From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
Newsgroups: comp.dcom.cell-relay
Subject: Re: ATM over T1; use of header address bytes
Date: 7 Oct 92 11:51:15 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

brobbins@qualcomm.com (Barry Robbins) writes:
+---------------
| How are [VPI & VCI] used?  How do they differ?
+---------------

The VPI is primarily for internal long-haul telco use, at the higher levels
of their switching hierarchy. Loosely speaking, the idea is to decrease the
difficulty of multi-gigabit/sec switching, by partitioning the network into
a "core" and a "periphery", where the "core" has 256 switches or less. If
they decide to use it that way then within the core, cells can be routed
based only on the VPI, the virtual "path" (trunk) between two core switches.
Only when a cell leaves the core would its VCI need to be looked at.

The VPI will probably be zero on all subscriber links.

[Aside: I expect that the current VPI/VCI division will eventually be
 transcended by divisions that are more appropriate to the actual telco
 backbone switches that get built, much like "IP subnets" allow a more
 flexible division of IP addresses into nets & hosts than is hardcoded
 into the original Class A/B/C format. For that reason, many people
 including myself consider the VPI/VCI to really be a single 24-bit
 virtual circuit number. This still allows a form of the above-mentioned
 "switch only on VPI in the core" strategy to be used, only with a more
 flexible decision on which of the 24 bits are actually the "VPI" and
 which the "VCI".]

+---------------
| Are they defined for end-to-end addressing? Or can the switch and/or
| network stomp on them? 
+---------------

No, and yes. That is, VPI/VCI are *not* end-to-end, but hop-by-hop. They can
and almost certainly *will* change at every switch the cell goes through.

Think of traditional circuit-switching. You place a phone call. As a call
leaves your PBX, it's multiplexed into, say, time-slot (circuit) #17 of the
T1 on your PBX. At the CO, it's routed into time-slot #237 of a T3 trunk
going to the toll switch across town... and so on... and finally is found in
time-slot #03 in the T1 going to your callee's PBX, completely independently
of the phone numbers of you or the callee.

The next call between those two numbers will almost certainly use different
circuits at each hop along the way. There is no attempt to match the circuit
numbers used in any cable, trunk, or multiplexing level anywhere in the path
with any other circuit numbers -- certainly not end-to-end!

Now replace time slots in a framed multiplexing hierarchy with the VPI/VCI
tags in cells. Just as in the circuit case, there will be no need (nor attempt)
to preserve the VPI/VCI further than one switch-to-switch (or host-to-switch)
link. (...EXCEPT possibly while the cell is inside the telco's "core".)

However, once a call is set up, even though the VPI/VCI will change at each
switch along the way, the VPI/VCI corresponding to that call will remain
constant on any *given* switch-to-switch link for the duration of the call,
just like with "real" circuits.


-Rob

-----
Rob Warnock, MS-9U/510		rpw3@sgi.com
Silicon Graphics, Inc.		(415)390-1673
2011 N. Shoreline Blvd.
Mountain View, CA  94043
Newsgroups: comp.dcom.cell-relay