Data Communications Networking Devices - 4th Ed.pdf

14 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSon the local loop to boost the strength of the signal. Both the local loop and thecentral of®ce are operated by the telephone company serving each subscriberlocation. If the leased line is routed outside the local telephone company's servingarea it must be connected to an interexchange carrier IXC),such as AT&T,MCI,or Sprint. The location where this interconnection takes place is called the point ofpresence POP),which is normally located in the central of®ce of the local telephonecompany. Although data on an analog leased line ¯ows in an analog format,by the early 1990s most interexchange carriers digitized analog signals at the POP.Thus,between POPs most analog data is actually carried in digital form. Since thelocal loop is still an analog medium,it is the local loop which limits the datatransmission rate obtainable through the use of an analog leased line. By 1998modems permitting a 33.6 kbps data transmission rate on leased lines were commonlyavailable,and some vendors had introduced products that allow data ratesof up to 56 kbps in one direction and up to 33.6 kbps in the opposite direction,atechnique referred to as asymmetrical transmission.Digital facilitiesIn addition to the analog service,numerous digital service offerings have beenimplemented by communications carriers over the last decade. Using a digital service,data is transmitted from source to destination in digital form without the necessityof converting the signal into an analog form for transmission over analog facilitiesas is the case when modems or acoustic couplers are interfaced to analog facilities.To understand the ability of digital transmission facilities to transport datarequires an understanding of digital signaling techniques. Those techniquesprovide a mechanism to transport data end-to-end in modi®ed digital form onLANs as well as on wide area networks that can connect locations hundreds orthousands of miles apart.Digital signalingDigital signaling techniques have evolved from use in early telegraph systems toprovide communications for different types of modern technology,ranging inscope from the data transfer between a terminal and a modem to the ¯ow of data ona LAN and the transport of information on high speed wide area network digitalcommunications lines. Instead of one signaling technique numerous techniques areused,each technique having been developed to satisfy different communicationsrequirements. In this section we will focus our attention upon digital signalingused on wide area network transmission facilities,deferring a discussion of LANsignaling until later in this book.Unipolar non-return to zeroUnipolar non-return to zero NRZ) is a simple type of digital signaling which wasoriginally used in telegraphy. Today,unipolar non-return to zero signaling is used