Data communications are the exchange of data between two devices via a transmission medium such as a wire cable or wireless media. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of devices (physical equipment) and software (programs).
The term telecommunication, which includes telephony, telegraphy, and television, refers to long-distance communication (tele is Greek for “far”).
Fundamental characteristics of data communication
The effectiveness
of a data communications system depends on 4 fundamental characteristics:
1. Delivery. The system must convey data to its intended destination. Data can only be received by the intended device or user.
2. Accuracy. The systems must transfer correct data. Data that has been altered during transmission and remains incorrect is unusable.
3. Timeliness. The system must deliver data in correct time. Data delivered late holds no value. Specifically for video and audio, this means data should be delivered as it's produced, in the correct sequence, and without notable delays. This kind of delivery is called real-time transmission.
4. Jitter. Jitter is the inconsistency in packet arrival times. It's when there's an uneven delay in how packets are delivered. Imagine sending video packets every 50 milliseconds, but they don't all make it on time—some show up 30 milliseconds late, others 40. This kind of delay mix-up can mess with your video's flow, causing buffering, dropped frames, and sync issues.
Components of a data communications system
A data communications system has 5 components
1. Message. The message is the information (data) to be communicated. Popular
forms of information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.
4. Transmission medium. The transmission medium serves as the physical channel that carries a message from the sender to the receiver in data communication. Examples of such media are twisted-pair wires, coaxial cables, fiber-optic cables, and radio waves.
5. Protocol. A protocol defines the rules that manage the exchange of data. It's like a common language for devices, ensuring they understand each other. Without a protocol, even if devices are connected, they won't communicate effectively, similar to how a Bengali speaker and a Japanese speaker can't understand one another without a common language.
Data Representation
Information today comes in different forms such as text, numbers, images, audio, and
video.
Text.
In data communications, text is represented in a computer using character encoding schemes that assign a unique binary code to each character. The most common encoding schemes are ASCII (American Standard Code for Information Interchange), which uses 7 or 8 bits to represent characters, and Unicode, which supports a much larger range of characters from different languages and symbols by using 8, 16, or 32 bits. Each character in the text is stored as a sequence of binary numbers corresponding to its code in the chosen encoding scheme.
Numbers.
Numbers are also represented by bit patterns. ASCII code isn't utilized for representing numbers; instead, numbers are directly translated into binary form to streamline mathematical computations.
Images. Images are also represented by bit patterns. An image is composed
of a matrix of pixels (picture elements), where each pixel is a small dot. After an image is divided into pixels, each pixel is assigned a bit pattern. The size
and the value of the pattern depend on the image. For an image made of only black-and-white dots (e.g., a chessboard), a 1-bit pattern is enough to represent a pixel.
If an image is not made of pure white and pure black pixels, we can increase the
size of the bit pattern to include gray scale. For example, to show four levels of gray
scale, we can use 2-bit patterns. A black pixel can be represented by 00, a dark gray
pixel by 01, a light gray pixel by 10, and a white pixel by 11.
Audio. Audio inherently differs from text, numbers, or images as it is continuous rather than discrete. In computers, audio is represented as digital signals, which are transformed from analog sound waves via a process known as sampling. This process involves measuring the sound wave at consistent intervals, known as the sample rate, and converting each measurement into a digital value.
Video. A video is a series of still frames, with each frame being a static image made up of pixels. These frames are shown in quick succession, usually at a rate of 24 to 60 frames per second, to produce the appearance of movement. Frames are compressed using algorithms like H.264 or H.265 to minimize the file size while preserving quality. Video files also contain audio tracks, metadata, and synchronization details to facilitate correct playback.
Data Flow: Data Transmission Modes
Data Transmission Modes Based on direction of exchange of Information
Device communication can occur in three modes according to the direction of exchange of Information: simplex, half-duplex, or full duplex.
In simplex mode, the communication is unidirectional, as on a one-way street. Only one
of the two devices on a link can transmit; the other can only receive.
Keyboards and traditional monitors serve as examples of simplex devices. A keyboard is limited to input only, while a monitor solely displays output. Simplex mode dedicates the full capacity of the channel to transmit data in a single direction.
b. Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the same time.
When one device is sending, the other can only receive, and vice versa. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever of
the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios
are both half-duplex systems.
c. Full-Duplex
In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously. In full-duplex mode, signals traveling in one direction share the link's capacity with those going in the opposite direction. This sharing can happen in one of two ways:
- the link may have two physically distinct transmission paths, one for sending and the other for receiving.
- the channel's capacity may be split between signals moving in both directions.
The telephone network is a typical example of full-duplex communication, where both parties can simultaneously talk and listen during a call.
