Data Communications Networking Devices - 4th Ed.pdf

1.16 INTEGRATED SERVICES DIGITAL NETWORK ___________________________________ 151continue to transmit frames until the maximum number of frames outstanding isreached; or,it can issue a poll to the secondary station to query the status of itspreviously transmitted frames while it continues to transmit frames up until themaximum number of outstanding frames is reached.The primary station polled the secondary in frame three and then sent framefour while it waited for the secondary's response. When the secondary's responsewas received,it indicated that the next frame the secondary expected to receiveNR) was two. This informed the primary station that all frames after frame onewould have to be retransmitted. Thus,after transmitting frame four the primarystation then retransmitted frames two and three prior to retransmitting frame four.It should be noted that if selective rejection is implemented,the secondary couldhave issued a selective reject SREJ) of frame two. Then,upon its receipt,theprimary station would retransmit frame two and have then continued its transmissionwith frame ®ve. Although selective rejection can considerably increase thethroughput of HDLC,even without its use this protocol will provide the user witha considerable throughput increase in comparison to BISYNC.For comparison purposes Table 1.35 compares the major features of BISYNC,DDCMP,IBM's SDLC and the ITU HDLC protocols.Other protocolsMost of the previously mentioned protocols are restricted to use on wide areanetworks. Other protocols that are considerably more popular than thosepreviously discussed operate on both LANs and WANs and will be covered indetail in Chapter 2 and 3. These protocols include Novell's NetWare IPX/SPXand TCP/IP,the latter being the only protocol that can be used on the Internet.1.16 INTEGRATED SERVICES DIGITAL NETWORKIn this section we will examine both data and voice communications in the form ofthe Integrated Services Digital Network ISDN),which at one time was expectedto replace most,if not all,existing analog networks. Although ISDN has not livedup to the hype that surrounded its introduction,its service availability hasconsiderably expanded since its introduction during the 1980s. One of the mainlimitations of ISDN availability was the requirement for local telephonecompanies to upgrade their switching infrastructure to support the technology.Many telephone companies postponed the upgrade of their switches through themid-1990s as an economy measure while waiting for consumer demand for theservice to develop. Since 1995 the upgrade of telephone switches has proceeded ata very high rate,resulting in ISDN service becoming available to approximately80% of all telephone users in the United States by 1998. Thus,the limitedavailability of ISDN which acted as a constraint on its usage has considerablydiminished over the past few years.ISDN offers the potential for the development of a universal international digitalnetwork,with a series of standard interfaces that will facilitate the connection of a