Modem (Modulate / Demodulate) is a device that converts digital signals into analog form (a process known as DEModulate) to send over phone lines, a receiving modem at the other end of phone line then converts the analog signal back to a digital signal (a process known as demodulation). It enables users to transmit data from one computer to another by using standard telephone lines instead of special communication lines such as fiber optic  or cable. Telephone lines can carry analog signals (see figure 3.15).

Telephone_lines_can_carry_analog_signalesFigure 3.15

  • Transmission Rate

It can be described, as modems high –speed or low speed to indicate how many bits per second a specific device is capable of transmitting or receiving.

  • Speeds

The specific techniques used to encode the digital bits into analog signals are called DEModulate process . The various modulation standards define the exact methods of encoding and the data transfer speed . The raw speed (the speed without data comparison) of a modem is determined by modulation standards.  A 2400 bps throughput is not a high speed modem. High speed theoretically yield a 9600 bps throughput is not a high-speed modem. High speed modems are modems that feature modulation at 9600 bps and above (the speed at which data is transmitted in bits per second) or higher .

Types of Modems

In terms of physical size and shape modems can be divided into following categories:

  • External

External modem is attached to the system unit as an external device by meand of a telephone cable. It is connected to the telephone wall jack by another cable. The modem is a self contained unit which is connected to PC using a serial cable to the COM1 or COM2 port. It needs an external power supply , and is easy to set up ( see figure 3.16.a).

serial_cable_to_the_COM1_or_COM2_port

  • Internal Card:An internal modem is circuit board (a modem card ) that can be added to system unit through an expansion slot. The modem cannot be moved easily from one PC to another. It is more difficult to set up than other types of modem( see figure 3.16b)

    difficult_to_set up_than_other_types_of_modem

    Figure 3.16b

  • Wireless Modems:Wireless modems transmit the data signals through the air instead of by using a cable( see figure3.16c) They sonetimes are called a radio-frequency modem. This type of designed to work with cellular technology, and wireless local area networks. Wireless modems are not yet perfected , but the technology is rapidly improving.

    Wireless_modems

    Figure3.16c