Cell Relay Archive

["Greg Sack" <greg_sack@neca.nec.com.au>]

Walt Weber <waltweber@sprintmail.com> wrote in article
<360D0627.1DB0932A@sprintmail.com>...
> Can anyone give me a source for information on ATM T1 Circuit Emulation?
> 
> Specifically, I'm trying to figure out the difference between Basic and
> CAS structured CES.  I've looked at the ATM Forum site but didn't find
> what I'm looking for.  THX.
> 
> 

Have you looked at the ATMF Circuit Emulation Service spec,
af-vtoa-0078.000, available from the ATM Forum's web site?

Section 2 describes structured CES with CAS and without CAS (basic).  Basic
is used where you are not interested in carrying CAS bits from end to end
(eg for PBX-PBX using some form of common channel signalling, or where
signalling is of no interest) while CES with CAS is obviously used for
transporting CAS information end to end.

Both forms of structured service pack the Nx64Kbps time slot data into the
start of AAL1 structure using the same method.  Then if CAS mode is used,
an additional signalling substructure is appended containing the aggregated
CAS signalling bits for the Nx64Kbps data carried.

I presume that this signalling substructure is used to provide a common AAL
structure for the various different forms of circuit being emulated (eg T1,
E1 etc) which use different schemes for providing the CAS bits (eg robbed
bit vs Time slot 16).  This may also simplify interworking of different
circuit types (eg E1 to T1).

- Greg