Data Communications Networking Devices - 4th Ed.pdf

36 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTS1.10 TRANSMISSION RATEAnalog serviceMany factors can affect the transmission rate at which data is transferred on a widearea network. The types of modems and acoustic couplers used on analog facilitiesor the types of DSUs or CSUs used on digital facilities,as well as the linediscipline and the type of computer channel to which a terminal is connected via atransmission medium,play governing roles that affect transmission rates.However,the transmission medium itself is a most important factor in determiningtransmission rates.Data transmission services offered by communications carriers such as AT&T,MCI,Sprint,and British Telecom are based on their available plant facilities.Depending upon terminal and computer locations,two types of transmissionservices may be available. The ®rst type of service,analog transmission,is readilyavailable in the form of switched and leased telephone lines. Digital transmission isavailable in most large cities; however,the type of digital service offered can varyfrom city to city as well as between different locations within a city. For example,AT&T's DDS service,while available in over 100 cities in North America,is onlysupported by certain telephone company of®ces in each city. Thus,an analogextension is required to connect to this service from non-digital service locations aspreviously illustrated in Figure 1.16. Within each type of service several grades oftransmission are available for consideration.In general,analog service offers the user three grades of transmission: narrowband,voice-band and wideband. The data transmission rates capable on each of thesegrades of service depend upon the bandwidth and electrical properties of each typeof circuit offered within each grade of service. Basically,transmission speed is afunction of the bandwidth of the communications line: the greater the bandwidth,the higher the possible speed of transmission.Narrowband facilities are obtained by the carrier subdividing a voice-bandcircuit or by grouping a number of transmissions from different users onto a singleportion of a circuit by time. Transmission rates obtained on narrowband facilitiesrange between 45 and 300 bps. Teletype 1 terminals that connect to messageswitching systems are the primary example of the use of narrowband facilities.The primary use of analog narrowband transmission facilities is by informationdistribution organizations,such as news services and weather bureaux. Althoughthe utilization of analog narrowband transmission facilities was popular throughthe 1970s,since that time many organizations that used that medium havemigrated to transmission facilities that permit higher data transmission rates.While narrowband facilities have a bandwidth in the range of 200 to 400 Hz,voice-band facilities have a bandwidth in the range of 3000 Hz. Data transmissionspeeds obtainable on voice-band facilities are differentiated by the type of voicebandfacility utilizedÐswitched dial-up transmission or transmission via a leasedline. For transmission over the switched telephone network data rates up to56 kbps may be obtainable in one direction and up to 33.6 kbps in the oppositedirection when communicating with an information utility or corporate remote