RE: [PWE3] re: Proposed way to move forward for ATM/MPLS in pwe3

["Rutemiller, John" <John.Rutemiller@marconi.com>]

Khalid,

The way to move forward is to simplify. Not to expand.


I've been looking at PDU mode and trying to figure out if there
is an easier way to provide the same functionality. What I've
come up with (thanks to input from others) follows at the end of
this message.

There are some important characteristics to note with what I
propose compared to PDU mode:

1) It is a variation of Cell Mode
2) Works for all AAL types (not AAL5 specific)
   - When a frame is sent can be triggered by max_delay (any AAL)
     or by max_MTU (for AAL5 if desired), or by the PTI bit value
3) The arrival of an ATM cell causes the transmission of at
   most one frame
   - Admin/OAM cells are sent as the last cell of a frame
   - PDU mode will send two frames in response to the arrival
     of an admin/OAM cell
4) Transmission is at least as efficient as PDU mode
5) Like PDU mode, is not transparent to user cell EFCI and CLP


As a way to move forward, I would like to replace PDU mode with
what I propose. It is simpler than PDU mode, provides equivalent
functionality and is not AAL specific.

John


-----------------------------------------------------------------


==> Procedures at the Ingress Side

The procedures for processing arriving cells preserves the value of the ATM
user-to-user bit located in PTI bit position 3 and all PTI bits for
administrative cells. Processing is based on the following rules:

1)	All cells, except the last cell of a frame, MUST be a user cell with
the PTI bit value PTI=0x0.

2)	The last cell in a frame may be either a user cell with any PTI
value (PTI=0xx), or an administrative cell (PTI=1xx).

3)	Administrative cells MUST be transmitted only as the last cell in a
frame.

4)	User cells with PTI=0x1 MUST be transmitted only as the last cell in
a frame.





The pseudo code for processing an arriving cell is as follows:

If starting a new frame
		Initialize the A-PTI/C and U-PTI/C fields to zero

Append arriving cell payload to frame buffer

If PTI=0xx then
	U-PTI = arriving-PTI
		U-CLP = U-CLP | arriving-CLP
Else If PTI=1xx then
	A-PTI = arriving-PTI
		A-CLP = arriving-CLP
Endif

If PTI != 0x0 or (FrameSize => MaxFrameSsize)
		Send Frame
	Endif

The value of the user cell EFCI is always the value of the last arriving
cell.

The value of the user cell UU bit is always set to zero when the PTI=0x0. If
the PTI=0x1, the UU bit is set to one and the frame will be sent. Only the
last cell in a frame can set the PTI=0x1 value.

The value of the admin cell PTI bits are always preserved separately and the
frame is always sent immediately after appending the admin cell payload.


==> Procedures at the Egress Side

The procedures for processing arriving frames generates ATM cells from using
the PTI/CLP values carried in the first byte of the frame.



The pseudo code for processing an arriving frame is as follows:

While (not last cell payload)
		PTI = U-PTI & 010
		CLP = U-CLP
		Prepend VPI/VCI
		Send cell
	End

	If A-PTI/CLP != 0000 then
		PTI = U-PTI
		CLP = U-CLP
		Prepend VPI/VCI
		Send cell
	Else
		PTI = A-PTI
		CLP = A-CLP
		Prepend VPI/VCI
		Send cell
	Endif



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