coordinates the applications that communicate through hosts. A communication between computers suppose a lot of shorts conversations (parcels commutation), and more of that, others communications to make sure of the efficacy of the communication. Those conversations necessitate that hosts play by turns roles of client (service applicant) and server (service tradesman).

Communication between hosts Management of sessions

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10.1. Dialog control The session layer decides if the conversation will be simultaneous or alternate bidirectional type. This decision arises from the control of dialogue. • If the simultaneous bidirectional communication is permitted: o The management of the communication is assured by others layers of computers in communication. • If these collisions into the session layer are intolerable, the control of dialogue disposes of another option : the alternate bidirectional communication o This type of communication is making possible by the utilization of a data counter at the session layer level who permit to each host to transmit in turn.

10.2. Dialogue synchronization This step is the most important; it permits to communicants hosts to make a pause to save the communication in progress and resynchronize the dialogue for example. For that a « point of control » is used, send by one of the interlocutors to another to save the conversation, verify the hour of the last portion of dialogue executed. This process is called “the synchronization of the dialogue”. As the human language; it is important in a discussion to show to your interlocutor the beginning of a conversation (“hello” in a telephonic conversation) and signified that you’re going to stop the conversation. The two principal controls are: • well-ordered throwing • end of the communication

10.3. Dialog division The division of the dialogue includes the throwing, the end and the well-ordered management of the communication. Our diagram represents a small synchronization. To the level of the check point, the session layer of host A sends a synchronization message to host B, and the two hosts execute the sequence which follows: • to save the given files • to save network parameters • to save synchronization parameters • to note the point of end of the conversation.

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Checkpoints are similar to the way the text processing software makes a second pause to make the automatic document’s saving on an independent computer. However these checkpoints are used in order to separate the different parties of a session, previously called dialog. We have just seen how the hosts organize themselves around the communication; we now will see how the data are generated so that hosts understand themselves.

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11. Layer 6: Presentation layer In order to make two machines understand each other, they have to speak in the same language: this is Presentation layer’s job.

11.1. Functions and norms One of the roles of the presentation layer is to present the data in a format that the receiving device can understand. The presentation layer plays a role of interpreter between the units who have to communicate by the intermediate of a network. The layer 6, the presentation layer, assures three principal functions: • Data formatting (presentation) • Data encryption • Data crushing After receiving data from the application layer, the presentation layer executes some or all of these functions before forward them to the session layer. To the level of the reception station, the presentation layer receive data from the session layer and execute the necessary functions before forward them to the application layer. Norms of presentation layer describes also the diagrams presentation. The three principals graphical format are: • BMP (BitMaP) is an old format still largely spread, it is now replaced by the JPEG, which provided files with a better rate compression/size • JPEG (Joint Photographic Experts Group) – graphic Format more used for compression of images. • PNG (Portable Network Graphics) is a relatively new graphic format on the Internet with texture compression option. The others norms of presentation layer concern sounds and videos presentation. The following norms belong to this category: • MPEG (Motion Picture Experts Group) –Video compression and encoding format for CD or other numerical stocking support. • MP3 (MPEG To bush-hammer 3) - Format of music compression very used for the moment. It uses the study of the human ear thus algorithm of compression. • Divx (MPEG 4) format of compression created starting from MPEG 4 format developed by Microsoft and allowing a compression much better than the MPEG 1 or 2 (example: to make hold a film on CD instead of a DVD).

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• • •

Representation of data Comprehension by the receiver Formatting of data Syntax check

Presentation layer norms establish standards of files format to make hosts able to understand the different information.

11.2. Data encryption Encryption permits to protect the confidentiality of information during the transmission. Encryption is the use of an algorithm that encodes the message so that only the host to which one addresses may understand it. An encryption key is used to encrypt data at the source and to decipher them at destination. An algorithm is used to make these data incomprehensible for anyone who doesn’t have the key. An encryption key is used to encrypt data at the source and to decipher them at destination. An algorithm is used to make these data incomprehensible for anyone who doesn’t have the key.

11.3. Data compression The presentation layer also assures files compression. Compression apply algorithm (complex mathematical formula) to reduce files size. The algorithm searches some repetitive bits sequences into files and replaces them by a “token”. The token is a short bit sequences substituted to the full sequence. Example: Replacing "Cisco laboratory" by "Lab" We can also use a dictionary to replace some words too long: they are consisted of words or sequences generally returning as well as sequences of replacement, so as to reduce the files considerably.

