Communication Networks Problem Set 3: Ethernet Timing, Access, Window Size, Token Ring | Assignments Organizational Communication

CS/ECE 438: Communication Networks for Computers Fall 2005

Problem Set 3 Due: start of class, Monday, October 3rd

Direct Link Networks (Part II)

Assigned reading: Peterson and Davie: Chapter 2.5 – 2.9. All problems carry equal weight. For full credit, show all

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 transmitting 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 jamming sequence; (ii) Else the host finishes transmitting the 64 bit

preamble and then sends a 32 bit jamming sequence. For simplicity, assume a collision is detected as soon as an

interfering signal first begins to reach a host. Suppose the packets are 512 bits long, which is the minimum length

allowed. Hosts A and B are the only active hosts on a 10 Mbps Ethernet and the propagation time between them is

3µS, or 30 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 a jamming signal? What time does B finish sending a jamming

signal?

c. What time does A first 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 hearing 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 durations to retransmit (if it hears silence

after that long) and B decides to retransmit immediately after hearing a silent channel. Is the

transmission of host B successful?

f. At the time A was planning to send its second retransmission, it senses a carrier present. Suppose at

that particular time A decides to wait 3 x 51.2µs more until its next retransmission. What time does

host A finish sending its packet?

2. Multiple Access

Suppose two nodes 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. The slot duration is 51.2µs. (For simplicity, we

assume that collision slots and silent slots all have the same duration.) Let qi be the probability of collision in

the ith round, given that there are collisions in all the previous i – 1 rounds. (so q1 = 1 and q2 = ½).

a. Find qi as a function of i for all i ≥ 1.

b. Find the probability pi that exactly i rounds are needed for the first success, and compute p2, p3, p4 and

p5.

c. Using your answer to part 2, give an upper bound on the probability it takes more that 20 ms until the

beginning of a successful packet transmission.

3. Window Size

Consider an error-free 512-kbps satellite channel used to send 1024-byte data frames in one direction, with very

short acknowledgements coming back the other way. Assume an earth-satellite propagation delay of 270 msec.

a. What is the maximum throughput for window size of 1, 7, 15, 127 and 255?

b. At what minimum window size can the protocol run at the full rate of the channel?

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CS/ECE 438: Communication Networks for Computers Fall 2005

Problem Set 3 Due: start of class, Monday, October 3

Direct Link Networks (Part II)

Assigned reading: Peterson and Davie: Chapter 2.5 – 2.9. All problems carry equal weight. For full credit, show all 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 transmitting 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 jamming sequence; (ii) Else the host finishes transmitting the 64 bit preamble and then sends a 32 bit jamming sequence. For simplicity, assume a collision is detected as soon as an interfering signal first begins to reach a host. Suppose the packets are 512 bits long, which is the minimum length allowed. Hosts A and B are the only active hosts on a 10 Mbps Ethernet and the propagation time between them is 3 μS, or 30 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 a jamming signal? What time does B finish sending a jamming signal? c. What time does A first 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 hearing 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 durations to retransmit (if it hears silence after that long) and B decides to retransmit immediately after hearing a silent channel. Is the transmission of host B successful? f. At the time A was planning to send its second retransmission, it senses a carrier present. Suppose at that particular time A decides to wait 3 x 51.2μs more until its next retransmission. What time does host A finish sending its packet?

2. Multiple Access

Suppose two nodes are ready to send a packet at the same time a third node ends transmission on a 10 Mbps Ethernet. In the i th^ round after i – 1 collisions have already occurred, the two nodes wait 0, 1, …, 2 ( i –1)^ -1 slots until the next attempt, all 2 i -1^ choices having equal probability. The slot duration is 51.2μs. (For simplicity, we assume that collision slots and silent slots all have the same duration.) Let q (^) i be the probability of collision in the i th^ round, given that there are collisions in all the previous i – 1 rounds. (so q 1 = 1 and q 2 = ½). a. Find q (^) i as a function of i for all i ≥ 1. b. Find the probability p (^) i that exactly i rounds are needed for the first success, and compute p 2 , p 3 , p 4 and p 5. c. Using your answer to part 2, give an upper bound on the probability it takes more that 20 ms until the beginning of a successful packet transmission.

3. Window Size

Consider an error-free 512-kbps satellite channel used to send 1024-byte data frames in one direction, with very short acknowledgements coming back the other way. Assume an earth-satellite propagation delay of 270 msec. a. What is the maximum throughput for window size of 1, 7, 15, 127 and 255? b. At what minimum window size can the protocol run at the full rate of the channel?

4. Sequence Number Space

Consider the Go-Back-N protocol with a send window size of 10 and a sequence number range of 1024. Suppose that at time t , the next in-order packet that the receiver is expecting has a sequence number of k. Assume that the medium does not reorder messages. a. What are the possible sets of sequence number inside the sender’s window at time t? Justify your answer. b. What are all possible values of the ACK field in the message currently propagating back to the sender at time t? Justify your answer. c. With the Go-Back-N protocol, is it possible for the sender to receive an ACK for a packet that falls outside of its current window? Justify your answer.

5. Token Ring

a. In a token ring network, like FDDI, 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.

In terms of THT and RingLatency, express the efficiency of the network when only one station is active.
What setting of THT would be optimal for a network that had only one station active (with data to send) at a time?
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 200 Mbps, a ring latency of 500 μsec, and 1024 bit packets.
Assuming only one host wants to transmit and the delayed token release scheme is used, what is the maximum effective throughput rate that can be achieved? What is the efficiency?
Now assume N hosts want to transmit on the token ring and the token holding time (THT) is 50 μsec. What is the token rotation time? What is the maximum effective throughput rate that can be achieved? What is the efficiency?
Under the assumptions of part b, and using the immediate release scheme, what is the throughput rate that can be achieved?

Communication Networks Problem Set 3: Ethernet Timing, Access, Window Size, Token Ring, Assignments of Organizational Communication

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Download Communication Networks Problem Set 3: Ethernet Timing, Access, Window Size, Token Ring and more Assignments Organizational Communication in PDF only on Docsity!

CS/ECE 438: Communication Networks for Computers Fall 2005

Problem Set 3 Due: start of class, Monday, October 3

Direct Link Networks (Part II)

1. Ethernet Timing

2. Multiple Access

3. Window Size

4. Sequence Number Space

5. Token Ring