Subject: M- and V-bits in ATM VCC cell transport service

From: Akiva Sadovski <[email-masked]>
Date: Sun, 27 Jul 2003 16:10:20 +0200

Matthew and all,
  being a newcomer to atm encapsulations, i failed to understand the meaning
of M- and V-
bits in 

 7.3. ATM VCC Cell Transport Service.

<quote>
* M (transport mode) bit

       Bit (M) of the control byte indicates whether the packet contains
       an ATM cell or a frame payload. If set to 0, the packet contains
       an ATM cell. If set to 1, the PDU contains an AAL5 payload.

     * V (VCI present) bit

       Bit (V) of the control byte indicates whether the VCI field is
       present in the packet. If set to 1, the VCI field is present for
       the cell. If set to 0, no VCI field is present. In the case of a
       VCC, the VCI field is not required. For VPC, the VCI field is
       required and is transmitted with each cell.
<endquote>

but the draft states:"VCC cell transport service is characterized by the
mapping of a
single ATM VCC"; therefore frames will never be carried in this encap, and
there's no need to 
carry VCI along with payload. Why can't M- and V- bits be set to 0 at
ingress and ignored at 
egress in this mode ?

thanks in advance,
  Akiva

---

From: Mustapha Aissaoui <[email-masked]>
Date: Mon, 28 Jul 2003 09:38:47 -0400

Akiva,
although the type of encapsulation can be inferred from the label, the M-bit is
required in the AAL5 PDU mode because you need to switch to cell mode to carry
an OAM cell.

Although the V-bit is not really required in this draft, it is there for
backward compatiblity reason. The ATM Forum specification for ATM-MPLS network
interworking allows for a VCI optimization mode in the case of VPs with
concatenation (see Section 6.3.1 of str-aic-mpls -niwf-02.02, “ATM-MPLS Network
Interworking Version 2.0”.

Regards,
Mustapha.
-----------
Akiva Sadovski wrote:

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