goldstein%delni.DEC@decwrl.dec.com (Fred R. Goldstein dtn226-7388) (12/29/88)
The topic is no longer Siemens' acquisition of Rolm from IBM, but
Rolm's alleged dropping of calls during a switchover.
A redundant Rolm switch won't switch over if the other CPU is down.
That's pretty obvious, and Rolm's designers aren't really so foolish
as to let a system crash because one of its redundant CPUs is down!
Now it's probably possible to set it up wrong so that such things
happen, but it's definitely not the way it's designed.
A more likely problem for voice-band data users is the way the Rolm
switch handles "connections". Imagine, if you will, the way most PBXs
and COs operate. You pick up the phone, the switch decodes your digits,
the switch tells the matrix which ports to connect together, and the
CPU goes back to doing other things (until you hang up or something).
If the CPU were to fall asleep, the connection would remain intact.
Rolm is different. (I don't know if the 9750 is the same here but
I've worked with the older switches.) Connections are established
in software, in a "connection table". The main program periodically
reads the connection table out onto the bus, causing connections to
be made for a period somewhat longer than the program's loop timing.
(The loop was, I _think_, 150 ms. on the 7000.) If the CPU stops
operating, though, the connection goes away in a few hundred
milliseconds. The calls are not torn down or anything, but the
voice path itself drops.
Switchover between CPUs takes a couple of seconds. During this
interval, voice callers may hear a brief dropout of a second or so.
Calls in progress (dialing, etc.) are lost, but established calls
remain up, with the dropout. Modems, however, often have loss of
carrier disconnect on them. This dropout is of course a loss of
carrier, so they hang up!
Again from memory, switchover isn't supposed to happen automatically
at 2AM IF there are connetions up; it may wait until everything is
idle. But maybe not, at least in some cases. If you never have all-
idle time, then it may be desirable to force a switchover, for
maintainability reasons, even if somebody is on line.
When I was in charge of these things, I went to great efforts to
discourage modem users from using PBX extensions...
fred
(speaking for myself, not my employer or anyone else)