Cabling For Voice Communication
over structured cabling is a real cost-saver in the
longer term, but it can turn out to be an installation
nightmare. Here are some pointers on using structured
cabling for voice communications
many companies it has been a usual practice to install
separate cabling for voice and data communication systems.
But for most installations the added flexibility of
full structured cabling, with the ability to use any
outlet for analogue voice, digital phones, feature phones,
Ethernet, ISDN, or whatever, greatly outweighs the additional
cost of using CAT 5e or CAT 6 data grade UTP cabling
Lets assume that you've already managed to persuade
your customer of the advantages of cabling the entire
horizontal, in structured CAT 5, CAT 5e or CAT 6. After
all, the cost difference is minimal when you think that
standard CW1308 telephony cable is designed for only
3.4kHz whereas CAT 5/5e cable is rated nearly 30,000
times higher at 100MHz! Connecting phones through structured
cabling ought to be simple, and ultimately it is. But
first, you have to clear a minefield to reach there.
There are the different wiring schemes and wire-maps
to worry about. Once that's sorted out, it's plain sailing!
Let's start at the beginning. From any telco's local
exchange, telco's cables are routed, mostly underground,
until they come into the customer's building where they
are connected to the network-terminating device (NTD).
This is their property and the point at which they can
disconnect their network from the customer's network
for testing their safety, and for determining whether
a fault condition is due to their network or the customer's.
Depending on the configuration, there may be a DP (distribution
point) element combined with the NTD, which you are
allowed to connect to, or there may be a separate telco
owned DP or telco blocks on the MDF (main distribution
frame) to which you must connect. After this point you
can cable to your own DP or MDF if you so desire.
At this DP, some of the telco lines may be PBX exchange
lines to be cabled to a PABX whilst others might be
private lines to go to the PBX or data routers. Then
there might be DELs (direct exchange lines) to go to
payphones or equipment that should not route through
Some might also be ISDN2e basic rate lines for digital
equipment, terminal adapters and data routers. All of
these may need to be delivered through the structured
cabling system and most of them have different wiring,
termination or pin-out requirements.
The main distribution in terms of numbers will undoubtedly
be for the PBX extensions, so let's start there.
This comes out of the PBX as a two wire circuit with
the two wires known as the A leg and B leg. These need
to end up on the A leg and B legs on the BT socket that
the phone plugs intoand they need to be the right
way round! The analogue phone is usually a three-wire
device (if earth recall is used it may need four wires).
The third wire is generated by a Master Jack, which
contains two or three electronic components. A PBX Master
contains a capacitor, which generates the third "bell"
wire. It also contains a resistor, which allows the
line to be tested from the exchange-end, even when no
phone is plugged in. A Full Master contains the capacitor,
the resistor and a gas discharge tube, which acts as
a surge protector.
Normally, in telephony wiring, these components would
be fitted in the BT-style phone socket. We're using
RJ45s for our structured cabling which contain no such
entry components. So the normal solution is to fit an
RJ45-to-BT converter at the user-outlet end, which changes
connector type and, where necessary, contains the PBX
Master or Full Master components.
Phone And Digital Feature Phone
A digital phoneas some of the POTS (plain old
telephony service) phones are on certain digital PBXsuses
either two or three wires back to the PBX. Feature phones
however use at least four wires and some use six. None
of these digital devices need the resistor and capacitor
found in PBX master and Full Master converters. For
these 'phones' we would use a secondary converter that
converts from RJ45 to BT-style socket but has no additional
electronic components. Simple though this may sound,
there are some seven different wire-map variants of
secondary converters to choose from
Basic rate ISDN often needs to be delivered to the desktop,
or to other locations distant from the communications
room. There are a number of ways in which this can be
done. The most common is to install the telco's ISDN2e
terminal equipment in the comms room. It's output is
called the "S"/T bus, which has an RJ45 connector
and can be routed directly through the patching system
and structured cabling network to where it is needed.
But, because it is a bus system, it will not work correctly
without terminating resistors. These terminating registers
must be provided either by using special ISDN RJ45 outlets
(not very flexible) or by using ISDN converters that
contain them. This method does not take full advantage
of ISDN, which is actually capable of sharing itself
between eight devices (digital phones, digital faxes,
terminal adapters, routers, etc) each with a unique
address (phone number).
