Analog Equipment

Because you cannot pass an analog signal directly to a digital device or vice versa, you need analog-digital (A-D) and digital-analog (D-A) converters.

The modem is a classic example of an A-D/D-A converter. The modem converts a digital signal into an analog audio signal when sending (originating) data over a phone line. It then converts the signal from analog to digital for the terminating (receiving) end of the line.

Modem is an abbreviation for MOdulator-DEModulator. A modem is a device connected to a PC that enables the PC to transmit data over a telephone line. Modems convert digital signals (from the PC) to analog signals for transmission across a phone line. Wireless modems convert digital data into radio signals and vice versa.

Modems came into existence in the 1960s as a way enabling communication terminals to connect to computers over telephone lines. A typical arrangement is illustrated in Figure 4-6.

Figure 4-6. Modem Operation (with Terminal and Mainframe)

As faster modems were invented and deployed, the concept of gradual degradation was introduced. Gradual degradation is the modem's capability to test the phone line and fall back to slower speeds if the line cannot support the modem's fastest possible speed. Gradual degradation is why a 56 Kbps dial-up modem will sometimes connect at 53.3 Kbps or 48 Kbps, and 33.6 Kbps other times.

The next step in modem evolution was the asymmetric digital subscriber line (ADSL) modem. This is called asymmetric because these modems send data faster in one direction than the modem sends in the other. ADSL modems operate over dedicated copper facilities between the residence/business and the Local Exchange Carrier's DSL Access Multiplexer (DSLAM).

Technical Note: DSL Modems

DSL modems function by sharing, or dividing, the frequency spectrum. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe.