TRANSMISSION

Data Transmission

     There is a maximum limit to the amount of data that can be transmitted using any transmission medium (Shannon's Law). Each medium has its own maximum. Data transmitted as signals. The number of signals per second is the frequency of the signal. This is measured in Hertz (Hz). One cycle per second is 1 Hz. One million cycles per second is 1 MHz.

Bandwidth

     The maximum range of frequencies that can be transmitted is the bandwidth of the medium. This determines the maximum amount of data that can be transmitted. A coaxial cable might have a bandwidth of 10 to 100 MHz. An optic fiber has a bandwidth of up to 10^8 MHz. The higher the bandwidth the more data that can be transmitted per second, hence the popularity of optic fibers. In fact, computers cannot yet utilize the full capacity of optic fibers, that is , they cannot transmit at the speed which the fiber is capable of carrying.

Channel Capacity

     The speed of transmission of information is said to be the channel capacity. We count it as data rate in digital world. It depends on numerous factors such as:

• Bandwidth : The physical limitation of underlying media.
• Error-rate : Incorrect reception of information because of noise.
• Encoding : The number of levels used for signaling.

Transmission Media

     Transmission media in data communication refers to the path through which data is transmitted from one computer (Sender) to another computer (Receiver). Transmission media can be broadly classified into two types.

• Guided Media : All communication wires/cables are guided media, such as UTP, coaxial cables and fiber Optics. In this media, the sender and receiver are directly connected and the information is send (guide) through it.
• Unguided Media : Wireless or open air space is said to be unguided media, because there is no connectivity between the sender and receiver. Information is spread over the air, and anyone including the actual recipient may collect the information.

Guided Media

     As the name indicates, the data to be transmitted is guided along a solid physical path. So the characteristics of the transmitting media are an important factor that determines the quality of the received data. A solid physical path exists in guided media transmission. There are mainly three types of guided media.

• Coaxial Cable
• Twisted Pair
• Optical Fiber

Coaxial Cable

     Coaxial cables are basically copper cables. It consists of a cylindrical outer conductor and an inner copper wire. The inner copper wire is the core and is surrounded by an insulating material. The outer cylindrical conductor surrounds this inner copper conductor. The outer conductor is also insulated by a protective sheath. This construction provides coaxial cable better performance in the presence of noise and other interference. Coaxial cables are capable of transmitting high frequency signals. They are also known as coax. This is because a coaxial cable consists of two conductors that run concentrically with each other along the axis of cable, that is , inner conductor and outer conductor. The two conductors in the cable share a common axis. Coaxial cables are mostly used in cable TV networks. They are also widely used in local area networks. Coaxial Cables carry electrical signals. It has a high bandwidth and good noise immunity. The original Ethernet standard was based on 10 Mbps coaxial cable. Ethernet is the most popular LAN standard. There are two forms of coax :

• Thick Coaxial : Operates over distances of up to 500 m based on 10-Base-5 standard.
• Thin  Coaxial : Operates over distances of up to 200 m based on 10-Base-2 standard.

[10-Base-5 --- 10 Mbps base band signaling up to 500 m]

Components of Coaxial Cable

1. Core : It is the inner conductor of a coaxial cable. Copper cables are usually used as core. Stranded wires can also be used. The advantage of using stranded wires is that they will make the cable more flexible and easier to connect to equipment. Solid copper wires are usually used for permanent wiring. Core actually carries the electromagnetic waves or data signals.
2. Insulation Layer : It provides electrical insulation between inner and outer conductors of coaxial cable. It is also known as dielectric layer.
3. Outer Conductor : The outer conductor is made up of braided wires, metal foil or a combination of both. The presence of this shield makes the coaxial cable highly resistant to electromagnetic interference (EMI). This conductor completes the circuit in the coaxial cable. That is, current flow between the inner and outer conductor will be maintained.
4. Outer Jacket or Sheath : The outer jacket is made up of Teflon or plastic. It prevents the physical damage of the coaxial cable.

Categories of Coaxial Cables

    Coaxial cables differ in their impedance values, Coaxial cables are categorized as follows:

Type	Impedance	Use
RG-8	50 Ohms	Thick Ethernet
RG-11	75 Ohms	Broadband LANs
RG-58	50 Ohms	Thin Ethernet
RG-59	75 Ohms	Cable TV

