DATA COMMUNICATION

We have discussed about networks and their basic terminologies. Certain important concepts about computer networks and the ways they are established were also described. However the question that what are mechanisms behind working of computer network or simply a network of computers?  was not answered. Now in this article we will be discussed all those criteria that are used for working of computer based information or data communication networks. In this discussion data communication is a most frequently used term. It is the process of data transmission from one computer to another across a network. This transmission of data involves sending and receiving data signals to or from other computers with the help of communication channels. Communication channels may be some sort of cable i.e. a telephone line or a wireless media such a microwave or a communication satellite link. The entire system responsible for such transmission of data among computers is called Data Communication System.

A data communication system involves a sender computer or device, channel encoder (or MODEM), a set of specific type of communication channel, channel decoder (or MODEM) and the receiving computer or device. The word device means that data communication phenomena is a very wide concept that is not limited to computer networks only rather this term may be used for any type of communication e.g. radio or television transmission, telephonic calls, mobile communications and so many other types of transmissions. The diagram representation of a typical data communication system may be given as under:

The following steps are accomplished during this communication:

• Sender forwards data signals to channels encoder. These signals are in digital form at this time. 
• The channel encoder converts signals from digital to a form which can pass through channels.
• Communication channel passes the encoded data signals from sender point to receiver point.
• Channel decoder again converts signals of data from channel-encoded form into digital form which is understandable for receiving computer.
• The decoded signals of data are passed to receiving computer.

Analogue & Digital Data:

Computer is capable to process the digital and analogue data. Let us see basic differences between these two types of data. Analogue data means data in a continuous wave from so that there are infinite points between any two positions on the wave. Analogue data is not cent percent accurate as it is based on approximation however it can represent any measurement.
Digital data on the other hand is discrete in nature and there are zero points in between two well-known positions i.e. 0s and 1s. Accuracy in digital data is 100% however it can be used to represent certain mathematical measurements like infinity etc. Digital computer use digital data that is composed of binary 0s and 1s. However the transmission of digital data requires its conversion in to analogue form or another form that is capable to pass through channels of the networking media. This conversion is performed with the help of channels encoder that is shown in the above diagrammatic representation. At the receiver end, data is converted again to its digital form. Channel decoder performs this for decoding the data
Both the analogue and digital data are diagrammatically represented below:

Types of Data:

Another key factor that has its impacts on transmission of data is the size of data i.e. number of bits occupies by a specific amount of data. If a specific data occupies more bits in memory then its transmission will be time-consuming  and vice versa. Depending on transmission speed due to size there are four main types of data that can be communicated from one computer to another. These types are described one by one below.

1. Textual Data:
   Textual or character data includes characters i.e. numbers, alphabets and symbols present on key board or coded into memory. This data include numbers from 0,1,2,3,4,5,6,7,8,9; alphabets from A,.............Z and a,.......................z and special character like +,_,*,/,(),{},[],^,;, and so on. Communication of textual data takes place within a short span of time because it doesn't increase the network traffic as compared to other types of data. The reason behind its fast transmission is that no extra information bits are required reason behind its fast transmission is that no extra information bits are required alongwith it there are some coding representation like ASCII and EBDIC for this type data. It can be easily represented in binary number system and this representation doesn't enlarge the number of bits too much.
2. Graphical Data:
   Graphical data is composed of pixels. Pixels are tiny dots or computer screen and are called picture elements. These dots are plotted with specific colours to make a graphical image, textual effect or animation.There are thousands of pixels in a graphical shape and a very large number of information bits are associated with each pixel describing its colour code, coordinates on x-axis and y-axis of the screen and other features. The transmission of such data is slow due to large number extra information bits and requires a high capacity of medium to communicate it. This capacity of communication medium is called band width i.e. number of data bits transmitted per second. The reason for higher bandwidth is that extra information bits are needed which can represent pixels colours and location where they are to be plotted on the screen. Each pixel in the picture must have these information bits due to which the number of bits increases enormously for all pixels. Such a high number of bits considerably enhance the network traffic over communication medium.
3. Phonic Data:This data may also be called voice or audio data and is most commonly communicated in case of voice chatting, telephone calls, radio or television news and mobile communication. Communication in this case demands more medium capacity or bandwidth as in addition to voice; bits representing the frequency or pitch of sound and loud or whispering voice are also needed.
4. Video Data:
   This data include bits associated with live images, Video chatting taking place through cameras at sending and receiving points is a good example of communicating video data. In such circumstances; cameras at both sender and receiver ends catches continuously images of the persons or other objects in front and passes them to computers. These computers then communicate bits of video data to each other.
   As video data is composed of live images and a very huge amount of bits in needed for transmission of such data, the communication is very slow in case of conventional telephone lines. An ISDN or a DSL line is more suitable for the better transmission this type of data.

