COMMUNICATION MEDIUM

COMMUNICATION MEDIUM

Twisted Pair

Coaxial Cable

Fiber Optics

Microwave and Satellite Transimission

• Guided Media

• Unguided media

Guided Media

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 sentences and is not meant to be interpreted as copper wire cabling only. Some of the guided media used in networking is as following:

• Twisted Pair
• Coaxial Cable
• Fiber Optics

1. Twisted Pair

It is one types of copper cables, it consists of two insulated copper wires that are twisted together in helical form. There are two types of twisted pairs, Unshielded Twisted Pair (UTP) cable and Shielded Twisted Pairs (STP). Both of types are used Ethernet LANs, they consists of four pairs of color coded wires that have been twisted together and then encased in a flexible plastic sheath.
The color codes identify the individual pairs and wires in the pairs and aid in cable termination. The purpose of twisting the wire is to reduce electrical interference from similar pairs closed by.
Twisted pairs can run approximately 100 meter without amplification, but for longer distances, repeaters are needed. Twisted pairs can be used for either analogue or digital transmission. The bandwidth depends on the thickness of wire and the distance traveled. Twisted pair can carry 10Mbps, 100Mbps or 1000MBps, this variation depends on the category of cable (Cables are placed into categories according to their ability to carry higher bandwidth rates, For example, Category 5 (Cat5), Category 6 (Cat6).

2. Coaxial Cable

It is one type of copper cables. It consists of a stiff copper conductor as a core surrounded by a layer of flexible insulation, the insulator is incased by cylindrical conductor often as a closely woven braided mesh. The outer conductor is covered in a protective plastic sheath.
The constriction and shielding of coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable length. For 1-Km cables, a data rate of 1 to 2 Gbps is feasible. Longer cables can also be used but at lower data rate or with periodic amplifiers.
At one time, coaxial cable was the most widely used network cabling. There were a couple of reasons for coaxial wide usage. Coaxial was relatively inexpensive, and it was light, flexible, and easy to work with. It was so popular that it became a safe, easily supported installation. Coaxial cables are widely used in telephone system but have now largely replaced by fiber optics on long-land routers. However coaxial cables are still widely used for TV cables and some LAN. In Nepal SUBISU Network company is using this cable to supply TV channels and Internet.

3. Fiber Optics

Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabling is capable of very large raw data bandwidth rates. With current optical fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps).
An optical transmission system has three components the light source, the transmission medium and the detector. The pulse of light indicates "1" bit and the absence of light indicates a"0" bit. The transmission medium is ultrathin fiber of glass. The light source attached to one end, the light moves over the fiber optics, the detector will generate an electrical signal when the light falls on it.
By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accept an electrical signal, convert and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.

Unguided Media

Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media. Some of the unguided media used in networking is as following:

• Microwave System
• Communication Satellites

1. Microwave Transmission

Above the 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. In addition, this directionality allows multiple transmitters lined up in a raw to communicate with multiple receivers in a raw without interference. Before fiber optics, for decades these microwaves formed the heart of long distance telephone transmission systems.
Since the microwaves travel in a straight line, if the towers are far apart, the earth will get in the way. Consequently, repeaters are needed periodically. The distance between repeaters goes up very rough, with the tower height. For 100 M tower, repeater can be spaced 80 Km a part. Unlike radio waves at lower frequencies, microwave does not pass through building wall. In addition, even through the beam may be well focused at the transmitter; there is still some divergence in space.

2. Communication Satellite

Communication satellites have some interesting properties that make them attractive for many applications. A Communication satellite can be thought to be as big microwave repeater in the sky. It contains several "Transponders", each of which listens to some portion of the spectrum, amplify the incoming signal and then rebroadcast it at another frequency to avoid interference with the incoming signal. The down word beams can be broad covering a substantial fraction of the earth surface.

Satellite versus Fiber

• The bandwidth of Fiber is more than the bandwidth of satellite.
• For mobile communication, fiber optics link are of no use to them, but Satellite links are potentially are.
• The Satellite has a broadcast nature.
• With Satellite, communication is available in a hostile terrain or poorly developed terrestrial infrastructure.