Data Transmission Modes Based on Synchronization Between the Transmitter and the Receiver
The data transmission modes can be characterized in two types according to the synchronization between the transmitter and the receiver in data communication:
Synchronous
In a Synchronous mode of data transmission, data is transmitted as a continuous stream of bits. The bits are sent one after another without any start/stop bits or gaps between the bytes. Both the sender and receiver are paced by the same system clock to achieve synchronization. The receiver is responsible for grouping the incoming bits correctly into bytes. It does this by counting the bits and grouping them into units of eight bits (one byte). The receiver receives data at the same rate as the transmitter sends it. This requires precise timing and synchronization between the transmitter and receiver. The receiver continuously listens for data, even when no bits are currently being transmitted ensuring that it can pick up the transmission as soon as data starts arriving.
Since there is no separation between each character, it becomes necessary to insert synchronization elements with the message, this is called " Character-Level Synchronization ". It employs special characters, often called "SYN characters" to identify the start of a new character and adjust its internal clock to match the sender's clock. These are predefined bit patterns that both the transmitter and receiver recognize. For example, a common sync pattern might be "01.
The receiver looks for the predefined synchronization pattern in the bit stream. Once the sync pattern is detected, the receiver aligns the subsequent bits into bytes based on the known length of the byte (usually 8 bits).
Imagine a data transmission where each byte consists of 8 bits. To help the receiver maintain synchronization, a special 2-bit sync pattern "01" is inserted before each byte. The transmission might look like this:
... 01 11001010 01 00110101 01 11100011...
Asynchronous
In asynchronous transmission mode a start and the stop bit is introduced in the message during transmission. The start and stop bits ensure that the data is transmitted correctly from the sender to the receiver. Typically 0 as start bit and 1 as stop bit is used.
Asynchronous here means 'asynchronous at the byte level', but the bits must be still synchronized. The time duration between each character is the same and synchronized.
Data bytes can be sent at any time, making this mode suitable for systems where data transmission is sporadic or unpredictable. While each byte is independently framed with start and stop bits and can be sent at irregular intervals, the order in which bytes are sent and received still matters. Asynchronous transmission ensures that each byte is correctly identified and interpreted, but it does not inherently reorder bytes. If we send 2 bytes in a specific sequence, the receiver will interpret them in the order they arrive.
Data Transmission Modes Based on number of bits sent
The data transmission modes can be characterized in the following two types based on the number of bits sent simultaneously in the network:
Serial
The Serial data transmission mode is a mode in which the data bits are sent serially one after the other at a time over the transmission channel. It needs a single transmission line for communication. The data bits are received in synchronization with one another. So, there is a challenge of synchronizing the transmitter and receiver.
Transmission occurs over several clock cycles, with each bit being sent in one clock cycle. The clock rate (frequency) determines the speed of data transmission. Data integrity is maintained, as it transmits the data bits in a specific order, one after the other.
This type of transmission mode is best suited for long-distance data communication, or the amount of data being sent is relatively small.
Parallel
The Parallel data transmission mode is a mode in which the data bits are sent parallelly at a time. In other words, there is a transmission of n-bits at the same time simultaneously.
Multiple transmission lines are used in such modes of transmission. So, multiple data bytes can be transmitted in a single system clock. This mode of transmission is used when a large amount of data has to be sent in a shorter duration of time. It is mostly used for short-distance data communication.
For n-bits, we need n-transmission lines. So, the complexity of the network increases but the transmission speed is high.
Communication Channels
Communication channels are the medium that connects two or more workstations for data communication. Workstations can be connected by either wired media or wireless media for data communication. It is also known as a transmission medium. The transmission medium or channel is a link that carries messages between two or more devices. We can group the communication media into two categories:
- Guided media transmission
- Unguided media transmission
1. Guided Media: In such transmission medium, the link between communicating devices are created using physical wires and then the data is transmitted using these cables in terms of signals. Example types: Twisted pair cable, Coaxial Cable, Optical fibers
2. Unguided Media: In unguided transmission media signals are propagated from one device to another device wirelessly. Signals can wave through the air, water, or vacuum in the form of Microwave, Radio wave, Infrared etc.
Measuring Capacity of Communication Media
The capacity of a channel is the
maximum amount of signals or traffic that a channel
can carry. It is measured in terms of bandwidth and
data transfer rate as described below:
Bandwidth
Bandwidth of a channel is the range of frequencies
available for transmission of data through that channel.
Higher the bandwidth, higher the data transfer rate.
Normally, bandwidth is the difference of maximum and
minimum frequency contained in the composite signals.
Bandwidth is measured in Hertz (Hz).
1 KHz =1000 Hz
1 MHz =1000 KHz = 1000000 Hz
Data Transfer Rate
Data travels in the form of signals over a channel. One
signal carries one or more bits over the channel. Data
transfer rate is the number of bits transmitted between
source and destination in one second. It is also known
as bit rate. It is measured in terms of bits per second
(bps). The higher units for data transfer rates are:
