16-sol - Distributed System Structures CHAPTER 16 Practice...

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16 CHAPTER Distributed System Structures Practice Exercises 16.1 Most WANs employ only a partially connected topology. Why is this so? Answer: Cost. A fully connected network requires a link between every node in the network. For a WAN, this may be too costly as com- munication links between physically distant hosts may be expensive. 16.2 Under what circumstances is a token-passing network more effective than an Ethernet network? Answer: A token ring is very effective under high sustained load, as no collisions can occur and each slot may be used to carry a message, providing high throughput. A token ring is less effective when the load is light (token processing takes longer than bus access, so any one packet can take longer to reach its destination) or sporadic. 16.3 Why would it be a bad idea for gateways to pass broadcast packets between networks? What would be the advantages of doing so? Answer: All broadcasts would be propagated to all networks, causing a lot of network traf±c. If broadcast traf±c were limited to important data (and very little of it), then broadcast propagation would save gateways from having to run special software to watch for this data (such as network routing information) and rebroadcast it. 16.4 Discuss the advantages and disadvantages of caching name transla- tions for computers located in remote domains. Answer: There is a performance advantage to caching name transla- tions for computers located in remote domains: repeated resolution of thesamenamefromd if feren tcompu
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60 Chapter 16 Distributed System Structures name translations when updates are made in the mapping of names to IP addresses. These consistency problems could be solved by invalidat- ing translations, which would require state to be managed regarding which computers are caching a certain translation and also would re- quire a number of invalidation messages, or by using leases whereby the caching entity invalidates a translation after a certain period of time. The latter approach requires less state and no invalidation messages but
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This note was uploaded on 11/03/2009 for the course IT OS taught by Professor Dr.stephan during the Winter '09 term at Abu Dhabi University.

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16-sol - Distributed System Structures CHAPTER 16 Practice...

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