Consider a short link with a transmission rate of 100 bits/sec in either directi

Question

Consider a short link with a transmission rate of 100 bits/sec in either direction. Packets containing data are 100Kbits long, and packets containing only control (for example, ACK) are 100 bits long. Assume that N parallel connections each get 1/N of the link bandwidth. Now consider the HTTP protocol, and suppose that each downloaded object is 100Kbits long, and that the initial downloaded object contains 20 referenced objects from the same sender.

a) Would parallel downloads of non-persistent HTTP make sense in this case?

b) With respect to persistent HTTP, would you expect significant gains over non-persistent HTTP?

Explain your answer.

Explanation / Answer

Solution:

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 using non-persistent connections. Parallel downloads would allow 10 connections to share the 150 bits/sec bandwidth, giving each just 15bits/sec. Thus, the total time needed to receive all objects is given by:

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

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

= 11033 + 8*Tp (seconds)

Now consider a persistent HTTP connection. The total time needed is given by:

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

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

=11013 + 24*Tp(seconds)

Assuming the speed of light is 300*106m/sec, then Tp=10/(300*106)=0.03 microsec. Tp

is therefore negligible compared with transmission delay.

Thus, we see that persistent HTTP is not significantly faster (less than 1 percent) than the non-persistent case with parallel download

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