Communication Networks (2nd Edition) Chapter 6 Solutions Solutions to Chapter 6 1. Why do LANs tend to use broadcast networks? Why not use networks consisting of multiplexers and switches? Solution: The computers in a LAN are separated by a short distance (typically 100m) so high speed and reliable communication is possible using a shared broadcast medium. The cost of the medium is negligible and the overall cost is dominated by the cost of the network interface cards in each computer. In addition, the LAN users usually belong to the same group where all users are generally trusted, so broadcast does not pose much security danger. The original reason for avoiding a multiplexer and switch approach to LANs is that a centralized, expensive “box” is required. The availability of Application Specific Integrated Circuits (ASICs) has reduced the cost of switching boxes and made switch-based LANs feasible, and in some environments the dominant approach. 2. Explain the typical characteristics of a LAN in terms of network type, bit rate, geographic extent, delay- bandwidth product, addressing, and cost. For each characteristic, can you find a LAN that deviates from the typical? Which of the above characteristics is most basic to a LAN? Solution: Type: broadcast network in a bus, ring or star topology. Bit rate: from 1Mbps to 100 Mbps. Delay-bandwidth product: small. Addressing: flat. Geographical extent: up to 1000 m (small). Cost: low. 10 Gbps Ethernet deviates from the above characteristics in that it is non-broadcast, of much higher bit rate, large delay-bandwidth product, larger geographic extent and high cost. The most basic characteristic of a LAN is small geographical extent. 3. Compare the two-channel approach (Figure 6.4) with the single-channel approach (Figure 6.5) in terms of the types of MAC protocols they can support. Solution: Figure 6.4 consists of two unidirectional channels, one outbound from a central node to secondary nodes, and another inbound from the secondaries to the central node. The bandwidth that is available in each direction is fixed. This arrangement can support polling protocols as well as contention protocols in the inbound direction. Figure 6.5 provides a single channel that is shared by all stations. This arrangement also supports polling and contention-type MAC protocols. The bandwidth available in each direction can be controlled dynamically. 4. Suppose that the ALOHA protocol is used to share a 56 kbps satellite channel. Suppose that frames are 1000 bits long. Find the maximum throughput of the system in frames/second. Solution: Maximum throughput for ALOHA = 0.184 Maximum throughput in frames/sec = (56000 bits/sec) x (1 frame/1000 bits) x 0.184 = 10.304 The maximum throughput is approximately 10 frames/sec. Leon-Garcia/Widjaja 1 每一个抄通信网答案的学生都是头疼的天使-La o f u z iCommunication Networks (2nd Edition) Chapter 6 Solutions 5. Let G be the total rate at which frames are transmitted in a slotted ALOHA system. What proportion of slots goes empty in this system? What proportion of slots go empty when the system is operating at its maximum throughput? Can observations about channel activity be used to determine when stations should transmit? Solution: Proportion of empty slots = P0 transmission = G0/0!eG= eGMaximum throughput = 0.368; Gmt = 1 Proportion of empty slots at maximum throughput = e1= 0.368 Any attempt to decrease the proportion of empty slots below e-1 is counterproductive as this action will push the throughput below its maximum value. 6. Modify the state transition diagram of Stop-and-Wait ARQ to handle the behavior of a station that implements the ALOHA protocol. Solution: transmit timeout 7. Suppose that each station in an ALOHA system transmits its frames using spread spectrum transmission. Assume that the spreading sequences for the different stations have been selected so that they have low cross- correlations. What happens when transmissions occur at the same time? What limits the capacity of this system? Solution: The transmitted signals from different stations occupy the whole frequency band simultaneously. To each receiver, the aggregate of the other signals appears like noise after the receiver applies its spreading sequence in the demodulation. Consequently, the receiver can reliably extract its desired signal as long as the signal to noise ratio remains sufficiently high. The system capacity is limited by the maximum amount of signal interference allowed at the receiver. Increasing the number of signals in the system increases the interference power level. As the number of signals is increased, the SNR decreases and the bit error rate in each receiver will increase. 8. Consider four stations that are all attached to two different bus cables. The stations exchange fixed-size frames of length 1 second. Time is divided into slots of 1 second. When a station h