LGW2ECh7solutions - Communication Networks(2nd Edition...

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Communication Networks (2 nd Edition) Chapter 7 Solutions Solutions to Chapter 7 7.1. Explain how a network that operates internally with virtual circuits can provide connectionless service. Comment on the delay performance of the service. Can you identify inefficiencies in this approach? Solution: The connection-oriented (co) network can present a network sublayer interface to the upper layer to make itself appear as a connectionless (cl) network. The upper layer sends packets in connectionless fashion, but the network resolves the packet destination’s cl address into the corresponding co address (e.g. ATM address), and then establishes a virtual circuit between the source and destination. Any subsequent packets with the same source and destination are transmitted through this virtual circuit. The interface hides the internal operation of the network from the upper layer. The cl-to-co address resolution is performed for the transmission of every packet, and hence incurs extra processing delay. Address caching can speed up the process, but cannot eliminate the delay. The first cl packet incurs extra delay because of the time required to set up the virtual circuit. This cl-service-over-co-network approach is inefficient in that the QoS support of the connection- oriented network cannot be made available to the network layer. ATM LAN emulation provides an example that involves providing connectionless service over a connection-oriented service. 7.2. Is it possible for a network to offer best-effort connection-oriented service? What features would such a service have, and how does it compare to best-effort connectionless service? Solution: Best-effort connection-oriented service would involve the transfer of packets along a pre-established path in a manner that does not provide mechanisms for dealing with the loss, corruption or misde- livery of packets. Best-effort connection-oriented service would require some means for establishing a path prior to the transfer of packets. Best-effort connectionless service would involve the transfer of packets in a datagram fashion, where routing decisions are made independently for each packet. The path setup requirement makes connection-oriented service more complex than connectionless service. On the other hand, once a path is established, less processing is required to decide how a packet is to be forwarded. Connectionless service is more robust than connection-oriented service since connectionless service readily reroutes packets around a failure while VC service requires that new paths be established. 7.3. Suppose a service provider uses connectionless operation to run its network internally. Explain how the provider can offer customers reliable connection-oriented network service.
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