CHAPTER 1 Communication Systems Concepts 352 شال Banan Mahmoudi Introduction By the end of this course, we should be able to: – Understand basics of communication systems – Understand basics of communication channels – Understand types of data and signals – Differentiate among the basics of some communication techniques – Apply some communication techniques in solving simple problems Introduction • Telecommunication is the transmission of information, over significant distances, for the purpose of communication. Introduction • In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded drumbeats, lung-blown horns, or sent by loud whistles, for example. • In the modern age of electricity and electronics. Telecommunications now also includes the use of electrical devices such as telegraphs, telephones, and teletypes, the use of radio and microwave communications, as well as fiber optics and their associated electronics, plus the use of the orbiting satellites and the Internet. 1. BASIC TELECOMMUNICATION SYSTEM • The fundamental purpose of a communication system is the exchange of data between two parties. A very simple telecom system is shown in Figure 1. 1. BASIC TELECOMMUNICATION SYSTEM • At the transmitting end, there will be a source that generates the data. • and a transducer that converts the data into an electrical signal. • The signal is sent over a transmission medium. • At the receiving end, the transducer again converts the electrical signal into data and is given to the destination . 1. BASIC TELECOMMUNICATION SYSTEM • For example, if two people want to talk to each other using this system, the transducer is the microphone that converts the sound waves into equivalent electrical signals. At the receiving end, the speakers convert the electrical signal into acoustic waves. Similarly, if video is to be transmitted, the transducers required are a video camera at the transmitting side and a monitor at the receiving side. The medium can be copper wire. 2. TYPES OF COMMUNICATION • Based on the requirements, the communications can be of different types: I. Point-to-point communication: In this type, communication takes place between two end points. For instance, in the case of voice communication using telephones, there is one calling party and one called party. Hence the communication is point-to-point. II. Point-to-multipoint communication: In this type of communication, there is one sender and multiple recipients. For example, in voice conferencing, one person will be talking but many others can listen. 2. TYPES OF COMMUNICATION III. Broadcasting: In a broadcasting system, there is a central location from which information is sent to many recipients, as in the case of audio or video broadcasting. In a broadcasting system, the listeners are passive, and there is no reverse communication path. IV. Simplex communication: In simplex communication, communication is possible only in one direction. There is one sender and one receiver; the sender and receiver cannot change roles. 2. TYPES OF COMMUNICATION V. Half-duplex communication: Half-duplex communication is possible in both directions between two entities (computers or persons), but one at a time. A walkie-talkie uses this approach. The person who wants to talk presses a talk button on his handset to start talking, and the other person's handset will be in receive mode. When the sender finishes, he terminates it with an over message. The other person can press the talk button and start talking. These types of systems require limited channel bandwidth, so they are low cost systems. 2. TYPES OF COMMUNICATION VI. Full-duplex communication: In a full-duplex communication system, the two parties—the caller and the called—can communicate simultaneously, as in a telephone system. However, note that the communication system allows simultaneous transmission of data, but when two persons talk simultaneously, there is no effective communication! The ability of the communication system to transport data in both directions defines the system as full-duplex. 3. A Communications Model • We begin our study with a simple model of communications, illustrated by the block diagram in Figure l.la. A Communications Model • The key elements of the model are : 1. Source. This device generates the data to be transmitted; examples are telephones and personal computers. 2. Transmitter - Transforms and encodes the information to produce electromagnetic signals that can be transmitted across transmission medium. A Communications Model 3. Transmission system : - transmission line - Complex network 4. Receiver : accepts the signals from the transmitter system and converts it into a form that can be handled by the destination device. 5. Destination. Takes the incoming data from the receiver. Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control 4. DATA COMMUNICATIONS • The following is a simplified data communications model. • Suppose that the input device and transmitter are components of a personal computer. • The user of the PC wishes to send a message m to another user. • The user activates the electronic mail package on the PC and enters the message via the keyboard (input device). • The character string is buffered in main memory (bit stream). • The PC is connected to some transmission medium, such as local network or a telephone line, by an I/O device (transmitter) such as modem. • The input data are transferred to the transmitter as a sequence of voltage shifts. 4. DATA COMMUNICATIONS • The transmitter is connected directly to the medium and converts the incoming stream into a signal suitable for transmission. • The transmitted signal is subject to some impairment before it reaches the receiver. Thus, the received signal may differ from the transmitted signal. • The receiver will attempt to estimate the original signal, based on the received signal and its knowledge of the medium, producing a sequence of bits. • These bits are sent to the output personal computer, where they are buffered in memory. • In many cases, the destination system will attempt to determine if an error has occurred and, if so, try to correct it. • The data are then presented to the user on screen or printer. TRANSMISSION OF INFORMATION: • The basic building block of any communications system facility is the transmission line. • The transmission line must have the required capacity, acceptable reliability and minimum cost. TRANSMISSION OF INFORMATION: • The most commonly used transmission media are: Twisted-pair lines, Coaxial cables, Optical fiber cables, Satellite microwave. TRANSMISSION OF INFORMATION: • The data rates that can be achieved and the rate at which errors can occur depend on: The nature of the signal Type of the medium. Communication techniques: Inserting a signal on the medium Encode the information into an electromagnetic signal Interface between a device and the transmission medium Controlling the flow of information Recovering information from its loss or corruption TRANSMISSION EFFICIENCY: • The cost is a very important factor in any communication system. The amount of information that can be carrier over a given resource must be maximized by two ways: Multiplexing Compression NETWORKS • A computer network, often simply referred to as a network, is a collection of computers and devices interconnected by communications channels that facilitate communications among users and allows users to share resources. • Networks are often classified as local area network (LAN) wide area network (WAN). NETWORKS • A local area network (LAN) is a computer network that connects computers and devices in a limited geographical area such as home, school, computer laboratory or office building. The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased line. NETWORKS • A wide area network (WAN) is a computer network that covers a broad area. WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet.