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.

The standardisation of cabling and cables is presently in violent progress and it may therefore be expected that people who are not actively involved in development of the new categories and requirements may feel confused or even lost. Company marketing is in some cases adding to the general confusion by using non-defined names as "level" or "Gigabit" cables.

The background for the development is double. First, a technical improvement of cable performance which far exceeds the Cat. 5 requirements. Second, development of applications like Gigabit Ethernet which stresses the Cat. 5 performance over the limit making new performance ratings desirable or necessary. For this reason standardisation groups are working to prepare new cable standards so that the following categories will be covered in the future:

Cat. 3 Low performance cable rating which is disappearing. Not included in the 3P Certification programme.

Cat. 5 The traditional rating of cables for data installations. Has been requested by IEEE application committee to be extended in the future with new transmission parameters like equal level far end crosstalk, power sum requirements and delay skew. Expected publication of requirements is mid 1999. Rated frequency is 100 MHz.

Cat. 5+ Ne rating now being developed in the US, which will probably become the future Cat. 5 rating. Rated frequency is 100 MHz but possibly requirements will be specified in the future to 125 MHz in order to show graceful degradation at frequencies where ACR (signal to noise ratio) may be negative. Prepared to support Gigabit Ethernet. Please note that the name Enhanced Cat. 5 is used in the US.
Cat. 6 New rating which is presently being developed in both the US, ISO/IEC and CENELEC. Is intended to reflect the max. technical performance of unscreened and overall screened cable types. Expected publication of requirements is late 2000. Rated frequency is 200 MHz with requirements specified to 250 MHz in order to show graceful degradation at frequencies where ACR (signal to noise ratio) may be negative.

Cat. 7 A rating for individual pair screened cables derived from the german DIN 44312-5 standard requirements. Is intended to reflect the technical performance of normal individual pair screened cable types (commonly called STP, S-STP or PIMF cables). Expected publication of requirements is late 2000. Rated frequency is 600 MHz. Upper test frequency is presently being discussed and so far requirements only apply to 600 MHz.

The following standardisation groups are responsible for the development of international cable specifications:

ISO/IEC JTC 1, SC 25, WG 3:
An international cabling committee which is driving cable requirements, for instance based on cooperation with application committees. Cable standards for ISO/IEC JTC 1, SC 25, WG 3 are prepared by IEC SC 46C.

CENELEC TC 215:
An international cabling committee for European countries working in parallel with ISO/IEC JTC 1, SC 25, WG 3. Many persons are members of both cabling committees. Efforts are being made to secure consistency between the work of the two cabling committees, but national traditions between countries may cause differences in the resulting standards. Cable standards for CENELEC TC 215 are prepared by CENELEC SC 46XC.

IEC SC 46C:
The international cable committee which developed cable standards, for instance according to requirements of ISO/IEC JTC 1, SC 25, WG 3. Cable standards are to be referenced in the new edition of the cabling standard developed by ISO/IEC JTC 1, SC 25, WG 3.

CENELEC TC 46X (SC 46XC):
The CENELEC cable committee which developed cable standards, for instance according to requirements of CENELEC TC 215. Many persons are members of both cable committees. Efforts are being made to secure consistency between the work of the two cable committees, but presently very big differences exist in the structure of the two proposals of cable standards.

Similar work is ongoing in the US to develop cable standards for the future cabling specified in ANSI/TIA/EIA-568 and Addendums.

It should be noted that the cabling standards ISO/IEC 11801 and CENELEC EN 50173 define the categories of cables while the cable standards only specify the frequency ratings of the cables.

3P is a member of all the cable and cabling committees referenced above in order to secure compliance of 3P Verified cables with future requirements of developing standards.

Until recently most cables for LAN installations were PVC sheathed, and halogen free cables were only applied in a few special cases. However, a significant change of attitude and regulation is now developing in the market, for instance in CENELEC standards EN 50167, EN 50168 and EN 50169 by direct specification that screened cables must be halogen free. These new requirements to halogen free cables have impact on the safety and especially the fire rating of the cables, as discussed in the present 3P Newsletter.

Arguments for turning to Halogen Free Cables

PVC is in most respects an ideal sheath material. Superior mechanical characteristics are combined with high reliability. However, two main drawbacks have forced the development of alternative, halogen free sheath materials.
First issue concerns environmental considerations in connection with use of PVC. Key words like "acid rain", "dioxine formation", "pollution with heavy metals", "fertility of man and male animal" and "cancer risk" are popular environmental arguments connected with PVC. Discussion of relevance of the various claimed environmental hazards is interesting, but outside the scope of the present 3P Newsletter.

