Data Communications Networking Devices - 4th Ed.pdf

412 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTTable 4.18 V.34 Bandwidth Requirements, Maximum Operating Rates, and CarrierFrequenciesSymbol Modem Carrier Bandwidth Maximumrate protocol frequency requirements bit rate2400 V.34 1600 Hz 400±2800 Hz 21 600V.34/VFC 1800 Hz 600±3000 Hz 21 6002473 V.34 1646 Hz 274±3018 Hz 24 000VFC V.34 1829 Hz 457±3200 Hz 24 0002800 V.34 1680 Hz 280±3080 Hz 24 000VFC/V.34 1867 Hz 467±3267 Hz 24 0003000 V.34 1800 Hz 300±3300 Hz 26 400V.34/VFC 2000 Hz 500±3500 Hz 26 400VFC 1875 Hz 375±3376 Hz 26 4003200 V.34 1829 Hz 229±3429 Hz 28 800VFC 1920 Hz 320±3250 Hz 28 8003429 V.34 1959 Hz 244±3674 Hz 28 800In examining the entries in Table 4.18, you will note that the variety of symbolbaud) rates provides the possibility of having a non-integral number of bits persymbol. When this occurs, a shell-mapping algorithm is used to generate theconstellation points.OptionsIn addition to three baud rates being speci®ed as options, the V.34 standard allowsasymmetrical transmission, an auxiliary channel, non-linear encoding, precoding,and two trellis-state codes to be optionally included in a modem's receiver. Toappreciate the value of each option, as well as to understand why some V.34modems implementing all options can cost considerably more than other V.34modems that defer implementing one or more options, requires a discussion of eachoption. That discussion follows.Asymmetrical transmissionThe asymmetrical transmission capability of a V.34 modem enables it to send andreceive at different operating rates. This feature removes the slowest commondenominator acting as a break on the data exchange.To appreciate the value of asymmetrical transmission, assume two users wish tocommunicate over the PSTN. Suppose one user's of®ce is very close to a servingcentral of®ce while the second user is located on a very long local copper loop thatlimits maximum data transfer capability. Without an asymmetrical transmissioncapability, communications would occur at the lower rate in both directions.Auxiliary channelThe V.34 standard speci®es a separate transmitted signal for the exchange ofinformation on line quality and modem operating conditions. Unless the auxiliary