Bandwidth Bottlenecks
Caches also can reduce network bottlenecks. Many networks provide more bandwidth to local network clients than to remote servers (Screenshot 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.
(Screenshot 7-1.)
In Screenshot 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. 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.
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 |