Another Bottleneck

We've looked at the packets, compression, and latency. We've seen that each takes a toll on throughput. The following shows the effect of this at various modem speeds.

Let's assume a typical web page is 8KB in size and contains 8 small graphics. HTML is highly compressible, since it is plain ASCII text.

Small art files on the web average 1KB (8000 bits) in size. Most are GIFs or JPEGs, which are already compressed. Except in rare instances, the compression routines in your modem will do nothing to speed their transmission and may actually slow it. The following table shows transmission time in seconds and throughput in kbps for 8KB of source code, for a single 1KB graphic, and to download an entire page with 8 images. Numbers assume a 110 ms latency and 115kbps serial connection to the modem. (56k assumes 53kbps connection, which tests have shown is unlikely to occur in the real world.)

Real world results will vary with line condition and serial port speed. Many older computers have serial ports that top out between 19.2 and 57.6 kbps. Serial port should be set to at least twice modem speed (115k for a 56k modem). More on this topic on the web.

              2400bps   14.4kbps   28.8kbps   33.6kbps    56kbps
HTML         27.1/2.4k  4.6/14.0k  2.4/27.3k  2.1/31.6k  1.4/48.3k
1KB GIF       3.5/2.3k  0.7/11.9k  0.4/20.3k  0.4/22.6k  0.3/29.9k
Page + GIFs  55.4/2.3k 10.1/12.8k  6.0/23.1k  5.0/26.2k  3.5/36.7k

The following table assumes 2:1 compression of HTML source code, but no compression (or overhead) for GIFs.

              2400bps   14.4kbps   28.8kbps   33.6kbps    56kbps
HTML         13.6/4.8k  2.4/27.3k  1.3/51.8k  1.1/59.4k  0.7/87.8k
Page + GIFs  41.9/3.1k  7.9/16.4k  4.5/28.9k  4.0/32.4k  2.9/44.3k

The following table assumes 3:1 compression of HTML source code, but no compression (or overhead) for GIFs. Note that for the 56k modem, the assumed 115k serial port becomes a bottleneck. This is barely faster than 2:1 compression, but only because 115k is a bit more than twice the maximum 53k speed of the modem. A faster serial port or internal modem would overcome this throughput restriction.

              2400bps   14.4kbps   28.8kbps   33.6kbps    56kbps
HTML          9.1/7.1k  1.6/39.8k  0.9/73.9k  0.8/84.2k  0.7/94.9k
Page + GIFs  37.4/3.5k  7.1/18.2k  4.1/31.6k  3.7/35.3k  2.9/45.1k

Some interesting facts emerge.

• Larger files show higher throughput than smaller files, particularly with faster modems.
• Assuming equal download of text and graphics on the web, a 56k modem is only 35-40% faster than a 33.6k modem and 50-60% faster than a 28.8k modem.
• As we approach the speed of serial ports (here assumed at 115kbps, but often slower on older computers), serial port speed negatively impacts throughput. This is more pronounced with highly compressible data. This makes a strong argument for internal modems, faster serial ports, and serial port alternatives (ethernet, Firewire, universal serial bus, etc.)
• Although compression is helpful, the benefit of a faster modem is more pronounced with uncompressed data. With an 8KB file, a 56k modem runs 77% faster than 28.8, but with compression, it is only 64% faster. Latency and serial port speed account for the difference, since these figures make no allowance for actual compression or decompression time.
• Latency takes a greater toll on faster modems. While a 53k connection should have 22 times greater throughput than a 2.4k connection, it is only 20 times better for an 8KB file and just 13 times better for a 1KB file.