Data Communications Networking Devices - 4th Ed.pdf

1.3 TYPES OF SERVICES AND TRANSMISSION DEVICES _______________________________ 5Table 1.2General line selection guideDistance between Speed of Use ofLine type transmission points transmission transmissionDedicated Local Limited by Short or long durationdirect connect)conductorSwitched Limited by telephone Normally up to Short-durationdial-up) access availability 33 600 bps analog), transmission1.544 Mbps digital)Leased private) Limited by telephone Limited by type of Long duration orcompany availability facility numerous shortduration calls1.3 TYPES OF SERVICES AND TRANSMISSION DEVICESDigital devices which include terminals,mainframe computers,and personalcomputers transmit data as unipolar digital signals,as indicated in Figure 1.1a).When the distance between a terminal device and a computer is relatively short,thetransmission of digital information between the two devices may be obtained bycabling the devices together. As the distance between the two devices increases,thepulses of the digital signals become distorted because of the resistance,inductance,and capacitance of the cable used as a transmission medium. At a certain distancebetween the two devices the pulses of the digital data will distort,such that they areunrecognizable by the receiver,as illustrated in Figure 1.1b). To extend thetransmission distance between devices,specialized equipment must be employed,with the type of equipment used dependent upon the type of transmission mediumemployed.Figure 1.1 a) Digital signaling. Digital devices to include terminals and computerstransmit data as unipolar digital signals. b) Digital signal distortion. As the distance betweenthe transmitter and receiver increases digital signals become distorted because of theresistance, inductance, and capacitance of the cable used as a transmission medium