11 Aug 1996

The Storm Before the COM

PC Communcations over Modems and ISDN

You have a big problem when, at the start, you don't even know what to call the main subject of an article. DOS gave the names COM1, COM2, ... to the devices. In other contexts, they are called serial ports because they transmit bits one at a time (in contrast to the "parallel port" which sends data one byte at a time to the printer or Zip drive). When you plug an internal modem card inside the machine, it behaves like a COM2 or COM3 port. However, it is also possible to connect serial mice to one of these ports. Initially the COM port was used by terminal emulators, but today it is used by the Dial Up Networking support to connect a home or laptop computer to the corporate network.

The people who designed the first IBM PC did not set out to establish a standard that would last for decades. They selected the chips that were cheapest or easiest to find. Programs were then written to use those specific chips, and subsequent generations of hardware found that it was easier to emulate the earlier hardware rather than try and change things.

The chip IBM selected for its COM port was made by a company called National Semiconductor. There was nothing special about the chip, except perhaps that it was cheap enough that the optional serial port card could be sold for $35. In those days, modems transferred data at 2400 baud (bits per second). Today there are modems that run ten times faster, and the effective data transfer rate can be doubled again by compression. Yet the COM port in a PC still duplicates the programming interface of the original 1981 chip.

DOS originally provided no meaningful support for the COM chip. Communications programs (which in those days emulated some type of computer terminal) were written to manipulate the serial port chip directly. However, direct use of hardware devices is regarded as bad form under Windows 95, OS/2, and other modern operating systems. Windows provides a rich set of communications services, including "telephony" and fax modem support, that modern application programs are forced to use. In Windows 95, the operating system is configured with all the special characteristics of a particular modem. Then application programs do not have to figure out what special sequence and options should be used to dial the phone.

Although modem speeds have increased every few years, the current generation of 28800 baud modems is close to the end of the line. The phone system itself has an internal speed limit of 64000 bits per second, and because data is converted into sound and then back into data without synchronizing your computer to the phone company, the best that is available in data transfer is half that speed. This is a mathematical limit and cannot be broken by advances in chip manufacture.

However, ten years after the digital Compact Disk replace old phonograph records, traditional voice telephones are an obsolete technology. Internally, the local phone companies, long distance, and international carriers have converted to digital service. There is an international standard for digital phone service to the home or business. It is called ISDN and is just beginning to be widely offered in the US. With ISDN, computers can transfer data at the full internal phone company speed of 64000 bits per second on each phone line, and higher effective data rates can be achieved by using two or more phone lines in parallel.

In the new deregulated environment, customers have to purchase their own phone equipment. This has not been a problem for traditional phone service, because everyone went to the store and bought phones that looked and worked just like the units that the phone company used to install. However, ISDN requires new types of unfamiliar equipment. This article will also introduce the most general and least expensive ISDN equipment and will provide an outline of the steps to exploit this new service.


How Modems and ISDN Work

Modems and Asynchronous Communication

Installing ISDN

Universal Serial Bus (USB)

Copyright 1995 PCLT -- The Storm Before the COM -- H. Gilbert