This set of Computer Networks Multiple Choice Questions & Answers (MCQs) focuses on “Multiple Access Links and Protocols”.
1. Which of the following are the types of basic protocols that are present in multiple access control in data link layer?
a) Random, controlled, channelization
b) Controlled, channelization
c) Random, channelization
d) Random, controlled, contributed
View Answer
Explanation: In random access control protocol, the medium can be shared randomly depending on the state of the medium. In controlled access, stations share the medium depending on authorization. Channelization allows the stations to share the bandwidth of the link.
2. Which aspect of the random-access protocols makes them contention methods?
a) No rules for accessing medium
b) Shared bandwidth
c) Authorized access for medium
d) Scheduling time for medium
View Answer
Explanation: Random access protocol also called as contention methods have no rules or time for stations to place their frames on the medium. For this reason, stations compete among themselves for accessing the medium.
3. Why station need to wait random amount of time for re-transmission of the frame after collision in pure aloha?
a) To prevent other stations
b) To avoid more collisions
c) To detect other collisions
d) To get acknowledgment from the receiver
View Answer
Explanation: Collision includes two more stations, if those stations again re-transmit the frames at a time; it causes collision again. Back-off time is to avoid this collision by making the stations to wait for some random times.
4. Which strategy is used by the pure aloha to prevent the congestion caused by continuous re-transmitted frames?
a) Aborts the re-transmission after 5 attempts
b) A station is allowed to re-transmit for 2 times
c) Restriction on the attempts for re-transmission
d) Re-transmission has no restriction
View Answer
Explanation: If a station re-transmits a frame more than the times, that is defined by the pure aloha, then pure aloha will abort the process re-transmission of the particular frame. The station needs to try after sometime later to send the frames.
5. What is the vulnerable time of the pure aloha for fixed length of frames (T=frame time)?
a) 2*T
b) 3*T
c) 1/2*T
d) 5*T
View Answer
Explanation: Vulnerable time is time in which, there is a possibility for a collision. T(frame) is the time taken by a frame to be sent to destination. Vulnerable time will be 2 times the fixed length of frames.
6. Which of the following controlled access protocol is known as master slave method?
a) Polling
b) Reservation
c) Token passing
d) Scheduling
View Answer
Explanation: Polling is the method for channel allocation where one station acts as a server and remaining stations acts according to the decision of master station. This master station also known as primary device, which uses two functions called poll function and select function to send and receive data within the network.
7. Which of the following type of multiple access protocols uses a scheme called “Authorization from other stations”?
a) Random access protocol
b) Controlled access protocol
c) Channelization
d) Persistent method
View Answer
Explanation: In controlled access protocols, stations send frames only if they get the authorization from the other stations to use the channel. This authorization can be obtained using methods such as reservation, polling, token passing.
8. Which of the following persistent method of random-access protocol reduces the collisions and efficiency of the network?
a) 1 – persistent
b) Non persistent
c) Persistent
d) 1 – persistent and non-persistent
View Answer
Explanation: Unlike 1- persistent in non-persistent methods, stations wait for some random time even if it finds the channel idle. This random time reduces the collision at the same time it reduces the efficiency because random times may keep the channel unused even the stations have frames to send.
9. What is the vulnerable time in carrier sense multiple access?
a) T propagation
b) T frame
c) 2*T frame
d) 2*T propagation
View Answer
Explanation: In carrier sense multiple access, the stations sense the medium whether it is idle or not. This information is sent by the station which is using the medium, using propagation of a signal. The propagation delay in the signal causes the collision.
10. Slotted aloha has vulnerable time that is half of the vulnerable time of pure aloha.
a) True
b) False
View Answer
Explanation: In slotted aloha, the time is divided into time slots, that are of T(frame) length. The stations are instructed to send the frames at the starting of the time slot. Still there is a chance for collision, if 2 frames are started at the same it. This is why vulnerable time is equal to the time taken by a frame to be sent to the destination.
11. Which of the following is not a channelization protocol?
a) Frequency division multiple access
b) Time division multiple access
c) Code division multiple access
d) Bandwidth division multiple access
View Answer
Explanation: The channelization protocols use frequency, time and code to divide the bandwidth of the channel. Bandwidth is divided into frequency bands in FDMA, same bandwidth is used by the stations in TDMA.
12. What technique is used by the TDMA to accomplish synchronization?
a) Preamble bits
b) Synchronization bytes
c) Guard bits
d) Start points
View Answer
Explanation: Time division multiple access allocates the time slots for station to use the channel. These time slots are synchronized by preamble bits, which acts as guard time between two time slots.
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