Tuesday, 20 May 2014

TRANSMISSION MEDIA(CN)


The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.
  • BOUNDED/GUIDED MEDIA
  • UNBOUNDED/UNGUIDED MEDIA

1. BOUNDED MEDIA:
Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up oa external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are
  • Coaxial Cable
  • Twisted Pairs Cable
  • Fiber Optics Cable

COAXIAL CABLE:
Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.
Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.
Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.
Outside the copper mesh is the final protective cover. (as shown in Fig)
The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.
Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.
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.

  • Low cost
  • Easy to install
  • Up to 10Mbps capacity
  • Medium immunity form EMI
  • Medium of attenuation

ADVANTAGESOF  COAXIAL CABLE
  • Inexpensive
  • Easy to wire
  • Easy to expand
  • Moderate level of EMI immunity
  • DISADVANTAGE OF COAXIAL CABLE---
    • Single cable failure can take down an entire network

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps.Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling
1. Unshielded twisted pair (UTP)
2. Shielded twisted pair (STP)

1. Unshielded twisted pair (UTP)
UTP is more common. It can be either voice grade or data grade depending on the condition. UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available due to its many use. There are five levels of data cabling
Category 1
These are used in telephone lines and low speed data cable.
Category 2
These cables can support up to 4 mps implementation.
Category 3
These cable supports up to 16 mps and are mostly used in 10 mps.
Category 4
These are used for large distance and high speed. It can support 20mps.
Category 5
This is the highest rating for UTP cable and can support up to 100mps.
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.
Characteristics of UTP
      • low cost
      • easy to install
      • High speed capacity
      • High attenuation
      • Effective to EMI
      • 100 meter limit

Advantages of UTP
      • Easy installation
      • Capable of high speed for LAN
      • Low cost

Disadvantages of UTP
      • Short distance due to attenuation

2.Shielded twisted pair (STP)
It is similar to UTP but has a mesh shielding that's protects it from EMI which allows for higher transmission rate.
IBM has defined category for STP cable.
Type 1
STP features two pairs of 22-AWG
Type 2
This type include type 1 with 4 telephone pairs
Type 6
This type feature two pairs of standard shielded 26-AWG
Type 7
This type of STP consist of 1 pair of standard shielded 26-AWG
Type 9
This type consist of shielded 26-AWG wire
Characteristics of STP--
      • Medium cost
      • Easy to install
      • Higher capacity than UTP
      • Higher attenuation, but same as UTP
      • Medium immunity from EMI
      • 100 meter limit

Advantages of STP:
      • Shielded
      • Faster than UTP and coaxial

Disadvantages of STP:
      • More expensive than UTP and coaxial
      • More difficult installation
      • High attenuation rate
Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into "1's" and "0's" at the other end.
In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known asCLADDING. No light escapes the glass core because of this reflective cladding.
Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)
Characteristics Of Fiber Optic Cable:
      • Expensive
      • Very hard to install
      • Capable of extremely high speed
      • Extremely low attenuation
      • No EMI interference

Advantages Of Fiber Optic Cable:
      • Fast
      • Low attenuation
      • No EMI interference
Disadvantages Fiber Optics:
      • Very costly
      • Hard to install
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