Data Communication & Data Transmission

Data Communication System:

Data communication in computers is the process of exchanging data (like text, audio, or video) between two or more networked or connected devices over a transmission medium, such as cables or wireless airwaves. This exchange relies on a communication system comprising a sender, a receiver, a message, a transmission channel, and established protocols (rules) to ensure accurate, timely, and correctly delivered data. 

Components:

Key components of data communication are as follows: -

1. Sender – the device that originates and transmits the data.
2. Receiver – the device that receives the transmitted data. The location of receiver computer is generally different from the sender computer. The distance between sender and receiver depends upon the types of network used in between.
3. Message – the actual data being exchanged, which can be text, numbers, images, audio or video.
4. Transmission Medium – the physical or wireless channels that carries the data between the sender and the receiver. This includes: -
5. Guided Media – physical links like twisted-pair cables, coaxial cables, or fibre optic cables.
6. Unguided Media – wireless channels using electromagnetic waves, such as microwaves, radiowaves, satellites or infrared.
7. Protocols – a set of rules that govern the communication process, ensuring data is understood and processed correctly by both devices.

Data Transmission:

Movement of data in form of bits between two devices is data transmission. When we enter data into the computer via keyboard, each keyed element is encoded by the electronics within the keyboard into an equivalent binary coded pattern, using one of the standard coding schemes that are used for the interchange of information. To represent all characters of the keyboard, a unique pattern of 7 or 8 bits in size is used. The use of 7 bits means that 128 different elements can be represented, while 8 bits can represent 256 elements. A similar procedure is followed at

the receiver that decodes every received binary pattern into the corresponding character.

Examples include –

1. Extended Binary Coded Decimal (EBCDIC)
2. American Standard Code for Information Interchange codes (ASCII)

Both coding schemes cater to all the normal alphabetic, numeric, and punctuation characters, collectively referred to as printable characters and a range of additional control characters, known as non-printable characters.

Types of Data Transmission:

Types of Data Transmission include: -

1. Parallel Transmission
2. Serial Transmission
3. Synchronous
4. Asynchronous

1. Parallel Transmission:

Within a computing or communication device, the distances between different subunits are too short. Thus, it is normal practice to transfer data between subunits using a separate wire to carry each bit of data. There are multiple wires connecting each sub-unit and data is exchanged using a parallel transfer mode. This mode of operation results in minimal delays in transferring each word.

1. In parallel transmission, all the bits of data are transmitted simultaneously on separate communication lines.
2. In order to transmit n bits, n wires or lines are used. Thus each bit has its own line.
3. All n bits of one group are transmitted with each clock pulse from one device to another i.e. multiple bits are sent with each clock pulse.
4. Parallel transmission is used for short distance communication.

Advantage of Parallel Transmission –

It is speedy way of transmitting data as multiple bits are transmitted simultaneously with a single clock pulse.

Disadvantage of Parallel Transmission –

It is costly method of data transmission as it requires n lines to transmit n bits at the same time.

2. Serial Transmission:

When transferring data between two physically separate devices, especially if the separation is more than a few kilometres, for reasons of cost, it is more economical to use a single pair of lines. Data is transmitted as a single bit at a time using a fixed time interval for each bit. This mode of transmission is known as bit- serial transmission.

In serial transmission, the various bits of data are transmitted serially one after the other. It requires only one communication line rather than n lines to transmit data from sender to receiver. Thus all the bits of data are transmitted on single line in serial fashion. In serial transmission, only single bit is sent with each clock pulse.

Advantage of Serial Transmission –

Use of single communication line reduces the transmission line cost by the factor of n as compared to parallel transmission.

Disadvantages of Serial Transmission –

1. Use of conversion devices at source and destination end may lead to increase in overall transmission cost.
2. This method is slower as compared to parallel transmission as bits are transmitted serially one after the other.

Types of Serial Transmission:

Serial Transmission is further categorised as –

1. Synchronous
2. Asynchronous

Both these transmissions use 'Bit synchronization'.

