2.4 Fibre Optic

2.4 Fibre Optic Cable

Fibre-optic cable consists of glass or plastic fibres that carry data in the form of light signals. Unlike copper cable there is no electricity, only light. This adds a level to security to fibre that is missing from copper, i.e. the cable cannot be tapped to detect signals.

Fibre-optic cable is perfect for high speed, high quality data transmission and although it suffers from a form of attenuation, it is nowhere near as severe as that for copper cable.

The reliability, security and distances covered by fibre optic cable make it the natural choice as backbone cabling within buildings and between buildings.

How Fibre-Optic Cable Works

The glass or plastic used in fibre-optic cable is manufactured to be pure and without any imperfections so that the light signal can travel without hindrance over long distances, e.g. miles. The fibres or threads are extremely thin strands; about as thick as human hair, coated with two layers of highly reflective plastic that creates an internal mirror around the strand. The outer sheath works as mirror causing an internal reflection that light cannot escape from and is thus guided along bouncing off at shallow angles. This principle is known as total internal reflection.

At both ends of the fibre link there are photodiodes which must be in exact alignment in order to prevent signal reflection. The means of placing the signal on the fibre can be carried out by Light Emitting Diode (LED) or an Injection Laser Diode (ILD).

Advantages and Disadvantages of Fibre

Advantages:

  • It is immune to EFI and RFI
  • It can cover greater distances more reliably than wire
  • It can handle greater speeds, e.g. 1000Mbps and upwards
  • It can not be compromised by signal tapping

Disadvantages:

  • It is not as robust as wire
  • It is more expensive to buy
  • It is more expensive to install
  • It has be installed by specialists

Multi-Mode Fibre

Multi-Mode uses a thicker core than single mode which allows more than one frequency of light to be transmitted at the same time. This type of transmission works by using Wave Division Multiplexing (WDM).

Light signals on multimode cable are generated by Laser (Light Amplification by Simulated Emission of Radiation).

Generally, multimode cable is cheaper to make and install than single mode fibre and as such is a popular choice for backbones within buildings with an accepted range of up to 2km, however, it is not suitable for links between buildings.

Single Mode (Mono Mode)

Single mode fibre has a thinner core than multimode and uses a single beam of light for data transmission. The light pulses are generated by Light Emitting Diodes (LEDs).

Single mode is faster and can work over longer distances than multimode and as such it is suitable for use as backbones between buildings. Single mode has a 50% greater data carrying capacity than multi mode cable with an acceptable transmission range is up to 3km.

Even although fibre is manufactured to a high quality there can still be imperfections that could led to signal loss. To overcome this fibre cable is tested by a device called an optical loss meter and the measurement that shows whether the cable is fit to use is referred to as the optical link loss budget.

Fibre Connectors

There are many different connectors used with fibre-optic cable, however, two of the most popular and recognisable are:

  • ST – Straight Tip
  • SC – Subscriber Connector/Standard Connector/Square Connector

The ST connector, created by AT & T, uses a twist attachment similar to that of Thinnet BNC connectors. This mechanism provides for a quick and secure connection and makes it quick a popular choice.

The SC connector is a latched connector where the latch clicks into place upon connection given a very secure connection that can only be disconnected by pressing a button to release the latch.

Balanced and Unbalanced Lines

For a signal to be sent on guided media two wires are needed; one to carry the data and the other to carry the reference.

There are two methods used to do this:

  • Unbalanced Transmission
  • Balanced Transmission

The two wires in unbalanced lines consist of one for the signal and the other for the reference. The wire used for reference is typically the ground and can be used as a shield against interference, i.e. noise. Unbalanced lines suffer from more problems with noise than balanced. An example of an unbalanced line is coaxial cable.

Balanced lines use both wires to carry data and therefore one is not used for the function of a reference or ground. One wire carries the signal as a positive and the other carries it as a negative. The negative signal is 180 degrees out of phase with the positive; therefore, equal noise occurs on both and can cancel each other. This is known as cancellation and an example of a balanced line is UTP cable.

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