3170103456-应承峻-作业一1
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2022-08-03
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浙江大学《计算机网络》课程课后作业一
应承峻 3170103456
1. Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8-mm
tapes instead of a flask of brandy. (When your disk fills up, you consider that an
emergency.) These tapes each contain 7 gigabytes. The dog can travel to your side,
wherever you may be, at 18 km/hour. For what range of distances does Bernie have a
higher data rate than a transmission line whose data rate (excluding overhead) is 150
Mbps? How does your answer change if (i) Bernie’s speed is doubled; (ii) each tape
capacity is doubled; (iii) the data rate of the transmission line is doubled
Answer：
(0) Assume the time consume between the dog and myself is t. we have
We can get
(1) from qustion (0) we can get s is also doubled so that s = 11.2KM
(2) from qustion (0) we can get s is also doubled so that s = 11.2KM
(3) from qustion (0) we can get s is also halved so that s = 2.8KM
2. What are two reasons for using layered protocols? What is one possible disadvantage of
using layered protocols?
Answer:
(1) Reasons: ① It can devide the datas into many small packets, which can be easy to
transmit. ② When the protocol changed, it will not affect other layers.
(2) Disadvantage: It difficult to implement and need the all programmers obey the
protocol.
3. In some networks, the data link layer handles transmission errors by requesting that
damaged frames be retransmitted. If the probability of a frame’s being damaged is p,
what is the mean number of transmissions required to send a frame? Assume that
acknowledgements are never lost.
Answer:
Assume that after k-1 times, we success at the k-th time and we denote it as
. For
each time, the probability of success is 1-p and p for failed.
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