Bit Synchronization is a function that is required to determine when the beginning and end of the data transmission occurs. It helps the receiving computer to know when data begin and end during a transmission. Therefore, bit synchronization provides timing control.

a. Asynchronous Transmission:

1. Asynchronous transmission sends only one character at a time where a character is either a letter of the alphabet or number or control character i.e. it sends one byte of data at a time.
2. Bit synchronization between two devices is made possible using start bit and stop bit.
3. Start bit indicates the beginning of data i.e. alerts the receiver to the arrival of new group of bits. A start bit usually 0 is added to the beginning of each byte.
4. Stop bit indicates the end of data i.e. to let the receiver know that byte is finished, one or more additional bits are appended to the end of the byte. These bits, usually 1s are called stop bits.
5. Addition of start and stop increase the number of data bits. Hence more bandwidth is consumed in asynchronous transmission.
6. There is idle time between the transmissions of different data bytes. This idle time is also known as Gap
7. The gap or idle time can be of varying intervals. This mechanism is called Asynchronous, because at byte level sender and receiver need not to be synchronized. But within each byte, receiver must be synchronized with the incoming bit stream.

Applications of Asynchronous Transmission –

1. Asynchronous transmission is well suited for keyboard type-terminals and paper tape devices. The advantage of this method is that it does not require any local storage at the terminal or the computer as transmission takes place character by character.
2. Asynchronous transmission is best suited to Internet traffic in which information is transmitted in short bursts. This type of transmission is used by modems.

Advantages of Asynchronous Transmission –

1. This method of data transmission is cheaper in cost as compared to synchronous e.g. If lines are short, asynchronous transmission is better, because line cost would be low and idle time will not be expensive.
2. In this approach each individual character is complete in itself, therefore if character is corrupted during transmission, its successor and predecessor character will not be affected.
3. It is possible to transmit signals from sources having different bit rates.
4. The transmission can start as soon as data byte to be transmitted becomes available.
5. Moreover, this mode of data transmission in easy to implement.

Disadvantages of Asynchronous Transmission –

1. This method is less efficient and slower than synchronous transmission due to the overhead of extra bits and insertion of gaps into bit stream.
2. Successful transmission inevitably depends on the recognition of the start bits. These bits can be missed or corrupted.

b. Synchronous Transmission:

1. Synchronous transmission does not use start and stop bits.
2. In this method bit stream is combined into longer frames that may contain multiple bytes.
3. There is no gap between the various bytes in the data stream.
4. In the absence of start & stop bits, bit synchronization is established between sender & receiver by 'timing' the transmission of each bit.
5. Since the various bytes are placed on the link without any gap, it is the responsibility of receiver to separate the bit stream into bytes so as to reconstruct the original information.
6. In order to receive the data error free, the receiver and sender operates at the same clock frequency.

Application of Synchronous Transmission –

Synchronous transmission is used for high speed communication between computers.

Advantage of Synchronous Transmission –

This method is faster as compared to asynchronous as there are no extra bits (start bit & stop bit) and also there is no gap between the individual data bytes.

Disadvantages of Synchronous Transmission –

1. It is costly as compared to asynchronous method. It requires local buffer storage at the two ends of line to assemble blocks and it also requires accurately synchronized clocks at both ends. This lead to increase in the cost.
2. The sender and receiver have to operate at the same clock frequency. This requires proper synchronization which makes the system complicated.

Comparison between Serial and Parallel Transmission:

Comparison between Asynchronous and Synchronous (Types of Serial Transmission):

Modes of Transmission:

The term Transmission Mode defines the direction of the flow of information between two communication devices i.e. it tells the direction of signal flow between the two devices.

There are three ways or modes of data transmission: Simplex, Half duplex (HDX), Full duplex (FDX).

1. Simplex – under this technique, data is transmitted in only one direction. Eg.: T.V. Broadcasting.

Examples include:

1. A Communication between a computer and a keyboard involves simplex duplex transmission. A television broadcast is an example of simplex duplex transmission.
2. Another example of simplex transmission is loudspeaker system. An announcer speaks into a microphone and his/her voice is sent through an amplifier and then to all the speakers.
3. Many fire alarm systems work the same way.

