GATE PYQ

Data Link Layer

Q1.
Consider a network using the pure ALOHA medium access control protocol, where each frame is of length 1,000 bits. The channel transmission rate is 1 Mbps (=10^6 bits per second). The aggregate number of transmissions across all the nodes (including new frame transmissions and retransmitted frames due to collisions) is modelled as a Poisson process with a rate of 1,000 frames per second. Throughput is defined as the average number of frames successfully transmitted per second. The throughput of the network (rounded to the nearest integer) is ______________

Q2.
Consider a 100 Mbps link between an earth station (sender) and a satellite (receiver) at an altitude of 2100 km. The signal propagates at a speed of 3 \times 10^8 m/s. The time taken (in milliseconds, rounded off to two decimal places) for the receiver to completely receive a packet of 1000 bytes transmitted by the sender is

Q3.
Suppose two hosts are connected by a point-to-point link and they are configured to use Stop-and-Wait protocol for reliable data transfer. Identify in which one of the following scenarios, the utilization of the link is the lowest.

Q4.
Assume that a 12-bit Hamming codeword consisting of 8-bit data and 4 check bits is d_8d_7d_6d_5c_8d_4d_4d_3d_2c_4d_1c_2c_1, where the data bits and the check bits are given in the following tables: Which one of the following choices gives the correct values of x and y?

Q5.
Consider the sliding window flow-control protocol operating between a sender and a receiver over a full-duplex error-free link. Assume the following: The time taken for processing the data frame by the receiver is negligible. The time taken for processing the acknowledgement frame by the sender is negligible. The sender has infinite number of frames available for transmission. The size of the data frame is 2,000 bits and the size of the acknowledgement frame is 10 bits. The link data rate in each direction is 1 Mbps (= 10^6 bits per second). One way propagation delay of the link is 100 milliseconds. The minimum value of the sender's window size in terms of the number of frames, (rounded to the nearest integer) needed to achieve a link utilization of 50% is _____________

Q6.
Consider the cyclic redundancy check (CRC) based error detecting scheme having the generator polynomial X^3+X+1. Suppose the message m_4m_3m_2m_1m_0=11000 is to be transmitted. Check bits c_2c_1c_0 are appended at the end of the message by the transmitter using the above CRC scheme. The transmitted bit string is denoted by m_4m_3m_2m_1m_0c_2c_1c_0. The value of the checkbit sequence c_2c_1c_0 is

Q7.
StationA uses 32 byte packets to transmit messages to Station B using a sliding window protocol. The round trip delay between A and B is 80 ms and the bottleneck bandwidth on the path between A and B is 128 kbps. What is the optimal window size that A should use?

Q8.
Consider the following two statements. S1: Destination MAC address of an ARP reply is a broadcast address. S2: Destination MAC address of an ARP request is a broadcast address. Which one of the following choices is correct?

Q9.
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:

Q10.
Consider that 15 machines need to be connected in a LAN using 8-port Ethernet switches. Assume that these switches do not have any separate up link ports. The minimum number of switches needed is ___________.