CN Data Link Layer

Question 1

Determine the maximum length of the cable (in km) for transmitting data at a rate of 500 Mbps in an Ethernet LAN with frames of size 10,000 bits. Assume the signal speed in the cable to be 2,00,000 km/s.

• 1

• 2

• 2.5

• 5

Question 2

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?

• G(x) contains more than two terms

• G(x) does not divide 1+x^k, for any k not exceeding the frame length

• 1+x is a factor of G(x)

• G(x) has an odd number of terms.

Question 3

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 (i) that will be required to represent the sequence numbers distinctly? Assume that no time gap needs to be given between transmission of two frames.

• i = 2
• i = 3
• i = 4
• i = 5

Question 4

Consider the data of previous question. Suppose that the sliding window protocol is used with the sender window size of 2^i where is the number of bits identified in the previous question and acknowledgments are always piggybacked. After sending 2^i 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.)

• 16ms
• 18ms
• 20ms
• 22ms

Question 5

In Ethernet when Manchester encoding is used, the bit rate is:

• Half the baud rate.
• Twice the baud rate.
• Same as the baud rate.
• None of the above

Question 6

There are n stations in a slotted LAN. Each station attempts to transmit with a probability p in each time slot. What is the probability that ONLY one station transmits in a given time slot?

• (1-p)^(n-1)
• np(1-p)^(n-1)
• p(1-p)^(n-1)
• 1-(1-p)^(n-1)

Question 7

In a token ring network the transmission speed is 10^7 bps and the propagation speed is 200 meters/micro second. The 1-bit delay in this network is equivalent to:

• 500 metres of cable.
  

• 200 metres of cable.
  

• 20 metres of cable.
  

• 50 metres of cable.
  

Question 8

The message 11001001 is to be transmitted using the CRC polynomial x^3 + 1 to protect it from errors. The message that should be transmitted is:

• 11001001000
• 11001001011
• 11001010
• 110010010011

Question 9

The distance between two stations M and N is L kilometers. All frames are K bits long. The propagation delay per kilometer is t seconds. Let R bits/second be the channel capacity. Assuming that processing delay is negligible, the minimum number of bits for the sequence number field in a frame for maximum utilization, when the sliding window protocolis used, is:

• A
• B
• C
• D

Question 10

Consider a token ring network with a length of 2 km having 10 stations including a monitoring station. The propagation speed of the signal is 2 × 10⁸ m/s and the token transmission time is ignored. If each station is allowed to hold the token for 2 μsec, the minimum time for which the monitoring station should wait (in μsec)before assuming that the token is lost is _______.

• 28 to 30
• 20 to 22
• 0 to 2
• 31 to 33