Cell Relay Archive

[riwells@my-deja.com]

Here's my take on this extremely interesting posting (by Jeff Wilson)
and
the subsequent response (by Baris Aksoy).  KINDLY FEEL FREE TO CORRECT
ME,
SHOULD ANY OF MY OBSERVATIONS BE WRONG.  To keep this posting short and
readable, I am splitting it into multiple postings.  Here's part II:


Point 4
=======
I have often wondered myself whether there was a need to make the
structure of AAL2 so radically different from other AALs.  Consider
this (based on I.362 and I.363):

AAL 1
|---Convergence Sublayer (CS)
|---Segmentation and Reassembly Sublayer (SAR)


AAL 3/4, 5
|---Convergence Sublayer (CS)
    |---Service Specific Convergence Sublayer (SSCS)
    |---Common Part Convergence Sublayer (CPCS)
|---Segmentation and Reassembly Sublayer (SAR)

If you make a tree structure from this hierarchy, and do a post-order
traversal of that tree, you get a good indication of the various
adaptation processing steps, in the correct order


                               AAL
                              /   \
                             /     \
                            CS     SAR
                           /  \
                          /    \
                        SSCS   CPCS


You begin with the service specific convergence operation [SSCS] (which
is optional), following it up with a common convergence operation [CPCS]
(where the service type is not discernable).  This completes the
convergence operation [CS].  This is followed by the
segmentation/reassembly operation [SAR], and once that is done, the
entire adaptation process is completed [AAL]


In AAL2, the sub-layer hierarchy makes a complete departure from I.362
section 2.1, which states that:
"To support services above the AAL, some interdependent functions must
be performed in the AAL. These functions are organized in two logical
sublayers, the convergence sublayer (CS) and the segmentation and
reassembly sublayer (SAR). These sublayers may be further subdivided."


For AAL2, the sublayer structure looks this:


                                    AAL
                                   /   \
                                  /     \
                                 /       \
                                /         \
                               /           \
                              /             \
                             /               \
                          SSCS               CPS
                          /  \              /  \
                         /    \            /    \
                        /      \          /      \
                       SAR   Trunking   CPS      CPS
                       /|\              Packet   PDU
                      / | \
                     /  |  \
                    /   |   \
                   /    |    \
                SSSAR SSTED SSADT


As you can see, my "postorder traversal" theory collapses for AAL2
(or doesn't it?)

To date, no one has been able to explain to me satisfactorily as to why
such a radical departure from I.362, section 2.1 was necessary.  At the
very least, a corrigendum should be issued for I.362, explicitly
mentioning that this section does not hold true for AAL2.

Flames welcome.....

----Richard



In article <388031CB.2693ED6D@gte.net>,
  baris.aksoy@gte.net wrote:
>
> --------------BAAADC14172621049E638683
> Content-Type: text/plain; charset=us-ascii
> Content-Transfer-Encoding: 7bit
>
> Hello,
>
> Either I'm missing something here or, you miss that AAL2 layer also
has
> SAR layer. AAL2 consists of a Common Part Sublayer (CPS) and a Service
> Specific Convergence Sublayer (SSCS):
>
>       -The AAL2 CPS specification, ITU-T I.362.2 - 09.97
>
>      - The AAL2 SSCS consists of two elements:
>           Segmentation and Reassembly (I.366.1) - 06.98
>           I.Trunk (I.366.2) - 02.99
>
> The SAR-SSCS sublayer for the AAL2 supports a bandwidth-efficient
> transmission of low-rate, short, and variable length packets in delay
> sensitive applications, such as voice.In this way, it's different form
> AAL5- SAR function, which targets transmission of bulk of variable
length
> non-real time packets.
>
> Baris
>


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