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14. What Speed Should I Use with My Modem?

By "speed" we really mean the "data flow rate" but almost everybody incorrectly calls it speed. For all modern modems you have no choice of the speed that the modem uses on the telephone line since it will automatically choose the highest possible speed that is feasible under the circumstances. If one modem is slower than the other, then the faster modem will operate at the slower modem's speed. On a noisy line, the speed will drop still lower.

While the above speeds are selected automatically by the modems you do have a choice as to what speed will be used between your modem and your computer (PC-to-modem speed). This is sometimes called "DTE speed" where "DTE" stands for Data Terminal Equipment (Your computer is a DTE.) You need to set this speed high enough so this part of the signal path will not be a bottleneck. The setting for the DTE speed is the maximum speed of this link. Most of the time it will likely actually operate at lower speeds.

For an external modem, DTE speed is the speed (in bits/sec) of the flow over the cable between you modem and PC. For an internal modem, it's the same idea since the modem also emulates a serial port. It may seem ridiculous having a speed limit on communication between a computer and a modem card that is directly connected inside the computer to a much higher speed bus. But it's usually that way since the modem card probably includes a dedicated serial port which does have speed limits (and settable speeds). However, some software modems have no such speed limits.

14.1 Speed and Data Compression

What speed do you choose? If it were not for "data compression" one might try to choose a DTE speed exactly the same as the modem speed. Data compression takes the bytes sent to the modem from your computer and encodes them into a fewer number of bytes. For example, if the flow (speed) from the PC to the modem was 20,000 bytes/sec (bps) and the compression ratio was 2 to 1, then only 10,000 bytes/sec would flow over the telephone line. Thus for a 2:1 compression ratio you would need to set the DTE speed to double the maximum modem speed on the phone line. If the compression ratio were 3 to 1 you would need to set it 3 times faster, etc.

14.2 Where do I Set Speed ?

This DTE (PC-to-modem) speed is normally set by a menu in your communications program or by an option given to the getty command if someone is dialing in. You can't set the DCE modem-to-modem speed since this is set automatically by the modem to the highest feasible speed after negotiation with the other modem. Well, actually you can set the modem-to-modem speed with the S37 register but you shouldn't do it. If the two modems on a connection were to be set this way to different speeds, then they couldn't communicate with each other.

14.3 Can't Set a High Enough Speed

Speeds over 115.2kbps







How speed is set in hardware: the divisor and baud_base





Setting the divisor, speed accounting




Crystal frequency is higher than baud_base



14.4 Speed Table

It's best to have at least a 16650 UART for a 56k modem but few modems or serial ports provide it. Second best is a 16550 that has been tweaked to give 230,400 bps (230.4 kbps). Most people still use a 16550 that is only 115.2 kbps but it's claimed to only slow down thruput by a few percent (on average). This is because a typical compression ratio is 2 to 1 and for downloading compressed files (packages) it's 1 to 1. There's no degradation for these cases. Here are some suggested speeds to set your serial line if your modem speed is:

All the above speeds may use V.42bis data compression and V.42 error correction. If data compression is not used then the speed may be set lower so long as it's above the modem speed.

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