COMPUTER NETWORKS

Bounded/Guided Transmission Media

It is the transmission media in which signals are confined to a specific path using wire or cable. The types of Bounded/ Guided are discussed below.


Twisted Pair Cable

This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed easily, and they support many different types of network. Some important points :

  • Its frequency range is 0 to 3.5 kHz.
  • Typical attenuation is 0.2 dB/Km @ 1kHz.
  • Typical delay is 50 ┬Ás/km.
  • Repeater spacing is 2km.

Twisted Pair is of two types :

  • Unshielded Twisted Pair (UTP)
  • Shielded Twisted Pair (STP)

Unshielded Twisted Pair Cable

It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which consists of two conductors usually copper, each with its own colour plastic insulator. Identification is the reason behind coloured plastic insulation.

UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable use RJ-45 connector.

Unshielded Twisted Pair Cable

Advantages :

  • Installation is easy
  • Flexible
  • Cheap
  • It has high speed capacity,
  • 100 meter limit
  • Higher grades of UTP are used in LAN technologies like Ethernet.

It consists of two insulating copper wires (1mm thick). The wires are twisted together in a helical form to reduce electrical interference from similar pair.

Disadvantages :

  • Bandwidth is low when compared with Coaxial Cable
  • Provides less protection from interference.

Shielded Twisted Pair Cable

This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk (explained in KEY TERMS Chapter).

It has same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is more expensive than coaxial and unshielded twisted pair.

Shielded Twisted Pair Cable

Advantages :

  • Easy to install
  • Performance is adequate
  • Can be used for Analog or Digital transmission
  • Increases the signalling rate
  • Higher capacity than unshielded twisted pair
  • Eliminates crosstalk

Disadvantages :

  • Difficult to manufacture
  • Heavy

Coaxial Cable

Coaxial is called by this name because it contains two conductors that are parallel to each other. Copper is used in this as centre conductor which can be a solid wire or a standard one. It is surrounded by PVC installation, a sheath which is encased in an outer conductor of metal foil, barid or both.

Outer metallic wrapping is used as a shield against noise and as the second conductor which completes the circuit. The outer conductor is also encased in an insulating sheath. The outermost part is the plastic cover which protects the whole cable.

Here the most common coaxial standards.

  • 50-Ohm RG-7 or RG-11 : used with thick Ethernet.
  • 50-Ohm RG-58 : used with thin Ethernet
  • 75-Ohm RG-59 : used with cable television
  • 93-Ohm RG-62 : used with ARCNET.

Coaxial Cable

There are two types of Coaxial cables :

BaseBand

This is a 50 ohm (Ω) coaxial cable which is used for digital transmission. It is mostly used for LAN's. Baseband transmits a single signal at a time with very high speed. The major drawback is that it needs amplification after every 1000 feet.

BroadBand

This uses analog transmission on standard cable television cabling. It transmits several simultaneous signal using different frequencies. It covers large area when compared with Baseband Coaxial Cable.


Advantages :

  • Bandwidth is high
  • Used in long distance telephone lines.
  • Transmits digital signals at a very high rate of 10Mbps.
  • Much higher noise immunity
  • Data transmission without distortion.
  • The can span to longer distance at higher speeds as they have better shielding when compared to twisted pair cable

Disadvantages :

  • Single cable failure can fail the entire network.
  • Difficult to install and expensive when compared with twisted pair.
  • If the shield is imperfect, it can lead to grounded loop.

Fiber Optic Cable

These are similar to coaxial cable. It uses electric signals to transmit data. At the centre is the glass core through which light propagates.

In multimode fibres, the core is 50microns, and In single mode fibres, the thickness is 8 to 10 microns.

The core in fiber optic cable is surrounded by glass cladding with lower index of refraction as compared to core to keep all the light in core. This is covered with a thin plastic jacket to protect the cladding. The fibers are grouped together in bundles protected by an outer shield.

Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)

Fiber Optic Cable

Advantages :

  • Provides high quality transmission of signals at very high speed.
  • These are not affected by electromagnetic interference, so noise and distortion is very less.
  • Used for both analog and digital signals.

Disadvantages :

  • It is expensive
  • Difficult to install.
  • Maintenance is expensive and difficult.
  • Do not allow complete routing of light signals.