NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/comp/dcom/cellrel/924

Internet Message Format | 1993-01-04 | 1.8 KB

Path: sparky!uunet!spool.mu.edu!enterpoop.mit.edu!eru.mt.luth.se!lunic!sunic!sics.se!craig
From: [e-mail] (Craig Partridge)
Newsgroups: comp.dcom.cell-relay
Subject: two more questions re: SONET and ATM
Message-ID: <[e-mail]>
Date: 4 Jan 93 17:32:29 GMT
Organization: Swedish Institute of Computer Science, Kista
Lines: 26

Thanks to all the folks who replied to my previous inquiry.  I've got what
I hope are my two final questions on this topic.

First, one of the books I'm reading (Sexton and Reid, Transmission Networking)
notes that one could load the sync sequence (bytes A1 and A2) into the payload
of an ATM cell and screw up the SONET multiplexors.  They note that as a result
the data in the ATM cells have to be scrambled.  But it wasn't clear to me
that the scrambler would always provide protection.  Is the scrambler
guaranteed not to generate the two byte sequence 0xF6 0x28, or is some
fancy bitstuffing done too?  What about for digital voice -- how is it
protected against sending a sync sequence?

Second, I'm getting conflicting information about how ATM cells get packed.
My understanding is that originally cells were packed into VC-4 payloads,
with bytes and octet boundaries in sync, but possibly offset within the
VC-4 payload (i.e. the H4 pointer may be non-zero).   Furthermore, in the
worst case, a cell could span two VC-4 payloads (end of one and start of
the next).  But more recently I'd been told that the rules have been
changed so that the ATM cells always start in the first byte of the VC-4
payload and there's no splitting cells across VC-4 payloads.  Has this
change indeed taken place?  [As a related side question, so far as I can
tell, SONET uses VC-4 packing for cells, just like SDH -- is that correct?]

Thanks!

Craig Partridge