Serial Ports

Typical PC serial ports can run at an absolute maximum of 38.4K reliably (all speeds are in bits per second). This is fine for older modem standards, but not for faster modem standards or ISDN.

A few Typical Speeds

Older modems are rated as V.32bis, which translates to a speed of 14.4K. Most of these modems also supported compression, by means of a standard called V.42bis. The effect of modem compression is hard to estimate. With a file that is already compressed, it will usually have no effect at all (or even slow it down); but with typical English text, a compression of about 150% is probably reasonable. This means that a V.32bis modem can get 2,000 - 2,500 cps (characters per second). A character per second is roughly equivalent to about 10 bits per second, allowing for start and stop bits, and is close enough for estimating. Binary files usually compress between 10 - 20%, so they come inbetween English and compressed files. Files that consist of entirely repeated characters could get almost infinite compression, theoretically. dBASE files are not very space efficient, so a factor of 10 to one for these files isn't unusual. As you can see, any figures available for performance with compression are very loose estimates at best. I quote below figures for downloading English text, which come from various sources, such as newsgroup downloads with slurp, or scratchpad transfers from CIX.

More modern modems support V.34, which is 28.8K, or around 3,000 - 3,500 cps with compression. ISDN comes in a number of different forms: V.110, which doesn't really specify bandwidth, but is most often 57.6K; and V.120, which should be 64K. Each of these can have V42bis compression applied, but this isn't by any means as universal as it is with modems. The two D channels available on a BRI (domestic) ISDN can also be bundled, if talking to compatible equipment. As with compression, not all equipment will support this, and most commercial services don't support channel bundling. So ISDN rates can vary from 57.6K, by way of 64K, to about 80K, to 128K, up to 180K - 200K.

If you compare that to the speed of a typical ethernet network, there is still a long way to go. Modern ethernet systems support 10M, and the latest equipment supports 100M. Most ethernet cards in reality work around 4M - 6M for 8 and 16 bit cards, 6M - 8M for 32 bit. And the network software will overload if you run it much greater than about 30% load (which is a common network engineers rule of thumb, and as such can be misleading). This is still 100x as fast as ISDN.

The Port

Most PCs and comms equipment use a serial port for connection. As I said above, a serial port on any PC can be expected to perform at 38.4K. This isn't good enough for V.34 modems, so faster serial port addin cards are available. These use a 16550 chip (or equivalent), which push up the speed that the hardware can do to about 100K - 120K. This solves the analogue modem problem, and works for single channel ISDN, but isn't good enough for channel bonding.

There are a number of solutions to this problem: one is to use a version of the 16550 that support speed doubling or even quadrupling. This lets you push data through the serial port at 230.4K or even 460.8K, by deceiving the PC about the speed you are really using. Again, these are commonly available as add-in cards for PC systems. Another solution is to use the parallel port instead of the serial port.

The parallel port can supply 16 data lines at the same time, instead of dribbling along with one bit at a time, so it has to be faster. This isn't as smart as it sounds, as you need a bidirectional parallel port to pull off this trick, and there is as yet no commonly accepted standard for PC parallel port hardware, unlike the serial port. So throughput with the parallel port can be a guessing game.

The Operating System

Although you can get the data running through your comms lines at a phenomenal speed, with the comms equipment and your hardware all specially selected to work at this speed, there is a good chance that your operating system can't accept the data as fast as it is being pushed at it.

Curiously enough, the less capable your operating system, the faster it can accept data. MS-DOS, which does very little for you and is nothing more than a glorified motherboard driver, is the best at this; Windows, which also does next to nothing, is almost as good. Unix or NT systems may not be able to find the time from servicing all the tasks they are running to beat 19.2K to 38.4K. It should therefore come as no surprise that Windows NT is worse than Windows, but very slightly better than Unix.

Words and design by:
Paul Lynch
Last updated: 27 November 1997
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