Selection of the right cable for a data installation is as difficult as it is vital. Future applications aimed to run on the network must be considered and different technical issues as screening / non- screening, EMC, fibre vs. copper, data speed vs. transmission frequency, earthing and bonding, fire safety and nominal impedance have to be weighted against cable and installation costs. As always, quality costs and a compromise must therefore normally be drawn where one will not pay more for any additional performance.

Different LAN owners will make different decisions based on future network requirements, budgets and, most of all, on information level of decision makers.

The present discussion is intended as a brief introduction to aid selection of the optimum cables for data installations.

Cable performance depends on various transmission parameters like NEXT (near end crosstalk), attenuation, ACR (attenuation to crosstalk ratio = signal to noise ratio), FEXT (far end crosstalk), impedance or return loss, delay skew (relative time delay for propagation of bits along the different pairs of the cable) and EMC performance. Significant parameters for one application, like 10Base-T, may be very different from other applications (like Gigabit Ethernet). Therefore it is extremely difficult to look into the future and predict the necessary minimum performance of cabling to support next generation applications. Cabling committees try to support the market by including into the cable standards minimum requirements to all relevant parameters necessary to operate foreseen new future applications.

However, no one can look more than 2 - 3 years ahead, and therefore it is generally recommended always to select the best performing cables in order to avoid, as far as possible, future surprises.

The following facts should help in making the optimum cable selection.

Fibre versus Copper

Fibre cabling has an evident advantage of safe EMC performance and long distance transmission. Counting against fibre are costs (total installation costs) and tradition to use copper cabling to already existing telephones, PCs and other equipment. Data speed is by many people believed to be a significant advantage for fibre cabling.
However, with the new advanced codings and high quality copper cabling several gigabit/sec will be obtainable on copper cabling which makes this argument somewhat unimportant.

Consequently, fibre cabling is the natural choice for backbone cabling, but will not take over the horisontal cabling from copper unless the total costs of fibre installations are reduced to below the corresponding copper level.

The present Survey of Qualified Components for Generic Cabling does not contain any fibre cables.

Screened versus Unscreened

Unscreened installations are generally less expensive than the corresponding screened. This cost benefit is mainly due to the sometimes very expensive grounding of the building installation where max. 1 Vdc between the different parts of the earthing system must be present unless special measures can be taken. Also the installation time for terminating any cable screens adds to the screened installation costs.

However, this cost benefit for unscreened installations has to be weighted against performance sell-out. EMC performance of screened installations is better than for unscreened (provided, of course, that screens are correctly terminated and an acceptable grounding is present). It is a common misunderstanding that the balance of unscreened cables can compensate for the missing screening. Even the best UTP cables have 15 - 50 dB (30 X - 100.000 X) worse EMC performance than screened cables.

The interesting point is only how much EMC performance is really needed. No general answer can presently be given other than the common sense advice: The better the EMC performance - the smaller the risk that future high speed network applications go down due to EMC disturbances. It should be noted that the biggest problem is the disturbances to the network (mobile phones, other PCs, etc.) for long links and channels running high speed applications. No problems are for instance foreseen when running low speed applications (like 10Base-T) on unscreened LANs. Potential emission problems for installations are often included in EMC discussions. This is probably not a significant issue as the PCs will often be the worst performing parts of the network with respect to emission problems.

Consequently screened cabling can always be recommended if the additional expenses can be covered by the budget. However, if future high speed applications are intended to run on the network one would indeed be very brave to install less than well screened cabling.

Data Speed versus Transmission Frequency

It is a common confusion point that two different definitions of transmission speed or velocity exist on the market, i.e. Mbit/sec and MHz.
The difference is easy to understand. MHz is the unit used to characterise cabling and is independent of the application running on the network. Contrary to this Mbit/sec is identifying the data speed on any cabling and cable, and is only determined by the coding. A well performing cable has a high rating in MHz, while a high bit rate (in Mbit/sec) can be found on even an inferior cable. However, to run high bit rates on a low "MHz" rated cable the application needs to be stressed by having complex coding (i.e. more expensive chips and netcards).

Cats

The number of cable ratings have been extended by three new categories and standardisation of these new performance ratings is in rapid progress. Presently the four cable categories Cat. 5, Cat. 5+ (Enhanced Cat. 5), Cat. 6 and Cat. 7 are included in the 3P verification programme.
Discussion of the different categories of cables is found in the section about standardisation.

When evaluating the need for a specific category of cable for an installation it should be noted that any higher category will always cover all categories having a lower rating. This means, for instance, that a Cat. 5+ cable will cover all Cat. 5 requirements and have some additional performance benefits, which are again improved for Cat. 6 and Cat. 7 cables. However, since installation costs are usually expected to increase when going to a higher cable category it should in every case be evaluated if the additional performance benefits of a higher rating will justify the associated installation costs.

Safety

Two main safety classes of cables exist outside the US, i.e. PVC sheathed and low smoke, zero halogen cables (LS0H). Furthermore plenum cables are widely used in the US, but these cable types are, for costs and toxicity reasons, neither popular nor specified in Europe.

PVC cables are still extensively used, but due to the generally limited fire retardancy and unpleasant smoke and gases developed during fire LS0H cables are now always being specified in CENELEC cable standards. The fire retardancy ratings of LS0H cables is presently being discussed and 3P certifies the LS0H cables to four different ratings, IEC 60332-1(flame retardant), IEC 60332-3, Cat. C, IEC 60332-3, Cat. D and IEC 60332-3, Cat. E, as discussed in the standardisation section.

Impedance

Three different nominal impedance levels are possible for data installations, i.e. 100 Ohm, 120 Ohm and 150 Ohm.

100 Ohm cables are having the predominant market share and therefore the development of hardware and software is mainly focusing on 100 Ohm systems. However, 120 Ohm and especially 150 Ohm cables are having significantly improved attenuation performance, which is essential as the significance of this parameter is increasing for future applications. 150 Ohm cables are mainly 2-pair cables which means that the present 4-pair version of the Gigabit Ethernet application will not run on one 150 Ohm cable.