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Cabling best practice: your guide

Danny Bradbury

Your server is working slowly and the accounts department is complaining that queries are taking three times as long as normal. You start by checking everything from the top of the network stack down.

The applications seem to be performing alright, and there are no viruses or unexpected packets in the system. There are no Trojans on the server, and no one is downloading gigabytes of music to their iPod. You are stumped. But did you think of going to the communications room and checking the server's patch cable?

It is easy to forget that cables are the arterial systems of our networks. They can get clogged by too much data, and occasionally they can become degraded or corroded, forcing network managers to perform the equivalent of a triple bypass to get things running smoothly again.

Maintaining a healthy cabling system is crucial to the performance of the network, but how do you measure the health of your infrastructure?

Cabling: the basic structures

There are three main parts to most cabling structures: the core backbone cabling infrastructure is usually a vertical cabling system (running between floors) the structured cabling runs to the desk (normally horizontally along each floor) and the third part, the patch cabinet, is where computers on the local area network are connected to each other and hooked to incoming wide area network connections.

Patch cabinets are where things can go wrong, because cables are being pulled and replaced all the time.

Kelvyn Dale, managing director at network consultancy and cabling firm Q2 Communications, says, "When there is a problem, it tends to be the patch cords. The cables that you see in buildings that get pulled through risers, and so on, are solid, single-core copper, but patch cords are stranded, with strands of copper in each of the wires in the cord, so they tend to break more easily."

But just because it stays put when installed, backbone and structured cabling is not invulnerable. Even external cabling designed to run between buildings has unique vulnerabilities. Some suppliers have introduced anti-rat cables to stop data-hungry rodents doing their worst, for example. And Dale has seen other cabling that has given out after sitting in water-flooded ducts for months on end.

"We often find that electrical contractors pull the cables around as they would with electrical cable, and that is wrong," he says.

Cables have a certain bend radius beyond which they are not meant to flex. To see why, bend a thin piece of copper backwards and forwards and see what happens to it.

Cables get trodden on and bent backwards, all of which can degrade the information-carrying capabilities of the wire. Even having cable ties tied too tightly can cause problems.

Mapping and tracking cabling

When the experts do it, they do it properly. Mobile network operator T-Mobile is working with supplier Computacenter Services to lay new cable at T-Mobile's corporate campus and 14 switching centres - and there is a lot of cable.

The campus roll-out covers 48,000 outlets on the site and involves enough cabling to stretch from London to Moscow. Computacenter is currently working on upgrading T-Mobile's 14 switching offices, which handle voice and data services to customers.

"We have a complete strategy document that covers the way that cables are laid in buildings," says Tony Alger, manager for central and data products at T-Mobile. Alger says that such missives come from the engineering and planning department. "We have colour codes detailing what carries what service."

Cables carrying customer data, for example, use a different colour from those carrying support data or telephone voice call data. "If you drop something down into the void and it breaks through the protective coating and hits the cable, then, depending on what colour it is, you know how important it is to get things fixed," Alger says.

The void is the area of the datacentre where structured cabling runs and rarely gets changed. Experts carry out best practice everywhere, including here, but some amateur installers do not bother colour coding or documenting patch cabinets, where the environment changes frequently. That can quickly turn it into a cable installer's worst nightmare.

"When you get into the cabinet and you find the 'bowl of spaghetti' syndrome, that takes a long time to do current state analysis," says Computacenter's IT facilities solution unit director Neil Silverstein.

Sometimes, network teams will simply add another cord of any colour for expediency when carrying out a change on the network, and before long they will begin looking for another cabinet because their current one is full.

Often, they could recover 50% of a cabinet's capacity by pulling redundant cables, says Silverstein - if only they knew which was which.

Why and how to upgrade cabling

But why do companies such as T-Mobile want to upgrade their backbone and structured cabling? Depending on the infrastructure and application requirements, such upgrades can come around more often than you think.

"The industry-prescribed renewal rate is about six years, but that varies widely in practice," says Rey Geelani, managing director at cabling specialist Network Interlinks.

Renewal rates depend partly on the situation of the building. A building with tenants that change regularly may need more upgrades to suit their particular needs, for example, and existing cabling configurations or types of cable may not be adequate.

Suppliers suggest installing cable with enough capacity to support two to three generations of active electronics.

Which cable?

The types of cable have multiplied over the years. Copper and optical fibre are the two basic types. Fibre is used mainly for vertical backbone deployments, and copper is generally used for structured cabling.

Copper breaks down into multiple categories, and the categories are numbered. The categories are referred to as "Cat", which gives us Cat 1 cable, through to Cat 6 and beyond.

"Pre-Cat 5 was all based on telephone wiring. Cat 3 was a telephone-grade cable and was a cheap and cheerful way of driving a 10mbps network," says Geelani.

Cat 4 was better quality, but was short-lived. "It did not really do any­thing apart from deliver 10mbps a bit better."

Cat 5 was the quantum leap in cabling, because it delivered 100mbps, but this was superseded by Cat 5e, which offered up to 1gbps by using a higher grade of copper and rearranging the twists of the cable inside the shielding.

When Category 6 came along, it supported higher data rates using a thicker copper core, and supported data rates up to 10gbps - sort of.

Pre-Cat 6 wiring was able to support distances up to 90m (plus 10m extra for patch cabling) before it needed a repeater, but Cat 6, when pushed to 10gbps , may only be able to cover half of that.

"New technologies such as 10Gbase-T do not support Cat 5e, and Cat 6 only does so to certain distances and with much mitigation to lessen the effects of alien crosstalk," says Carrie Higbie, global network applications market manager at cabling manufacturer Siemon. Alien crosstalk refers to signal interference between cables, which presents both a reliability and security issue.

Companies must also choose between shielded and unshielded cable, depending on the electrical interference that they expect from other cables and mechanical systems in areas such as lift shafts.

The other problem with Cat 6 cabling is that it can be harder to run through containment systems, says Dale. "Cables tend to be about 7mm in diameter for Cat 5e and 9mm-10mm for Cat 6. It does not sound a great deal, but when you have 100 of them in a container and you have to get them around a corner, those extra millimetres are important," he says.

In spite of its disadvantages, Cat 6 is the recommended requirement in the US-based TIA-942 ­datacentre cabling standard. However, experts recommend using the next grade of standards as they become available.

Cat 6A (augmented) operates at double the frequency of Cat 6, and Cat 7 drives frequencies even higher.

Fibre cables are just as fragile as copper. Bending them beyond their allowed radius can introduce hairline cracks that can impede the signal, says Dale.

There are two main types of fibre - single mode and multi-mode. The single-mode fibre, which allows just one light source to pass along the cable, is typically used outside the datacentre for covering longer distances. The multi-mode version, which allows multiple light sources down the cable for increased bandwidth, covers shorter distances and is ideally suited for the datacentre.

Fibre is graded using the Optical Multimode (OM) standard, and experts recommend the highest grade, OM3.

That is the problem with cabling: it is expensive, because you are constantly advised to buy the latest, most future-proof materials. And the volatile price of copper of late has driven up prices across the board, says Dale.

But experts argue that you are better off putting in the best possible cabling and amortising the cost over a longer period, rather than having to go back and recable sooner because you used inferior materials.

Options for building high-speed networks >>

Fibre optic networks vulnerable to attack >>


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