Wiring a Building for Telecom
Older homes usually have very simple wiring for their telephones:
A four-wire "station wire" cable winds around the house, connecting all
outlets in parallel to the "Network Interface Jack" where the
wires from the telephone company come into the house.
Office buildings are generally wired very differently, and since
newer homes should be wired the same way, it is worthwhile to
get familiar with this system.
Whether you are finding out how the building your employer occupies
is currently wired, or you are planning a new or expanded installation,
you need to learn the system.
The first two places you need to locate are:
- The building's "telecom entrance": Where do the telephone
company's cables enter the building.
- The "plywood board": Where do the cables come together.
If you are lucky, these happen to be in the same room. If not, you need to
add cabling to bring the outside lines from the entrance to the
plywood board. (Typically by installing a "66 block" at each of the
two locations, and pulling a 25 pair cable between them.
The plywood board is the heart of the system. Typically, a 4 foot
by 8 foot 1/2 inch plywood board is mounted on the wall, and everything
is then screwed into this: The telephone switch, the voicemail system,
the punchdown block with the outside lines, the punchdown blocks with
wires to the offices. Ideally, you will also find room for your network
(ethernet) hubs here. In the following, I will discuss these.
Cable types and Punchdown Blocks
You will find some or all of the following cable types. These are
all useful and each has its place.
- Station wire: A thin (3-4 mm or 1/8 inch) round cable, typically
with a beige outer jacket. 4 wires, preferably solid 24 AWG;
typically the strands are Red and Green (twisted together) and
Yellow and Black (twisted together). Sometimes you will instead
see the strands colored Blue/White and Orange/White (i.e. one pair
is a blue wire with white bands twisted with a white wire with blue
bands; the other pair is an orange wire with white bands, twisted
with a white wire with orange bands).
If the cable is old, the wires may not be twisted in pairs.
- Satin cord: A silver colored flat cable with 4 stranded 26 AWG
wires. No twists. Should only be used for 6-8 foot cord from a
wall jack to a telephone set.
- Category 5 wire: A 5 mm (1/4 inch) round cable, typically with a grey
outer jacket. 8 wires, in twisted pairs: Blue/White, Orange/White,
Green/White and Brown/White. Use for 2 telephone jacks or one LAN
outlet jack. (See this page for more
about category 5 wiring.)
- 25-pair cable: 1/2 to 3/4 inch round cable with grey,
beige or pink outer jacket. 25 twisted pairs, usually fitted
with AMP CHAMP connector (or Amphenol). Used for connecting punch
down blocks in different parts of a building. Often used between
PBX and punchdowns on plywood board.
- Punchdown blocks, typically "type 66". A plastic block 3" wide by
about 10" high with 50 rows of 4 little forked clamps sticking out.
The 4 clamps in each row form two paired terminals; you can
punch a wire into the forked clamp on the left, then another into
the forked clamp on the right, then install a "bridge clip"
across the two middle clips. You can now easily disconnect the
bridge clip to separate the circuits for test and measurement.
Usually, a 50-pair connector is mounted on one or both sides,
so that you only punch down the wires on on side of the block
individually. A special tool is required for punching the
connections down.
Category 5 cable is used for both telephone wiring and data network
wiring. It exists in two very different types: Solid conductor and
stranded conductor. Unfortunately, you can buy a 1000 foot spool,
and it won't say on the box which kind it is; you have to feel
the cord, flexing it to see how stiff it is.
Solid conductor cable is meant to go in the wall. It is stiffer
than stranded, and the solid conductors work well with the 110-type
punchdown connectors on the back of wall jacks and with the
66-type punchdown blocks on a plywood board. It does not work well
with the modular plugs that you crimp on the end of a patch cord,
and the solid conductors are prone to break when a loose cable
is handled, kinked and stepped on.
Stranded conductor cable is softer, and is meant to be used for
cords running around in user space. It does not work well with
punch-down connectors, but works fairly well with crimp-on plugs.
(There is a flat version of the stranded cable, but it should not
be used for data networks, because it is not twisted pairs,
and thus subject to noise injection and noise radiation. Use it
only for patch cords less than 10 feet, and not for 100 Mbps.)
For more information, see the page about
Category 5 wiring and connectors.
A wiring installation must be designed, installed, tested and documented.
To ensure that you discuss these issues with your contractor,
reference a
Design Brief.
$Log: wiring.htm,v $
Revision 1.4 2000/06/21 05:09:59 lars
Added cross-references, and supporting links.
Added 25-pair color code.