Introduction 1-5Using ISDN to Support Leased Line WAN CircuitsISDN provides an ideal service to connect remote LANs. To beeffective, the connecting bandwidth needed is at least 56 Kbps toachieve a realistic throughput. Slower speed links can be used butusually only when usage is low and infrequent, or if higher speedcircuits cannot be provided.Leased digital point-to-point circuits can still be cost effective if usagespans many hours per day. However as ISDN tariffs reduce, this balancealso changes. ISDN can be used to provide effective backup of thesepoint-to-point WAN circuits in two ways.n Firstly, if the point-to-point circuit fails, an ISDN channel can bedialled-up automatically and quickly, to provide an alternative path tothe remote unit.n Secondly, if the leased circuit becomes heavily loaded due to peaks inthe traffic between remote bridges or routers, additional bandwidthcan be automatically dialled-up to supplement the bandwidth of theleased circuit. The interconnected bridges would then treat the leasedline and ISDN channel as parallel links, sharing the load across the two.
