3Com AccessBuilder 500, Руководство пользователя

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