Introduction

Data communication is the process of exchanging information or data. With due regard to computer networks, data communication is done between two or more devices over a network and sent through a transmission medium such as cable, Wi-Fi, radio wave, etc. This process involves the combination of hardware and software to enable communication to take place. The hardware components include the sender and receiver devices, signal transmission devices, switches, and other intermediate devices through which data have to pass. The software components involve algorithms and protocols that specify what type of data is to be communicated, how the data is to be communicated, and when it should be communicated.

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Components of Data Communication

There are the five fundamental components that are needed for the effective data communication to take place. Without any one of these components, communication cannot happen.

a. The Sender:

The sender is any device that is capable of transmitting data (the message). Without the sender, there will be no way one can send any message.

b. The Message:

This is the information that is needed to be communicated by the sender to a receiver. The message is usually encrypted before being transmitted in order to protect the privacy and security of the sender and the receiver. It could be in form of text audio, video, etc.

c. Transmission Medium:

The transmission medium generates the signal or provides the means through which the message travels from the sender’s device to the receiver’s device. The medium can be wired (cable) or wireless (radio wave) or variants of both.

d. Protocol:

This is a standard set of rules used by the sender and receiver to communicate data preset before the data is sent. A protocol is a set of rules that define how data communication will be executed. Without protocols, devices from various vendors and networks will not be able to communicate, since they all use different languages.

e. Receiver:

The receiver is the device that receives the data sent by the sender. The device is often programmed to ring, beep, vibrate, or light up in order to alert its owner that a message has been received.

Data Representation

Data can be referred to as a collection of raw facts that has to be processed through various means before it can yield useful information. There are many different forms in which data can be represented. They include:

a. Texts:

Texts are a combination of alphabets in small case as well as upper case written in various forms and languages. They are stored as a pattern of bits and they can be encoded in Unicode or ASCII forms.

b. Numbers:

Numbers are an array of digits from 0 to 9, roman figures, and other algorithms. They are stored as a pattern of bits and they can also be encoded in Unicode or ASCII forms.

c. Images:

Images are used to convey expressions through the picture/image that it is sent. For example, the image of a branded airplane with a beautiful air hostess by the side is an advertisement to attract potential travelers to fly with the airline. A pixel is the smallest element of an electronic image. In other words, a picture is an array of pixel elements. Pixels are also represented in form of bits. The size and quality of a picture depends on the number of pixels/resolution and the bit pattern used to indicate the value of each pixel. In electronic devices, images are digitally stored. Common electronic image formats include: jpg, bmp, png, etc.

d. Audio:

This is the data represented in form of audible sound that can be recorded and transmitted.

e. Video:

Video refers to broadcasting of data in forms of motion pictures or movies.

Modes of Data Transmission

Data flows in three different ways, namely:

a. Simplex Mode:

In simplex mode, the data flow is unidirectional, i.e., communication only flows in one direction. Only one of the two communicating machines on a line can transmit data; the other can only receive and process data, but cannot transmit or reply to a message.

b. Half- Duplex:

In half-duplex transmission mode, the communicating devices can both transmit and receive data, but not simultaneously. E.g., when one device is sending Data, the other device can only receive, and vice versa.

c. Full-Duplex:

With full-duplex transmission mode (also called duplex), both communicating devices can transmit and receive data simultaneously.

Data Transmission Networks

A network is a set of devices and nodes that are connected by various wired or wireless means to enable them communicate effectively. A node is any electronic device, e.g., a computer, mobile phone, printer, etc., that is capable of sending and/or receiving data generated and broadcast by other nodes on the network. A reliable network should be able to meet the following three requirements:

a. Performance:

This is measured using transit time and response time; transit time is the time required for data to travel from the sending device to the receiving device, while response time is the elapsed time between a service request/inquiry from a device and a response from the server.

b. Reliability:

This is the frequency at which network service is available and the time it takes the network to recover from a failure.

c. Security:

This is concerned with securing data from unauthorized access, prevention of data losses, and ensuring privacy of the network users.

Types of Network

There are many different types of networks. They are mostly classified according to their size. They include: LAN, WAN, MAN, wireless network and internetwork.

a. Local Area Network:

This network is small and is meant to enable a few devices to share resources (printer, data, storage, and application program) in order to save costs. LAN networks are usually in simplex mode, i.e., only one device on the LAN transmits data while the others only receive.

b. Wide Area Network:

This facilitates the transmission of data over long-distance areas that may comprise a district or even a country. A WAN can as simple as a dial-up connection and a tower connection between two far-spaced buildings or as complex as the backbones that connect many places to the Internet.

c. Metropolitan Area Network:

This is a network designed to be used within a local area, town, or city only. They are often based on point-to-multipoint network architecture.

Data Communication Equipment

These are the general networking equipment types and the types of connections that can be established using them.

a. Data Terminal Equipment (DTE):

This is the end user’s sending/receiving equipment that converts outgoing data into a transmittable signal, and convert incoming signal into meaningful data. DTEs come in many different forms and shapes. Examples include personal computers, mobile devices, terminal adapters, desktop servers, etc. DTEs are usually owned and managed by the end users.

b. Data Circuit Equipment (DCE):

This is the network equipment that connects DTEs to the network service. A DTE and the network may use different types of signals—or a DTE may not have the signal necessary to enable it connect to the network, hence it requires a DCE. For example, a computer cannot connect to a GSM/CDMA network on its own and would require DCE equipment to do so. A DCE may be a part of a DTE, such as a smartphone that provides hotspot service to many computers, or it may be an entirely separate device like a modem or multiplexer.

c. Data Switching Equipment (DSE):

This equipment is used to connect DCEs together in order to provide switching capability to the network. DSEs are the main nodes in a network and are responsible for routing data across the network. DSEs can be referred to as switches. Digital switches are examples of DSE equipment.

