Computer Networks

Network can be classified into 3 major types depending upon the geographic spread. They are:
1. LAN (Local Area Network)
2. MAN (Metropolitan Area Network)
3. WAN (Wide Area Network)

LAN
A LAN or a Local Area Network is a network that is restricted to a small physical area. A small LAN might connect a few computers in an office or in a home; a large LAN could extend over an office park or university campus, connecting computers and other devices in a number of buildings. The major benefit of a local area network is that it can help to reduce cost by allowing people and microcomputers to share expensive resources.
There are three types of LANs. They are discussed below:
1. Dedicated Server LANs: Dedicated server LANs account for more than 70 percent of all installed LANs. A dedicated server LAN can connect with almost any other network, can handle very large databases and have a dedicated nework server.
2. Peer-to-Peer LANs: This network is a local area network that allows all users access to data on all workstations. In such networks, any computer on the network shares its resource such as hard disk and printer with any other computer on the same network.
3. Zero-slot LANs: This LAN operates like peer to peer LAN, but offers limited simple abilities such as sharing files and printers, transfer of files and transmission of e-mail. It is inexpensive. It does not require a network interface circuit card. Its adapter plug can be plugged into a serial or parallel port. This network usually can handle up to 30 computers

MAN
This is larger than a LAN and stands for Metropolitan Area Network. A MAN usually spans a geographical area that encompasses a city or country area. It interconnects various buildings or other facilities within this area. For example, linkages can be established between two commercial buildings. MAN technology has been rapidly developing in the area of cellular phone systems.

WAN
A wide area network (WAN) is one that operates over a vast distance (e.g., nationwide). Its nodes may span cities, states, or national boundaries. This network interconnects computers, LANs, MANs and other data transmission facilities. A WAN will employ communications circuits such as long distance telephone wires, microwaves, and satellites. Nationwide automated teller machines used in banking represent a common application of a wide area network.

Communication Media : Satellite

The problem with microwave communications is of line of sight. Because of the curvature of earth, mountains and other high structures often block the line of sight. So you require several repeater stations for long-distance transmission, which increases the cost of data transmission. This problem is overcome by using satellites.
A communication satellite is an electrical device positioned in an orbit around the earth. It can be thought of a big microwave repeater in the sky. I contains one or more 'transponders', each of which listens to some portion of the frequency spectrum, amplifies the incoming signal and then rebroadcasts it at another frequency. Different frequencies are used for 'uplinking' and 'downlinking' to avoid any interference of signals. Uplink refers to data flow from the earth to the satellite. Here the earth station works as a transmiter and the satellite transponder as a receiver. Downlink refers to data flow from the satellite to the earth. Here the saellite works as a transmitter and the earth station as a receiver.

Advantages
There is no line of sight restriction, so transmission and reception is possible between any two randomly chosen places.
Disadvantages
Launching a satellite into an orbit costs a lot.
A signal sent to a satellite is broadcasted to all receivers within the satellite's range. So, measures are required to prevent unauthorized tampering of information.

Communication Media : Microwave Link

Microwave radiation is also a popular medium of transmission. It does not require the laying of expensive cables. Microwave links use  very high frequency radio waves to transmit data through space. Microwave links use repeaters at intervals of about 25 to 30 km in between the transmitting and receiving stations

Parabolic antennas are mounted on towers to send a beam to another antenna which could be tens of kilometers away, but should be in the line of sight. The higher the tower the greater is the range. Microwave radio transmission is widely used for long-distance communication. It overcomes the problem of weak signals.

Advantages
  • Building two towers is cheaper than digging a 100 km trench and laying cables in it.
  • It can permit transmission rates of about 16 giga (1 giga bit = 10 to the power of 9 bits) bits per second.
Disadvantages
  • Repeaters, if used along the way, are to be maintained regularly.
  • Physical vibration will show up as signal noise.

Communication Media : Radio Frequency Propagation


Data transmission through air (and not through any channel) is called unguided transmission. Data is carried over electro-magnetic radiation in the form of radio waves. Such propagation is classified by the type of wave used for propagation. There are three types of RF (radio frequency) propagation.
  1. Ground wave
  2. Ionospheric
  3. Line of Sight (LOS)
Ground Wave Propagation
Ground wave propagation follows the curvature of the earth. Ground waves have carrier frequencies up to 2 MHz. The AM radio is an example of ground wave propagaiton.
Ionospheric Propagation
Ionospheric propagation bounces off the Earh's ionspheric layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with the weaher and the time of the day. The signal sent from a radio tower bounces off the ionosphere and comes back to earth to a receiving station.

Line of Sight Propagation
Line of sight propagation transmits exactly in the line of sight. The receiving station must be in the view of the transmit station. It is someimes called space wave or tropospheric propagation. It is limited by the curvature of the Eartth for ground-based stations (100 km. from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are FM radio, and satellite microwave.

