CS 455 - Computer Networks
Homework 6

Due: See class webpage.

• Question 1 (Token ring) - 20 pts

• We consider a token ring protocol operating on a ring network with a data rate of 10 Mbps.

There are N = 10 nodes connected to the token ring network.

A node is allowed to transmit 1 packet when it receives the token.

When a node receives the token and transmits a message, the message must return back to the sending node and then the node will pass the token to the next node in the ring.

We assume that all messages have the same length and each message is 1000 bytes (= 8000 bits) long.

The distance between any 2 nodes is the same and it takes 1.0 msec (milli sec) for a message to travel to the next node in the token ring.

We define the token rotation time Trot to be the time between 2 consecutive arrivals of the token at the same node.

For example:

• Suppose node 1 received the token at time T1

Node 1 could now transmit a message or if node 1 has no message to transmit, it will pass the token to the next node

• Then later, at time T2, node 1 received the token again.

• Then the token rotation time is equal to:

 ``` Trot = T2 − T1 ```

Questions:

• What is the minimum value of the token rotation time in this token ring network ? (10 pts)

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• What is the maximum value of the token rotation time in this token ring network ? (10 pts)

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• Question 2 (Token bus) - 30 pts

• Consider the variables in a node in a token bus network:

 ``` ID = 4444 predeccessor = 5678 successor = 1234 ```

Suppose the node 4444 wish to invite other nodes to join the token bus network

Question:

• What control message will the node 4444 transmit: (5 pts)

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• Give the range of node IDs that are invited: (5 pts)

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Suppose this node receives the token frame from the node 5555

Questions:

• Give a scenario where the node 4444 can receive a token from node 5555 (that is not its predecessor) (5 pts)

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• Show the values of the variables of this node after it has processed the token frame: (5 pts)

 ``` ID = 4444 predeccessor = _________ successor = _________ ```

Use the original values in the node's variables to answer this question

Suppose this node receives a Who_follows(5678) from node 9999

Questions:

• Give a scenario where the node 4444 can receive such a control frame (5 pts)

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• What is the response of node 4444 to this control message ? (5 pts)

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• Question 3 - 20 pts

• Consider the following wireless network with 4 nodes (A, B, C and D)

Questions:

• List the hidden node(s) of each node below: (10 pts)

 ``` Hidden node for A: ____________ Hidden node for B: ____________ Hidden node for C: ____________ Hidden node for D: ____________ ```

• List the exposed node(s) of each node below: (10 pts)

 ``` Hidden node for A: _____________ Hidden node for B: _____________ Hidden node for C: _____________ Hidden node for D: _____________ ```

• Question 4 (10 pts)

• Suppose 2 wireless nodes A and B uses the following chips:

 ``` chip of A = 1010111 chip of B = 1101000 ```

The nodes received the following transmissions:

 ``` A B --------------------------------- 1111111 _______ _______ 1000000 _______ _______ 0111111 _______ _______ 0000000 _______ _______ ```

Write in the table above the bit value that each node will assign to each transmission (10 pts)

• Question 5 (20 pts)

• Let A and B be two IEEE 802.11 hosts attempting to transmit on an wireless network. Each host has a steady queue of frames ready to send.

A's frames will be numbered as A1, A2 and so on, and B's frames will be numbered as B1, B2 and so on.

Suppose the IEEE 802.11 MAC algorithm using this value for the initial back off window size:

 ``` CWmin = 15 (i.e.: initial choice of random numbers from [0 .. 15]) ```

Suppose A and B simultaneously (= at the same time) attempt to send frame 1 and the transmissions collide (= did not receive an ACK frame).

Assumes that A chooses backoff time 2×T (i.e., A picked the random number 2) and B chooses backoff time 14×T.

According to the DCF protocol (see: click here ), A will wins the race and transmits its first frame A1 successfully.

Notice that when A starts transmitting its first frame A1, the node B will pause its count down (see the DCF protocol: click here ) !!!

When node A finishes transmitting the frame A1, the node A first perform a back off (while B will resume the count down)

Therefore, nodes A and B will be competing compete to transmit their frames.

Questions:

• Find the probability that A will win the second backoff race (10 pts)

 ``` Probab(A wins) = ___________________ ```

• Find the probability that B will win the second backoff race (10 pts)

 ``` Probab(B wins) = ______________ ```