Data Communications Networking Devices - 4th Ed.pdf

476 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTRTS/CTS delay time of 50 ms. Assuming the internal delay time remainedconstant at 5 ms, the total delay time to transmit the data block and receive anacknowledgement would become2 …50 ‡ 5† ˆ110 msWhen the modems operate at 4800 bps, the time to transmit a 400-bit block is400/4800 or 83.3 ms. Then the overhead attributable to the delay times is 110/83.3 + 110) or 56.9%. Given the modem operating rate of 4800 bps, the effectivedata transfer rate becomes4800 …4800 0:569† ˆ2069 bpsBased upon the preceding analysis, it is obvious that the doubling of a multipointmodem's operating rate only resulted in an insigni®cant increase in the effectivedata transfer rate. If this situation were allowed to persist, most data communicationsusers would never bene®t from upgrading the operating rate of modems usedon multipoint circuits. Fortunately, modem developments resulted in an increasein the effective data transfer rate that has a high correspondence to the increase in amodem's operating rate.Multipoint modem developmentsUntil the early 1970s, users were forced to trade off the bene®ts derived fromautomatic equalization with the longer RTS/CTS delay time that was obtainablethrough the use of a modem equipped with this feature. Fortunately, many modemmanufacturers incorporated techniques that reduce the RTS/CTS delay timewhile permitting automatic equalization.One manufacturer implemented a so-called `gearshift' technique where datatransmission begins at a rate of 2400 bps, using a modulation technique that doesnot require extensive equalization. This reduces the RTS/CTS delay to a level ofabout 9 ms, which is the delay time normally associated with manually equalizedmodems transmitting at 2400 bps. Next, as transmission proceeds at 2400 bps, thereceiving modem automatically equalizes on the incoming signal. After an initialtransmission of 64 bits of data is received at the 2400 bps data rate, the trainingcycle is completed and both the sending and receiving modems `gearshift' up to thefaster 4800 bps data rate to continue transmission. While this technique will reducethe RTS/CTS delay time, the actual number of data bits transmitted during aninterval of time will depend upon the size of the message transmitted. This isbecause the ®rst 64 bits of each message are transmitted at 2400 bps prior to themodem gearshifting to the 4800 bps data rate.Another technique used to increase the number of bits of informationtransmitted was obtained by incorporating a microprocessor into a modem. Thismicroprocessor is used to perform equalization and provides a very fast pollingfeature which increases the data traf®c transmitted during a period of time whencompared to the standard Bell System equivalent 4800 bps modems or thegearshift-type modem. This comparison is illustrated in Table 4.29.