计算机网络第二章 作业

1. Television channels are 20 MHz wide. How many bits/sec can be sent if 32-level digital signals are used? Assume a noiseless channel.
A.  Mbps

Nyquist Theory 2*20MHz*log₂32 = 2*20*5 = 200Mbps

2.7 signals, each requiring 4000 Hz, are multiplexed on to a single channel using FDM. How much minimum bandwidth is required for the multiplexed channel? Assume that the guard bands are 500 Hz wide.使用FDM将7个信号（每个都需要4000 Hz）复用到单个通道上。 多路复用通道需要多少最小带宽？ 假设保护频带为500 Hz宽。

7 * 4000HZ + 6 * 500HZ = 31000HZ

【使用FDM：n个通道需要n-1个保护频带把它们隔开】 

3.Suppose that A, B, and C are simultaneously transmitting 0 bits, using a CDMA system with the chip sequence of figure following:

What is the resulting chip sequence? give your answer as (+x,-x,-x, …)

传0即传它的反码， （+3，+1，+1，-1，-3，-1，+1）

4.A CDMA receiver gets the following chips: (-1 +1 -3 +1 -1 -3 +1 +1). Assuming the chip sequences defined in figure following,

which stations transmitted, and which bits did each one send?
Choise the best answer

• Station A send sent bit 1 sent bit 0 silence
• Station B send sent bit 1 sent bit 0 silence
• Station C send sent bit 1 sent bit 0 silence
• Station D send sent bit 1 sent bit 0 silence

用CDMA的特性S*A*1/8 = CODE

得A = 1 B = -1 C = 0 D = 1

5.Q. A signal is transmitted digitally over a 4-kHz noiseless channel with one sample every 125 µsec. How many bits per second are actually sent for each of these encoding methods?
A. 1) CCITT 2.048 Mbps standard:  kbps
A. 2) DPCM with a 4-bit relative signal value:  kbps
A. 3) Delta modulationard:  kbps

64 32 8

6.Q. What is the percent overhead on a T1 carrier; that is, what percent of the 1.544 Mbps are not delivered to the end user? How about the E1 carrier ?
A. For the T1 carrier:  % (give your answer as an integer)

Hint: A T1 frame consists of 24 channels plus one extra bit for framing, each channel has 1 bit for signaling, 7 bits for data

A. For the E1 carrier:  % (give your answer as an integer)

Hint: Thirty of the E1 channels are used for information and two are used for signaling

13 6

7.Q. A simple telephone system consists of two end offices and a single toll office to which each end office is connected by a 1-MHz full-duplex trunk. The average telephone is used to make four calls per 8-hour workday. The mean call duration is 6 min. Ten percent of the calls are long-distance (i.e., pass through the toll office). What is the maximum number of telephones an end office can support? (Assume 4 kHz per circuit.)
A. 

Hint: Do you get how much time will a telephone occupy a circuit for an hour?

50000

8.What is the transmission unit for the physical layer?