The second disadvantage forcing substitution of PVC concerns the fire behavior. In a fire situation burning or extensive heat causes:

Development of a heavy black smoke

Development of hydrochloric acid and some poisenous gasses.
Together these two factors affect human safety in case of fire. The smoke causes panic as escape routes cannot be seen. The poisenous gasses cause asphyxiation if people cannot escape the smoke in a short time.
Development of the hydrochloric acid may destroy both electronic equipment, machinery and buildings. The chlorine will contaminate all surfaces exposed to the smoke and may cause severe corrosion in a very short time. There are numerous examples of multi million dollar damages after even a very small PVC fire, in spite of the positive effect of an immidiate cleaning operation.

Benefits and Drawbacks of Halogen Free Sheath Materials

For the above reasons the search for an alternative to PVC has been intensive and proved to be successful. A number of compounds are available today, mainly based on the plastic material "EVA" filled with aluminium or magnesium hydroxyde. Fire retardancy comes from generation of water during fire.
The halogen free materials are normally more expensive than PVC to buy and to process. Consequently the halogen free cables will normally be more expensive than the corresponding PVC sheathed cables. Furthermore, some cables, especially with early compounds, may be more stiff than the corresponding PVC cables.

Fire and Flame Retardancy of Halogen Free Cables

A benefit for the halogen free cables is a better fire performance than possible for PVC cables. Unfortunately a large number of fire and flame tests exist and it is therefore not always clear which fire or flame performance a specific cable offers.
The following ratings can be found for communication cables:

Flame retardancy according to IEC 332-1 (Corresponds to HD 405.1) is verified by burning one cable with a single flame. IEC 332-1 is and will be the fundamental flamability rating for all cable types, including also PVC sheathed cables. All international communication cables must pass this requirement.

Fire Retardancy according to IEC 332-3, Cat. C (corresponding to HD 405.3) is verified by burning a bunch of cables with a large burner. IEC 332-3, Cat. C is the additional flammability rating that presently is mandatory for halogen free screened cables. However, the position with respect to the IEC 332-3, Cat. C requirements is both clear and confused:

Clear requirement: If you install screened cables and need compliance to EN 50173 (and therefore EN 50167, EN 50168 or EN 50169) you must install fire retardant, halogen free cables.

Confusion: No formal fire retardancy requirement exists for unscreened cable. Not because of international disagreement that unscreened cables should also be fire retardant and halogen free, but simply because standardization of unscreened cables is missing due to open EMC discussions.

Furthermore the fire retardancy requirements of screened cables might be reconsidered in the future updates of the standards. The valid argument is being discussed: Why should the halogen free cables be forced to have added costs in order to pass fire retardancy requirements (because of thicker sheath needed), when PVC sheathed cables have never passed this requirement?

Flame and Fire Retardancy according to, or covered by American Ratings like CM, CMX, CMR and UL VW-1
The above American safety ratings cover different flammability tests ranging from a single flame applied to a single cable (but different to the international IEC 332-1 test!) to a very severe testing of bunched cables in a tunnel (Steiner Tunnel Test).
Designations like "CMX" are often found on cables, but for most countries the American safety testing is a tradition from earlier days and should be replaced by the international flammability ratings with associated additional safety and material requirements.

Identification of Halogen Free Cables
There is confusion about the designations for halogen free cables. The most frequent identifications are found below, but abbreviations can be found pairwise in any combination or order:

• LS0H Means "Low Smoke, Zero Halogen"
• LSZH Means "Low Smoke, Zero Halogen"
• HFFR Means "Halogen Free, Fire Retardant"
• FRZH Means "Fire Retardant, Zero Halogen"
• LSFRZH Means "Low Smoke, Fire Retardant, Zero Halogen"

All cable designations are describing the same cable type, except for "FR" since halogen free cables may be either fire or flame retardant.
Additional Safety Requirements for Halogen Free Cables
Halogen free cables are in practice always passing requirements to halogen emission and low smoke generation specified in the relevant safety standards. Furthermore, the mechanical, thermal and ageing characteristics of the halogen free sheath, insulation and total cable are specified for communication cables.
All safety and material requirements are of course verified for 3P qualified halogen free cables. We always identify our qualified halogen cables as LS0H EN & ISO/IEC Communication Cables