Consider hosts in an institutional network accessing content via the following network. Suppose each object is 80K bits, hosts in the institutional network generate 18 requests/sec, and hosts from other parts of the Internet (not shown) generate 100 requests/sec. Suppose that the total delay for Router2 to send a request to Router3 and Router3 to send the response back to Router2 is 2 seconds. Ignore propagation delays for other links. Ignore delays for Conditional GET messages and their responses. Approximate the access link delay (including queuing and transmission delays) by , where is the service rate and is the arrival rate (unit: objects/sec).

1. Find the total delay for obtaining one object from the origin server to a host in the institutional network.
2. Suppose that 20% of requests generated by the institutional network are served by a local cache (i.e., the cache hit rate is 0.2). Repeat a).
3. What is the minimum cache hit rate to make the delay no greater than 100 ms?

public Router3 Router4 R = 20 Mbps origin Internet servers Content provider Rc = 1.54 Mbps 1 Gbps LAN Router2 RTT = 2 sec Router1 1 Gbps LAN 1 cache cache institutional network