Data communication refers to the exchange of data between two devices via some form of transmission medium. This process involves several key elements and faces various challenges, all managed by specific rules known as protocols.
Data Communication Components
For data communication to occur, five fundamental components are necessary:
- Sender: The device that sends the data message. It can be a computer, workstation, telephone handset, etc.
- Receiver: The device that receives the data message. It can be a computer, workstation, telephone handset, etc. The receiver does not necessarily have to be at the same location as the sender.
- Message: The information (data) to be communicated. This can include text, numbers, pictures, audio, video, or any combination of these.
- Medium: The physical path by which a message travels from sender to receiver. Examples include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves (for wireless communication).
- Protocol: A set of rules that governs data communication. It represents an agreement between the communicating devices on how communication should proceed. Without a protocol, two devices may be connected but unable to communicate.
Characteristics of Data Communication
Effective data communication systems are characterized by four key aspects:
- Delivery: The system must deliver data to the correct destination. The data must be received by the intended device or user and only by that device or user.
- Accuracy: The system must deliver data accurately. Data that has been altered in transmission and left uncorrected is unusable. Errors introduced during transmission must either be detected and corrected or discarded.
- Timeliness: The system must deliver data in a timely manner. For video and audio, timely delivery means delivering data as it is produced, without significant delay. This is often referred to as real-time transmission.
- Jitter: Refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. A high jitter can cause poor quality in audio/video streaming.
Transmission Impairments
Signals transmitted over any medium are subject to impairments that can degrade their quality. The main transmission impairments are:
- Attenuation: A loss of energy in the signal as it travels through the medium. The signal strength decreases over distance. To compensate for this loss, amplifiers or repeaters are used to boost the signal strength.
- Distortion: This occurs when the signal changes its form or shape. It can happen in a composite signal when different frequency components have different propagation speeds, meaning they arrive at the receiver at different times.
- Noise: Any unwanted energy that contaminates the signal and degrades its quality. Noise can be thermal noise (random motion of electrons), induced noise (from motors or appliances), crosstalk (effect of one wire on another), or impulse noise (sudden spikes of energy).
Transmission Modes
Transmission mode defines the direction of signal flow between two linked devices. There are three types:
| Mode | Description | Direction of Data Flow | Usage Examples |
|---|---|---|---|
| Simplex | Communication is unidirectional. Data flows only in one direction from sender to receiver. The receiver cannot send data back to the sender. | One-way | Radio broadcasting, television broadcasting, traditional keyboards to computer monitors. |
| Half-Duplex | Communication is bidirectional, but not simultaneously. Data can flow in both directions, but only one device can transmit at a time. Both devices can send and receive, but not at the same moment. | Both ways, but alternately | Walkie-talkies, CB radios. |
| Full-Duplex | Communication is bidirectional and simultaneous. Data can flow in both directions at the same time. Both devices can send and receive data concurrently. | Both ways, simultaneously | Telephone conversations, internet connections with modems. |
Protocol
A protocol is a set of rules that governs data communication. It dictates how data is formatted, transmitted, received, and interpreted. Without protocols, devices would not be able to understand each other.
Composition of a Protocol
Protocols typically address three key aspects:
- Syntax: Refers to the structure or format of the data, meaning the order in which data is presented. For example, a simple protocol might expect the first 8 bits of data to be the sender's address, the second 8 bits to be the receiver's address, and the rest of the stream to be the message itself.
- Semantics: Refers to the meaning of each section of bits. It dictates how a particular pattern of bits is interpreted, and what action should be taken based on that interpretation. For example, does an address identify the route to be taken or the final destination of the message?
- Timing: Refers to two characteristics: when data should be sent and how fast it can be sent. For example, if a sender produces data at 100 Mbps but the receiver can only process it at 1 Mbps, the transmission will lead to data overflow and loss if not regulated. Timing also addresses synchronization.
Functions of a Protocol
Protocols perform numerous functions to ensure reliable and efficient data exchange:
- Data Sequencing: Ensures that data packets arrive in the correct order at the destination. If packets arrive out of order, the protocol reassembles them correctly.
- Data Routing: Determines the optimal path for data to travel from the source to the destination across a network.
- Flow Control: Manages the rate of data transmission between two devices to prevent a fast sender from overwhelming a slow receiver, thus avoiding data loss.
- Error Control: Detects and often corrects errors that may occur during data transmission. This can involve retransmission of corrupted data or forward error correction.
- Connection Management: Establishes, maintains, and terminates connections between communicating devices. This ensures that communication resources are properly allocated and released.
- Multiplexing: Allows multiple data streams to be combined and transmitted over a single physical medium, thereby sharing the transmission capacity.
- Security: Provides mechanisms to protect data from unauthorized access, modification, or denial of service during transmission. This includes encryption and authentication.
- Addressing: Defines a unique identifier for each device on a network, allowing messages to be directed to the correct recipient.