**CN |Gate-2009| Computer Networks**

- While opening a TCP connection, the initial sequence number is to be derived using a time-of-day (ToD) clock that keeps running even when the host is down. The low order 32 bits of the counter of the ToD clock is to be used for the initial sequence numbers. The clock counter increments once per millisecond. The maximum packet lifetime is given to be Which one of the choices given below is closest to the minimum permissible rate at which sequence numbers used for packets of a connection can increase?
**[GATE – 2009]**

a. 0.015/s

b. 0.135/s

c. 0.064/s

d. 0.327/s

*Answer : a)*

- Let G(x) be the generator polynomial used for CRC checking. What is the condition that should be satisfied by G(x) to detect odd number of bits in error?
**[GATE – 2009]**

a. G(x) contains more than two terms

b. G(x) does not divide 1+x^{k}, for any k not exceeding the frame length

c. 1+x is a factor of G(x)

d. G(x) has an odd number of terms

*Answer : c)*

- Frames of 1000 bits are sent over a 10
^{6}bps duplex link between two hosts. The propagation time is 25ms. Frames are to be transmitted into this link to maximally pack them in transit (within the link). What is the minimum number of bits (l) that will be required to represent the sequence numbers distinctly? Assume that no time gap needs to be given between transmission of two frames.**[GATE – 2009]**

a. I = 2

b. I = 3

c. I = 4

d. I = 5

*Answer : d)*

**4.** Frames of 1000 bits are sent over a 10^{6} bps duplex link between two hosts. The propagation time is 25ms. Frames are to be transmitted into this link to maximally pack them in transit (within the link).

Suppose that the sliding window protocol is used with the sender window size of 2^{l}, where l is the number of bits identified in the earlier part and acknowledgements are always piggy backed. After sending 2^{l} frames, what is the minimum time the sender will have to wait before starting transmission of the next frame? (Identify the closest choice ignoring the frame processing time.) **[GATE – 2009]**

a. 16ms

b. 18ms

c. 20ms

d. 22ms

*Answer : c)*

CN |Gate-2009|