Question 1

Consider a simple communication system where multiple nodes are connected by a shared broadcast medium (like Ethernet or wireless). The nodes in the system use the following carrier-sense based medium access protocol. A node that receives a packet to transmit will carrier-sense the medium for 5 units of time. If the node does not detect any other transmission in this duration, it starts transmitting its packet in the next time unit. If the node detects another transmission, it waits until this other transmission finishes, and then begins to carrier-sense for 5 time units again. Once they start to transmit, nodes do not perform any collision detection and continue transmission even if a collision occurs. All transmissions last for 20 units of time. Assume that the transmission signal travels at the speed of 10 meters per unit time in the medium.

Assume that the system has two nodes P and Q, located at a distance d meters from each other. P starts transmitting a packet at time t=0 after successfully completing its carrier-sense phase. Node Q has a packet to transmit at time t=0 and begins to carrier-sense the medium.

The maximum distance d (in meters, rounded to the closest integer) that allows Q to successfully avoid a collision between its proposed transmission and P’s ongoing transmission is ___________.

       Computer-Networks       Ethernet       Gate 2018
Question 1 Explanation: 
Node senses the medium for 5 unit time. it means, any packet which arrives within 5 unit will be sensed and keep the channel busy.
Now signal travels at the speed of 10 meters per unit time.
Therefore, in 5 unit time, it can travel a maximum distance (d) of 50 m (5*10), which allows the receiver (Q) to sense that the channel is busy.
Question 2

In an Ethernet local area network, which one of the following statements is TRUE?

A station stops to sense the channel once it starts transmitting a frame.
The purpose of the jamming signal is to pad the frames that are smaller than the minimum frame size.
A station continues to transmit the packet even after the collision is detected.
The exponential backoff mechanism reduces the probability of collision on retransmissions.
       Computer-Networks       Ethernet       GATE 2016 set-2
Question 2 Explanation: 
An Ethernet is the most popularly and widely used LAN network for data transmission.
It is a protocol of data link layer and it tells how the data can be formatted to transmit and how to place the data on network for transmission.
Now considering the Ethernet protocol we will discuss all the options one by one
I. In Ethernet the station does not required, stops to sense the channel prior frame transmission.
II. A Jamming signal used to inform all the other devices or stations about collision that has occurred, so that further data transmission is stopped.
III. No, stations sends jamming signal if collusion is detected
IV. To reduce the probability of collision on retransmissions an exponential back off mechanism is used.
This is only True.
Question 3
The minimum frame size required for a CSMA/CD based computer network running at 1 Gbps on a 200 m cable with a link speed of 2 × 10m/s is
125 bytes
250 bytes
500 bytes
None of these
       Computer-Networks       Ethernet       Gate 2008-IT
Question 3 Explanation: 
For CSMA/CD protocol to work, the transmission time of the frame must be more than minimum value. This minimum value is the RTT.

Question 4
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.
       Computer-Networks       Ethernet       Gate-2007
Question 4 Explanation: 
Bit rate is half the baud rate in Manchester encoding as bits are transferred only during a positive transition of the clock.
Question 5
Suppose the round trip propagation delay for a 10 Mbps Ethernet having 48-bit jamming signal is 46.4 μs. The minimum frame size is:
       Computer-Networks       Ethernet       Gate-2005
Question 5 Explanation: 
Given RTT = 46.4 μs, B.w. = 10 Mbps
Round trip propagation delay is RTT = 2*Tp
Minimum frame size of Ethernet can be found by using formula Tt = 2*Tp
Let L is minimum frame size. Then L / 10Mbps = 46.4 μs
L=464 Kbits
It has nothing to do with jamming signal.
Question 6

A and B are the only two stations on an Ethernet. Each has a steady queue of frames to send. Both A and B attempt to transmit a frame, collide, and A wins the first backoff race. At the end of this successful transmission by A, both A and B attempt to transmit and collide. The probability that A wins the second backoff race is

       Computer-Networks       Ethernet       Gate-2004
Question 6 Explanation: 
A has 5 chances to win out of 8 combinations.
The probability that A wins the second back-off race = 5/8 = 0.625
More explanation in the video.
Question 7
A 2km long broadcast LAN has 107 bps bandwidth and uses CSMA/CD. The signal travels along the wire at 108 m/s. What is the minimum packet size that can be used on this network?  
50 bytes
100 bytes
200 bytes
None of the above
       Computer-Networks       Ethernet       Gate-2003
Question 7 Explanation: 
Minimum packet size for a CSMA/CD LAN is the frame which cover whole RTT(round trip time). i.e. Tt=2Tp
d= 2 km = 2 x 103 m, v = 2 x 108 m/s, B= 107
Tp= d / v = 2 x 103 /(2 x 108 ) seconds= 10-5 seconds
Let L bits be minimum size of frame, then Tt=t L / B = L / 107 seconds
Now, Tt=2Tp
L/107 = 2 x 10-5 = 200 bits = (200 / 8) bytes = 25 bytes
There are 7 questions to complete.
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