Cell Relay Archive

[Pete Flugstad <pete@see.sig.for.address>]

Kathleen Kuypers wrote:
> 1. About the VPI and VCI field in the ATM cell header
> -----------------------------------------------------
> According to what I've read:
> at UNI   VPI:8,  VCI:16 bits
> at NNI   VPI:12, VCI:16 bits
> Someone told me that it was perfectly according the ATM standards when
> an ATM-switch uses only let's say 4 bits for the VPI and 8 bits for the
> VCI. He says that to be compliant with the standard it is only needed
> that the switch supports a continuous VP-VC address range up to its
> maximum. So address space at UNI 0-(2^4-1) for the VP and 0-(2^8-1)
> would be sufficient.

Yes, this is common.  The bigger the supported ranged, generally the 
more expensive the hardware is going to be to deal with it.  Hence 
cheaper hardware means restricted ranges. 

> 2. consequences of reduced VPI/VCI range
> ----------------------------------------
> If we have a reduced rangen for our VPI and VCI, let's assume again 4
> bits for the VPI and 8 bits for the VCI, do we get in trouble when we
> use UNI 3.0/3.1, and later on UNI 4.0 and PNNI signalling with an
> ATM-switch which supports the complete range? Is there always a VPI/VCI
> negotiation-phase?

There is no VPI/VCI negotiation.  At every UNI or NNI interface, 
one side is designated the User, and one side the Network.  The 
Network side always picks the VPI/VCI.  If the switch that has 
the reduced range is the Network side, it will always pick 
VPI/VCI in the range it can handle.  Not a problem unless you 
want to setup more connections than it can handle.  

However, if the reduced range switch is the User, then it is not 
picking VPI/VCI, the other side is.  However, the specs say that 
the network is supposed to allocate VPI/VCI from the lowest possible 
number on up, and reuse them, rather than just having a counter 
that increments.  Again, this generally means that only if you set 
up more connections than you have VCIs do you have a problem.  

The reduced-range switch will reject all calls specifying a VPI/VCI
it can't handle, so there is no possibility for anything to go 
truely wrong.  You simply won't be able to get a call through. 

Pete
-- 
Peter J. Flugstad              Ascom Nexion
pete at stl dot nexen dot com  1807 Park 270 Drive, Suite 350
+1 XXX XXX XXXX                St. Louis, MO 63146  USA
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