# 通信网络代写｜Data Communication & Networks Assignment #4

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## V. Assignment

### Problem 1:

a. Describe the difference between router and switch
b. List three broad classes of Medium Access (MAC) Protocols
c. How big the MAC address space?

### Problem 2:

a. Consider the following data to be sent. Using two-dimensional
even bit parity, calculate the row and column parity bits.

11100101
10101001
00101101

b. Assume an 8-bit data to be sent via a link as: 11001011, assume G
as 1001. Using Cyclic redundancy check calculate the R.

c. In CSMA/CD, after the 4th collision what is the probability that a node
choosesK=3?

Problem 3: Considering an 11-bit data sequence 10111010100 and generator
polynomial P(X) = X4 + X3 + X2 + 1.

a. Calculate CRC and indicate bit sequence transmitted

b. Assume that the 9th data bit (from left) has been changed in the
transmission system. Show how the detecting algorithm will identify the
error.

Problem 4: Consider a CSMA/CD network running at 100 Gbps over a 15
kilometer long cable (without repeaters). Assume that the signal speed in the
cable is 550,000 kilometers/sec. What is the minimum frame size?

Problem 5: Consider a network that consists of two nodes A and B that are
located on opposite ends of a 2 kilometer cable, and both nodes have a single
1,500 bits frame (inclusive of all headers and preambles) to send to each other.

Assume that both nodes attempt to start transmission at time t = 0 and that there
are four repeaters between A and B, each inserting a 35 bit delay. We will assume
for the purpose of this exercise that the transmission rate is 150 Mbps and that
CSMA/CD is used for medium sharing with backoff intervals of multiples of 512
bits. Also assume that after the first collision, node A draws K = 0 and node B
draws K = 1 as part of their exponential back-off step. You can ignore the use of a
Jam signal and a 96 bit time delay.

a. Assuming a propagation speed of 2*108 meters/second, what is the
(one way) propagation delay between A and B in seconds? (Note: the
propagation delay should include the repeater delays)

b. At what time will the packet coming from node A be completely
delivered to node B? (Note: the time computed should be expressed in
seconds)

c. If node A is the only node that has a packet to send and the repeaters
are replaced by switches, at what time will the packet coming from
node A be completely delivered at node B? (Note: you may assume
that each switch has a twenty bit processing delay in addition to a store
and forward delay, and the time computed should be expressed in
seconds)

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