Communication Media : Optical Fibre

An optical fibre is a piece of hair-thin glass material of a different refractive index. Such a fibre is cabable of transmiting data at the speed of light with no significant loss of intensity over long distances.
Fibre-optic links are based on the principle of  ' total internal reflection '. When an electromagnetic wave, travelling in a medium with high refractive index, falls on the boundary of the surrounding medium of lower refractive index, a special phenomenon takes place. Up to a cerain angle of incidence, light passes through the boundary and enters the medium that has a lower refractive index. But if the angle of incidence is more than the critical angle, the light is reflected from the boundary and comes back to the first medium. This is called ' total internal reflectiont'. 
An optical transmission system based on the fibre-optics has three components, the transmission medium, the light source and the detector. The transmission medium is the ultra-thin glass fibre. The light source is either an LED (light emitting diode) or a laser diode, which emits light pulses when electric current is applied. the detector is a photo diode which generates electric pulses when light falls on it.
By attaching the LED or the laser diode to one end of an optical fibre and a photo diode at the other end, we can have an uni-directional data transmission system that accepts an electrical signal, converts and transmits it by light pulses and then reconverts the output to an electrical signal at the receiving end.

Advantages
It has very high rate of transmission of data.
It has better noise immunity.
It can transmit data over long distances.
Data is transmitted with high security.
The fibres are small in size.

Disadvantages
Limited physical arc of cable. Bend it too much and it will break!
Difficult to splice.


The cost of optical fibre is a trade off beween capacity and cost. A higher transmission capacity, it is cheaper than copper. At lower transmission capaciy, it is more expensive.

Communication Media : Coaxial Cable

Coaxial Cables are a group of specially wrapped and insulated wires capable of transmiting data at a very high rate. They consist of a central copper wire (inner conductor) surrounded by a PVC (polyvinyl chloride) insulation over which a sleeve of copper mesh (second conductor) is covered.

The metal sleeve is again shielded by an outer shield of thick PVC material. The signal is transmitted by the inner copper wire and is electrically shielded by the outer metal sleeve. Coaxial cables are extensively used in long-distance telephone lines and in closed-circuit TV.




Advantages
  • They are capable of transmitting digital signals at a very high rate of approximately 10 mega bits per second.
  • They have a higher noise immunity.
Disadvantages
  • These are compariively costly.
  • Such cables can easily be tapped, posing security problems.

Communication Media : Twisted Pair Cable

A twisted cable is the oldest and most common medium of transmission. It is generally used in telephone systems. A twisted pair consists of two insulated copper wires, typically 1 mm thick, twisted together just like a DNA molecule. The wire is twisted so as to reduce the electrical interference from and to the adjacent copper pairs. (The two conducting wires, which run parallel to each other, may cause electrical interference.)
When many twisted pairs run in parallel over a long distance, they are bundled togeher and enclosed in a proective sheath, so as not to interfere with each other. twisted pair is also used in a LAN (Local Area Network)

Advantages
It can be used for analog as well as digital transmission.
It is one of the cheapest media of transmission and has adequate performance.
Disadvantages
It is more prone to pick up noise signals.
A twisted pair can transmit dara only upto a certain distance.

Serial and Parallel Transmission

Serial Transmission
In, serial transmission, data is transmitted at one bit at a time in a continuous stream along the communications channel. For each direction of data flow, only one wire is used.
This pattern is analogous to the flow of traffic down a one-lane residential street. Most data transmitted over telephone lines uses a serial pattern. Serial Transmission is typically slower than parallel transmission, because data are sent sequentially in a bit-by-bit fashion.

Parallel Transmission
In the case of parallel data transmission, several bits of data are transmitted concurrently through separate communication lines. This resembles the flow of traffic on a multi lane highway. Internal transfer of binary data in a computer uses a parallel mode. If the computer uses a 32-bit internal structure, all the 32 bits of data are transferred simultaneously on 32-lane connections. Parallel data transmission is commonly used for interactitons between computer and is printing unit.

Digital and Analogue Transmission

Data is propagated from one point to another by means of electrical signals, which may be in the digital or analog form. As shown in the figure below,




analog signals are continuous in nature. They have continuous amplitude levels. Since they are continuous, it is very difficult to remove physically any noise and distortion from them, which is added during transmission or otherwise. The telephone lines used for data communication in computer networks are usually meant for carrying analog signals.





A digital signal is a sequence of voltage pulses represented in the binary form. These signals are well-defined with discrete amplitude levels. Computer generated data is digital in form.

Data Communication

Message and Structured Data
The term data communication describes the transmission of computer related records, which have a structured format, from source to destination over transmission media like telephone line, optical fibre, microwave link, etc.
Other forms of digital transmission such as digitized voice, facsimile or video do not use structured data except in the case of telemetry, which can use fixed data field and formats.
Each record of a structured data type is identified by a transaction code; the data is organized into predefined fields, each of which contains a specified maximum number of characters.

Bits Streams and Symbols
The characters which make up a message are transmitted one after another as a bit stream, where a bit is the fundamental data unit represented as 1 or 0, mark or space in digital systems.
A symbol is the basic transmission element, which is used to transmit a group of bits.

Data Rate
Data Rate is measured as bits per second. When group of k bits is combined to form a transmission symbol which has a transmit duration T, then
Data Rate = k/T

Baud Rate
Baud rate is the transmit speed of a communication channel in bits per second. For example, 4800 baud is 4,800 bits/second of data transfer.