Most new PCs offer two universal serial bus (USB) ports, either of which can be used to attach a modem. USB is a hot swap (the device can be added or removed without powering down the PC), Plug and Play interface (See Tutorial 10, "Expansion Buses,") well suited to this task. To install a modem this way, usually all that is required is to attach a USB cable between the modem and PC, connect the phone-line cable between the modem and a wall jack, and load the modem-driver software from the manufacturer's configuration disk when prompted.
External Analog Modems
The original modems used a pair of cups to cradle a telephone handset over a built-in speaker and microphone; in this way, the modem would send and receive tones acoustically, and the telephone handset would relay the tones. Today, the external modem is usually a rectangular box with a row of status lights on the front, a speaker to give audible feedback, and a number of ports on the back. Two of those ports are telephone jacks-one to connect to the wall line and the other to pass the telephone signal to a phone for regular voice conversations when the modem is not in data mode. A third port on the back of the modem is a serial port using a standard 25-pin RS-232 connector that passes data to and from a serial port on the PC.
ISDN Terminal Adapters
Until about 30 years ago, the North American telephone network was an analog system connecting phones by means of a grid of copper wires. Today, the long line sections (intercity telecom lines) are part of a packet-based, digital switching system, but the final run from the local switch to most homes is the aged copper-wire POTS line.
ISDN is an all-digital phone connection that uses special high-quality phone lines to ensure clean, high-speed, data transfers-directly to the user's home or business. Both voice and data are carried by bearer channels (B channels) with a maximum speed per channel of 64 Kbps. A companion data channel (D channel) handles signaling at 16 Kbps (or 64 Kbps, depending on service provided by the carrier).
In the context of ISDN communications, "K" means 1000; in other computing contexts, "K" means 1024.
ISDN connections do not make use of a modem. Instead, a device called a terminal adapter (TA) serves as the interface for both computers and analog phones served in a location. Most small-business and residential customers make use of a TA that has a 25-pin serial connection to attach to a computer serial port and that also provides analog telephone connections for two lines.
ISDN is complicated to install and should be set up using the help of a vendor or the local telephone company. After installation, ISDN functions like a high-speed modem, offering not only faster data transfers, but faster connections to remote ISDN providers such as ISPs. Because each TA unit is completely digital, there is no testing of the nature of the remote source by the hardware to establish the maximum connection rate (as with a modem), and links are typically established in under three seconds.