Q. A bit string, 0111101111101111110, needs to be transmitted at the data link layer. What is the string actually transmitted after bit stuffing?
五个1插个0
011110111110011111010
Q. What is the remainder obtained by dividing x^7 + x^5 + 1 by the generator polynomial x^3 + 1? (give your answer as bit string)
10100001 / 1001 = 101110 … 111
111是余数
Q.A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?
A. bits
发送一帧的时间等于信道传播延迟的2倍，信道利用率为50.
现在发送速率为4kbp/s，发一位需要0.25微秒
（20*10-3*2）/（0.25*10-6） = 160000 bits
Q.Consider an error-free 64-kbps satellite channel used to send 512-byte data frames in one direction, with very short acknowledgements coming back the other way. What is the maximum throughput for window sizes of 1, 7, 15? The earth-satellite propagation time is 270 msec. (give your answer as an integer)
A.A. for window size=1: bps
A.A. for window size=7: bps
A.for window size=15: bps
注意512-byte = 512 * 8 bits
Tframe = 512 * 8bits/64kbps = 64ms
Transmission cycle: T = 270ms * 2 + 64ms = 604ms
Transmission window: 604ms/64ms = 9.4, 大于9的时候channel就满了
W = 1 ; throughout = 512 * 8b / 604ms = 6781bps = 6.78kbps
W = 2 ; throughout = 7 * 512 * 8b / 604ms
W = 15 > 9 ; throughout = 65kbps
Q.A 100-km-long cable runs at the T1 data rate. The propagation speed in the cable is 2/3 the speed of light in vacuum. How many bits fit in the cable?
A. bits
电缆中传播速度是每秒200000km，每毫秒200km，100km将会在0.5ms内填满。
T1速率125微秒传送193位帧，0.5ms可以传送4个T1帧，即193*4 = 772bits
Box 1: Enter your answer as a number. Examples: 3, -4, 5.5
Enter DNE for Does Not Exist, oo for Infinity
序列号有4位，回退N协议，最多没有完成发送帧有多少？
四位可以表示0~15共16个数字 最多没有完成15个。
Box 1: Enter your answer as a number. Examples: 3, -4, 5.5
Enter DNE for Does Not Exist, oo for Infinity
序列号有4位，选择重传协议，最多没有完成发送帧有多少？
五位可以表示0~31共32个数字 最多没有完成（max+1）/2 = 33/2 = 32/2 = 16
- If station X received RTS of station A, X must remain silent for a short time
- If station X received RTS, but did not receive CTS, then X may not transmit its data.
- If station X has not received RTS, but received CTS, then X may not transmit its data
- If station X has received both RTS and CTS, then X may not transmit its data
Box 1: Select the best answer
B
- 1
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