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12. Layer 7: Application Layer 12.1. Introduction This layer plays the role of interaction with the software applications. He provides services to the communication of the application module with the insurance of: Identification and verification the availability of the communication partner wanted. • Synchronization of the applications that should cooperate. • Understanding on error correction procedure. • Control of the data integrity. In OSI model, the layer application is closest from the terminal system. It defines if the necessary resources for communication between systems are available. With out this application layer, there wouldn’t be any support for network communications. It doesn’t give any services to other layers from the OSI model, but it collaborates with the enforcement process out of the OSI model. This enforcement process can be: spreadsheet, word processor, ATM software, etc. Indeed, the layer application creates a direct interface with the rest of the OSI model true network applications (Web navigator, e-mail, FTP protocol, Telnet, etc…) or an indirect interface true selfgoverning applications (word processor, presentation software or spreadsheet), with network redirection software.

12.2. DNS 12.2.1. Presentation of the protocol Each station posses is own IP address. However, users don’t want to work with IP address, but name of station or address more understandable. For example: http://www.labo-cisco.com To answer for it, the DNS protocol can associate usual names to numeric address. Domain name resolution: Connection between IP address and associate domain name.

12.2.2. Host name, « domain name system » From the beginning of TCP/IP, since the network are not much extend; there are few computers connected to the same network, the network administrator created files called manual conversion table (files generally named hosts or hosts.txt), these, with ASCII characters, makes, on one line, association between IP address and the associate name, called host.

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The major inconvenient of this system is the updating of the table in each computer in case of modification, new computer or name modification. So, with the explosion of network extend and there interconnection, a more central system of name management has been put on. This system is named Domain Name System. The system is made up by hierarchy name so it can guarantee the only one name in an arborescence structure. It is called domain name, there are two components. The first is the correspondent name of the organization or enterprise, the second is its domain classification (.fr, .com,…) each domain machine is called host. The hostname must be unique in the considerate domain (the domain Web service has generally the name WWW). The overall is done by: the host name, a dote, the domain name called FQDN address (Fully Qualified Domain) this address permit do find a unique machine. So, www.cisco.com is a FQDN address. Machine called domain name server, can establish correspondent between domain name and IP address for the machine network. So each domain posses a domain name server connected to a higher domain name server. So, the system name is built on a distributed architecture. It means that there are no organisms in charge of all the domain names. But there is an organism the InterNIC for the domain name in .com,.net,.org and .edu, for example. The domain system is clear for the user but the following points must not be forgotten: Each computer must be configuring with a machine address that can be transform in IP address. This machine is called Domain Name Server. The IP address of secondary Domain Name Server can also be introduce so it can relieve the first one in case of malfunction.

12.2.3. Internet domain codes The domain classification is sometime called TLD (Top Level Domain,); it’s generally a geographical repartition. However, some names exist, first created for the United States, classifying domain by activities, for example: • .com for commercial enterprises ((it doesn’t mean much now it is international). • .edu for education organism • .gov for government organism • .net for the network organism • .org for the non-profits enterprises • .biz corresponds to companies in general • .info reserved with websites of information

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12.3. FTP and TFTP 12.3.1. FTP Ftp is a reliable and oriented -connection protocol which employs TCP to transfer files between systems which support this protocol. The principal goal of FTP is to transfer files from a computer to the other while copying and/or by moving files from the servers to clients, and clients to servers. FTP is assigned on port 21 by default. When files are copied from a server, FTP establishes a control connection between clients and servers. Then the second connection is established, which is a link between the computers by which the data are transferred. The data transfer can be done in ASCII mode or binary mode. These modes determine the encoding used for the data file, which in OSI model is a task of presentation layer, as we saw previously. After the file transfer is finished, the data connection is close automatically. The control connection is closed when the user disconnects and closes the session

12.3.2. TFTP TFTP is a connectionless protocol which employs UDP. TFTP (Trivial FTP) is employed on a router to transfer configuration files and IOS images and also to transfer files between systems which support TFTP. TFTP is conceived to be light and simple to use. Nevertheless TFTP can read or to write files on a server but it cannot list the directories and does not support users authentication. He is useful in some LANs because he functions more quickly than the FTP.

