How Modem Latency Impacts Throughput

1998: I recommend you read It’s the Latency, Stupid by Stuart Cheshire to get a good understanding of latency, then come back to this overview.

In real-world tests, the following latencies are typical (most results from Cheshire):

  • <4 ms for ethernet (intranet, no modem)
  • 5-8 ms using a cable modem
  • 21-34 ms with an ISDN-ethernet bridge
  • 110-159 ms with a standard modem (regardless of rated throughput)
  • 221-228 ms with Apple GeoPort modem and Express Modem 3.0 software

In the language of seconds, an ethernet link is established in under 1/250 sec., cable modem in about 1/125 sec., ISDN in roughly 1/40 sec. But your phone modem regardless of rated speed takes 1/6 to 1/9 sec. to establish a connection. Even if your modem could send 53,000 bits per second (FCC-mandated maximum and theoretically possible on a clean phone line), at least one-ninth of a second is wasted per file. Assuming a 53k connection, your modem will process no more than 47,100 bits during the first second of transmission. If your browser is downloading three or four files concurrently (which is not unusual), even more time is spent establishing communication.

Manufacturers probably could reduce the latency of analog modems, but it might come at the expense of compatibility with older hardware. And since users haven’t made a big deal of these delays, modem makers will continue to focus on total capacity instead of latency.

Until they address latency, there’s nothing you can do about bad latency other than switch to a lower latency connection such as ISDN, ADSL, or cable.

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