STRUCTURED CABLING: Compliance Is Golden

What should be the performance of a structured cabling system? Some

prioritize link performance, others quote channel performance. How can

installers and end users understand what performance they're really buying —

and be sure that that's what they get? First lets just recap what the

standards define by the words channel and link — and how they're not

necessarily what you might think.

The channel is the entire cabling system comprising all of the cable,

connecting hardware such as outlets and patch panels, and all the cords between

(but not including) the mated plug/socket in the network interface card (NIC),

used in your PC at one end — and the comms room equipment, typically a switch,

at the other. The performance requirement of the channel is actually handed down

to the cabling industry by the groups who define 'applications' such as

Ethernet, ATM, SDH and the like. These specifications basically say "If the

d.c. resistance is better than A Ohms, the insertion loss is better than B dB,

the signal to noise ratio is better than C dB, crosstalk is better than D dB,

the return loss is better than E dB, etc... then the application will work over

the channel". These groups that define the applications are only interested

that an RJ45 plug is at each end of the channel to fit into the RJ45 sockets

provided. They don't really care how the connection is made or configured as

long as the channel performance requirements are met.

Even

the different structured cabling standard bodies use different definitions! In

ISO/IEC11801:2002 and EN 50173-1:2002, a compliant channel can be any collection

of cables, connectors and cords that, as a cabling system, meet the channel

performance requirements. Whereas under the American standard, TIA/EIA-568-B ,a

channel must be constructed of individually compliant cabling components. And,

of course, if the individual components are not Cat 5e or Cat 6 component

compliant, then there is very little chance of the channel continuing to meet

its specified performance requirement should a different manufacturer's patch

cord, outlet jack or patch panel be introduced later on — leading to network

performance problems and reduced throughput.

A permanent link, as defined in ISO/IEC and EN, includes the horizontal

cabling from the horizontal patch panel jack in the communications room through

to the telecommunications outlet jack at the workplace — including the mated

connectors at each end. The permanent link is defined because, in most cases

when a cabling system is being installed, the channel (which also includes all

the additional cords and connections) simply doesn't exist at that point in

time.

Classes versus Categories



ISO/IEC 11801:2002 and EN 50173-1:2002 define various channels in terms of

cabling media and performance levels. For twisted pair cabling, there are three

high performance channels specified: Class D which is specified to 100 MHz

bandwidth and which is constructed using Cat 5e cabling components within what's

called a 'prescribed design'. A Class E channel is specified to 250 MHz, and

is constructed using Cat 6 cabling components. Similarly, a Class F channel is

specified to 600 MHz and is constructed using Cat 7 cabling components. The

possibility exists that channels can be constructed using proprietary cabling

components (cables, connecting hardware and cords), or from cabling components

that may look like Cat5e or Cat 6 components but are in fact not component

compliant. This can lead to poor (non- compliant) jacks and patchcords being

compensated for by better quality cable so that the overall channel meets or

exceeds the Class D, E or F performance requirements.

The

whole point of cabling standards is to be able to plug together cabling

components from different manufacturers and still meet the overall channel

requirement. You don't want a situation where an end user is mixing compliant

and non-compliant products. For instance, if manufacturer A makes its jacks with

a performance that is worse than the Cat 5e standard, but makes up for it in the

permanent link or channel by using a better performance cable, and manufacturer

B makes its jacks and cables so that they are only just compliant to Cat 5e (an

acceptable and very common practice), what happens when you mix manufacturer A's

jacks with manufacturer B's cable? Exactly — it's not going to work

properly! So, to ensure that cabling components from different manufacturers

will interoperate, we have Cat 5e and Cat 6 component requirements. Now, if all

the cable, outlet jacks, cords, and patch panels in a network are truly

component compliant, even if these cabling components are from mixed

manufacturers, not only do you have a system that will by definition give Class

D or Class E channel performance, in an ideal world you wouldn't even need to

test the system! Of course in the real world, it's quite possible for poor

terminations and wire map errors to occur or for jacks and plugs to become

damaged. And cables are all too easily crushed, kinked or damaged on site during

installation. So cabling components can cease to be component compliant with the

result that channel performance is jeopardised! In reality, it is imperative to

test on site after installation.

Of course there's a big problem here. Testing the channel or permanent link

does not reveal whether the components used are Cat 5e or Cat 6 compliant — it

can only tell you if, as installed, it meets the channel or permanent link

performance requirements. Do Cat 5e and Cat 6 cabling components guarantee Class

D and Class E performance respectively? Essentially, yes they do. It is the

intention of the standards that if you construct a permanent link using Cat5e or

Cat 6 cabling components it will meet the Class D or Class E requirements

respectively. If you then add fully component compliant Cat5e or Cat 6 cords and

connecting hardware respectively then you will have a Class D or E channel.

(And, in fact, if you mix Cat 5e and Cat 6 cabling components you will achieve a

minimum of Class D channel performance overall).

By definition, the electronics used with a Class D channel will never go past

100 MHz, and those using Class E will never go past 250 MHz. So manufacturers'

claims of 300/350/450 MHz are completely and totally irrelevant. It's just

pure, unnecessary, confusing hype. But let's be clear, although the standards

allow for a great diversity of proprietary solutions, 100% of customers want to

buy a system that is true Cat 5e or Cat 6 i.e. it's the fully interoperable,

fully component compliant option. And they can only achieve this by specifying

that all of the cabling components in the solution are fully component compliant

to the standards. If they are, and the permanent link and cords are fully

tested, the overall system will be exactly what they wanted to buy — a fully

compliant solution.

There

is a defining way to measure product capability and create a level playing field

and it's quite simple. Ensure that all of the cabling components in the system

offered compliant to Cat 5e (or Cat 6) according to ISO/IEC 11801:2002, EN

50173-1:2002 or TIA/EIA-568-B. That's what really matters. Users should decide

for themselves whether they value the comfort of a safety margin or headroom and

should totally discount irrelevant claims about additional bandwidth and channel

performance.

Bala K Chandran managing director Krone Communications India