Problem 1
Design and describe an application-level protocol to be used between an automatic teller machine and a bank's centralized computer. Your protocol should allow a user's card and password to be verified, the account balance (which is maintained at the centralized computer) to be queried, and anprotocol entities should be able to handle the all-too-common case in which there is not enough money in the account to cover the withdrawal. Specify your protocol by listing the messages exchanged and the action taken by the automatic teller machine or the bank's centralized computer on transmission and receipt of messages. Sketch the operation of your protocol for the case of a simple withdrawal with no errors, using a diagram similar to that in Figure \(1.2\). Explicitly state the assumptions made by your protocol about the underlying end-to-end transport service.
Problem 1
What is the difference between a host and an end system? List several different types of end systems. Is a Web server an end system?
Problem 3
Consider an application that transmits data at a steady rate (for example, the sender generates an \(N\)-bit unit of data every \(k\) time units, where \(k\) is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Answer the following questions, briefly justifying your answer: a. Would a packet-switched network or a circuit-switched network be more appropriate for this application? Why? b. Suppose that a packet-switched network is used and the only traffic in this network comes from such applications as described above. Furthermore, assume that the sum of the application data rates is less than the capacities of each and every link. Is some form of congestion control needed? Why?
Problem 3
Why are standards important for protocols?
Problem 4
List six access technologies. Classify each one as home access, enterprise access, or wide-area wireless access.
Problem 6
This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate \(R\) bps. Suppose that the two hosts are separated by \(m\) meters, and suppose the propagation speed along the link is \(s\) meters/sec. Host A is to send a packet of size \(L\) bits to Host B. a. Express the propagation delay, \(d_{\text {prop }}\), in terms of \(m\) and \(s\). b. Determine the transmission time of the packet, \(d_{\text {trans }}\), in terms of \(L\) and \(R\). c. Ignoring processing and queuing delays, obtain an expression for the endto- end delay. d. Suppose Host A begins to transmit the packet at time \(t=0\). At time \(t=d_{\text {trans }}\). where is the last bit of the packet? e. Suppose \(d_{\text {prop }}\) is greater than \(d_{\text {trans }} .\) At time \(t=d_{\text {trans }}\), where is the first bit of the packet? f. Suppose \(d_{\text {prop }}\) is less than \(d_{\text {trans }} .\) At time \(t=d_{\text {trans }}\), where is the first bit of the packet? g. Suppose \(s=2.5 \cdot 10^{8}, L=120\) bits, and \(R=56 \mathrm{kbps}\). Find the distance \(m\) so that \(d_{\text {prop }}\) equals \(d_{\text {trans }}^{-}\)
Problem 7
What is the transmission rate of Ethernet LANs?
Problem 7
In this problem, we consider sending real-time voice from Host A to Host B over a packet-switched network (VoIP). Host A converts analog voice to a digital \(64 \mathrm{kbps}\) bit stream on the fly. Host A then groups the bits into 56 -byte packets. There is one link between Hosts A and B; its transmission rate is 2 Mbps and its propagation delay is \(10 \mathrm{msec}\). As soon as Host A gathers a packet, it sends it to Host B. As soon as Host B receives an entire packet, it converts the packet's bits to an analog signal. How much time elapses from the time a bit is created (from the original analog signal at Host A) until the bit is decoded (as part of the analog signal at Host B)?
Problem 8
What are some of the physical media that Ethernet can run over?
Problem 8
Suppose users share a 3 Mbps link. Also suppose each user requires \(150 \mathrm{kbps}\) when transmitting, but each user transmits only 10 percent of the time. (See the discussion of packet switching versus circuit switching in Section 1.3.) a. When circuit switching is used, how many users can be supported? b. For the remainder of this problem, suppose packet switching is used. Find the probability that a given user is transmitting. c. Suppose there are 120 users. Find the probability that at any given time, exactly \(n\) users are transmitting simultaneously. (Hint: Use the binomial distribution.) d. Find the probability that there are 21 or more users transmitting simultaneously.
