Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 411V.34 modemThe ITU V.34 standard represents a quantum leap in both complexity andoperating rate capability in comparison to the V.32 bis standard. From anoperational perspective, the V.34 standard's complexity can be judged by itsdocumentation, which is approximately twice as thick as the V.32 bis standard. Thedoubling of modem documentation results from the addition of several newfeatures such as an asymmetrical transmission capability, an auxiliary channel,non-linear encoding, precoding, and a signi®cant increase in the trellis-code state tosupport trellis coding. When such previously developed features as error correction,compression, and different baud rate operations are considered, the V.34standard offers well over 100 combinations of modulation schemes, baud rates,and other operating parameters. Unfortunately, many of the new features, whilestandardized for use in the transmitter section of the modem, are speci®ed asoptions for implementation in the modem's receiver. Thus, interoperability can bea problem as the standard allows chip manufacturers to pick and choose from anextensive array of options.ModulationThe original V.34 standard speci®ed three mandatory baud ratesÐ2400, 3000, and3200. Under ideal conditions, a V.34 modem maps nine bits into each symbol,resulting in an operating rate of 3200 baud 9 bits/baud, or 28.8 kbps. At thisoperating rate there are 960 points in the modem's signal constellation. To significantlyreduce the probability of errors due to impairments causing a small shift in apoint in the signal constellation, the V.34 modem includes non-linear coding,precoding, and a 16-state trellis-coding capability as well as optionally speci®ed 32-and 64-state trellis coding. Each of these features will be discussed later in thissection.In addition to the mandatory baud rates, the V.34 standard speci®es optionalrates of 2743, 2800, and 3429. The 2743 and 2800 baud rates can be an importantconsideration when transmission occurs via an infrastructure where voice isdigitized using Adaptive Differential Pulse Code Modulation ADPCM), whichuses prediction to reduce the voice digitization rate from PCM's 64 kbps to 32kbps. You typically encounter ADPCM when communicating via satellite, onterrestrial circuits that are communicating via satellite, or on terrestrial circuits thatare routed overseas or to Hawaii, Alaska, and other non-contiguous US locations.Since ADPCM fails when baud rates exceed 3000, the optional 2743 and 2800 baudrates permit relatively high-speed communications to occur over an ADPCMinfrastructure.The V.34 standard speci®es two carrier frequencies for both mandatory andoptional baud rates. Table 4.18 summarizes the carrier frequencies, bandwidthrequirements, and maximum bit rate for the six baud rates included in the V.34standard.Also included in Table 4.18 is a column labeled `Modem protocol', whichindicates the difference in the use of carrier frequencies between a V.34 and a V.FCmodem.