Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 373transmission through the use of echo cancellation. In addition, trellis coding of datais an optional mode of operation for V.32 modems which, when in effect, results inthe probability of a bit error occurring less than that of most modems operating atsigni®cantly lower data rates than a V.32 modem.Types of modems and featuresModems can be categorized based upon a large number of features, their physicalconstruction, the type of data they can modulate, and the type of telephone facilitythey can operate upon. To become familiar with the basic types of modems, we willexamine many of the more popular features that can be used to de®ne differenttypes of this communications device.Mode of transmissionIf the transmitter or the receiver of the modem is such that the modem can send orreceive data in one direction only, the modem will function as a simplex modem. Ifthe operations of the transmitter and receiver are combined so that the modem maytransmit and receive data alternately, the modem will function as a half-duplexmodem. In the half-duplex mode of operation, the transmitter must be turned off atone location, and the transmitter of the modem at the other end of the line must beturned on before each change in transmission direction. The time interval requiredfor this operation is referred to as turnaround time. If the transmitter and receiveroperate simultaneously, the modem will function as a full-duplex modem. Thissimultaneous transmission in both directions can be accomplished by the use ofecho cancellation, splitting the telephone line's bandwidth into two channels on atwo-wire circuit, or by the utilization of two two-wire circuits, such as are obtainedon a four-wire leased line.Transmission techniqueModems are designed for asynchronous or synchronous data transmission.Asynchronous transmission is also referred to as start-stop transmission and isusually employed by unbuffered terminal devices where the time between charactertransmission occurs randomly.In asynchronous transmission, the character being transmitted is initialized bythe character's start bit as a mark-to-space transition on the line and terminated bythe character's stop bit which is converted to a `space/marking' signal on the line.The digital pulses between the start and stop bits are the encoded bits whichdetermine the type of character which was transmitted. Between the stop bit of onecharacter and the start bit of the next character, the asynchronous modem placesthe line in the `marking' condition. Upon receipt of the start bit of the nextcharacter the line is switched to a mark-to-space transition, and the modem at theother end of the line starts to sample the data.