Data Transmission Signals

Transmission is the act of sending information from one electronic device to another via analog or digital signal. With an analog signal, information appears as a continuous variation of the originating source. For example, human speech produces a continuous variation of air pressure. Therefore, we can say voice speech is analog.

With a digital signal, data appears as a sequence of binary values, 0 or 1. To represent these two values, a signal is used with only two wave lengths. One depicts the binary value 0 and the other depicts the binary value 1. Both types of signal can be converted into the other using special procedures as explained below:

a. Modulation:

Digital data can be transmitted over an analog line through modulation. The digital bit streams are modulated over the analog carrier signal. An example of a modulation device is a modem (modulator/demodulator), which converts an outgoing digital bit stream from a device into am analog signal and converts the incoming analog signal into a digital bit stream for use on the receiver’s device. There are also different types of modulation. They include amplitude modulation, frequency modulation and phase modulation.

b. Digitization:

Digitization is basically the opposite of modulation. In digitization, an analog signal is converted into digital form through a process of sampling. The analog signal is sampled, converted into digital data and transmitted over digital lines. It is then converted back to an analog signal at the receiver’s end. These sampling functions are performed by a device called a codec (coder/decoder). Digital signal is resistant to distortion and it is easier to efficiently transmit over a long distance than an analog signal.

Switching Methods

Switching is the method by which nodes establish a path for point-to-point communication in a network. Nodes in the network utilize their direct communication links to other nodes in order to establish a communication path. Each node has the ability to “switch” to another node if required to further stretch the data communication path until it is completed. There are two methods of switching:

a. Circuit Switching:

In circuit switching, two communicating parties are connected by a dedicated communication line allocated to two devices in a point-to-point connection. The communication path is maintained intact for the duration of the transmission between the communicating nodes and then closed. Circuit switching is mainly used in voice communications. With computer-based data, circuit switching would be overwhelmed, since it has a fixed amount of bandwidth and therefore cannot support spikes in service demand. When the network’s limit is reached, other simultaneous communications are blocked until some channels are released.

b. Packet Switching:

Packet switching was developed to address the inadequacies of circuit switching. Unlike circuit switching, in which data is transmitted along a dedicated circuit path, in packet switching, communication is in form of data packets. A message is divided into small packets. When the message, e.g., a requested video clip is sent and the receiving device is busy, the intermediate nodes will temporarily store the packets and wait for the receiving node to become available to receive it. Paused or interrupted transmission can be resumed when desired. As a result, each packet carries additional information, such as the receiver’s address/header, to enable the network to route the data to its final destination. A packet is usually composed of the header Length to indicate where data starts, its assigned lifetime limit, and encrypted user data. When the lifetime value reaches zero, the packet is discarded. This is meant to prevent network congestion by packets that would wander aimlessly around the network because the receiving device/address is not available.

Congestion Control

Every network has a carrying capacity, the maximum number of packets that it can accommodate at any point in time. When the limit is being approached, considerable delays in packet delivery are experienced resulting in network congestion. Unregulated congestion can lead to outright failure of the network. The best way to prevent network congestion is to avoid it. This is done by setting in place programmed measures that prevent buffer overflow. These measures include the following:

a. Disposing of Packets:

As stated earlier, a data packet is discarded when its set lifetime has elapsed. More radical ways of discarding packets may also be employed when the network is congested. For example, any packet whose delivery address cannot be reached by the shortest possible route can be discarded or when there is no more buffer space within the node, it can discard incoming packets immediately.

b. Reducing Traffic:

Nodes monitor traffic on their outgoing routes and can ask the source server to reduce its transmission rate when a route is approaching its full capacity. The request can be sent to the server host using a special packet.

c. Imposing Packets Limit:

This requires some means of keeping a count of the number of live packets in the network. When the limit is almost approached, additional incoming packets are discarded on arrival. Although this method ensures that the network is not overloaded with too many packets, it does not prevent an individual node from being congested unless a packet limit is set for each node, too.

Characteristics of Data Communication

The effectiveness and reliability of any data communication system depends upon the following four fundamental characteristics:

a. Service Delivery:

The data must be delivered to the correct user/destination address.

b. Accuracy:

The network must deliver the data accurately without any omission or errors and it must not become corrupted during transmission.

c. Timeliness:

Audio and video data that is being streamed must delivered timely without any delay if required, as with applications that require real-time transmission of data.

d. Jitter:

This is the variation in the delivery time of data packets. Uneven jitter will affect the timeliness of data packets being transmitted.

Reference

  • Data Communication & Network Standards: F.Y.B.Sc. (IT) (Semester II)
  • Module Communication Networks; Version 1 ECE , IIT Kharagpur
  • Sharam Hekmat: Communication Networks; PragSoft Corporation, www.pragsoft.com
  • S. Kimbleton, G. Schneider: Computer Communication Networks: Approaches, Objectives, And Performance Considerations; USC/Information Sciences Institute, 4676 Admiralty Way, Marina Del Rey, California 90291