Thick Ethernet
     This is the original Ethernet cable standard which is 1 cm thick and consists of a solid copper core. The IEEE name of thick-net is 10 Base 5. '10' indicates the speed of data transmission as 10 Mbps. Base band transmission is represented by 'Base' and '5' represents maximum segment length of thick-net cable, which is 500 meters. Due to its thickness, thick-net is heavy and difficult to bend and install.
Thin Ethernet
     Thin-net uses RG-58 type coaxial cable. Thin-net has a thickness of about 0.65 cm which makes it more flexible and easier to install than thick-net. The IEEE name of thin-net is 10 Base 2. Thus a base band signal, transmission speed of 10 Mbps through a segment length of 200 meters is possible in this type of cables. Thin-net is easy to install because of its light weight.
Base band and Broad band Coaxial Cables :-
     1. Base band Coaxial cables

• Cables used in Ethernet LANs.
• Digital signals are handled by base band cables.
• Base band cable is a 50 ohms cable.
• Noise immunity is high and reflection is less.
• Long distance communication is not feasible.
• Base band cables are less expensive and easy to install

     2. Broadband Coaxial cables

• Cables used in cable TV networks.
• Analog signaling is employed in broad band cables.
• Impedance of the cable is 75 ohms.
• Noise immunity less compared to base band cables and also suffers from intense reflections.
• With the help of analog amplifiers, broad band cables carry signals over large distances.
• Expensive and difficult to install.

BNC Connectors
     Connectors are required for connecting coaxial cables to a device. The most common type of connector used for coaxial cables is the Bayone-Neill-Concelman (BNC) connector. Different types of adapters are available for BNC connectors, including a T-connector, barrel connector and terminator. BNC cable connector is commonly used to connect a cable to a television. It is usually soldered or crimped to the end of a cable. BNC-T connector is used to join the network interface card in the computer or other devices in the network to the network cable. BNC barrel connector is used to join two thin-net cables of different lengths so as to increase the length of the cable. BNC terminal is used at the end of the cable to absorb unwanted (stray) signals and also to prevent reflection of signals.
Coaxial cables are used in:

• Local Area Networks
• Cable Television
• Analog and Digital telephone systems
• Digital Transmission
• Thick and Thin Ethernet

Twisted Pair Cable

     This is the oldest, least expensive and most commonly used form of guided transmission media. A twisted pair consists of two insulated copper wires twisted together to form a spiral pattern. Data is transmitted in the form of electrical signals through these copper conductors. Both of the wires will not be carrying the data. Only one wire is used to carry data signal, the other acts as ground. Twisted pairs mainly come in two forms, Unshielded twisted pair and Shielded twisted pair.

Unshielded Twisted Pair (UTP) cable

     UTP cables are the most commonly used twisted pair cables. They are commonly used in LANs. Two cables are usually twisted to form twisted pair. Each cable includes two copper wires or more than two copper wires. Commonly used UTP cables contain four pairs of wires inside the jacket.

According to the Electronics Industries Association (EIA)/Telecommunication Industry Association (TIA), there are mainly 7 categories of UTP cabling:

1. Category 1 (CAT 1) - Used as telephone wires. Only voice signals are transmitted through it. Data rate is less than 0.1 Mbps.
2. Category 2 (CAT 2) - Used in T-lines. This type contains four wire pairs. Data rate is 2 Mbps.
3. Category 3 (CAT 3) - Used in LAN applications. Data rate is 10 Mbps.
4. Category 4 (CAT 4) - Used in 10 Mbps Ethernet and 16 Mbps token ring.
5. Category 5 (CAT 5) - Becoming standard for pre-installation in office buildings. It is presently used in 100 Mbps fast Ethernet LANs.
6. Category 6 (CAT 6) - Used in gigabit Ethernet LANs. Data rate is 1 Gbps.

Shielded Twisted Pair (STP) cables

     The structure of STP cable is similar to that of a UTP cable. One difference is that the individual conductors are enclosed in a foil shielding. One conductor carries current and the other one acts as ground. The foil shield between the two conductors prevents leakage currents from flowing between the conductors. Thus STP cables are highly resistant to EMI and cross-talk. STP cables are usually employed in token-ring networks.

Twisted Pair Cable Connectors

     Registered Jack (RJ) -45 connector helps to connect UTP and STP cables. RJ-45 is a keyed connector since it can be inserted in only one way.

Optical Fiber

     An optical fiber is a thin glass medium capable of transmitting signals in the form of optical rays. An optical fiber consists of one or more thin fibers, made of glass or plastic known as core, which is the innermost section of the fiber cable. Each fiber is surrounded with a glass or plastic coating known as cladding.The optical properties of cladding will be different from that of the core. These fibers are surrounded by an outermost layer known as the jacket, which is usually made up of plastic or other material.

Unguided Media

     In unguided media transmission, messages or electromagnetic signals travel through air or space. Cables, connectors and other solid conductors are not required in unguided media systems. Wireless communication is particularly useful in those regions where cables are difficult to implement or install. The main types of unguided media are:

• Radio waves
• Microwaves
• Infrared waves

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