Mon, 30 Mar 87 09:58:07 EST
I confess to not having thought about it too much, but my impression
is frame relay simply permits the passage of level 2 packets on the
basis of level 2 address through the switch to some place in the phone
network where full level 2 processing can be handled. This means that
the switch can basically concentrate on tdm switching problems while
the level 2 termination point can concentrate statistical muxing
switching problems. Such a division makes sense as a way to optimise
CSC switch or packet switch performance.
In the asynchronous world LAPD could per passed on a 64 KBPS
unrestricted data circuit to a terminal concentrator and each LAPD
virtual level 2 could correspond to an asynchronous terminal. While I
consider asynchronous terminal concentrators a backward-looking
technology, in the business world lots of firm want such devices.
Obviously this is not an OSI application, but the separation of the
problem into a physical channel, multiple virtual level 2s, and a flow
control layer on top of this has clearly been influenced by concepts
of layering, and LAPD terminal multiplexing seems more sensible than
the PAD protocols.
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