Modems, An Audio Modem Riser Slot

Modems

The modem is a device used by a PC to communicate over a phone line. A modem can be an external device or a modem card (see the figures below). A modem card uses either an ISA or PCI slot.

To reduce the total cost of a computer system, some systemboards have a small expansion slot, less than half the length of a PCI slot, called an audio/modem riser (AMR) (see the figure below) slot or a communication and networking riser (CNR) slot. The small slot accommodates a small, inexpensive type of modem card called a modem riser card. In addition, the AMR slot can support an audio riser card, and the CNR slot can support an audio riser card or a networking riser card. A riser card has part of the audio, modem, or networking logic on the card and part on a controller on the systemboard.

An Audio/Modem Riser Slot

Regardless of the type of modem card or external device, a modem is both hardware and firmware. On the device is firmware on ROM chips that contains the protocol and instructions needed to format and convert data so that it can be transported over phone lines to a receiving modem on the other end. In general, modems are considered to be hardware, but it is fundamental to an understanding of communications to also consider them to be firmware. Computers are digital; regular phone lines are analog. Data is stored inside a PC and communicated to a modem as binary or digital data—0s and 1s. A modem converts this binary, or digital, data into an analog signal (the process is called modulation) that can travel over a phone line, and then the modem at the receiving end converts it back to digital (this process is called demodulation) before passing it on to the receiving PC. The two processes of Modulation / Demodulation led to the name of the device: modem.

An analog signal is made up of an infinite number of possible values in its range of values, but a digital signal has only a finite number of values in its range of values. Remember also that phone lines were designed to transmit sound (that is, the human voice, which is analog). Sound traveling over regular phone lines is transmitted as analog signals, meaning that there are an infinite number of sound values, just as there are an infinite number of sound values in the human voice.

When data is transmitted over phone lines, even though the data from a PC is inherently digital, it, too, must be converted to an analog signal in order to use telephone technology. Think of PC data as being converted from two simple states or measurements (0 and 1, or off and on) to waves (like sound waves), which have a potentially infinite number of states or measurements. Modems use different characteristics of waves to correspond with the 0s and 1s of digital communication.

Two PCs communicate over phone lines by using either internal or external modems. Either way, the modem provides a connection for a regular phone line called an RJ-11 connection, which is the same type of connection that you see for a regular phone wall outlet (see the figure below). In addition to a line-in connection from the wall outlet, a modem also has an extra RJ-11 connection for a telephone.