HTTP: The Definitive Guide

Bandwidth Bottlenecks

Caches also can reduce network bottlenecks. Many networks provide more bandwidth to local network clients than to remote servers (Figure 7-1). Clients access servers at the speed of the slowest network on the way. If a client gets a copy from a cache on a fast LAN, caching can boost performance—especially for larger documents.

Figure 7-1. Limited wide area bandwidth creates a bottleneck that caches can improve

In Figure 7-1, it might take 30 seconds for a user in the San Francisco branch of Joe’s Hardware, Inc. to download a 5-MB inventory file from the Atlanta headquarters, across the 1.4-Mbps T1 Internet connection. If the document was cached in the San Francisco office, a local user might be able to get the same document in less than a second across the Ethernet connection.

Table 7-1 shows how bandwidth affects transfer time for a few different network speeds and a few different sizes of documents. Bandwidth causes noticeable delays for larger documents, and the speed difference between different network types is dramatic.^[1] A 56-Kbps modem would take 749 seconds (over 12 minutes) to transfer a 5-MB file that could be transported in under a second across a fast Ethernet LAN.

Table 7-1. Bandwidth-imposed transfer time delays, idealized (time in seconds)

	Large HTML (15 KB)	JPEG (40 KB)	Large JPEG (150 KB)	Large file (5 MB)
Dialup modem (56 Kbit/sec)