Wiring Scheme Should You Use?
A nightmarish eight wiring scheme is currently in use.
If you're working with a legacy system it may not even
comply with one of these. You'll need to refer to whatever
PBX documentation and site documentation you can find
to glean which system has been used. It should be simple,
but it usually isn't. Make sure that the site documentation
states clearly and frequently which scheme you've used
to give your successors a chance!
In ascertaining which system to use, remember the trick
is to ensure that every wire coming from the PBX or
wherever ends up on the right pin of the BT-style socket
in the converter at the user-outlet end not just some
of them. The reason that this matters is simple, the
user may be using a two-wire analogue phone now, but
next week he might want to change it to a six-wire digital
feature-phone or a four-wire ISDN. And there's a lot
of rewiring to be done if that doesn't work when he
patches it through!
Besides getting the wires out on the right pins, it's
also important to ascertain whether the phone, modem
or whatever needs a third "bell" wire and
to make sure the converter brings that out on the right
pin, otherwise you'll often get either no ringing or
continuous ringing. And, if the phone needs an old-style
earth for its recall function (instead of the normal
timed break), that needs to come all the way from the
comms room on the correct wire and final pin.
Make sure that the A leg and B leg don't get crossed
over. Although you'll be able to make calls, the ringer
won't work on some phones (though to make life more
confusing it will work on others) and earth-recall won't
The Right Converter
There are two elements to consider for choosing the
correct converter. One is cost technical and the other
is technical cost. Starting with cost, if a secondary
converter does the job, use it because it doesn't
have the added cost of electronic components. The PBX
Master is the next choice cost-wise because it does
not need a gas discharge tube surge arrester. The Full
Master is the most expensive. On the technical front,
we need to get the correct wires onto the correct pins
and that's where it gets complicated.
There are nine wiring schemes and by the time you've
accounted for Full Masters, PBX Masters and Secondaries,
there are actually 21 different types to choose from.
Worse still, as we all know, old site documentation
rarely tells the truth, PBXs are proprietary and have
their own idiosyncrasies and so the chances of working
out which type(s) you need is fairly slim.
Start with the secondary types first since these are
cheaper, but make sure the ringer works properly on
a phone that you know needs the third "bell"
wire. Check that earth-recall works. Then try it with
digital feature-phones and you should be getting close.
If the secondary types don't work move up to the PBX
Masters until you find the solution repeating the same
tests. You only need to use the Full Master if you found
that a PBX Master worked but you know that local surge
protection is advisable.
Remember, too, that you're likely to end up with several
different types of converters out on the office/shop
floor maybe Full Masters for DELs, PBX Masters for phone
extensions, secondaries for digital phones, and ISDN
converters as well. One thing you can be sure of users
will mix them up.
So whenever you need to investigate a user fault report,
checking the type of converter is the equivalent of
checking the fuse in the plug. Essential!
There is another wiring method used that can do away
with the need for converters and the problems of users
mixing them up.
To achieve this, voice patch panels are used in the
comms room and the cables from the DP and PBX are terminated
onto Full Master jacks, PBX master jacks or secondary
jacks in the voice patch panels. Working out which type
to use is the same as I've described above, but once
they're wired that's it! You'll need to make-up patch
leads, with BT-style plugs one end and RJ45 plugs on
the other, to connect them into the structured cabling.
You need to ensure that you map all wires correctly
in the patch leads so that all the possible user devices
work. Finally, you need to use phones with RJ45 plugs
rather than BT-style ones, so you'll either need to
buy them with RJ45s or you may have to put new leads
on them yourself.
You'll find that time spent at the start of the job
making sure you've got the correct wire-map and the
right choice(s) of converters for every conceivable
type of analogue phone, digital phone, fax, modem, ISDN
device, etc. will save you lots of time and heartache
later in the job.
point, some of the telco
lines may be PBX exchange lines to be cabled to a PABX
whilst others might be private lines to go to the PBX
or data routers
elements to consider
when choosing the
one is cost technical and the other is
Nagendra, Manager-Premise Networks, KRONE Communications
Ltd., India, can be reached at firstname.lastname@example.org