12.4. HTTP The HTTP protocol (Hyper Text Transfer Protocol) works with the World Wide Web, which is the greatest part of Internet. One of the principal reasons of this extraordinary growth is the facility with which it gives access to information. A Web browser is a client – server application, which means that it requires from client and server a specific component installed on the 2 machines in order to function. A Web browser presents data in multi-media format, i.e. contents reacting to the actions of the user. The contents can be text, graphs, sound, or video. The web pages are written using HTML (Hypertext Markup Language): a web browser get the web page in HTML format and interpret it in order to print the page with a presentation better than a plain text one. To determine IP address of a remote HTTP server, the web browser uses DNS protocol to find IP address starting from the URL. Then the transport layer, the network layer, the data link layer, and the physical layer work together to start a session with HTTP server. The data which are transferred from HTTP server contain localization of the Web page on the server. The server answers the request by sending to web browser the HTML code and the various objects multi-media which decorates the page (sound, video, picture) and which is indicated in the instructions of HTML web page. The web browser gathers all the files to create visual Web page, and finishes the session with the server. If another page is required, the whole process starts again.

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12.5. SMTP Email servers communicate between them by employing SMTP (Simple Mail Transfer Protocol) to send and receive mail. SMTP send messages email in the ASCII format by using TCP. One often uses it as a protocol of sending of mail, rarely as a protocol of recovery of email, because it is insecure and especially does not offer any authentication.

12.6. SNMP Simple Network Management Protocol (SNMP) is a protocol of application layer which facilitates exchange of management information between network devices. SNMP makes it possible to network administrators to control the state of the network, to detect and solve problems of network, and to preview development of the network, if ever this one arrives at her maximum. SNMP employs UDP protocol as a protocol of transport layer. • A network controlled by SNMP includes three following key components: System of network management (NMS/Network Management System): NMS execute applications which monitor and control the managed devices. One or more NMS must exist on any managed network. • Managed devices: The managed devices are nodes of network which contain an SNMP agent and which are on a managed network. The managed devices gather and store information of management and make this information available to NMS using SNMP devices. The managed devices, sometimes called elements of network, can be routers, servers, switch, bridges, concentrators, host-computers, or printers. • Agents: Agents are modules of network software – management which reside in managed devices. An agent has the local knowledge of management information and translated this information into SNMP format.

12.7. Telnet protocol 12.7.1. Presentation of the Telnet protocol The Telnet protocol is a standard Internet protocol that can interface terminal and application through the net. This protocol gives basics rules to relieve the client (system with a screen and a keyboard) to a command interpreter (server). The Telnet protocol is based on a TCP connection to send data in ASCII format coded in 8 bits in which Telnet control sequence come in between. It provides a system oriented on communication, half-duplex, coded in 8 bits so easy to get down to work. The Telnet protocol is based on three fundamental concepts: • Paradigm of the Network virtual terminal (NVT). • The principle of negotiates options. • The rules of negotiations.

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This protocol is a basic protocol, on which is bases other protocol from the TCO/IP suite (FTP, SMTP, POP3, .etc.). The Telnet specification don’t mention authentication because Telnet is separate from the other application it uses (the FTP protocol definite an authentication sequence over Telnet.) Besides, the Telnet protocol transfers unsafe data, that is to say these data circulate clear on the network (not encoded). When Telnet protocol is used to connect a distant host, it is implant as server; the protocol is assign on port 23. Except the options and the associate negotiation rules, the Telnet protocol’s specifications are basic. Data transmission through Telnet uniquely consists in byte transmission over the TCP flux (Telnet protocol precise that data must be by default, if any options don’t say so, to be stamp before being sent. This means that by default, data must be sent line by line.) When the 255 octets are transmitting, the next one must be interpreted as a command. The 255 octets are named IAC (Interpret as Command). Commands are describe further more on the document.

12.7.2. Notion of virtual terminal At the beginning of the Internet, the network (ARPANET) was composed of very different machine configuration (keyboards, characters, resolution, and display line). Moreover, the terminal sessions had their way of controlling incoming/outgoing data flux. So, instead of creating adaptor for each terminal, a standard interface call NVT (Network Virtual Terminal) was put down. It gives a standard communication base composed of: • ASCII Characters 7 bits which is added the extend ASCII code. • Three control characters. • Five optional control characters. • A set of basic control signals. The Telnet protocol consists in creating a terminal abstraction, so any host (client or server) can communicate with any other hot with out knowing about his characteristics.