PRINCIPLES OF DATA COMMUNICATION AND NETWORKING

CHAPTER #1

PRINCIPLES OF DATA COMMUNICATION AND NETWORKING

Data communications and networking are changing the way we do business and the way We live. Business decisions have to be made ever more quickly, and the decision makers Require immediate access to accurate information. The development of the personal computer brought about tremendous changes for Business, industry, science, and education. A similar revolution is occurring in data Communications and networking. Technological advances are making it possible for Communications links to carry more and faster signals. As a result, services are evolving to allow use of this expanded capacity.

DATA COMMUNICATIONS
Definition: Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.  

When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance. The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance (tele is Greek for "far").                                                                                                                                             

The word data refers to information presented.        

The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter.

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.                                                                                                                 

2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.                                                                                                                                                3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.                                   4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay, an uneven quality in the video is the result.

COMPONENTS OF DATA COMMUNICATIONS:

                                                                                A data communications system has five components:

1: Message, 2: sender, 3: Receiver, 4: Transmission medium, 5: Protocol.

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 is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5: Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese.

 data representation:                                                                                                                                                                                                             A data can be represented in the form of text, numbers, images, audio, and video.                                            In data communications, text is represented as a bit pattern, a sequence of bits (Os or is). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or character used in any language in the world.

1: Number: Numbers are also represented by bit patterns. However, a code such as ASCII is not used to represent numbers; the number is directly converted to a binary number to simplify mathematical operations. Appendix B discusses several different numbering systems.

2: Images: Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the resolution. For example, an image can be divided into 1000 pixels or 10,000 pixels. In the second case, there is a better representation of the image (better resolution), but more memory is needed to store the image.

There are several methods to represent color images. One method is called RGB, so called because each color is made of a combination of three primary colors: red, green, and blue. The intensity of each color is measured, and a bit pattern is assigned to it. Another method is called YCM, in which a color is made of a combination of three other primary colors: yellow, cyan, and magenta.

3: Audio: Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal

4: Video: Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion.

Data Flow

Communication between two devices can be simplex, half-duplex, or full-duplex.

1: Simplex: 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 are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.                                           

2: 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. The half-duplex mode is like a one-lane road with traffic allowed in both directions. In half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time.

3: Full-Duplex: In full-duplex mode (also called duplex), both stations can transmit and receive. The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time. In full-duplex mode, signals going in one direction share the capacity of the link: with signals going in the other direction.

 EXAMPLE: One common example of full-duplex communication is the telephone network.

When two people are communicating by a telephone line, both can talk and listen at the same time.

NETWORKS: A network is a set of devices (often referred to as nodes) connected by communication links.  A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.

Analog signals: Analog signals can have an infinite number of values in a range. It is a continuous signal. An analog signal is a continuous wave denoted by a sine wave (Pictured below) and may vary in signal strength (Amplitude) or frequency (time).

Analog signal represented as a sine wave.

Digital signals: Digital signals can have limited number of values. In a digitals signals the information has in discrete form a digital signal a must for computer processing is described as using binary (Os and 1s), and therefore, cannot take on any fractional values.

Periodic signal: A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one full pattern is called a cycle. A non periodic signal changes without exhibiting.