Thursday, 4 July 2013

Physical communication media.

Physical communication media.

Means by which a communication signal is carried from one system to another (for example, twisted-pair wire, co axial cable, fiber-optics cable) and wireless links (for example, satellite, microwave and radio and infrared systems) is Transmission Media
There are two types of transmission media :
  • Guided
  • Unguided
Guided Transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the “Cabling” system. Guided Media is also known as Bound media. Cabling is meant in a generic sense in the previous sentence and is not meant to be interpreted as upper wire cabling only.
The unguided media is the wireless media. It simply transports electromagnetic waves without using any physical conductor. Signals are normally broadcast through the air and thus are available to any one who has the device capable of receiving them.
Guided
(i) Twisted pair cable
(ii) Co-axial cable
(iii) Fiber optic cable.
Unguided (wireless)
(i) Radio wave
(ii) Microwave
(iii) Infrared.

Guided
Guided Transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the “Cabling” system. Guided Media is also known as Bound media. Cabling is meant in a generic sense in the previous sentence and is not meant to be interpreted as upper wire cabling only.
  1. Twisted pair - Wire twisted to avoid crosstalk interference. It may be shielded or unshielded.
    • UTP-Unshielded Twisted Pair. Normally UTP contains 8 wires or 4 pair. 100 meter maximum length. 4-100 Mbps speed.
    • STP-Shielded twisted pair. 100 meter maximum length. 16-155 Mbps speed. Lower electrical interference than UTP.
  2. Coaxial - Two conductors separated by insulation such as TV 75 ohm cable. Maximum length of 185 to 500 meters.
    1. Thinnet - Thinnet uses a British Naval Connector (BNC) on each end. Thinnet is part of the RG-58 family of cable*. Maximum cable length is 185 meters. Transmission speed is 10Mbps. Thinnet cable should have 50 ohms impedance and its terminator has 50 ohms impedance. A T or barrel connector will have no impedance. Maximum thinnet nodes are 30 on a segment. One end of each cable is grounded.
    2. Thicknet - Half inch rigid cable. Maximum cable length is 500 meters. Transmission speed is 10Mbps. Expensive and is not commonly used. (RG-11 or RG-8). A vampire tap or piercing tap is used with a transceiver attached to connect computers to the cable. 100 connections may be made. The computer has an attachment unit interface (AUI) on its network card which is a 15 pin DB-15 connector. The computer is connected to the transceiver at the cable from its AUI on its network card using a drop cable. Maximum thicknet nodes are 100 on a segment. One end of each cable is grounded. 3. Fiber-optic - Data is transmitted using light rather than electrons. Usually there are two fibers, one for each direction. Cable length of 2 Kilometers. Speed from 100Mbps to 2Gbps. This is the most expensive and most difficult to install, but is not subject to interference. Two types of cables are: 1. Single mode cables for use with lasers has greater bandwidth and costs more. Injection laser diodes (ILD) work with single mode cable. 2. Multimode cables for use with Light Emitting Diode (LED) drivers. All signals appear to arrive at the same time. P intrinsic N diodes or photodiodes are used to convert light to electric signals when using multimode. 

      3.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:
      1..Expensive
      2.Very hard to install
      3.Capable of extremely high speed
      4.Extremely low attenuation
      5.No EMI interference
    Advantages Of Fiber Optic Cable:
    1.Fast    2.Low attenuation   3.No EMI interference
Disadvantages Fiber Optics:
  • Very costly
  • Hard to install 
 Unguided Media
 The unguided media is the wireless media. It simply transports electromagnetic waves without using any physical conductor. Signals are normally broadcast through the air and thus are available to any one who has the device capable of receiving them.

1.Radio waves: Electromagnetic wave ranging in frequencies between 3 KHz and 1GHz are normally called radio waves.
Radio waves are omnidirectional when an antenna transmits radio waves they are propagated in all directions. This means that sending and receiving antenna do not have to he aligned. A sending antenna can send waves that can be received by any receiving antenna.
Radio waves particularly those waves that propagate in sky mode, can travel long distances. This makes radio waves a good candidate for long-distance broadcasting such as AM radio.
Radio waves particularly those of low and medium frequencies can penetrate walls. It is an advantage because; an AM radio can receive signals inside a building. It is the disadvantage because we cannot isolate a communication to first inside or outside a building. The radio waves band is relatively narrow just under I GHz, compared to the microwave band. When this band is divided into subband, the sidebands are also narrow, leading to a low data rate for digital communications.






Microwaves: Electromagnetic waves having frequencies between I and 300 GFL are called microwaves.
Microwaves are unidirectional, when an antenna transmits microwaves they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas.
On the other hand microwaves.
Propagation is line-of-sight. Since the towers with the mounted antennas needs to be in direct sight of each other, towers that are for apart need to he very tall, the curvature of the earth as well as other blocking obstacles do not allow two short towers to communicate using microwaves, Repeaters are often needed for long distance communication very high frequency microwaves cannot penetrate waIls.



Parabolic dish antenna and horn antenna are used for this means of transmission.






3.Infrared: Infrared signals with frequencies ranges from 300 GHz to 400 1Hz can be used for short range communication. Infrared signals, having high frequencies, cannot penetrate walls. This helps to prevent interference between one system and another. in this one room cannot be affected by the infrared waves in another room.
fl infrared band, almost 400 THz, has an excellent potential for data transmission. So this will transfer digital data with a very high frequency. There are no. of computer devices which are used to send the data through infrared medium e.g. keyboard mice, PCs and printers. There are some manufacturers provide a special part called the IrDA port that allows a wireless keyboard to communicate with a PC.

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