Data Communications Networking Devices - 4th Ed.pdf

1.3 TYPES OF SERVICES AND TRANSMISSION DEVICES _______________________________ 9Figure 1.5 Interrelationship of terminals, modems, acoustic couplers, computers andanalog transmission mediums. When using modems on an analog transmission medium,the line can be a dedicated, leased, or switched facility. Terminal devices can use modemsor acoustic couplers to transmit via the switched networktones at the earpiece of the telephone connected to the terminal's acoustic coupler.The coupler then converts those tones into the appropriate electrical signalsrecognized by the attached terminal. The interrelationship of terminals,acousticcouplers,modems and analog transmission media is illustrated in Figure 1.5.In examining Figure 1.5,you will note that a circle subdivided into four equalparts by two intersecting lines is used as the symbol to denote the public switchedtelephone network or PSTN. This symbol will be used in the remainder of thebook to illustrate communications occurring over this type of line connection.Analog facilitiesSeveral types of analog switched and leased line facilities are offered bycommunications carriers. Each type of facility has its own set of characteristicsand rate structure. Normally,for extensive communications requirements,ananalytic study is conducted to determine which type or types of service should beutilized to provide an optimum cost-effective service for the user. Common typesof analog switched facilities are direct distance dialing,wide area telephone service,and foreign exchange service. The most common type of analog line is a voice gradeprivate line.DDDDirect distance dialing DDD) permits a person to dial directly any telephoneconnected to the public switched telephone network. The dialed telephone may beconnected to another terminal device or mainframe computer. The charge for thisservice,in addition to installation costs,may be a ®xed monthly fee if no longdistancecalls are made,a message unit rate based upon the number and duration oflocal calls,or a ®xed fee plus any long-distance charges incurred. Depending upon