5.1 Cabling Practice and Standards
There are three main cabling standards:
- EIA/TIA 568A – This was the first standard published (1991) and is the US standard
- ISO/IEC 11801 – This is the International standard for structured cabling systems.
- CENELEC EN 50173 – This is the European standard for cabling (the British version is BS EN 50173).
The reason for having a ‘standard’ is to define a method of connecting all types of vendor’s voice and data equipment, over a cabling system that uses a common media, common connectors and a common topology. This means that a building can be cabled for all its communications needs without the planner or architect ever having to know what type of equipment will be used.
Cable Testing and Testers
Before looking in detail at structured cabling it may be a good idea to review the features and capabilities of the wide range of cable testing equipment available. Although cable testers are used to determine the overall capacity of a cable run they can also be used for many other important diagnostics tasks. These include finding bad connections, measuring attenuation and crosstalk, detecting incorrect wiring, and displaying detailed wire maps.
How Cable Testing Works
Cable testers are known as Time Domain Reflectometers (TDRs) because they make use of a technique known as Time Domain Reflectometry. This technique allows for the finding and locating of flaws and problems on all types of cable, including fibre-optic. A TDR works by sending a signal down the cable and measuring the time it takes for a signal or partial signal to return. If the cable is faulty the signal may not be returned or the signal may be returned sooner than appropriate. If no signal is returned this indicates a break in the cable whereas an early return may indicate another problem. It is the measured time that is crucial in pinpointing where the fault lies in the cable. A TDR test can provide distance readings that are accurate to within 61 cm. This type of testing indicates a problem but does not explicitly identify the fault, Note: The devices used for testing fibre are known as OTDRs with the O standing for Optical. These devices are classified as Optoelectronic instruments.
TDR Wire Maps
Most cable testers include a feature called a wire map which is used to visualise the connection of wire pairs. This map is usually created as part of the testing and can be viewed by pressing a button. The map can be used to display:
- Reversed Pairs
- Short Circuits
Reversed pairs, or crossed pairs, are a problem unique to UTP. If crossed pairs are detected the cable will not be able to work properly and the connections will have to be cut off and new connections made.
OTDR: Signal Loss
A fibre link consists of two separate fibres that work independently of each other, ie: one transmits and the other receives. Each fibre is protected by a sheath that external light cannot get through; therefore there are no crosstalk problems. Electromagnetic Interference also has no affect on fibre; however, fibre is still prone to attenuation although to a lesser extent than copper based media. The one area that fibre has in common as a problem with copper is impedance. In fibre, the form that impedance takes could be impurities or micro breaks in the fibre. This would prevent the entire signal passing purely along the cable and cause difficulties at the receiving end. The main cause of this type of loss of signal strength is connectors not being installed correctly.
Testing Fibre: Link Loss Budget
In order to make sure that a fibre link works, ie: enough light is getting to the receiver in order for accurate data transfer, a calculation that works out the acceptable amount of signal loss that can occur should be carried out. This calculation is known as the ‘optical link loss budget’. This test is carried out by a device called a light source and power meter. If the fibre fails the link loss budget test then an Optical TDR can be used to locate impurities and cable flaws. However, the usual problem is incorrectly attached connectors.
Although the decibel is most familiar as a measure of sound, it is also used to describe all networking signals, whether voltage waves on copper, optical pulses in fibre, or microwaves in a wireless system. Decibels are the key measure of signal and noise in all communications systems. Decibels are a relative measure. There is always some reference voltage or power level in the denominator of the formula.