Data Encoding:

Data encoding and decoding are very important processes that are performed at the sender and receiver points of a data communication system respectively.  Data encoding may be called as compression or packing of data whereas decoding may be called as decompression or unpacking of data whereas decoding may be called as decompression or unpacking of data. Encoding and Decoding are opposite of each other.
Data encoding or compression is frequently used when transmitting large quantities of data thereby reducing of number of blocks transmitted and thus reducing the cost as well as the probability of transmission errors. It basically works by replacing the repeated bytes by one copy of the byte plus an count of the repetitions.  However there are many more compression techniques with many more complicated procedures involved within them.
Graphical images can also be compressed. In order to compress a black and white image that is coded as collection of 0s (for black) and 1s (for white), the combination 0s and 1s may simply be replaced with a set of integers with a set of integers representing the total number of 0s and 1s explicitly. For example a matrix of graphical representation with black and white pixels is given below. It is compressed as 9 0s and 16 1s.



Compression ranges from 90% or more down to 5% depending on the type of file and the amount of repetition of characters or blocks of spaces or colour etc. Utility programs such as PKZIP and PKUNZIP for compressing and decompressing files are readily available. The data is compressed or packed when it is forwarded to the communication channel from the sender whereas it is decompressed or unpacked when it is forwarded to the receiving computer. Two important benefits of data encoding or compression are:

1. The packed data can be easily transmitted through all small bandwidth capacity of the medium and is mostly received in error free form at the receiving end of data communication system.
2. If an illegal person attempts to steal the transmitted data, the compressed data will, not be understandable for him / her. It is due to the anonymous encoding of data that can't be decoded without the respective decoder.

Transmission Media:

Transmission media is a very important  component of data communication system. It provides channels for the transmission in the form of a network. Media is the plural of medium and thus communication media is a collection of different medium. Media are used for the exchange of data among computers. A network is established with the help of communication media. The media is composed of communication channels that are links between computers and are followed by the data signals during their flow from one computer to another one. There are two broad categories of communication or transmission media. These are described belo:

1. Cabling System or Guided Media:A cabling system or guided media comprise cables-based data communication channels within a bounded area. Bounded area is the region confined by data communication capacities of specific cable media. Types of cabling has a major bearing on the network's speed, performance, cost and practically e.g. a very tick cable being much harder to lay in or along walls. Cabling system includes:


1. Twisted Pair Cable (both shielded (STP) and unshielded (UTP),. The shield is made of aluminum or polyester and is covering the original cable. An example of this may be the telephone wire that is the cheapest but has slow transmission rates and suffers from Electro-Magnetic Interference(EMI) easily if it is a UTP. The copper wire is used to provide communication channels. Copper wire transmits the data in the form of pulses of electric current.

  Merits:

• UTP has the lowest cost. STP is also less costly than thick coaxial and fiber optic cables.
• The bandwidth capacity of STP is theoretically upto 500 MbPs, few can support 155 MBPS and most of them support 16 MbPS whereas aUTP can support 1-155 MbPS. MbPS stands for Mega Bits Per and MBPS for Mega Bytes Per Seconds.
• The nod capacity of both STP and UTP cables is 2 computers.