2. Half Duplex – it allows transmission of data in both ways, but data can be transmitted in only one way at any point of time. Both the connected devices can transmit and receive but not simultaneously. When one device is sending the other can only receive and vice-versa.

Example include:

A walkie-talkie operates in half duplex mode. It can only send or receive a transmission at any given time. It cannot do both at the same time.

3. Full Duplex – it allows transmission of data in both ways at the same time.

Example include:

Telephone networks operate in full duplex mode when two persons talk on telephone line; both can listen and speak simultaneously.

Communication Medium:

Communication medium refers to the physical channel through which data is sent and received. The speed of data transmission or data rate depends upon the type of medium being used in the network.

Mediums of data communication can be classified as Wired and Wireless: -

1. Wired – it is the transmission used in which signals are confined to a specific path using wires or cables. It is also referred to as Guided or Bounded Transmission media.

Its features include:

1. High Speed
2. Secure
3. Used for comparatively shorter distances

Types of Wired Media are as follows: -

a) Twisted Pair Cable – It consists of 2 separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective sheath. They are the most widely used Transmission Media.

Twisted pair is of two types:

i. Unshielded Twisted Pair (UTP) – This type of cable has the ability to block interference and does not depend on a physical shield for this purpose. It is used for telephonic applications.

Its advantages include:

1. Cheap
2. Easy to install
3. High speed capacity

Its disadvantages include:

1. Susceptible to external interference
2. Lower capacity and performance in comparison to STP
3. Short distance transmission due to attenuation

ii. Shielded Twisted Pair (STP) – This type of cable consists of a special jacket to block external interference. It is used in fast-data-rate Ethernet and in voice and data channels of telephone lines.

Its advantages include:

1. Better performance at a higher data rate in comparison to UTP
2. Eliminates crosstalk
3. Comparatively faster

Its disadvantages include:

1. Comparatively difficult to install and manufacture
2. More expensive
3. Bulky

b) Coaxial Cable – it is called coaxial as it contains two conductors that are parallel to each other. It is surrounded by PVC installations.

There are two types of coaxial cables: Baseband (dedicated cable bandwidth) and Broadband (cable bandwidth is split into several ranges).

Cable TVs and analogue television networks widely use Coaxial cables.

Advantages of Coaxial Cable include:

1. Bandwidth is high.
2. Much higher noise immunity.
3. Data transmission without distortion.
4. Inexpensive.

Disadvantages of Coaxial Cable include:

1. Difficult to install.
2. Single cable failure can fail entire network.

c) Optical Fibre – these are similar to coaxial cable. It uses electric signals to transmit data. It has bandwidth more than 2 GBPS. It is used for transmission of large volumes of data.

Advantages of Optical Fibre include:

1. High Quality Transmission
2. High Speed
3. Used for both analogue and digital signals
4. No electromagnetic interference

Disadvantages of Optical Fibre include:

1. It is expensive.
2. Difficult to install.
3. Maintenance is expensive.
4. Unidirectional, i.e., will need another fibre, if we need bidirectional communication.

2. Wireless – wireless media send the data through air, which is available to everyone who has a device capable of receiving them. media. No physical medium is required for the transmission of electromagnetic signals. It is also referred to as Unguided or Unbounded Transmission media.

Its features include:

1. Signal is broadcasted through air
2. Less Secure
3. Used for larger distances

Types of wireless media are as follows: -

a) Microwaves – It is a line of sight transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. Frequency Range: 1GHz – 300GHz. These are majorly used for mobile phone communication and television distribution.

b) Radio Waves – These are easy to generate and can penetrate through buildings. The sending and receiving antennas need not be aligned. Frequency Range: 3 KHz – 1GHz. AM and FM radios and cordless phones use Radio waves for transmission. Further Categorized as: Terrestrial and Satellite.

c) Infrared – Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems. Frequency Range: 300GHz – 400THz. It is used in TV remotes, wireless mouse, keyboard, printer, etc.