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. (1) Bounded/Guided Media. (2) UnBounded/UnGuided 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.
- Coaxial Cable
- Twisted Pairs Cable
- Fiber Optics Cable
Coxial 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.
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.
-> 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.
Characteristics of Coaxial Cable:
-> Low cost
-> Easy to install
-> Up to 10Mbps capacity
-> Medium immunity form EMI.
-> Medium of attenuation
ADVANTAGES COAXIAL CABLE:
-> 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)
UHSHIELDED 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.
NOTE : 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
Disadvantage of UTP :
-> Short distance due to attenuation
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
Advantage Of STP
-> Faster than UTP and coaxial
Disadvantage of STP
-> More expensive than UTP and coaxial
-> More difficult installation
-> High attenuation rate
FIBER OPTICS CABLE : 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. Fibre optic cable has bandwidth more than 2 gbps (Gigabytes per Second)
Characteristics of Fiber Optics Cable
-> Very hard to install
-> Capable of extremely high speed
-> Extremely low attenuation
-> No EMI interference
Advantage of Fiber Optics Cable
-> Low attenuation
-> No EMI interference
Disadvantage of Fiber Optics Cable
-> Very costly
-> Hard to install
In this media the signal is not transferred through a medium in a particular direction but is transmitted in the complete medium and it flows in all directions.
TERRESTRIAL MICROWAVE : One of the unguided media is known as the terrestrial microwave where the signal is transmitted using a parabolic dish. A parabolic dish is used because with the receiver placed at the center. By the property of the parabola if a wave coming at any angle falls on the surface of the parabola then it is made to pass through the focus. Hence all the signals that fall on the parabola go to the receiver and this gives maximum concentration and a focused beam. This microwave works only in the line of sight hence the sender and the receiver have to be visible to each other. If the receiver and the transmitter are both placed very high where there is no obstruction in between then it can be used for longer distances. In this way it can be used for higher frequencies and get higher data rates.
SATELLITE MICROWAVE : Another unguided media that we use is satellite microwave. The relay station in this case is the satellite. The signal has to travel a very long distance in the space and hence it gets attenuated. Therefore the satellite receives the input signal at one frequency, amplifies it and transmits it on the other frequency. The receiver hence looks only for the frequency that is coming from the satellite and will not take the unamplified signal coming from the sender which is also readily available in the surroundings. The frequency at which the signal is sent to the satellite is called the uplink frequency and the frequency at which the signal is transmitted by the satellite is called the down link frequency.
This setup requires the satellite to be launched in the geostationary orbit. As the earth moves, this orbit moves with it. So the situation is like having a transmitter over a very high pole so that when the earth moves the transmitting point also moves along with the earth. If this condition the receiver always remains at a constant position above the earth.The height of the geostationary orbit is 35,784km above the surface of earth. The satellite microwave is commonly used for television transmission, long distance telephone and even private business network.The satellite links are formed as shown in figure 3.8 and 3.9. One way in which these links are formed is point to point link. In these links there is one transmitter and one receiver. The message that is transmitted in the space is received only by a particular receiver. This system is commonly used in a computer network. The other way in which the link is formed is the broadcast link. In this link there is one transmitter and multiple receivers. This is used in satellite T.V.
INFRARED : The other mode is infrared. Here we modulate non-coherent infrared light. The receiver and the transmitter have to be in the line of sight. This is blocked by walls. An example of this is a T.V. remote control.
WIRELESS PROPAGATION : In the wireless propagation the signal can travel through three routes. One is through ground wave, where it follows the contour of earth. The frequency in this case is up to 2Mhz. The example of it is AM radio. The second route is sky wave such as BBC, voice of America which is otherwise known as short wave.The third is the line of sight propagation (LOS). The frequency on this route is 30Mhz and more, this is possible due to refraction.For the line of sight even if we have very high tower we cannot really connect to everywhere in world. Refer figure 3.10. Due to the shape of the earth we have to put three geostationary satellites for complete transmission around the earth.