Data Communications Networking Devices - 4th Ed.pdf

1.3 TYPES OF SERVICES AND TRANSMISSION DEVICES ______________________________ 13The use of an FX line permits the elimination of long-distance charges thatwould be incurred by users directly dialing a distant computer facility. Since onlyone person at a time may use the FX line,normally only groups of users whoseusage can be scheduled are suitable for FX utilization. Figure 1.8 illustrates thepossible connections between remotely located terminal devices and a centralcomputer where transmission occurs over an analog facility.The major difference between an FX line and a leased line is that any terminaldialing the FX line provides the second modem required for the transmission ofdata over the line; whereas a leased line used for data transmission normally has a®xed modem attached at both ends of the circuit.Leased linesThe most common type of analog leased line is a voice grade private line. This lineobtains its name from its ability to permit one voice conversation with frequenciesbetween 300 and 3300 Hz to be carried on the line. In actuality,the bandwidth orrange of frequencies that can be transmitted over a twisted-pair analog switched oranalog leased line extends from 0 to approximately 1 MHz. However,to economizeon the transmission of multiple voice conversations routed between telephonecompany of®ces,the initial design of the telephone company cable infrastructureresulted in the use of ®lters to remove frequencies below 300 Hz and above3300 Hz,resulting in a 3000 Hz bandwidth for voice conversations. At telephonecompany of®ces voice conversations destined to another of®ce are multiplexedonto a trunk or high speed line by frequency,requiring only 300 Hz of bandwidthper conversation,enabling one trunk to transport a large number of voice conversationsshifted in frequency from one another. At the distant of®ce other frequencydivision multiplexing equipment shifts each conversation back into its originalfrequency range as well as routing the call to its destination. Although the use of®lters has considerably economized on the cost of routing multiple calls on trunksconnecting telephone company of®ces,they have resulted in a bandwidth limit of3000 Hz which makes high speed transmission on an analog loop most dif®cult toobtain. As we turn our attention to the operation of different types of modems laterin this book,we will also obtain an appreciation of how the 3000 Hz bandwidth ofanalog lines limits the communications rate to most homes and many of®ces.Figure 1.9 illustrates the typical routing of a leased line in the United States.The routing from each subscriber location to a telephone company central of®ceserving the subscriber is known as a local loop. Normally the local loop is a twowireor four-wire copper single or dual twisted-pair cable with ampli®ers insertedFigure 1.9 Leased line routing. Leased lines are routed from a local telephone companyserving a subscriber to an interexchange carrier at the point of presence POP)