Cell Relay Archive

[Geert Goossens <gagoosse@info.vub.ac.be>]

Hossain Sainul wrote:
> 
> Distribution: world
> 
> Respected folks
> I am studying ATM network.Sometimes I find that ATM does not
> permit cell-retransmission.But when I go through literaures
> on TCP over ATM, I find that retransmission of cells occur if
> any cell belonging to a packet is lost. This is always
> confusing me.Could someone pls explain---
> 1)whether cell-retransmission occurs in ATM
> 2)if the answer is negative,then how the retransmission of
> cells of any TCP-packet is possible in the same architecture
> of ATM?
> 
> Regards.
> With advanced thanks-
> Sainul

ATM on itself does not include a mechanism to retransmit lost cells. The
connection is non-reliable. There is nothing in the ATM lower layer that
allows the source to see that a cell did not arrive at the destination.
However to send packets (they can consist of several cells, up to 64KB),
you can use Adaption Layers. AAL 5 is also unreliable, the only thing it
does is assembling a number of cells into one packet. If you run a TCP
stack using AAL 5, you can of course retransmit TCP packets whenever a
cell is lost or wrongly received at the other end. The TCP packet holds
information (like a sequence number) in the ATM payload that the other
side can use to tell the source that he has received the packet
correctly.

The protocol stack looks as follows:

reliable, takes care of retransmission
++++++++++++++++++ 
| TCP/IP stack   | 
++++++++++++++++++

unreliable, encapsulates several cells into one packet
++++++++++++++++++
|    AAL 5       | 
++++++++++++++++++

sends cells at a certain rate (depending on the agreed quality of
service)
++++++++++++++++++
|     ATM        | 
++++++++++++++++++

There is also AAL 3/4 that provide a reliable connection over ATM. They
use a similar mechanism as TCP.
I think you were confused over the difference between cells and packets
(a packet consists of one or several cells).
I hope this answers your question.

	Geert

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