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Structured Cabling For Voice Communication

Voice 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

In 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 throughout.

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 the PABX.

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.

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 on the BT socket that the phone plugs into—and 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.

Digital Phone And Digital Feature Phone
A digital phone—as some of the POTS (plain old telephony service) phones are on certain digital PBXs—uses 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).

Which 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 work either.

Choosing 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!

An Alternative Approach
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.

In Conclusion
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.

At the distribution 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

There are two elements to consider when choosing the correct converter, one is cost technical and the other is technical cost

D.S Nagendra, Manager-Premise Networks, KRONE Communications Ltd., India, can be reached at dra.ds@krone.com

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