# 计算机通信代写｜CSDS 325/425 Computer Networks I

## Instructions

Please typewrite your answers and submit a pdf file to Canvas. You should complete this assignment individually.

## Questions

1. (6pts) Consider two nodes, A and B, that use the slotted ALOHA protocol to contend for a channel. Suppose node A has more data to transmit than node B, and node A’s retransmision probability pA is greater than node B’s retransmission probability pB.Suppose that R = 1 Gbps and Rc is 300 Mbps and Rs is 200 Mbps.

(a) (2pts) Provide a formula for node A’s average throughput. What is the total efficiency of the protocol with these two nodes.

(b) (2pts) If pA = 2pB, is node A’s average throughput twice as large as that of node B? Why or why not? If not, how can you choose pA and pB to make that happen?

(c) (2pts) In general, suppose there are N nodes, among which node A has retransmission probability 2p and all other nodes have retransmission probability p. Provide expressions to compute the average throughputs of node A and of any other node.

1. (2pts) Suppose nodes A and B are on the same 10 Mbps broadcast channel, and the propagation delay between the two nodes is 325 bit times. Suppose CSMA/CD and Ethernet packets are used for this broadcast channel. Suppose node A begins transmitting a frame and, before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Supose at time t=0, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of 512 + 64 bit times. So A would finish transmitting the frame at time t = 512 + 64 bit times. Thus, the answer is no, if B’s signal reaches A before bit time t = 512 + 64 bits. In the worst case, when does B’s signal reach A?

What is the end-end delay of a packet from when it first begins transmission on link 1,until is it received in full by the server at the end of link 3. Assume store-and forward packet transmission.

1. (2pts) Consider the 5-bit genreator, G=10011, and suppose that D has the value 1010101010.What is the value of R?
2. (4pts) In this problem, we explore some of the properties of CRC. For the generator G (=1001), answer the following questions

(a) (2pts) Why can it detect any single bit error in data D?

(b) (2pts) Can the above G detect any odd number of bit errors? Why?

1. (6pts) Consider the network shown below. The IP and MAC addresses are shown for hosts A, B, C and D, as well as for the router’s interfaces. Consider an IP datagram being sent from node A to node C.

(a) (2pts) What are the source and destination MAC addresses on the frame at point(4)?

(b) (2pts) What are the source and destination MAC addresses on the frame at point(2)?

(c) (2pts) What are the source and destination IP addresses of the datagram at point(6)? E-mail: itcsdx@outlook.com  微信:itcsdx 