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Communication Networks Problem Set 3: Ethernet Collisions, Multiple Access, Token Rings, Assignments of Organizational Communication

University of Illinois - Urbana-ChampaignOrganizational Communication

Problem set 3 for the cs/ece 438: communication networks course, focusing on ethernet collisions, multiple access, token ring networks, and bridges. Students are required to solve problems related to ethernet timing, collision probability, token ring network efficiency, and bridge port selection. Problems involve calculating transmission times, collision probabilities, and network efficiencies.

Typology: Assignments

Pre 2010

Uploaded on 03/16/2009

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CS/ECE 438: Communication Networks Fall 2006
Problem Set 3 Due Wednesday, October 4
All problems carry equal weight. Please justify your answers and show all
your work.
1. Ethernet Timing
This problem is about the Ethernet/IEEE 802.11 access protocol. To be
definite, suppose that if a host detects a transmission while it is trans-
mitting a frame, then: (i) if the host has already transmitted the 64-bit
preamble, the host stops transmitting the frame and sends a 32-bit jam-
ming sequence; (ii) else the host finishes transmitting the 64-bit preamble
and then sends a 32-bit jamming sequence. Suppose the packets are 512
bits long, which is the mimimum length allowed. Hosts A and B are the
only active hosts on a 10 Mbps Ethernet and the propagation time be-
tween them is 20 µs, or 200 bit durations. Suppose A begins transmitting
a frame at time t= 0, and just before the beginning of the frame reaches
B, B begins sending a frame, and then almost immediately B detects a
collision.
(a) Does A finish transmitting the frame before it detects that there was
a collision? Explain.
(b) What time does A finish sending the jamming signal? What time
does B finish sending the jamming signal?
(c) What time does A hear an idle channel again? What time does B
first hear an idle channel again?
(d) Suppose each host next decides to retransmit immediately after hear-
ing the channel idle. After the resulting (second) collision: When
does A next hear the channel idle? When does B next hear the
channel idle?
(e) Suppose after the second collision, A decides to wait 512 bit dura-
tions to retransmit (if it hears silence after that long) and B decides
to retransmit immediately after hearin ga silent channel. Is the trans-
mission of host B successful?
NOTE: For simplicity, please ignore the 9.6 µs inter-frame wait time in
the Ethernet protocol.
2. Multiple Access
Suppose nodes Aand Bare ready to send a packet at the same time a third
node ends transmission on a 10 Mbps Ethernet. In the ith round after
i−1 collisions have already occurred, the two nodes wait 0, 1, . . . , 2i−1−1
slots until the next attempt, all 2i−1choices having equal probability.
1
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Download Communication Networks Problem Set 3: Ethernet Collisions, Multiple Access, Token Rings and more Assignments Organizational Communication in PDF only on Docsity!

CS/ECE 438: Communication Networks Fall 2006

Problem Set 3 Due Wednesday, October 4

All problems carry equal weight. Please justify your answers and show all your work.

  1. Ethernet Timing This problem is about the Ethernet/IEEE 802.11 access protocol. To be definite, suppose that if a host detects a transmission while it is trans- mitting a frame, then: (i) if the host has already transmitted the 64-bit preamble, the host stops transmitting the frame and sends a 32-bit jam- ming sequence; (ii) else the host finishes transmitting the 64-bit preamble and then sends a 32-bit jamming sequence. Suppose the packets are 512 bits long, which is the mimimum length allowed. Hosts A and B are the only active hosts on a 10 Mbps Ethernet and the propagation time be- tween them is 20 μs, or 200 bit durations. Suppose A begins transmitting a frame at time t = 0, and just before the beginning of the frame reaches B, B begins sending a frame, and then almost immediately B detects a collision.

(a) Does A finish transmitting the frame before it detects that there was a collision? Explain. (b) What time does A finish sending the jamming signal? What time does B finish sending the jamming signal? (c) What time does A hear an idle channel again? What time does B first hear an idle channel again? (d) Suppose each host next decides to retransmit immediately after hear- ing the channel idle. After the resulting (second) collision: When does A next hear the channel idle? When does B next hear the channel idle? (e) Suppose after the second collision, A decides to wait 512 bit dura- tions to retransmit (if it hears silence after that long) and B decides to retransmit immediately after hearin ga silent channel. Is the trans- mission of host B successful?

NOTE: For simplicity, please ignore the 9.6 μs inter-frame wait time in the Ethernet protocol.

  1. Multiple Access Suppose nodes A and B are ready to send a packet at the same time a third node ends transmission on a 10 Mbps Ethernet. In the ith round after i − 1 collisions have already occurred, the two nodes wait 0, 1,... , 2 i−^1 − 1 slots until the next attempt, all 2i−^1 choices having equal probability.

(a) Find the probability qi of a collision in the ith round, given that there are collisions in the previous i − 1 rounds (i.e. q 1 = 1, q 2 = 1/2), for all i ≥ 1. (b) Find the probability pi that exactly i rounds are needed for the first success, and compute p 1 , p 2 ,... , p 5. (c) Now assume that after the first collision, node A “wins” the backoff and transmits successfully. After it is finished, both nodes try to transmit again (A has an infinite amount of traffic to send), causing a collision. After this collision, the A’s collision counter is at 1 and B’s is at 2. Compute the probability that A wins again. (d) Given that A “won” the first round, compute the probability that A captures the network for the next 10 frames.

  1. Token Ring Networks

(a) In a token ring network, a station is allowed to hold the token for some period of time, the token holding time, THT. Let RingLatency denote the time it takes the token to make one complete rotation around the network when none of the stations have any data to send. (Assume early token release for this subpart). i. In terms of THT and RingLatency, express the efficiency of the network when only one nstation is active. ii. What setting of THT would be optimal for a network that only had one station active (with data to send) at a time? iii. In the case where N stations are active, give an upper bound on the token rotation time, TRT, for the network. (b) Consider a token ring with a data rate of 10 Mbps, a ring latency of 150 μsec, and 1024-bit packets. i. Assuming only one host wants to transmit and the delayed token release scheme is used, what is the maximum effective through- put rate that can be achieved? ii. Now assume N hosts want to transmit on the token ring and the token holding time (THT) is 500 μsec. What is the token rotation time? iii. Under the assumptions of part (2), and using the immediate re- lease scheme, what is the throughput rate that can be achieved?

  1. Bridges

Consider the network configuration in Figure 1 for this question.

(a) What ports would be not selected by the spanning tree algorithm? (The ID of each bridge is its number, i.e. B1 has ID 1.) List ports as pairs (bridge, network), i.e. (B1,A).