In both UTP and STP cables, attenuation is 100 meters which means that the signal can travel without degradation up to 100 meters

Demerits:

• UTP is very much susceptible to Electro-Magnetic Interface i.t. EMI however STP is less susceptible to EMI due to its shielding effect. Anyhow more or less, the EMI may may its susceptibility on both UTP and STP cable.
• Coaxial Cable:Coaxial cables are high quality and well-installed cables. They can transmit data much faster and more accurately than twisted pair cables. The central cable shares a common axis and is therefore called coaxial or simply coax. There are two types of coaxial cables:
1. Baseband Coaxial:Baseband coaxial carries one signal at a time. A bit of 0 or 1 value is sent by the presence or absence of voltage in the cable. Base band signals can travel very fast but can only be send over short distances. Over about 1000 feet, special booster equipments are needed for Baseband coaxial cables.
2. Broadband Coaxial:Broadband coaxial can carry multiple signals on a fixed carrier wave, with signals for 0s and 1s send as variations on this wave. Data, Audio and Video transmission can take place simultaneously using broadband coaxial cables.

Merits:

• Coaxial cables (also called Coax) are relatively inexpensive. Thin Coax is less expensive than STP however it is less expensive than fiber optic cable.
• The installation is simple and it has higher potential of bandwidth capacity.
• The node capacity depends on the type of coaxial cable. Thick Coax can connect 100 devices. Thin Coax such as RG-58 can afford 30 computers.
• Attenuation of RG-8 that is used in thick Ethernet is 500 meters whereas the attenuation of RG-58 that is used in thin Ethernet is 185 meters.

Demerits:

• Though coaxial cable offers much better resistance to EMI due to its shielding effect, even then the transmission speed is less than that of fiber optic cable,
• Fiber Optics cables represent the latest technological developments, being very fine cables that can carry several hundred thousands voice communication simultaneously. A single fiber optic is a glass or hand transparent plastic having diameter equal to that of the human hair and it carries data in the form of pluses of light. A single fiber optic cable may contain thousands of fiber. The glass or plastic fiber is called a core and it has a cladding. Cladding protects the light traveling through core from external prismatic effects or any possible dispersion in the form of rainbow when the whether is rainy. These cables are capable of transmitting the data free of any Electro-Magnetic Interference (EMI) effecs

Merits:

• Fiber optic cable can provide extremely high bandwidth capacity which ranges from 100 MbPS to GbPS (Giga bits Per Second).
• Fiber optic has much lower attenuation because its light particle photon is chargeless and massless and can therefore transmit signals upto kilometers.
• Fiber optic is not subject to EMI as light particles photons are chargeless.

'Demerits:

• Though fiber optic is not subject to EMI, another problem that is known as the chromatic dispersion of light may occure due to the prismatic effects.
• Fiber optic cable and associated equipment such as hubs and cards are highly expensive. 
• Installation of fiber optic is much more diffixult because connections must be set carefully to avoid any loss of light in its path.Cabling System or Guided Media:

Wireless or UnGuided Media:

Cabling systems are used for communication wide a bounded area whereas using wireless or unguided media the data communication takes place across an unbounded area. In this case data communication is performed through waves such as:

• Satellite Transmission using satellite-dish and communication satellite. It is possible to send the signals over very long distances. Communication satellites are placed about 22000 miles above the earth in Geo-Synchronous orbit. In this orbit these satellites continuously rotate with the earth. 
• Microwaves Transmission is that type of transmission that takes place through microwaves are similar to Radio waves and can be used to transmit the data between microwaves stations. These microwaves travels in a straight line i.e. line of sight. Due to this reason the transmission distance between stations is limited to about 30 miles because of the earth surface's curature. The microwave stations are usually located at the hilltops or towers.
• Mobile Communication takes place through mobile sets. In modern mobile sets there are a number of facilities like digital camera in addition to the voice transmission facilities. Now a days mobiles may be used for a number of purposes such as mobile E-commerce, mobile telephone set and mobile set digital cameras for the transmission of video data.

