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 ones speed. On a noisy line, this 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 should 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 that way since the modem card probably includes a dedicated serial port which does have speed limits (and settable speeds).

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 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 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.

14.3 Can't Set a High Enough Speed

You need to find out the highest speed supported by your hardware. As of late 1998 (and late 2000) most serial ports only supported speeds up to 115.2k bps. Some 56k internal modems support 230.4k bps (but it may be hard to find out which ones do). Recent Linux kernels support high speeds (over 115.2k) but you might have some problems using it because of one or both of the following reasons:

1. Older versions of an application program (or stty) may not accept the high speed.
2. Setserial may show the wrong speed unless you use the baud_base option. In any case the high speed will work OK.

How speed is set in hardware: the divisor and baud_base

Here's a list of commonly used divisors and their corresponding speeds (assuming a maximum speed of 115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24 (4.8k), 48 (2.4k), 96 (1.2k), etc. The serial driver sets the speed in the hardware by sending the hardware only a "divisor" (a positive integer). This "divisor" divides the maximum speed of the hardware resulting in a slower speed (except a divisor of 1 obviously tells the hardware to run at maximum speed).

Normally, if you specify a speed of 115.2k (in your communication program or by stty) then the serial driver sets the port hardware to divisor 1 which obviously sets the highest speed. If you happen to have hardware with a maximum speed of say 230.4k, then specifying 115.2k will result in divisor 1 and will actually give you 230.4k. This is double the speed that you set. In fact, for any speed you set, the actual speed will be double. If you had hardware that could run at 460.8k then the actual speed would be quadruple what you set.

Work-arounds for setting speed

To correct this accounting (but not always fix the problem) you may use "setserial" to change the baud_base to the actual maximal speed of your port such as 230.4k. Then if you set the speed (by your application or by stty) to 230.4k, a divisor of 1 will be used and you'll get the same speed as you set. PROBLEM: stty and many communication programs (as of mid 1999) still have 115.2k as their maximum speed setting and will not let you set 230.4k, etc. So in these cases one solution is not to change anything with setserial but mentally keep in mind that the actual speed is always double what you set.

There's another work-around which is not much better. To use it you set the baud_base (with setserial) to the maximal speed of your hardware. This corrects the accounting so that if you set say 115.2k you actually get 115.2k. Now you still have to figure out how to set the highest speed if your communication program (or the like) will not let you do it. Fortunately, setserial has a way to do this: use the "spd_cust" parameter with "divisor 1". Then when you set the speed to 38400 in a communication program, the divisor will be set to 1 in the port and it will operate at maximum speed. For example:
setserial /dev/ttyS2 spd_cust baud_base 230400 divisor 1
Don't try using "divisor" for any other purpose other than the special use illustrated above (with spd_cust).

If there are two or more high speeds that you want to use that your communication program can't set, then it's not quite as easy as above. But the same principles apply. You could just keep the default baud_base and understand that when you set a speed you are really only setting a divisor. So your actual speed will always be your maximum speed divided by whatever divisor is set by the serial driver. See How speed is set in hardware: the divisor and baud_base

Crystal frequency is not baud_base

Note that the baud_base setting is usually much lower than the frequency of the crystal oscillator in the hardware since the crystal frequency is often divided by 16 in the hardware to get the actual top speed. The reason the crystal frequency needs to be higher is so that this high crystal speed can be used to take a number of samples of each bit to determine if it's a 1 or a 0.

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:

• 56k (v.90): use 115.2 kbps or 230.4 kbps (best)
• 33.6k (v.34bis): use 115.2 kbps
• 28.8k (v.34): use 115.2 kbps
• 14.4k (v.32bis): use 57600 bps
• 9.6k (v.32): use 38400 bps
• slower than a 9600 bps (v.32) modem: Set the speed to the same speed as the modem.

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.