TELECOM Digest OnLine - Sorted: Phone Multiplexer Failures, Power Outages, and VOIP Firewall Problems

Phone Multiplexer Failures, Power Outages, and VOIP Firewall Problems
Mon, 18 Jun 2007 14:07:08 -0700

It has been along time since posted to the Telecommunications Digest,
I was under the impression that like most Usenet groups this one had
died, I am surprised that Pat did not try to contact me?

Recently at a discussion with Steve Bellovin on cryptography in a
bar at Foggy Bottom close to George Washington University after an ACM
meeting, the discussion morphed into the question of SLIC's
(Subscriber Line Interface Circuits), or Pair-Gains, essentially
multiplexers to convert a one copper pair line to multiple lines, due
to a long line repairman in our group. (I do not know why we wandered
off of cryptography?)

He was stating that when you had an AC power failure, the batteries in
the SLIC's only last for about 8 hours, then everything will go
dead. This confused me because as someone with limited experience with
SLIC's only at the cross-connects near the Demarc, the F2, or the F3
(Demarc), that these things ran off of CO 48V, and usually there was
no electric power to be had at the F2, or F3. Through an alcoholic
haze when he started to talk about large currents it finally dawned on
me that he was talking about SLIC's used around the Central Office
(CO) or F1 that split one pair into thousands of lines? And I was
talking about SLIC's that split one line into 2, 4, 8, or 16, so that
an extra copper line did not have to be added on the street (F1 to
F2), or from the F2 to the residence. With the glut of broadband, I
was under the impression that all CO to CO, or CO to F1 traffic was
done via optical fiber, so I'm not sure if these devices are still in
use? And if this is a real risk?

I do know that during the massive East Coast long lasting power
failure in August 2003, that while there was no problem with landline
telephone service due to CO 48V generation, the Key Telephone Sets
(KTS) in all the New York City firehouses which are used to route the
calls within the firehouse were dependent on AC power and only had
batteries that lasted about 8 hours. So while the telephone lines
coming in to the buildings were working, the phones on the desks
stopped working after 4 to 8 hours. I believe this has now been fixed?

Both Cell Phone Towers and Cell Switches immediately failed when AC
Power went out, causing long cues at the Pay Phones around the
city. This problem to my knowledge has not been fixed?

The residential SLIC's or Pair-Gains that I was talking about do have
a very high failure rate. Recently a friend of mine was constantly
losing service at his Co-opt, Verizon did at least five service calls
and kept on telling him that nothing was wrong and it had to be
"inside wiring" if that. He asked me to look at it. It turns out that
when the previous tenant wanted an extra line for his apartment,
instead of running the line from the complex's basement junction box,
which is normally the Demarc, Verizon added a 4 line SLIC in the
closet of the co-opt apartment. When my friend moved in they still
kept his single line through the SLIC, and that started failing. I
disconnected the SLIC from the circuit and everything started working
fine, when I got Verizon out there, they agreed, but had no record of
the SLIC that they installed, including on the Line Maintenance
Configuration System (LCMS) computer?

One item that is relevant that Prof. Steve Bellovin, the father of
Firewalls, talked about, is that current Internet Firewalls do not
handle VOIP very well, are not suited or designed for voice, and there
are many security problems with VOIP. The industry is well aware of
this problem and has tried to develop standards for a specialized
Firewall just for Voice, VOIP, called a "Boundary System", which is
based on Telephony technology and terminology. For some reason Steve
has not participated in this effort.

Robert J. Perillo
Principal Telecomm. Engineer

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