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.
I. BOUNDED/GUIDED MEDIA
II. UNBOUNDED/UNGUIDED MEDIA
i. 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
1) Coaxial Cable
2) Twisted Pairs Cable
3) Fiber Optics Cable
1) 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
2) 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)
STP
UTP
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:
• 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
ii. UNBOUNDED / UN GUIDED MEDIA
Unbounded / Unguided media or wireless media doesn't use any physical connectors between the two devices communicating. Usually the transmission is send through the atmosphere but sometime it can be just across the rule. Wireless media is used when a physical obstruction or distance blocks are used with normal cable media. The three types of wireless media are:
1. RADIO WAVES
2. MICRO WAVES
3. INFRARED WAVES
1. RADIO WAVES:-
It has frequency between 10 K Hz to 1 G Hz. Radio waves has the following types.
• Short waves
• VHF (Very High Frequency)
• UHF (Ultra High Frequency)
SHORT WAVES:-
There are different types of antennas used for radio waves. Radio waves transmission can be divided into following categories.
• LOW POWER, SINGLE FREQUENCY.
• HIGH POWER, SINGLE FREQUENCY
• LOW POWER , SINGLE FREQUENCY:-
As the name shows this system transmits from one frequency and has low power out. The normal operating ranges on these devices are 20 to 25 meter.
CHARACTERISTICS LOW POWER , SINGLE FREQUENCY:-
• Low cost
• Simple installation with pre-configured
• 1 M bps to 10 M bps capacity
• High attenuation
• Low immunity to EMI
• HIGH POWER, SINGLE FREQUENCY:-
This is similar to low power single frequency. These devices can communicate over greater distances.
CHARACTERISTICS HIGH POWER, SINGLE FREQUENCY:-
• Moderate cost
• Easier to install than low power single frequency
• 1 Mbps to 10 Mbps of capacity
2. Satellite Micro waves
The main problem with aero wave communication is the curvature of the earth, mountains & other structure often block the line of side. Due to this reason, many repeats are required for long distance which increases the cost of data transmission between the two points. This problem is recommended by using satellites.
Satellite micro wave transmission is used to transmit signals through out the world. These system use satellites in orbit about 50,000 Km above the earth. Satellite dishes are used to send the signals to the satellite where it is again send back down to the receiver satellite. These transmissions also use directional parabolic antenna’ with in line of side.
In satellite communication micro wave signals at 6 GHz is transmitted from a transmitter on the earth through the satellite position in space. By the time signal reaches the satellites becomes weaker due to 50,000 Km distance. The satellite amplifies week signals and transmits it back to the earth at the frequency less than 6 GHz.
Characteristics Satellite Micro waves:
• High cost
• Extremely difficult and hare installation.
• Variable attenuation.
• Low immunity to EMI
• High security needed because a signal send to satellite is broadcasts through all receivers with in satellite.
3. Infrared
Infrared frequencies are just below visible light. These high frequencies allow high sped data transmission. This technology is similar to the use of a remote control for a TV. Infrared transmission can be affected by objects obstructing sender or receiver. These transmissions fall into two categories.
1. Point to point
2. Broadcast
1. Point to Point: -Point to point infrared transmission signal directly between two systems. Many lap top system use point to pint transmission. These systems require direct alignment between many devices.
Characteristics of Point to point:-
• Wide range of cost
• Moderately easy installation.
• 100 k bps to 16 Mb of capacity.
• Variable attenuation.
• High immunity to EMI
2. Broad Cast: -These infrared transmission use sprayed signal, one broad cast in all directions instead of direct beam. This help to reduce the problems of proper alignment and abstraction. It also allows multiple receiver of signal
Characteristics of Broad Cast:-
• In expensive.
• Single installation.
• 1M bps capacity.
• Variable attenuation.