Bay Networks Marlin, Reference Manual

Page: 41 / 185

T
UTORIAL
2-20
(there are only two B channels), but it means that various devices
(telephone, fax machine, bridge, PC, etc.) can all use one connection if they
do not need to be connected permanently.
An important feature of the ISDN service is that it can be provided using the
same wiring that was installed for the original telephone service, so it can
be provided relatively cheaply anywhere the analogue system was used
before.
The costs of installation and rental of Basic Rate ISDN lines have dropped
to the point where there is very little difference between an ISDN line and
two business lines (as ISDN provides two channels equivalent to two
analogue lines). The better performance and higher bandwidth of ISDN
also reduce the effective difference between the two.
2.6.2 ISDN
AND
LAN B
RIDGING
ISDN provides an ideal service for use with remote LAN bridges. To be
effective, remote bridges require at least 64 Kbps for realistic throughput.
Slower-speed links can be used, but usually only when usage is low and
infrequent or higher-speed circuits cannot be provided.
Leased digital point-to-point circuits still offer the most cost-effective
interconnection medium for remote bridging if usage spans a number of
hours; however, this may change as ISDN tariffs are reduced. ISDN can be
used to provide effective backup of these point-to-point circuits as follows:
•
If the point-to-point circuit fails, an ISDN channel can be dialled up very
quickly to provide an alternative path to the remote bridge.
•
If the leased circuit becomes heavily loaded, due to traffic peaks between
remote bridges, then extra bandwidth can be dialled up to add to the
leased circuit bandwidth. Each bridge handles the leased line and ISDN
channel as parallel links to the other bridge. As soon as the traffic load
goes down, the ISDN call can be dropped, thus saving further call
charges.
In some network configurations, only ISDN dial-up circuits can be used to
interconnect some or all of the bridges. If a given link is used infrequently,
for example when files are downloaded to a remote PC in the morning, then