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Connecting phones through structured cabling ought to be simple – and ultimately it is – but first there the minefield of wiring schemes to worry about.

In the public domain are BT's, C&W's and other telecom companies (telco) local exchanges. From there, the telco's cables are routed until they come into the customer's building where the telco connects them 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.

Depending on the configuration there may be a distribution point (DP) combined with the NTD to connect to. Or, there may be a separate telco-owned DP or blocks on the main distribution frame to which you must connect (see figure1).

At this DP, some of the telco lines may be PBX exchange lines to be cabled to a PABX, others might be private lines to go to the PBX or data routers while others are direct exchange lines to go to payphones or equipment that should not route through the PABX. Others are 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.

Analogue PBX extension

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 in the BT socket that the phone plugs into. The analogue phone is usually a three-wire device – though if earth recall is used it may need four. 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.

Normally, these components would be fitted in the BT-style phone socket, but if you are using RJ45s for structured cabling they contain no such components. The solution is to fit an RJ45-to-BT converter (see figure 2) at the user-outlet end which changes connector type and contains the PBX master or full master components. These are the PBX master converter and full master converter respectively.

Digital phones

A digital phone uses either two or three wires back to the PBX. Feature phones however generally use at least four and sometimes six wires. None of these digital devices need the resistor and capacitor. For these phones we would use a secondary converter which 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 converter to choose from.

ISDN

Basic rate ISDN often needs to be delivered to the desktop, or to other locations distant from the comms room. There are a number of ways of doing this. 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, 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 and these must be provided either by using special ISDN RJ45 outlets (not very flexible) or ISDN converters that contain the terminating resistors. This method does not take full advantage of ISDN which is actually capable of sharing itself between devices, each with a unique address (phone number).

Unfortunately the S/T bus is a daisy-chain type of topology and structured cabling system is a star. Krone & BT developed an ISDN patch panel that overcomes this problem and has eight RJ45 patch outlets each of which can be cabled anywhere in the structured cabling network to allow several devices to be used. This special ISDN patch panel also contains the ISDN terminating resistors.

Which scheme to use?

There are eight wiring schemes commonly 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 and site documentation to glean which system has been used. If you're installing new, then the preferred scheme in the UK is TIA/EIA 568 B.

It is important to ascertain whether the device needs a third 'bell' wire and to make sure the converter brings that out on the right pin, otherwise you'll get either no ringing or continuous ringing. And, if the phone needs an old-style earth for its recall function 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 because, although you can make calls, the ringer won't work on some phones.

Choosing the right converter

Starting with cost, if a secondary converter does the job, use it. 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, you need to get the correct wires onto the correct pins – and that is where it gets complicated. Not only are there nine wiring schemes but by the time you've accounted for full masters, PBX masters and secondaries, there are actually 21 different types to choose from. Worse still, PBXs are proprietary and have their own idiosyncracies and so the chances of working out which type(s) you need is fairly slim.

Many installers carry a full set of well-labelled converters and test on-site. Start with the secondary types, 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 find that a PBX master works but you know that local surge protection is advisable.

Maintenance

You are likely to end up with several different types of converter 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. 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 which 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 either full master jacks, PBX master jacks or secondary jacks in the voice patch panels. 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 will also need to make sure that you map all of the wires correctly in the patch leads so that all the user devices work. Finally, use phones with RJ45 plugs rather than BT-style ones.

Voice over structured cabling is a real cost-saver for the customer in the longer term, but it can be a bit of a pain for the installer. 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 type of analogue or digital phone, fax, modem, ISDN device, etc will save you time and heartache later in the job.