Latency Plus Compression: Another Modem Bottleneck

1998: We’ve looked at 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 8 KB in size and contains 8 small graphics. HTML is highly compressible since it is plain ASCII text.

Small art files on the web average 1 KB (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 8 KB of source code, for a single 1 KB graphic, and to download an entire page with 8 images. Numbers assume a 110 ms latency and 115 kbps serial connection to the modem. (56k assumes 53 kbps 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 (so 115 kbps 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 115 kbps serial port becomes a bottleneck. This is barely faster than 2:1 compression, but only because 115 kbps is a bit more than twice the maximum 53 kbps speed of the modem. A faster serial port or an 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 115 kbps, 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, USB, Apple’s Comm Slot, etc.)
  • Although compression is helpful, the benefit of a faster modem is more pronounced with uncompressed data. With an 8 KB 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 53 kbps connection should have 22 times greater throughput than a 2.4 kbps connection, it is only 20 times better for an 8 KB file and just 13 times better for a 1 KB file.

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