Data Communications Networking Devices - 4th Ed.pdf

358 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTThe data encoder is an option built into many modems. The encoder is used inconjunction with some modulation schemes, enabling each signal change torepresent more than one bit of information. We will discuss the use of data encodersand decoders later in this chapter when we examine the difference between a bit anda baud.ScramblersAs previously discussed in Chapter 1, synchronous modems provide clockingsignals on pins 15 and 17 of the RS-232 interface. When a modem receives amodulated synchronous data stream and passes the demodulated data to anattached terminal device, it also provides a clocking signal to the data terminal.This clocking signal tells the terminal device when to sample pin 3, the receiveddata circuit, and is produced by the modem from the received data. Thus, thereceived clocking signal is commonly referred to as a derived clocking signal as it isderived from the received data.For a synchronous modem's received clock to function correctly it must remainin synchronization with the data being received. This requires a suf®cient numberof changes in the composition of the data, e.g., 0 to 1 and 1 to 0, to permit thereceiving modem's circuitry to derive timing from the received data. Since the datastream can consist of any arbitrary bit pattern, it is quite possible that the data willrandomly contain long sequences of 0s or 1s. When these sequences occur the datawill not provide the modem's receiver with a suf®cient number of transitions forclock recovery, a condition which resulted in the incorporation of scramblers intosynchronous modems.A scrambler modi®es the data to be modulated based upon a prede®nedalgorithm. This algorithm is normally implemented through the use of a feedbackshift register, which examines a de®ned sequence of bits and modi®es theircomposition to ensure that every possible bit combination is equally likely to occur.At the receiving modem a descrambler employs the inverse of the prede®nedalgorithm to restore the data into its original serial data stream.Modulator, ampli®er and ®lterThe modulator acts upon a serial data stream by using the composition of the datato alter the carrier tone which the modem places on the communications line. Whena connection between two modems is established one modem will `raise' a carriertone that is heard by the distant modem. The reader is referred to `The modulationprocess' in this section for information concerning the operation of differentmethods used to modulate data.The ampli®er boosts the level of the modulated signal for transmission onto thetelephone line while the ®lter limits the frequencies of the tones placed on the lineto comply with federal regulations. At the receiver, modulated tones received fromthe telephone line are ®ltered to remove extraneous tones caused by noise and thenampli®ed to boost the received signal level.