Cell Relay Archive

[mstoen@pnni.com (Matthew Stoen)]

Here's a humble stab at your Qs.

LANE 1.0 uses only UBR although I woundn't be surprised if vendor 
enhancements exist. RFC1577 does not specify traffic or QoS. I've never 
done classical IP but I imagine many vendors rely upon UBR. UBR does not 
support "rate" negotiation other than call connect/reject.

CAC depends upon how your ATM network is configured and what traffic 
parameters or service categories are requested at call initiation. For 
example, PNNI 1.0 UBR GCAC relies only upon UBR MaxCR to screen usable 
PNNI links (trunks) [PNNI 1.0]. However, if the user requests a non-zero 
minimum acceptable PCR that the network cannot support then the call may 
be rejected [TM 4.0]. Thus, for UBR the path to a reachable destination 
with the best administrative weight with all links having a UBR MaxCR < 0 
is attempted, but a specific network implementation may reject a call 
with an unsupportable non-zero PCR.

GCAC approximates based on summarized topology information. PNNI 1.0 
supplies summarized topology information to network nodes.

TM 4.0 states that UBR connection conformance is characterized by PCR and 
CDVT for the CLP=0+1 flow. The use of PCR for CAC and UPC enforcement is 
network specific. UBR.1 does not allow the network to overwrite CLP 
during UPC. UBR.2 allows any cell to be tagged during UPC. Presumably, 
cells exceeding PCR or CDVT may be ignored, dropped or tagged. 

Any "fast packet" (PacBell speak) service (f/r, SMDS, ATM) used by 
customers with mismatched network interfaces can result in cell/frame 
loss under high sustained load. In the case of ATM UBR, upper layer PDU 
loss is often the only congestion feedback; EFCI is optional for CBR, rt-
VBR, nrt-VBR and UBR but not always implemented.

Hopefully, that helps and anyone who codes this stuff may elaborate and 
correct my errors.

Regards,
Matt

> how does the UNI/NNI negotiate the rate if at all?):
> · PC based LANE or Classical IP implementations today use UBR (or
> proprietary ABR).
> · In most implementations there is no CAC for UBR traffic.
> · If there is a CAC for UBR, the CAC function knows the entire network
> topology (how did it get to know the topology?).
> · If the CAC function doesn't know the entire network topology, it asks
> at connection setup (what signaling message is that?).
> · UBR specifies a PCR and CDVT, but in almost all implementations that's
> just set to the constraints of the physical interface for any VCC.
> · Barring any large buffers or packet discard schemes, UBR traffic from
> a high bit-rate source will suffer a high cell loss, if sent to a low
> bit-rate destination.  Any controls on the bit-rate are from the upper
> layer protocols/application at the ends of the connection.