Answers to the exercise #3 on Computer Networks - Christian
a user's card and password to be verified, the account balance (which is main- tained at the bank) to be queried, and an account withdrawal to be made. (money ...
Answers to the exercise #3 on Computer Networks Christian Rinderknecht 11 April 2008
Question. Design a protocol to be used between an automatic teller machine (ATM) and a bank’s centralised computer. Your protocol should allow a user’s card and password to be verified, the account balance (which is maintained at the bank) to be queried, and an account withdrawal to be made (money is given to the customer). 1. Specify your protocol by listing the messages exchanged and the action taken by the ATM or the bank’s centralised computer on transmission or receipt of each message: From ATM to Bank Message name and arguments
Meaning/Action
From Bank to ATM Message name and arguments
Meaning/Action
1
2. Sketch the operation of your protocol, using a diagram, for the cases of a simple withdrawal (a) with no errors, (b) with one error. Answer. One example is as follows. Messages from ATM to server: Message name HELO
PASSWD BALANCE WITHDRAWAL BYE
Purpose Let server know that there is a card in the ATM, then transmits user ID to server User enters PIN, which is sent to server User requests balance User asks to withdraw money User all done
Messages from server to ATM (display): Message name Purpose PASSWD Ask user for PIN (password) OK Last requested operation (PASSWD, WITHDRAWAL) OK Last requested operation ERR (PASSWD, WITHDRAWAL) resulted in error AMOUNT Sent in response to BALANCE request User done, display welcome BYE screen at ATM A correct withdrawal looks like this: HELO PASSWD BALANCE WITHDRAWAL ATM dispenses money BYE
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