Modems and COM Ports 26 April 97
The modern telephone system is designed to carry the sound of a human voice talking normally. The engineers save money by cutting out the high a low pitch sounds that occur only in music, or sounds that last for a very short period of time. Modems convert data into sound so that it can be sent through the phone system. Just as the telegraph sent data by alternating dots and dashes, the earliest modems sent data by alternating the sound on a phone between two tones. However, modern modems move much larger amounts of data by sending combinations of tones at different frequencies.
If you have access to all possible frequencies, there is an enormous amount of data you can carry. Each TV channel is about 3 Megabits of data per second, and a cable system carries nearly 100 channels. However, the telephone system is designed to only handle the frequencies of the human voice, and the maximum amount of data it can theoretically carry is 4000 bytes per second per phone line.
Trying to squeeze as much data as possible into the available frequencies makes the signal vulnerable to "noise" that is introduced from a bad connection, "crosstalk" from other phone conversations in the same bundle of wires, and external electrical sources. Computer chips made it possible to build a "smart" modem that transmits a block of data, waits for an acknowledgment, and retransmits the block if it something went wrong the first time. Then data compression was added. In current use, the information actually exchanged between the two modems may be much more complicated and sophisticated than the data that either computer sees.
Sound waves are a regular variation in air pressure. If pressure changes once, it is not sound. When you go up 20 stories in an elevator, the pressure changes but no "sound is produced." Bounce a ball and a much, much smaller pressure change occurs, but because the pressure varies up and down over time in a regular pattern there is a sound. As a result, there is no such thing as "instantaneous" sound. Sound can only occur over time. Furthermore, the lower the pitch, the longer the wavelength, and the longer the time needed before the sound can be clearly observed.
A telephone has a microphone and speaker. The microphone generates an electric wave from the sound waves that strike it. The speaker generates a sound wave from the electrical waves that it receives. This is an "analog" system, and is basically the same as the signal between components in a stereo system.
A telephone circuit has one pair of wires. The two wires are twisted around each other to reduce the effect of external noise. Normally, phone service runs two pair of wires from the pole to your house, allowing for two phone lines. The wires not only provide a signal, but they also power the phone.
Thirty years ago, the entire phone system transmitted amplified sound waves throughout the country. Then the same silicon chip technology that made computers possible changed the entire internal design of the phone system. Rather than dealing with analog waves, the modern phone equipment turns the sound into a stream of bytes.
The ability to turn sound into data should not surprise anyone. Music is recorded as digital data on Compact Disks, which provide better quality and more reliability than old analog tapes or records. The phone company "samples" the incoming electrical waves 8000 times a second and records one byte of data. This produces an 8000 byte per second stream of digital information. The modern phone "switch" is a large special purpose computer that moves bytes of data from one device (such as the sampler connected to your phone line) to another device (ether the phone of the neighbor you are calling or a trunk line to another exchange or a long distance carrier).
A telephone switch has to move a lot of data, but unlike a real computer it doesn't have to store any of it. The bytes move out as fast as they move in, so they don't have to move to memory or disk. Internally, all phone company equipment, including all the long distance carriers, operate on the digital streams of 8000 bytes per second. The data remains digital until it arrives at the telephone switch of the phone company that services the other end of the conversation. Then it is turned back into an analog electrical signal (a "sound wave") for transmission to the phone device.
Phone companies don't want to string a separate pair of phone wires between the central switches for every phone conversation that they carry. Once it has been reduced to digital data, several phone conversations can be combined on a wire. Companies that use a large number of phone lines, such as Internet Service Providers, find it simpler and less expensive to connect to the phone company through the internal digital signal conventions. A large phone user can rent a "T1" line that supports 24 separate digital phone lines each with an 8000 byte per second data stream.
Modems will never be much faster than they are today. This is not a technical limit, but a mathematical law. The modem converts the data to sound, and the phone company samples that sound 8000 times a second. If the modem knew when the samples were going to occur (if the transmitter and receiver are "synchronized"), then it would be possible to generate a different byte of data for every one of those 8000 samples. Without synchronization, the best you can do is to transmit a byte of data every two samples. That puts a limit (on current conventional voice grade analog phone circuits) of 4000 bytes per second of transmitted data. Some additional speed is lost to housekeeping, and the result is very close to modern V.34 modems.
This limit applies only to the unsynchronized conversion of sound to data through sampling. It doesn't apply when digital data is converted back to sound again. If you control the 8000 byte per second original digital data internally transmitted through the phone switch, then you can generate a full 8000 byte per second modulated sound signal from the phone company to a modem.
A new generation of "56K bps" equipment operates this way. To work the trick, a company or ISP must connect directly to the internal phone system digital signal through a T1 line. It can then generate an 8000 byte per second digital signal (or 56Kb rounded down for housekeeping) to the modem. However, in the opposite direction the modem can only generate sound that must be sampled. So the 56Kb speed applies in one direction (from the Internet to the home) but data transmitted in the other direction is limited to previous V.34 speed.
