Cell Relay Archive

["C. M. Heard/VVNET, Inc." <heard@vvnet.com>]

In article <356D33D9.142D41C5@polystar.se> Mikael Appelkvist
<mikael.appelkvist@polystar.se> wrote:
>
>I am wondering when the PLCP framing and when the cell mapping (G.804)
>in e.g. E1 is used and what are the benefits in having two different
>frame modes?

PLCP framing was originally developed for SMDS/CBDS.  It was never
officially a standard for ATM except at the DS3 rate, although some
manufacturers supported PLCP framing at DS1 rates before the G.804
direct mapping (sometimes called HEC framing) was adopted by the ATM
Forum (DS1 Physical Layer Specification, af-phy-0016.000, September 1994).
A little over two years ago the ATM Forum adopted a direct mapping for
DS3 also (DS3 Physical Layer Interface Spec, af-phy-0054.000, March 1996).
This spec did not completely destandardize the use of PLCP framing for
ATM at the DS3 rate but did deprecate it.  New implementations are urged
to use the direct mapping instead.

I do not know if any manufacturers supported PLCP framing for ATM
at E1 or E3 rates, but the ATM Forum specs for these two rates
(E3 UNI, af-phy-0034.000, August 1995;  E1 Physical Layer Interface
Specification, af-phy-0064.000, September 1996) specify the G.804
direct mapping.

It seems to be widely agreed that PLCP framing, which adds four bytes of
overhead for each cell, offers no advantage over the G.804 direct mapping
for all its costly overhead.  So it has largely fallen out of favour.

Mike
--
C. M. Heard/VVNET, Inc.
heard@vvnet.com