[VL2006] - 76: Don't forget the . . . .

[ViewsLetter on VoIP]

VL on VoIP <www.viewsletter.com>

*
When Planning for VoIP, Don't Forget the ....*

by William Flanagan

Our team has been surveying sites where the existing analog PBXs are
scheduled for replacement by VoIP installations.  We're looking at what
might be reusable in the conversion, and what needs doing.  In most
cases not much can be saved;  most sites need lots of preparation before
completing the changeover.  Everyone's eager for new technology, but we
must keep some key points in mind.

*Cable Lengths.*  Many of this sites we've examined are campuses where
multiple buildings spread as far as three miles.  No problem for analog
lines, where a local loop of up to 18,000 feet is standard (less for
some PBXs).  Copper cables for Ethernet, however, are limited to just
over 300 feet (100 meters, according to the standard).  Low-capacitance
cables can stretch that distance, but not much.  Fifty-pair copper
cables worked well for analog voice, and also for data using Digital
Subscriber Loop (DSL) technology.  One DSL carrier can't service a
building full of phones, so it's likely that optical fiber will need to
replace the twisted pairs, not difficult when small form-factor
pluggables (SFP) on routers and switches can carry GigEnet for miles on
fiber.

*Space for Remote Switches.*  We don't (yet) have a VoIP architecture
that delivers connections over "fiber to the phone."  Some day the
network might be all glass, all the way.  Now we rely on copper pairs,
typically four pairs in a Category 6 cable, to link the phone to .....
what exactly?  Not the PBX in another building, that's too far away, 
but to a switch somewhere within 100 m in the same building.  That's
where the fiber terminates too, on the switch.  Those switches need
power and an air conditioned space in each remote building.  In analog
days, the remote building needed only a few punch down blocks to cross
connect house wiring to the distribution cable.  Blocks were often
placed in uncontrolled environments like boiler rooms, external
electrical boxes, attics, and basements--not acceptable for active
electronics.   So don't forget to plan for a space with power and
climate control for the switch.

*Phone Power.*  Analog phones operate from the "loop current" injected
by the battery at the PBX.  With glass replacing copper in the
distribution cabling, each phone needs a power source to replace the
PBX's battery.  Most phones accept power from a local "brick in the
wall," but that can mean a lot of bricks that need attention, can fall
out, get borrowed, etc.  Best practice now is for the local switch to
inject power into the CAT6 wire to each IP phone.  Then, only the switch
needs local power. 

*Backup Power.*  We've grown accustomed to a working phone during a
blackout.  Some of us like to keep at least one analog phone line per
site (either from the local telco or a remote PBX) because the phone on
that line will work when the building loses all electrical power.  The
phone carries on as long as the battery (at the PBX or central office)
lasts.   But what if a remote building served by VoIP loses power?  If
the switch has a battery backup or uninterruptible power supply (UPS),
then it can continue to power phones and communicate over the fiber. 
Best to plan for how long you want backup to last when each phone could
be drawing up to 15 W in addition to the switch's own drain.  Than
upgrade the battery to anticipate declining power storage capacity over
the years of anticipated before replacing the battery.

* Lightning and** Surge Protection.*  On the bright side, replacing
copper cables with glass fiber means almost zero risk from
inter-building lightning strikes.  There's no over-voltage protection
needed on a fiber.   But with the introduction of LAN switches and
electronic phones into a building, consider the building's lightning
protection.  Could a strike reach the house telecom wiring or an
electrical line?  If so, that could burn both phones and switch.  Bring
back the surge protectors.

*Privacy.*  A butt set or plain telephone could tap an analog
conversation at a punch down block or connection point anywhere along a
line.  Historically, people dealt with that by locking telecom rooms and
pedestal boxes.  We watched for people digging around buried phone
cables or climbing poles.  But a clever hacker can tap an IP phone from
just about anywhere, over the network.  Openly posted software tools
make it easy to spoof addresses, change reported equipment types, jump
between virtual LANs, and do just about anything to a connection. 
Security deserves attention to avoid vulnerabilities and to balance
risks against protections.

There's more, of course.  Stay tuned.
__

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*  Find out now:  call +1.703.242.8381*  or email
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"Beware of false knowledge; it is more dangerous than ignorance."
--George Bernard Shaw

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