Consider a link, over which a sender can transmit at a rate of R=100bits/s in bo

Question

Consider a link, over which a sender can transmit at a rate of R=100bits/s in both directions. Suppose that packets containing data are of size LD=100Kbits and packets containing only control (e.g. ACKs, handshake and GET requests) are of size LA=200bits. Assume that N parallel connections get an equal portion of 1/N of the link bandwidth. Now consider the HTTP protocol and consider that following a website visit the client needs to download 10 objects each of size 100Kbits. Calculate the time to download the web page using parallel sessions and non-persistent HTTP. Now calculate the time to download the same web page using persistent HTTP. Provide a brief discussion of your results.

Note: Assume that the propagation and processing delays are negligible and that the sender and receiver buffers are not overloaded.

Explanation / Answer

Note that each downloaded object can be completely put into one data packet.
Let Tp denote the one-way.

propagation delay between the client and the server.

First consider parallel downloads via non-persistent connections.
Parallel download would allow 10
connections share the 100 bits/sec bandwidth, thus each gets just 10 bits/sec.
Thus, the total time needed to
receive all objects is given by:

(200/100+Tp + 200/100 +Tp + 200/100+Tp + 100,000/100+ Tp )
+ (200/(100/10)+Tp + 200/(100/10) +Tp + 200/(100/10)+Tp + 100,000/(100/10)+ Tp )

Then consider persistent HTTP connection. The total time needed is give by:
(200/100+Tp + 200/100 +Tp + 200/100+Tp + 100,000/100+ Tp )
+ 10*(200/100+Tp + 100,000/100+ Tp )

Assume the speed of light is 300*106 m/sec, then Tp=10/(300*106
)=0.03 microsec. Tp is negligible
compared with transmission delay.
Thus, we see that the persistent HTTP does not have significant gain (less than 1 percent) over the nonpersistent
case with parallel download.

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