Assignment #5

ECE 487 (Data Communications Networks)

Your Last Name: ____________________ Your Fist Name: _____________________

Your Student ID: ________________________

Due: Wednesday, Feb. 12, 2020, 4:00 PM, in the assignment box at 2nd Floor – Pedway

between ICE and ETLC

1. Consider that a communication system uses an automatic repeat request (ARQ) protocol.

The distance between the sender and receiver is 6,000,000 meters, and the propagation

speed is 3×108 meters per second. We only consider propagation delay and ignore all

other delay (and therefore, the transmission time of a frame can be infinitely small). The

time-out value of a timer is 1 second. At the sender side, there are an infinite number of

packets in Layer 3 to be sent. The size of a data frame is 2000 bits.

(a) Assume stop-and-wait ARQ is used and no frame is corrupted. What is the

throughput of the system? Here throughput means for a long duration (for example, a

day), on average how many bits of information per second can be delivered

successfully from the sender to the receiver. (2 points)

(b) Assume stop-and-wait ARQ is used. The channel from the sender to the receiver is

noiseless. The channel from the receiver to the sender is noisy in such a manner that,

for each particular data frame, its first ACK is always corrupted, while its second

ACK is always successfully received by the sender. Then what is the throughput of

the system? (2 points)

(c) Assume Selective Repeat ARQ with m=3 bits for the sequence number is used and no

frame is corrupted. What is the throughput of the system? (2 points)

2. Fifty stations on a pure ALOHA network share a 1-Mbps (106 bit per second) channel. If

frames are 1000 bits long, find the system throughput (in unit: frames/second) if each station is

sending 10 frames per second. Repeat this question for slotted ALOHA. (4 points)

3. (a) In a CSMA/CD network with a data rate of 5 Mbps, the minimum frame size is found to be

512 bits for the correct operation of the collision detection process. What should be the minimum

frame size if we increase the data rate to 30 Mbps? (b) In a CSMA/CD network with a data rate

of 5 Mbps, the maximum distance between any station pair is found to be 500 meters for the

correct operation of the collision detection process. What should be the maximum distance if we

increase the data rate to 50 Mbps? (4 points)

4. Consider a standard Ethernet (which implements CSMA/CD and 1-persistent method) with

only three stations in a line: the distance between Station A and Station B and between Station B

and Station C are both 150 meters. The propagation speed is the speed of light (3×108 meters per

second). The data rate is 10 Mbps. At time instant t1=0 microsecond, Station A has a frame with

size 2,000 bits to be sent. At time instant t2=0.2 microsecond, Station B has a frame with size

3,000 bits to be sent. At time instant t3=0.3 microsecond, Station C has a frame with size 4,000

bits to be sent. Please determine the number of bits each station can send during its first

transmission attempt. Note that 1 microsecond = 10-6 second. (6 points)

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