Unfortunately, there is currently no single standard for the 56K bps trick. Several different vendors have incompatible versions. To use this service, it is necessary to buy a modem at home which is compatible with the special equipment used by your company or ISP.
Digital phone equipment is used everywhere except at your home or office. Since the limit on data transmission is caused by the conversion of sound to a digital signal, the obvious direct solution to the problem is to extend digital signaling all the way to the home. A T1 line would provide this type of connection. It carries 24 phone circuits each with 8000 bytes per second, or a single circuit of around 1.5 Million bits per second. Prices vary depending on location. In Connecticut, they start at around $400 per month.
An international standard called ISDN has been established for an less expensive personal digital service. An ISDN line carries two digital phone circuits on a single pair of wires, and in Connecticut the starting price is $50 per month. This is called a Basic Rate Interface or BRI circuit.
Because it is designed for use in a home or small office, ISDN allows the lines to be shared by voice telephones, fax machines, and computers. If an incoming phone call originates from another ISDN device, the digital "ring" signal identifies the type of device that places the call. Only the fax machine would answer an incoming fax. An incoming voice call would ring the phone. However, only in Europe do they generally use native ISDN devices.
The BRI has two logical phone lines. They may have two phone numbers, but today phone companies are running out of numbers. A better practice is for them to share one number. If one line is busy and a second call comes in, it rings on the second line.
A conventional telephone line runs from the pole to a distribution point in the building. It may then be wired to many different jacks. More than one phone can be connected to the line. However, an ISDN line has to provide high speed and low noise, so it can only be wired to one jack and, initially, connect to one device.
ISDN Jargon:
In the US, home users select one of three types of ISDN devices.
In all three cases, it is probably a good idea to use a device that connects directly to the U-loop (that has a built-in NT1). It is also common for the devices to supply one or two ordinary telephone jacks that allow the user to make ordinary voice telephone calls over the ISDN line.
An ISDN line is a digital phone service. It provides a raw 8000 byte per second communication channel. You can dump digitized voice into it with little thought. However, to transfer data between computers there has to be a protocol. The data must be divided up into packets. There must be some type of error detection and recovery. If the speed of the sender doesn't match the speed of the receiver, or exactly match the 8000/sec clock, then there has to be some pacing or padding.
There are a bunch of standards. There is synchronous ISDN, asynchronous ISDN, frame-relay ISDN. Anything you can do with some other type of phone you can also do with ISDN. This stuff matters if you are planning to hook up to an IBM mainframe SNA network. Under the covers, two CISCO routers may talk to each other using any protocol they choose. However, the only thing that really matters today is Internet access, and for that the standard is PPP.
Now comes the part where the pieces don't exactly fit. Each PC comes with one or two COM ports that support asynchronous communication. Technically, this means that the COM port transmits each byte as a sequence of 10 bits (one start bit, eight data bits, and one stop bit) at one of the supported speeds (57600 or 115200 bits per second in this case). There can be an additional delay between any two bytes of any length, at the discretion of the sender.
ISDN, on the other hand, transfers 64000 bits per second per phone line. Every 1/64000th of a second, the sender has to provide another bit. Combine the two lines and you have 128000 bits per second, but there has to be a standard to say how the bits are divided between the lines at one end and combined back together at the other.
The most common solution is to let the PC communicate at 115200 bits per second asynchronous PPP to the ISDN card or external "modem". The device then converts the asynchronous version of the PPP protocol into the synchronous version of the protocol, and transmits the same data over the 128000 bit per second carrier. At the other end it can be converted back to an asynchronous signal, though it is more common for the central corporate, campus, or ISP equipment to run natively on synchronous protocols.
People use the phone occasionally. Even teenagers get off the phone to eat and sleep. However, computers can remain connected for hours. Since a local call is typically free, there is no pressure to hang up the phone when the computer user is working on another application. While the connection is active, the modems continue to exchange sounds (the "carrier" tone). Today, with the growth of interest in the Internet, local phone companies are complaining that their traffic load has increased substantially. All those long data calls place a burden on the central switching equipment.
A modem may take a minute to dial the line and establish a connection. An ISDN connection can be established in seconds. Many ISDN devices will hang up the line when there is no traffic, then dial back automatically when there is something to send. And with a digital connection, there is no meaningless noise continuously flowing between the two ends. Bytes flow only when there is meaningful data.
ISDN could be a win-win arrangement for the phone companies. They can install two phone lines in the same wire now used for a single line. ISDN uses the central office equipment more efficiently, and may limit the need for expensive upgrades of switching equipment to handle the increasingly popular internet services. Unfortunately, local phone companies have viewed ISDN as a premium service for which they could charge an unreasonable price.
Return to the Table of Contents
Copyright 1995 PCLT -- The Storm Before the COM -- H. Gilbert