Cell Relay Archive

["Dominic Richens" <dominic.richens@sympatico.ca>]

"anouch" <anouch_atm@yahoo.com> wrote:
> CBR is designed for Constant bit rate and VBR for Variable bit rate
> applications. But beside the bit rate nature of traffic there is also
> an urgency nature i.e. CBR is more urgent (have high priority) than
> VBR. So how should one configure a connection with low priority
> traffic and a constant bit rate?

I think you are reading into the standards a little too much.  That
standard's define low CDV for CBR and rt-VBR.  "high priority" is one
simplistic way to implementation low CDV.  Urgent is perhaps a poor synonym
for this.

So the answer depends on what you mean by "low priority CBR".  Do you mean
"best effort"?  Or do you mean loose, but bounded CDV and CLR?

The former is achieve with UBR or UBR+, and latter would be nrt-VBR.  The
UBR+ guarantees a minimum amount of bandwidth (or frame rate), but let's
more through if there is available B/W, the nrt-VBR guarantees the traffic
contract, and discards or tags above that.

> When multiplexing a number of low priority VBR connections into a
> single VC, the burstiness of this multiplexed-VC approches one
> (PCR/SCR=1). Indeed this is often the case in a carriers' carrier
> business. A long distance operator often aggregates ISP's traffic into
> few VCs and deliver them at another point.

So if it is really VCs aggregated into one VC, then the aggregation box can
do shaping to keep the outgoing VC behaved.  The aggregator would
essentially be a router.  If it is VCs into one VP, then you can try to
shape the VP, or rely on core ATM switches that can do sub-connection
scheduling to keep fairness (and guarantee QoS) between VCs within the VP.

> Regardless of the urgent nature of the traffic we have two choices:
> A) a CBR VC
> B) a VBR VC with PCR=SCR and MBS=1
>
> So my questions are:
>
> 1) Is B) a valid configuration (ATM Forum conforming), since the
> traffic is no more variable bit rate?

Yes, that would be the nrt-VBR example.

> 2) Do you know other methods for configuring low priority constant bit
> rate traffic?

That would be UBR or UBR+.

> It is worth noting that ISPs are used to performance of a dedicated
> circuit (Leased Line). So when they consider moving to an ATM style
> connection, they often ask for a CBR guaranteed connection. But the
> problem for a carriers' carrier is that if all ISPs want CBR, then how
> the carrier's network can be economically profitable?

The simple answer is: Charge more for it and do as above with nrt-VBR.  You
want to encorage the ISP to get out of the 1970's by squishing inherenetly
non-realtime frame-based VBR into a CBR stream.

Typically these kinds of facilities are negotiated through network SLA type
arrangements, where CLR, CDV and CTD can be negotiated, and it is up to the
carrier to figure out how to monitor/guarantee the agreed upon QoS (i.e.
using nrt-VBR, GFR, UBR+).

cheers!