origin servers public Internet 1.5 Mbps access link institutional network 100 Mbps LAN
Consider the figure used to illustrate Web caching below. Suppose that the access link capacity is C bits/sec and the average object size is S bits and that the average request rate from the institution’s browsers to the origin servers is A requests/sec. Also, suppose that the amount of time it takes from when the router on the Internet side of the access link forwards an HTTP request until it receives the response is T secondson average. Model the total average response time perceived by the institutional users as the sum of the average access delay (that is, the delay from the Internet router to institution router) and the average Internet delay. For the average access delay, use D/(1 − DB), where D is the average time required to transmit an object over the access link and B is the arrival rate of objects to the access link. Note that this access delay includes both the transmission and queuing delays. (Note also that B = A if there isno caching.) Ignore the details of persistent or non-persistent HTTP in this question as well as any LAN delays.
Now suppose a cache is installed in the institutional LAN and its hit rate is
p. What is the total average response time T2?
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