Media Gateway Due 22nd of December Benoit Bégué / 20066067 KAIST – Electrical Engineering

Table of contents Media Gateway ................................................................................................................... 1 Table of contents................................................................................................................. 1 Introduction: what is a Media Gateway? ............................................................................ 2 Operation principles............................................................................................................ 2 Related standard.................................................................................................................. 3 Signaling proptocols ....................................................................................................... 3 Voice codecs ................................................................................................................... 3 Terminations on a Gateway ............................................................................................ 3 Performance Analysis ......................................................................................................... 4 Problems in performance measurements ............................................................................ 5 Experiment environment..................................................................................................... 6 Tests and results.................................................................................................................. 7 Presentation of conducted tests....................................................................................... 7 Tests 1 and 2 ................................................................................................................... 7 Test 3............................................................................................................................... 8 Delay impact ............................................................................................................... 8 The theory ............................................................................................................... 8 The results............................................................................................................... 9 Jitter impact............................................................................................................... 10 The theory ............................................................................................................. 10 The results............................................................................................................. 10 Packet Loss impact ................................................................................................... 11 The theory ............................................................................................................. 11 The results............................................................................................................. 12 Others: quick market analysis........................................................................................... 14 Conclusion ........................................................................................................................ 14

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Introduction: what is a Media Gateway? A Media Gateway is a translation unit between disparate telecommunications networks such as: • PSTN • Next Generation Networks • 2G, 2.5G and 3G radio access networks • PBX Media Gateways enable multimedia communications across Next Generation Networks over multiple transport protocols such as ATM and IP. With the transition that is operating rights now to VoIP fashion of telecommunications, the need for Media Gateways is increasing more and more. We will look at their performance.

Operation principles The media gateways work in pair with another device called the Gatekeeper or Media Gateway Controller (MGC). This device deals with the signaling while the Media Gateway (MG) deals with the data and/or the voice.

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Related standard Signaling proptocols Signaling protocol between an MGC and gateways : • ITU-T Rec. H.323. This is employed where all network elements (NEs) have software intelligence. • SIP (Session Initiation Protocol, Ref. 2) is used when the end devices have software intelligence and the network itself is without such intelligence. • MGCP (media gateway control protocol) is another gateway control protocol. • MEGACO (ITU-T Rec. H.248, Ref. 13) is a gateway control protocol applicable when end devices are without software intelligence and the network has software intelligence.

Voice codecs The terminals may have different capabilities nin terms of voice codecs: • PCM • ADPCM • LPC • AMR • CECP • G.7xx That’s why we need a negotiation about terminal compatibility. H.245 is used to negotiate service capabilities between terminals and gateway controllers.

Terminations on a Gateway Terminations are logical entities on a GW: • Analog telephone line • Digital telephone (64 Kbps) • T1 trunk line (1.544 Mbps) • E1 (2.048 Mbps) • RTP (VoIP)

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Performance Analysis For this project, I used the following reference paper:

A Study of IP Network Impairments Impact on Media Gateway Performance Sherry Wang, Luis Nieto, Edward Zielinski Next Generation Networks, Bell-Labs, Lucent Technologies

The question asked and answered by people in this paper can be formulated as follows: Can voice quality be guaranteed in a connectionless and best effort data network? Delay can have two effects on speech quality. First, it increases the subjective effect of any echo impairment. Second, even when echo is controlled, delay above 150ms in each direction can interfere with the dynamics of a conversation and degree of interactions. Jitter, the variation of packets’ inter-arrival time, results primarily from queuing and processing delay of IP packets along a transmission path. A practical test has observed more than average 100ms jitter [5]. Usually, a media gateway has a jitter buffer to smooth out jitter. When severe jitter happens, which causes an accumulation of packets in a buffer, a number of packets will be dropped to re-synchronize the transmission. Packet Loss directly contributes to the intelligibility of a word or a sentence. Most of digital signal processing (DSP) units in a gateway can handle 3% bit error rate. However, “bursty” packet loss that is closely related to a data network has not been evaluated thoroughly.

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Problems in performance measurements Voice quality is defined as the qualitative and quantitative measurement of the sound and conversation quality. The problem is that this measurement is not a really easy thing to measure as a physical quantity such as volume, because this is about perception and, therefore, it is subjective and may chance for each different people. Two methods are mostly used for this measurement :

Mean Opinion Score (MOS) This method is to use several people and ask them to rate the quality of a phone conversation between 1 and 5. You put together the results of at least 20 different people. • Subjective opinion score, • Expensive in terms of time and human efforts • Perceptual Speech Quality Measurement (PSQM) PSQM compares an original voice signal to a received copy of this signal. It measures the dissimilarity of the received signal to the original signal from human auditory perceptions. A lower PSQM score implies higher similarity between two signals. The score of zero corresponds to equality of two signals to the human ear. • defined by ITU-T Recommendation P.861 • Repeatable • Objective • Reasonably inexpensive

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Experiment environment

Major system components are: • • • • •

performance measurement tools for initiating calls and measuring voice quality of the calls a telephone switch for conveying calls a softswitch for signaling among the switch and media gateways and set up a communication path two media gateways for packetizing and depacketizing voice frames, performing error corrections and smoothing jitter an IP network emulator for creating IP network impairments.

The G.711 voice codec was used through out this study. The choice of the number of voice frames per IP packet is a trade off between a fast delivery and network bandwidth efficiency. In this study, 10ms voice frame per RTP packet was used as recommended by the manufacture. An adaptive jitter buffer was used to accommodate jitter smoothing and reduce delay at the same time.

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Tests and results Presentation of conducted tests Three kinds of tests were conducted: 1st test: Analog communication This test is a circuit network analog voice quality baseline test It involves only a telephone switch and the measurement tools, which will not introduce any system impairments. The results can be used as the best-case reference. 2nd: Analog and IP communication (using switches and gateways) in ideal IP network condition This test is an IP network voice quality baseline test. Here we add gateways to the above analog system to quantify the delay and voice quality degradations caused by gateways’ processing without network impairments stress. 3rd: 2 with IP network impaired by Delay/Jitter/Packet Loss The test measures the voice quality to see if the gateways are adequate to correct or conceal IP network errors.

Tests 1 and 2

These tests reveal that the use of gateways, even in perfect IP networking conditions, affect the signal quality in the voice communication. But this degradation is really reasonable and should not be noticed to much by the users.

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Test 3 Delay impact

The theory The end-to-end delay or the speech latency can be summarized as a function of • gateway performance g • IP network hop behavior h

Where: • Dpkt is packet-processing delay • Th is transmission delay • Qh is queuing delay • Ph is propagation delay The hop behavior is replaced by a fixed delay D(i) generated by the IP network emulator. D(i) is a constant in ith test, i = 1, 2...

Where: • N is the number of frames per packet –constant in this study• f is the frame size

In the experiment, C was measured at approximately 26.5ms; that is about 6ms in switch and about 10ms in each gateway.

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The results

The IP network delay has a linear impact on the speech latency.

The IP network delay, when it is limited to relatively low value (