Fiber Optic Cable

Fiber Optic Cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.

Properties of light

Ø  Light travels in a straight line as long as it moves through a single uniform substance. If traveling through one substance suddenly enters another, ray changes its direction.

Bending of light ray

If the angle of incidence(the angle the ray makes with the line perpendicular to the interface between the two medium) is less than the critical angle the ray refracts and move closer to the surface.

If the angle of incidence is equal to the critical angle, the light bends along the interface.

If the angle of incidence is greater than the critical angle, the ray reflects and travels again in the denser substance. Critical angle differs from one medium to another medium.

Optical fiber use reflection to guide light through a channel.

A Glass or plastic core is surrounded by a cladding of less dense glass or plastic.

Propagation Modes

Multimode

In the multiple mode, multiple light beams from a source move through the core in different paths.

Ø  Multimode-Step-Index fiber: The density of core remains constant from the centre to the edge.

A ray of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface there is an abrupt change to a lower density that changes the angle of the beam’s motion.

Ø  Multimode- Graded -Index fiber: The density is varying. Density is highest at the centre of the core and decreases gradually to its lowest at the edge.

Single Mode

Single mode uses step-index fiber and a highly focused source of light

that limits beams to a small range of angles, all close to the horizontal.

The single mode fiber itself is manufactured with a much smaller

diameter than that of multimedia fiber.

Connectors

Ø  Subscriber channel (SC) connector  is used for cable TV.

Ø  Straight-tip (ST) connector is used for connecting cable to networking devices.

Advantages of Optical Fiber

Ø  Noise resistance

Ø  Less signal attenuation

Ø  Light weight

Disadvantages

Ø  Cost

Ø  Installation and maintenance

Ø  Unidirectional

Ø  Fragility (easily broken)

Unguided media

Unguided media transport electromagnetic waves without using a

physical conductor. This type of communication is often referred to as wireless communication.

Signals are normally broadcast through air and thus available to

anyone who has device capable of receiving them.

Unguided signals can travel from the source to destination in several ways:

Ø  Ground propagation – waves travel through lowest portion on

  atmosphere.

Ø  Sky propagation – High frequency waves radiate upward into

      ionosphere and reflected back to earth.

Ø   Line-of-sight propagation – Very high frequency signals travel

  in a straight line

Radio Waves

Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves.

Properties

Ø  Radio waves are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned.

Ø  A sending antenna sends waves that can be received

     by any receiving antenna.

Ø  Radio waves, particularly those of low and medium

frequencies, can penetrate walls.

Disadvantages

Ø  The omnidirectional property has a disadvantage, that the radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band.

Ø  As Radio waves can penetrate through walls, we cannot isolate a communication to just inside or outside a building.

Applications

Radio waves are used for multicast communications, such as radio and television, and paging systems.

Microwaves

Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves.

Properties

Ø  Microwaves are unidirectional.

Ø  Sending and receiving antennas need to be aligned

Ø  Microwave propagation is line-of-sight

Ø  Very high-frequency microwaves cannot penetrate walls

Ø  Parabolic Dish antenna focus all incoming waves into single point

Ø  Outgoing transmissions are broadcast through a horn aimed at the dish.

Disadvantage

Ø  If receivers are inside buildings, they cannot receive these waves

Applications

Ø  Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

Infrared

Ø  Electromagnetic waves with frequencies from 300 GHz to 400 THz are called infrared rays

Ø  Infrared waves, having high frequencies, cannot penetrate walls.

Applications

Ø  Infrared signals can be used for short-range communication

     in a closed area using line-of-sight propagation.

Channel Access on links

Multiple Access Techniques

      Various multiple access techniques are

Ø  Frequency Division Multiple Access(FDMA)

Ø  Time Division Multiple Access (TDMA)

Ø  Code Division Multiple Access(CDMA)

Frequency Division Multiple Access

Ø  In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands.

Ø  Each station is allocated a band to send its data.

Ø  In this method when any one frequency level is kept idle and another is used frequently leads to inefficiency.

Time Division Multiple Access

Ø  In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time.

Ø  Each station is allocated a time slot during which it can send data.

Ø  The main problem with TDMA lies in achieving synchronization between the different stations.

Each station needs to know the beginning of its slot and the location of its slot

Code Division Multiple Access

Ø  CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link.

Ø  It differs from TDMA because all stations can send data at the same time without  timesharing.

Ø  CDMA simply means communication with different codes.

Ø  CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips.

Ø  Chips will be added with the original data and it can be transmitted through same medium.