Cell Relay Archive

[Erik Oestlyngen <eoestlyn@online.no>]

Lawrence Wu (STCS) wrote:

> Hello,
>
> If a CBR user transmits below its PCR, is the reserved BW available to
> other users?
>
> Similarly, if a VBR user transmits below its SCR, is the reserved BW
> available to other users?
>
> Best Regards,
> Lawrence

I think it goes like this:
What actually using bandwith capacity on an ATM link are CELLS. No cells =
available bandwith, that is if you don't put restrictions on your link in
other ways by using CAC (Connection Acception Control or something like
that) rules with no "overbooking" (then new connection that may use
available bandwith will not be allowed).

What actually control cell streams are ADAPTION LAYERS.

Using CBR the communication normally are controlled by AAL 1. Once started
the AAL 1 continually sends cells at the peak rate known to the adaption
layer, empty or not empty, thus normally using its allocated bandwith.
Stopping the AAL 1 process will free the bandwith on the ATM link.

Using VBR, the communication for instance may be controlled by AAL 5,
which divide higher layer frames into the necessary numbers of cells and
forward them (if controlled by a SCR shaping mechanism in the
communication device) at PCR speed (within the network limits of CDVT,
cell delay variance tolerance) until MBS (max burst size) is filled, then
wait until SCR is reached in average, then send more cells at PCR if there
were more left. When no frames are received from higher layers, the
bandwith is available to other users.

If the communication device is not using SCR shaping, AAL 5 (or whatever)
will send at PCR as long as there are cells to send, the network policing
will then act according to its configuration (tag or discard), and
respectively tag or discard cells that transeed the PCR/MBS and SCR
scheme. Tagged cells will have low priorities in network queues and will
be thrown upon congestion.

Regards Erik