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hw5solution

# hw5solution - destination address They also use internal...

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1. A disadvantage of the CSMA/CD is the capacity wasted due to contention. Suppose that time is divided into discrete slots, with each of N stations attempting to transmit with probability p during each slot. What fraction of slots are wasted due to multiple simultaneous transmission attempts? The fraction of slots wasted due to multiple transmission attempts is equal to the probability that there will be 2 or more transmission attempts in a slot. Pr[2 or more attempts] = 1 – Pr[no attempts] – Pr[exactly 1 attempt] = 1 – (1 – p) N – Np(1 – p) N–1 2. In today's typical Ethernet, the stations are connected to an Ethernet switch using twisted pairs. In this case, do we still need CSMA/CD to control the medium access? Explain why. No, CSMA/CD is not needed. The reasons are: 1. Twisted pairs are full-duplex transmission medium, it supports bidirectional signal exchange. 2. Ethernet switches forward packets based on their
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Unformatted text preview: destination address. They also use internal buffers to resolve contention. Therefore, the given scenario does not have collision at all, thus no CSMA/CD is needed. 3. Station A, B and C are interconnected through an Ethernet switch. At a certain moment, A and B start sending packets to C simultaneously. Do you think this will cause a transmission collision at station C? Do we need CSMA/CD to handle this? If not, what is the solution? No collision will be caused at C, and CSMA/CD is not needed. In this network, when the switch receives two packets going to station C simultaneously, it uses its internal memory to resolve the contention. One of the packets is forwarded to the destination while the other is stored in the memory. After the first forwarding is done, the buffered packet is taken out of the memory for delivery....
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• Spring '10
• KANG XI
• Network switch, media access control, Ethernet switches, transmission attempts, simultaneous transmission attempts

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