Subject: RE: [Fwd: use ideas from, but not true aal]

From: Sasha Vainshtein <[email-masked]>
Date: Thu, 18 Apr 2002 11:52:37 +0200

Bob and Craig,
It seems to me that you are missing an important distinction
between carrying unstructured and structured circuits over a
PW.
1. If you deal with your unstrucrtured E1 or T1 circuits, you:
   a) Do not need any framers in the PEs nor do you need to know
       where your frames and multiframes begin - this is left to CEs
    b) Simply carry all the bits of the native E1 or T1 service
        including framing - the overhead is small enough (BTW,
        Bob's original proposal has been exactly that)
   c) Replace each lost packet with the equivalent amount of AIS
      ("all ones") - CEs withstand it quite well for as long as 50 ms
      (otherwise you cannot connect them with SONET/SDH which
      does not guarantee better APS times - see G.841, 
      Section 7.2.2). You MUST use fixed payload size packets 
      in order to know exactly the amount of AIS to be replayed 
      instead of each lost packet
  d) Let the CEs to resynchronize with the framing/multiframing
      structures - after a short AIS condition it takes them no more
      than 2 multiframes, i.e. up to 6 ms
2. If you deal with N*DS0 circuits extracted from E1 or T1, then:
    a) Framing/multiframing structures of the E1/T1 line that carried
       the incoming N*DS0 end service cannot be simply reproduced by
       on the E1/T1 line carrying the outgoing end service. 
      The hub-and-spokes topology use case for N*DS0 provides a simple
      counter-example
       o The incoming spokes end services all use the same clock, but 
          multiframing on their carrier E1/T1 lines has different phases
       o The outgoing hub end services are all carried over the same E1/T1
line
   b) These structures MUST be created by the egress PE autonomously
   c)  As a consequence, loss of packets does not affect framing and
       CE multiframe synchronization at all.
 
Hopefully this clarifies the situation.
 
With best regards,
                                   Sasha Vainshtein
email:     [email-masked] <mailto:[email-masked]> 
tel:       +972-3-7659993 (office)
           +972-8-9254948 (res.)
           +972-58-674833 (cell.)

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Wednesday, April 17, 2002 7:51 PM
To: [email-masked]
Cc: [email-masked]
Subject: RE: [Fwd: [PWE3] use ideas from, but not true aal]

Definitely if all packets are equal in length, this is true.  If you get
packet 15, then packet 19, and you know the length of the intervening
packets that were lost, then framing can be deduced.  If I were trading off
the cost of a single structure pointer per PW header  against the cost of
the above computation (and the associated packet length constraint), I don't
know how I'd come out on that issue.  It's certainly something worth
considering. 

-Craig 

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