Cell Relay Archive

[skasera@dl.in.dsqsoft.com]

Thanks for all those who have responded. To continue this thread,  the
doubts that I was having were
     1) where would an AAL2 switch be ideally located?
     2) would it not be possible to extend the VC to the ultimate AAL2
endpoint rather than having a concatenation of AAL2 type links?
     3) Why do we require a routing function when the ATM VCs are already
routed (using PNNI or for that matter configured)?
     4) Does each ATM switch also acts as an AAL2 switch. If yes, this
would considerably slow down cell switching.

To answer aforementioned questions, these are two possible scenarios that I
could think of:

Scenario a) ATM/AAL2 based trunks connecting multiple cities over an
existing ATM network.

Analysis)Suppose you want a trunk line between A and C, but you have leased
ATM VCs between (A,B) and (B,C). In this case, it is not at all possible
that the VCs are end to end. What you require here is an intermediate node
(called AAL2 switch) which does reassembly, and then forwards AAL2 frames.
So, the stack would look like:

AAL2           AAL2 AAL2           AAL2
ATM  ATM  ATM  ATM  ATM  ATM  ATM  ATM
PHY  PHY       PHY  PHY  PHY  PHY  PHY        PHY
(A)               (B)              (C)

In this case, B will act as an AAL2 switch. Even for this case, as Saso
pointed out, AAL operates end to end, but for a VC only.
The advantage of AAL2 signaling here is that it provides means to connect A
and C which otherwise was not possible through VCs. Also, it allows dynamic
connection establishment and release of AAL2 connections.

Scenario B) Consider a set of trunk nodes which only wish to have one ATM
VC.

Analysis) Now, this can be done through a point-to-multipoint topology
whereby all such trunk nodes communicate via a bundling server, which has
connections with all the nodes. This server will then act as AAL2 switch.

The above explains where AAL2 switch is located, and why it is not possible
to provide end-to-end connections. Also, as scenarion 1 explains, only
certain switches will switch AAL2 frames. For a VC, there will be
intermediate switches which only switch on ATM VPI/VCI. Finally, the need
for routing is explained in scenario 1, where in a partially connected
network, there may be a requirement to find the next hop for a given AAL
endpoint.


This is my understanding,
comments or further explanations are welcome,
regards,
sumit kasera