Bandwidth Bottlenecks

7.2 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 performanceespecially 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.

^[1] This table shows just the effect of network bandwidth on transfer time. It assumes 100% network efficiency and no network or application processing latencies. In this way, the delay is a lower bound. Real delays will be larger, and the delays for small objects will be dominated by non-bandwidth overheads.

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)	2.19	5.85	21.94	748.98
DSL (256 Kbit/sec)	.48	1.28	4.80	163.84
T1 (1.4 Mbit/sec)	.09	.23	.85	29.13
Slow Ethernet (10 Mbit/sec)	.01	.03	.12	4.19
DS3 (45 Mbit/sec)	.00	.01	.03	.93
Fast Ethernet (100 Mbit/sec)	.00	.00	.01	.42