Data Communications Networking Devices - 4th Ed.pdf

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630 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTAn FDM's aggregate data handling capacity depends upon the mixture ofsubchannels as well as on the type of line conditioning added to a circuit. A chart ofFDM subchannel allocations for a voice-grade leased line is illustrated in Figure6.3. This chart can be used to compute the mixture of data subchannels that can betransmitted via a single voice-grade leased line when frequency division multiplexingis employed. The ®gure is based upon data subchannel spacing standardsformulated by the ITU. As illustrated, 17 75-bps subchannels can be multiplexedon an unconditioned C0) circuit, 19 subchannels on a C1 conditional circuit, 22subchannels on a C2 circuit and 24 channels on a C4 conditioned circuit. For higherdata rates ITU standards allocate a fractional proportion of the bandwidth allocatedto the previously discussed data rate of 75 bps.With the development of terminals operating at speeds that were not multiples ofITU frequencies, such as 134.5 bps teleprinters, a number of vendors developedFDM equipment tailored to make more ef®cient use of voice-grade circuits thanpermitted by ITU standards.Since the physical bandwidth of the line limits the number of devices which maybe multiplexed, FDM is mainly used for multiplexing low-speed asynchronousterminals. An advantage obtained through the use of such equipment is its codetransparency. Once a data band is set, any terminal operating at that speed or lesscan be used on that channel without concern for the code of the terminal. Forexample, a channel set to carry 300 bps transmission could also be used to service anIBM 2741 terminal transmitting at 134.5 bps or a Teletype 110 bps terminal.Another advantage of FDM equipment is that no modems are required since thechannel sets modulate the line at speci®ed frequencies, as shown in Figure 6.2. Atthe main computer site, the FDM multiplexer interfaces computer ports throughchannel sets. The common logic acts as a summer, connecting the multiplexerchannel sets to the leased line.At each remote location, a channel set provides the necessary interface betweenthe terminal device at that location and the leased line. When using FDMequipment, individual data channels can be picked up or dropped off at any pointon a telephone circuit. This characteristic permits the utilization of multipoint linesand can result in considerable line charge reductions based upon developing a singlecircuit which can interface multiple terminals. Each remote terminal device to beserviced only needs to be connected to an FDM channel set which contains bandpass®lters that separate the line signal into the individual frequencies designatedfor that terminal. Guard bands of unused frequencies are used between eachchannel frequency to permit the ®lters a degree of tolerance in separating out theindividual signals.FDM normally operates in a full-duplex transmission mode on a four-wirecircuit by having all transmit tones sent on one pair of wires and all receive tonesreturn on a second pair. FDM can also operate in the full-duplex mode on a twowireline. This can be accomplished by having the transmitter and receiver of eachchannel set tuned to different frequencies. For example, with 16 channels available,one channel set could be tuned for channel 1 to transmit and channel 9 to receive,while another channel set would be tuned to channel 2 to transmit and channel 10 toreceive. With this technique, the number of data channels is halved.
6.1 MULTIPLEXERS _______________________________________________________________ 631Figure 6.3 ITU FDM subchannel allocations for a voice-grade leased line
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Copyright # 1986, 1989, 1992, 1999
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CONTENTSPrefaceAcknowledgementsxxii
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CONTENTS __________________________
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CONTENTS __________________________
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CONTENTS __________________________
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Data Communications Networking Devi
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1.2 LINE CONNECTIONS ______________
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1.3 TYPES OF SERVICES AND TRANSMISS
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1.4 TRANSMISSION MODE _____________
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1.4 TRANSMISSION MODE _____________
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1.5 TRANSMISSION TECHNIQUES _______
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1.5 TRANSMISSION TECHNIQUES _______
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1.7 LINE STRUCTURE ________________
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1.8 LINE DISCIPLINE _______________
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1.9 NETWORK TOPOLOGY ______________
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1.10 TRANSMISSION RATE ____________
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1.11 TRANSMISSION CODES ___________
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1.12 ERROR DETECTION AND CORRECTION
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1.13 STANDARDS ORGANIZATIONS, ACTIV
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1.16 INTEGRATED SERVICES DIGITAL NE
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REVIEW QUESTIONS __________________
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Data Communications Networking Devi
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4.1 ACOUSTIC COUPLERS _____________
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4.6 SECURITY MODEMS _______________
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4.9 BROADBAND MODEMS ______________
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4.10 DIGITAL SERVICE UNITS ________
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4.11 CHANNEL SERVICE UNITS ________
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4.12 PARALLEL INTERFACE EXTENDERS _
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