SE301计算机网络期末答案B.pdf

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中 山 大 学 软 件 学 院 软 件 工 程 专 业 2 00 8 级 (2010-2)

《 SE -

301 计算机网络

》 期 末 试 题 参 考 答 案 (B)

1） (10points) Compute the CRC for a given message (M) and a generator

polynomial (P). M is 0111101 and P is 1011.

答： 0111101000 除以 1011，商 0110110，余数 010，故CRC为010.

2） (10points) CSMA/CA

a) What is CSMA/CA?

signals (fading); can ’t sense all collisions in any case: hidden terminal, fading (3

points)

c) Sender:

1 if sense channel idle for DIFS then transmit entire frame (no CD)

2 if sense channel busy then start random backoff time, timer counts down

while channel idle, transmit when timer expires, if no ACK, increase random backoff

interval, repeat 2

3 to transmit data, sender will broadcast RTS to Access Point (AP), and wait for

CTS (broadcast from AP). Upon receiving CTS, sender can transmit the entire frame.

Receiver: if frame received OK, broadcast ACK after SIFS(ACK needed due to

CTS heard by all nodes: sender transmits data frame; other stations defer

transmissions. (5 points)

3） (10points) Where (input ports and/or output ports) can packet queuing occur in a

router? Briefly explain the conditions that lead to such queuing.