Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <74qt4l$c3i@bmdhh222.europe.nortel.com>,
  stevenp@bnr.co.uk (Steven Perryman) wrote:

[ ... ]

> These are the definitions I have :
>
> AAL Mode: This attribute indicates whether the AAL for the supporting VCC is
> operating in message mode or streaming mode, assured or unassured.
>
> SSCS Type: This attribute identifies the SSCS type for the AAL. Valid values
> are NULL, Data SSCS based on SSCOP (assured operation), Data SSCS based on
> SSCOP (non-assured operation)
>
> So my question is, is there a relationship between the terms 'assured' and
> 'unassured' , for above definitions ??
>
> If I set AAL mode as 'assured' and SSCS type to 'Data SSCS/SSCOP' , must the
> SSCS type use assured operation (similarly for non-assured) ??
>
> From all this, I guess the answer to the above is yes.

I agree the answer is yes.

What is happening here is that if you want to transfer data, you can choose to
have the transmission "reliable" or "unreliable." This is analogous to using
either TCP or UDP over IP, respectively.

ATM by itself is a best effort scheme, just like IP (in this one respect). So
to achieve reliable transfer, you need the Layer 4 protocol (SSCOP) to be used
in reliable mode, i.e. it sends ACKs back. In other words, reliable SSCOP over
AAL5 is analogous to TCP/IP.

If you want unreliable transfer, which makes most sense for streaming data
for example, use unreliable SSCOP over AAL5 (datagram service, analogous to
UDP/IP).

You can even leave the SSCOP layer null, in which case you could conceivably
put either UDP or TCP in its place. Why not?

In short, any "reliability" must be provided by the Layer 4 protocol, be it
SSCOP or something else, like TCP.

Bert
manfredi@arl.bna.boeing.com

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