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General questions for network communications

1. A user message is split into 15 frames, each of which has an 80% chance of arriving error-free. If no error control is used by the data link layer, how many times must the message be sent on the average to get the entire message through?

2. A communication channel has a bit rate of 14 kbps and an one-way propagation delay of 10 msec. If the sender sends one frame and waits for an acknowledgment before proceeding, what range of frame sizes gives an efficiency of at least 50%?

3. A 200 km long cable operates at T1 rate. The propagation speed in the cable is 2/3 of speed of light (200,000 km/sec). How many bits can fit in the cable?

4. A group of N stations share a 64 kbps pure ALOHA channel. Each station generates a 1000-bit frame on an average of once every 100 second, even if the previous frame has not yet been sent (i.e. frame is buffered). What is the maximum value of N?

5. 10,000 travel agents are competing for the use of a single slotted ALOHA channel. The average agent sends 36 requests / hour. A slot time is 125 usec. What is the appropriate total channel load?

6. Consider building a 10Gigabit Ethernet running over a 1-km optical cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?

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