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Modems (modulators/demodulators) are data communication devices that convert digital signals to analog signals, and vice versa. Modems allow digital transmissions over analog telephone lines. They allow people to connect their computers with other computers, corporate office LANs, and the Internet. Modems are used to establish connections of the telephone company's local loop, which is the analog copper cable that runs between homes and businesses to the telephone company central office (CO). The phone company then switches the call like any other voice call to create a point-to-point link to the destination.

Before the Internet, the usual connection was for one user to dial another user, or for a user to dial into a modem at a bulletin board, place of business, or a school (rather than connect through the Internet, as discussed next). In these cases, the connection is modem to modem across the PSTN, as shown at the top in Figure M-3. Note that in the figure, the computer's outgoing digital signal is converted to an analog signal for transmission across the copper local loop to the CO. At the CO, the analog signal is converted back to digital for transmission across its digital backbones. At the destination CO, the signal is once again converted from digital to analog and transmitted to the end user's modem, where it is converted back to digital. Remember all those conversions. Later in this topic, the problems they cause will be discussed.

Today, most users dial into the Internet, where they connect with other users via e-mail, collaborative applications, instant messaging, and so on. Users also connect with their home networks over the Internet by using secure VPN technology. This allows users to dial into a local number and connect to the corporate office across the Internet. The bottom half of Figure M-3 illustrates how most modem connections are made in today's Internet-connected environment. The important point of this configuration is that the ISP is connected to the telephone company's CO with a digital connection. This configuration makes high-speed 56-Kbit/sec modem technology possible, as explained later in this topic under "V.90 56-Kbit/sec Modems."

There are two types of modems:

• Consumer voice-grade modems    Most off-the-shelf modems are designed to allow PC users to communicate over the voice phone system. The modems employ compatible communication techniques that comply with several standards, most notably the ITU V series standards (previously called the CCITT standards).


• Broadband modems    These are modems for nontelephone system connections. A company may set up its own dedicated lines or microwave towers and use broadband modems to achieve very high data rates between those sites.

A modem puts an AC (alternating current) carrier signal on the line (in the 1,000-2,000-Hz range) and adds digital information to this signal. A modem at the receiving end then extracts the digital information from the carrier signal. Modulation and demodulation are covered more fully under "Modulation Techniques." Two other related topics are "Error Detection and Correction" and "Signals."

There are some restrictions in the local loop. Long ago, the telephone company established the frequency range of 300-3,300 Hz as the range for voice. The telephone switching system filters out higher frequencies to eliminate noise. Unfortunately, this filtering equipment prevents taking advantage of higher frequencies and wider bandwidth that could help boost data transmission rates. Therefore, a number of techniques, including encoding and compression, have been developed to squeeze as much digital data into the 300-3,300-Hz range as possible. As a side note, the digital subscriber services discussed under "DSL (Digital Subscriber Line)" allow higher frequencies (and higher data rates) on the local loop because they bypass the telephone company's traditional voice switching equipment (class 5 switches).

A voice-grade modem may be either an external box or an internally mounted circuit board. The PC Card format is also available for portable computers. The external modem connects to a computer serial port via an RS-232 serial cable. Serial connection methods are discussed under "Serial Communication and Interfaces."

Carriers and service providers often have racks of modems that users dial into. These are connected to remote access servers that manage the modems and authenticate users. Modems are usually integrated into RAC (remote access concentrator) devices. See "NAS (Network Access Server)," "RADIUS (Remote Authentication Dial-In User Service)," "Remote Access," and "Roaming."

Modems are asynchronous, synchronous, or both. An asynchronous transmission can be compared to the sporadic flow of cars on a highway, while a synchronous transmission can be compared to a steady stream of boxcars on a train track. In asynchronous mode, data is framed and each frame is sent independently. The receiver must be able to detect when a frame starts and ends. In asynchronous mode, a clock signal separates the beginning and end of each frame of data. Synchronous modems are generally more efficient and are typically used for dedicated links between two sites and long streaming data transfers. Most consumer modems are asynchronous. See "Asynchronous Communications" and "Synchronous Communications."

When one modem "calls" another, the destination modem answers and a signal exchange takes place that establishes the parameters for a communication session. The negotiation process determines the maximum signaling rate available between the two modems, as well as the use of compression. Negotiation and signaling are established by the modem standards discussed later.

A full-duplex modem will send signals in both directions at the same time and at the same rate. Newer high-speed (56-Kbit/sec) voice-grade modems such as the V.90 series are asymmetrical, meaning that the download channel has a higher rate than the upload channel.