Role of MODEM in Data Communication:

In the very beginning  of this chapter, we have studied that a data communication system needs channel encoder and channel decoder at its sending and receiving ends. Proper equipments are required for these activities. Such equipment may be MODEM that means Modulator-Demodulator. These are computer hardware specified for communication through telephone ISDN lines and through fiber optics also (where available). Thus the type of MODEM strictly depends on the channels of communication medium.
The telephone lines were originally designed for speech that is transmitted in the analogue or waveform. In order to send digital data through telephone line, it is first converted (modulated) into analogue form at the sending end and after its transmission it is converted back ( demodulated) into its digital form at the receiving end. This conversion i.e. modulation and demodulation takes place with the help of a MODEM. MODEMs can transmit data at speeds between and about 40000 bits per second. Following are the main categories of a MODEM:

• Internal MODEM is highly suitable for a standalone PC and it is fixed within the computer CPU.
• External MODEM is used for a LAN and it is attached to the server for providing the Internet services to the entire network. It is more powerful in terms of transmission speed than internal modem but is also more expensive.
• In the case of fiber optics the sending device must be equipped with a light source so that the transmission could occur, and the receiving device must be equipped with a photo sensitive cell (called photodiode) capable of translating the received light into current usable by a computer. The light source may be either a Light Emitting Diode (LED) or an Injection LASER Diode (ILD). This equipment may be considerably expensive but is high fast too.
• ISDN MODEM is the most powerful and the most expensive MODEM having extremely high bandwidth and most suitable for video conferencing.
  
  A MODEM contains both the modulator and demodulator in it. As the Internet service is mostly available through connections to telephone lines in our country that is why a MODEM is basically used for connecting us to the Internet. The above diagrammatic representation is used for the phenomenon of modulation and demodulation through a MODEM.

Transmission Modes:

The term transmission mode used to define the direction of signal flow between two linked devices. Depending upon the time intervals in the process of transmission and the number of bits to be transmitted, following transmission modes are the common:

1. Asynchronous Transmission Mode:
   In this mode the individual characters are transmitted a irregular intervals for example as soon as the user types them, they get transmitted. The bits representing individual characters are types them, they get transmitted. The bits representing individual characters are enclosed by start and stop bits. Parity bits are also added to the character code for providing a check against incorrect transmitted i.e. start bit, 7 bits for the character code (in case of 2⁷ or 128 characters), a parity bit and a stop bit. This mode of transmission is used for lower-speed transmission e.g. on a local area network consisting of a number of linked PCs. A major shortcoming of this data transmission mode is that start, stop and parity bits with every individual character considerably increase the network traffic and this mode is therefore not suitable for long-rang transmission of data..
   Here parity bit is a special bit that counts the numbers of 0s and 1s in the binary code of a character to be transmitted. Two types of parity may be even and odd parity e.g. in the 8-bit representation of a character, 10010011 has even parity of 0s as well as 1s whereas 110010011 has odd parity for 0s and 1s. This parity value is checked for verifying that if it remains the same after transmission as was before transmission then there is no transmission error at all however if the parity changes from even to odd or the number of 0s and 1s contained within the binary code changes, then the receiver requests sender to attempt for another transmission of same data as the current transmission was erroneous.
2. Synchronous Transmission Mode:
   In this mode of data transmission the timing signals synchronize the transmission at the sending and receiving ends so that there is no need for start stop bits alongwith each individual character. However these start and stop bits may be used at the beginning and end of the whole block of character. This mode of transmission is more suitable for longer transmission like remote job entry however it requires more expensive and sophisticated equipments.\

Type of Transmission Modes:

Depending upon the direction of data transmission, the given types of data transmission are described one by one below:

1. Simplex Transmission:
   Simplex transmission may also be called one-way transmission as it takes place only in a single direction. This type of transmission could be used in case when the sending devices such as temperature sensor never require a response from the receiving computer. Common examples of this type of transmission are television and radio transmissions that take place from their stations in one direction and are not responded to by a TV or radio set.
2. Half Duplex Transmission:
   In this case data transmission may take place in both directions but not at the same time as for example Walkie-Talkie. This type of data transmission may mostly be found between central computer and their terminals.
3. Full Duplex Transmission:
   In full duplex transmission data is sent in both directions at the same time such as phone calls. Most interactive computer applications use full duplex transmission of data.