TRANSMISSION MEDIA

TRANSMISSION MEDIA

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 o a 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.

CHARACTERISTICS OF COAXIAL CABLE

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

ADVANTAGES COAXIAL CABLE

  • Inexpensive
  • Easy to wire
  • Easy to expand
  • Moderate level of EMI immunity

DISADVANTAGE COAXIAL CABLE

  • Single cable failure can take down an entire network

Twisted Pair Cable

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

These are used in telephone lines and low speed data cable.

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.

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 Optics

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 as CLADDING. 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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2. Transmission Media - Unguided

Unguided Transmission Media is data signals that flow through the air. They are not guided or bound to a channel to follow. They are classified by the type of wave propagation

  1. RF Propagation

There are 3 types of RF (Radio Frequency) Propagation:

  • Ground Wave,
  • Ionospheric and
  • Line of Sight (LOS) Propagation.

Ground Wave Propagationfollows the curvature of the Earth. Ground Waves have carrier frequencies up to 2 MHz. AM radio is an example of Ground Wave Propagation.

  1. Radio Frequencies
    Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.
  1. Microwave

Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

4.    Satellite

Satellites are transponders that are set in a geostationary orbit directly over the equator. A transponder is a unit that receives on one frequency and retransmits on another. The geostationary orbit is 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earths rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out to space

5.   Iridium Telecom System

The Iridium telecom system is a new satellite sytem that will be the largest private aerospace project. It is a mobile telecom system to compete with cellular phones.

The following table compares the usable bandwidth between the different Guided Transmission Media

Cable Type Bandwidth
Open Cable 0 - 5 MHz
Twisted Pair 0 - 100 MHz
Coaxial Cable 0 - 600 MHz
Optical Fibre 0 - 1 GHz

SET (Maharashtra) Examination 16th April 2017

Sr.No.NameDate
1 Commencement of online Application 27 Dec 2016
2 Last date for Applying Online 10 Jan 2017
3 Availability of Admit Card on website 06 Apr 2017
4 Date of Examination 16 Apr 2017

 

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