Know your modem basics

Despite the superior speed of DSL, the lowly modem is still the dominant communications peripheral for remote users. Dinosaur or not, techs should make it a point to brush up on their modem know-how, starting with UART chips and stop and start bits.

Although modems have changed little over the last several years, they are still one of the most common peripherals. Even with the advent of broadband communications, more people still use modems to access the Internet than use broadband. Given this reliance on this older technology, support pros need to know the mechanics behind these still-dominant communication devices. The first step in understanding this peripheral is learning about basic modem functionality, from the UART chip to stop and start bits.

What's in a name?
The word modem is an acronym, short for modulation=demodulation. The reason that modems have this name is because they modulate and demodulate data. As you’re no doubt aware, computers are completely digital. Everything inside a computer works through binary code. (Refresher note: Binary code is a series of zeros and ones, which are represented to the system through either the presence of electrical current [one] or the absence of it [zero].)

Digital to analog and back again
In a nutshell, modems are used to allow two computers to communicate with each other over a phone line. However, standard phone lines, which provide plain old telephone services (POTS), can’t accept digital data. These phone lines were originally designed to carry only audio. Therefore, a modem’s job is to convert a purely digital signal into an analog signal (in this case, a series of tones) that can be sent across the phone line. The receiving modem must then be able to listen to these tones and convert the analog signal back into a digital signal.

Converting between analog and digital data is actually only half of the battle. Once the data has been converted, it must be placed into a format that the computer can understand. The conversion process results in a single stream of zeros and ones, known as serial data. However, your computer expects a stream that’s at least eight bits wide because of the computer’s bus design. This eight-bit data stream is known as parallel data. A single stream of data is completely useless to the computer. Therefore, the next step in the process is to convert the data from a single-bit data stream into an eight-bit data stream.

The UART chip
The process of converting the single-data stream into eight different data streams is handled by a universal asynchronous receiver-transmitter (UART) chip. The UART chip is the primary component in your PC’s serial port. Data flowing out of the serial port is converted from eight data streams into a single-data stream, while inbound data is converted from a single-data stream into eight different data streams. Of course, only external modems use a serial port. Internal modems have a UART chip built directly onto the modem card. This is why PCs sometimes see internal modems as being associated with serial ports that don’t exist.

In the UART acronym, the letter “A” stands for asynchronous. There are two types of communications that a modem can participate in: synchronous and asynchronous. Synchronous communications involve both computers remaining completely synchronized to each other, while asynchronous communications are unsynchronized. In asynchronous communications, either computer is free to transmit data at will. The best description of this that I’ve ever heard compares asynchronous communications to a voice phone conversation. During a voice conversation, either person is free to speak at any time, and the conversation doesn’t have to follow a rigid standard of who can speak when.

Start and stop bits
Because asynchronous communications are unregulated, the computers need a method for telling where one bit ends and the next one begins. This is accomplished through the use of start bits and stop bits. The computer places a known bit at the beginning and the end of each set of bits. The receiving modem looks for this known bit combination. Typically, the start bit and the stop bit will both be set to zero for each set of data bits.

Rarely, a parity bit is also appended to a data block just before the stop bit. The receiving computer can use the parity bit to determine whether the data bit was correct. However, almost no one uses parity bits anymore because the newer modems are much better at producing reliable communications than older modems were.

Stay tuned
Now that I’ve introduced you to the basic functionality of a modem, it’s time to move on to other issues such as speed, flow control, and handshaking. I’ll be discussing these topics in the next installment of this series on modem basics.

Broadband: The 56K killer?
With the growing availability of broadband service, will 56K modems soon be a thing of the past? How many more years will support techs be fiddling with dial-up connections and AT commands? Post a comment to this article and tell us whether you think the 56K modem is in its twilight years.


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