Structured Cabling Design  

Designing Structured Cabling Systems - Ten Step Guide

Below we have provided a ten step introductory guide for the Design of Structured Cabling Systems and IT Network Infrastructures. (see structured cabling schematic)

Step 1

Which group of standards will you conform to?

The three principle design standards give the details of how to design and specify a structured cabling standard, they are;

            ISO 11801

            EN 50173

            TIA/EIA 568-A

These standards in turn however refer to hundreds of other standards relating to component specifications, fire performance, testing methods, containment systems etc.

Step 2

Horizontal cabling - Basic rules

Four-pair cables are run from user positions to a patch panel.  At the patch panel, patchcords link into the active LAN equipment or into backbone cabling.  The user position has a wall outlet or floor outlet, and this links into the PC on your desk via another patchcord.  The outlet is a called a TO (Telecommunications Outlet) and contains an eight way plug meeting IEC  60603-7, more commonly referred to as an RJ-45.

• Two outlets per work area

• Two outlets per 10 square metres of useable floor space

• Outlets to be within 3 metres of the user station

• Both outlets to be RJ 45 

• Max cable run to be 90 m

• Max total length of patchcords at both ends of the link to be 10 m

• Cable and RJ45 to be Cat5e grade

Options

Cat 3 or optical fibre can be used

If optical fibre, select 50/125 or 62.5/125 multimode

If using fibre select SC or ST connectors

Cat 6/Class E can be specified

Cat 5e Cable can be unscreened, UTP,  Foil screened, FTP, or Foil and Braid screened S-FTP.

Cable fire performance can be:

1) IEC 332-1

2) IEC 332-1, IEC 754, IEC 1034

3) IEC 332-3-c.  IEC 754,  IEC 1034

4) UL 910 plenum

Each grade, in ascending order, has a better performance in fire situations but at a correspondingly higher price.

The exact density of cables, number of outlets and their position is up to the end user, or else at the proposal of the installer/designer

Step 3

Backbone Cabling

All of the horizontal cables are star-wired back to Telecommunications Closets or Floor Distributors where they are terminated in patch panels.  These patch panels are connected together via the building backbone cabling which can be up to 500 metres long.  It can be copper cable but is more likely to be optical fibre, either multimode or singlemode.  The kind of cables and the number of cores needs to be decided.

Step 4

Campus Cabling.  The campus cabling links different buildings together.  It can be up to 1500 m long. It can be copper cable but is more likely to be optical fibre, either multimode or singlemode.  The kind of cables and the number of cores needs to be decided.

Step 5

Positioning and design of Telecommunications Closets to link horizontal and backbone cabling.

Positioning and design of the equipment room as a central focus for the main computing, LAN and PABX equipment.

Positioning and design of the Service Entrance facility whereby outdoor cables are terminated and the point of demarcation between customer owned equipment and the PTT cables is defined.

Step 6

Cable containment system.  How will the cables be protected?  Within buildings the choices are;

-Cable trays

-wire basket/raceway

-cable ladders

-J hooks

-conduit

-dado rails

-PVC trunking

-built-in underfloor duct

-raised floors

-suspended ceilings

Step 7

Cable Administration system.  The cabling and its containment system need to be clearly identified and their locations, routes and capabilities recorded in a  cable administration system.  This usually involves a logical numbering scheme that can be applied to all cables, outlets, patch panels and even containment systems.  Various colour schemes are also available.

These schemes can be paper based but for the larger installations then a computer based system is advisable.  There are several proprietary solutions on the market which offer various database and graphical methods for keeping track of cabling assets.  Some systems are also active in that they can detect moves and changes and automatically update the database.

Step 8

Earthing Scheme.  All exposed metallic elements of the cable system and cable containment system need to be earthed (grounded) for safety and also electromagnetic compatibility requirements.  If screened cables are used then special attention must be given to effective bonding of the screening elements.  Poorly earthed screened cabling may behave worse than unscreened cabling.

An electrically ‘clean’ earth must be available at all points where the cabling is terminated, but especially within telecommunication closets, equipment rooms and service entrances.  A clean earth is usually defined as a conductive element with not more than 1 volt rms potential difference between it and the real earth down below.  Copper cabling linking two different buildings can suffer from earth loops if the ground potential is different.  Non-metallic optical cabling is usually picked for problem areas such as these.

Some useful standards are;

Step 9

Testing regime.  All cables must be tested to demonstrate compliance with the standards and specification to which they were bought.  Testing can be split into copper cable testing and optical fibre testing.  Ideally all cables should be 100% tested.

Copper cables.

There are five manufacturers of hand held copper cable testers that will automatically test the installed cable plant for all the expected parameters.  By the use of a remote injector, the cabling is tested from both ends, which is a condition of the standards.  The cabling has to pass all of the suite of tests to be awarded and overall pass.  Points to remember are;

What is being tested? the channel (i.e. end-to-end including all the patchcords) or the basic link (i.e. the permanently installed cable from outlet to patchpanel).  The test figures are different for each setting.  It is usually more practical to test the basic link (also referred to as the permanent link).

What level is being tested?  The tester should normally be set to Cat5e link or Class E link if Category 6 cable is being used.

The results are stored electronically and must be in a format recognisable by the cable management software that comes with the tester.  There are now numerous test standards and draft standards.  The most influential is likely to be;

IEC 61935     Generic specification for the testing of balanced generic cabling in accordance with ISO/IEC 11801

The tests required are;

Cable length and ACR are also useful additions to this set of tests.

Optical cables

All that needs to be tested with short distance multimode optical cables is attenuation.  This can be achieved by a device called a light source and power meter.  This device will simply measure the absolute loss across the optical link.  This then has to be compared with the design value of attenuation.  If the tested value is less than the design value then the link can be seen to be acceptable. 

Optical Time Domain Reflectometers can give a great deal of information about optical fibres, but for short haul multimode fibre they are an expensive overkill that gives results that need expert interpretation.  An OTDR remains an essential tool for fault finding.

Step 10

Final thoughts

Is the design of the cabling system in-step with the LAN aspirations of the end user?  For example, Cat5e is the minimum performance grade suitable for gigabit Ethernet.  Standard Cat5 cable may not have sufficient delay skew performance for RGB video systems however.  Cat 6 cabling will give a longer service life due to its higher performance, but at an initial higher cost.

Some optical fibre LANs, e.g. gigabit Ethernet cannot transmit over the full distance allowed in standards based optical structured cabling.  These LAN limitations have to be taken into account.  The next generation of 10 gigabit Ethernet will need a new generation of optical fibre to make it work.

The best way to ensure success in a structured cabling installation is to use properly trained people to design, implement and test the system.  The RCDD qualification from BICSI is the only qualification which covers all aspects of structured cabling design and implementation.

The above information is offered as a summary of ISO 11801 and related standards.  It is not a definitive design guide and does not replace study and implementation of the Standards themselves.  The publisher accepts no responsibility for inaccuracies or omissions.  To purchase the full Standards go to your national standards body, e.g. British Standards Institution, Nederlands Normalisatie Instituut etc. or ISO.

Further advice and a free consultation is available from one of our consultants John Laban RCDD/LAN