UNIVERSIDAD DE LAS AMÉRICAS-PUEBLA

COLLEGE OF ENGINEERING

DEPARTMENT OF COMPUTING, ELECTRONICS AND MECHATRONICS

IS 326 Computer Networks – Homework 4 (Source: Computer Networking a Top-Down Approach by J. F. Kurose & K. W. Ross)

1. Suppose the information content of a packet is the bit pattern 1010101010101011 and an even parity scheme is used. What would the value of the field containing the parity bits be for the case of a two-dimensional parity scheme? A minimum length checksum field should be used.

2. Suppose the information portion of a packet contains 10 bytes consisting of the 8-bit unsigned binary representation of the integers 0 through 9. Compute the Internet checksum for this data.

3. Consider the 4-bit generator, G = 1001, and suppose that D has the value 10101010. What is the value of R?

4. Consider a broadcast channel with N nodes and a transmission rate of R bps. Suppose the broadcast channel uses polling, with an additional polling node, for multiple access. Suppose the amount of time from when a node completes transmission until the subsequent node is permitted to transmit, the polling delay, is d_poll. Suppose that within a polling round, a given node is allowed to transmit at most Q bits. What is the maximum throughput of the broadcast channel.

5. With the CSMA/CD protocol, the adapter waits 512 K bit times after a collision, where K is drawn randomly. For K = 100, how long does the adapter wait until returning to step 2 for a 10 Mbps Ethernet? For a 100 Mbps Ethernet?

6. Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and the propagation delay between the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the same CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? Suppose A and B begin transmission at t = 0 bit times. They both detect collisions at t = 225 bit times. They finish transmitting a jam signal at t = 225 + 48 = 273 bit times. Suppose K_A = 0 and K_B = 1. At what time does B schedule its retransmission? At what time does A begin transmission? At what time does A’s signal reach B? Does B refrain from transmitting at its scheduled time?

7. Suppose two nodes, A and B, are attached to opposite ends of a 900 m cable, and hat they each have one frame of 1,000 bits (including all headers and preambles) to send to each other. Both nodes attempt to transmit at time t = 0. Assume the transmission rate is 10 Mbps, and CSMA/CD with backoff intervals of multiples of 512 bits is used. After the first collision, A draws K = 0 and B draws K = 1 in the exponential backoff protocol. Ignore tha jam signal and the 96-bit time delay.

a) What is the one-way propagation delay between A and B in seconds? Assume that the signal propagation speed is 2 x 10⁸ m/sec.

b) At what time, in seconds, is A’s packet completely delivered at B?