# Computer Networks | Set 9

Following questions have been asked in GATE CS 2007 exam.

1) Which one of the following uses UDP as the transport protocol?
(A) HTTP
(B) Telnet
(C) DNS
(D) SMTP

Answer (C)
DNS primarily uses User Datagram Protocol (UDP) on port number 53 to serve requests. DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server.

2) In Ethernet when Manchester encoding is used, the bit rate is:
(A) Half the baud rate.
(B) Twice the baud rate.
(C) Same as the baud rate.
(D) none of the above

Answer (A)
In Manchester encoding, the bitrate is half of the baud rate.

3) 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?
(A) (1-p)^(n-1)
(B) np(1-p)^(n-1)
(C) p(1-p)^(n-1)
(D) 1-(1-p)^(n-1)

Answer (B)
The probability that a particular station transmits and no body else transmits = p*(1-p)^(n-1)
The probability that any station can transmit = n*(probability that a particular station transmits) = n*p*(1-p)^(n-1). See this for details.

4) In a token ring network the transmission speed is 10^7 bps and the propagation speed is 200 metres/micro second. The 1-bit delay in this network is equivalent to:
(A) 500 metres of cable.
(B) 200 metres of cable.
(C) 20 metres of cable.
(D) 50 metres of cable.

Answer (C)
Transmission delay for 1 bit t = 1/(10^7) = 0.1 micro seconds.
200 meters can be traveled in 1 micro second. Therefore, in 0.1 micro seconds, 20 meters can be traveled.

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