CS 558 - Computer Networks
Homework 3

Due: See class webpage.

Question 1. (20 pts)

A sliding window protocol uses the cumulative ACK scheme (where ACK n means reception of all frames upto and including frame n).

The number of bits in the sequence number field is 3.

A receive windows size and send window size is 5.

The initial send and receiver windows are {0,1,2,3,4}.

The following events occurs at the receiver in the given sequence:

1. The receiver first receives the frame numbered with seq. no. 3. What ACK message (give sequence number) will the receiver send back to the sender ? (2 pts)

2. The receiver next receives the frames numbered with seq. no. 0 and 2. What ACK message (give sequence number) will the receiver send back to the sender ? (2 pts)

3. The receiver next receives the frame numbered with seq. no. 1. What ACK message (give sequence number) will the receiver send back to the sender ? (2 pts)

4. The receiver next receives the frame numbered with seq. no. 0. What ACK message (give sequence number) will the receiver send back to the sender ? (2 pts)

Is this frame for certain a retransmission or can this frame be a new frame ? Show your reason to get full credit. (5 pts)

5. The receiver next receives the frame numbered with seq. no. 1. What ACK message (give sequence number) will the receiver send back to the sender ? (2 pts)

Is this frame for certain a retransmission or can this frame be a new frame ? Show your reason to get full credit. (5 pts)

Question 2. (20 pts)

The maximum network diameter in Ethernet is 2500 m, i.e., the distance between any two nodes on an Ethernet is 2500 m. The speed of electrical signals in copper is 2x108 m/sec.

• What is the maximum end-to-end propagation delay ? (10 pts)

At time t = 0, an Ethernet host senses that the channel is idle and immediately begin to transmit (at t = 0). As you know, the Ethernet host will sense the channel (while it is transmitting) for possible collision.

• After how many seconds of sensing can this Ethernet host be certain that there will not be any collisions ? I.e., when can it stop sensing (because it is sure there won't be a collision). (10 pts)

Question 3. (10 pts)

• If in CDMA/CA, two pairs of senders and receivers use the following random keys:

• pair 1 uses 0000000
• pair 2 uses 0000001

Can these two pairs of senders and receivers share the channel effectively (without errors) with CDMA/CD ? Explain to get full credit. (10 pts)

Question 4. (10 pts)

• A pair of sender/receiver is currently using the random key 0100010

1. Give a (7 bit) random key that does not cause errors with the above key. (5 pts)

2. Give a (7 bit) random key (that is not equal to 0100010 or 1011101) that does cause errors with the above key. (5 pts)

Question 5 (20 pts)

Consider a number of inter-connected LAN connected in the following manner:

Questions:

1. Give the values of the initial state of the bridges when they are initialized in the following figure: (4 pts)

Write the state of the ports in their corresponding boxes.

2. Give the configuration control messages sent by each bridge in the state given above - if a bridge does not transmit any configuration message, enter NA as answer: (4 pts)

• Bridge 1: ________________________
• Bridge 2: ________________________
• Bridge 3: ________________________
• Bridge 4: ________________________
• Bridge 5: ________________________

3. Show the values of the state variables of the bridges after all configuration control messages that were transmitted in part 2 have been processed: (12 pts)

Question 6 (20 pts)

1. Given that network is in the state given the following figure:

Suppose that there are no blocked port in the above state.

Write in the above figure the state of each port (4 pts).

2. Give the configuration control messages sent by each bridge in the state given above - if a bridge does not transmit any configuration message, enter NA as answer: (4 pts)

• Bridge 1: ________________________
• Bridge 2: ________________________
• Bridge 3: ________________________
• Bridge 4: ________________________
• Bridge 5: ________________________

3. Show the values of the state variables of the bridges after all configuration control messages that were transmitted in part 2 have been processed: (12 pts)