Data Communications Introduction PDF

DATA COMMUNICATIONS BIT 243 LECTURER: ISAIAH BREW WEEK 1 INTRODUCTION TO COMPUTER NETWORKS The world of computer networks and data communications is a surprisingly vast and increasingly significant field of study. It is virtually impossible for the average person on the str...

DATA COMMUNICATIONS BIT 243 LECTURER: ISAIAH BREW WEEK 1 INTRODUCTION TO COMPUTER NETWORKS The world of computer networks and data communications is a surprisingly vast and increasingly significant field of study. It is virtually impossible for the average person on the street to spend 24 hours without directly or indirectly using some form of computer network. We define data communications as the transfer of digital or analog data using digital or analog signals. Once created, these analog and digital signals then are transmitted over conducted media or wireless media. INTRODUCTION TO COMPUTER NETWORKS INTRODUCTION TO COMPUTER NETWORKS Workstations are personal computers (or microcomputers, desktops, laptops, or tablets, to name a few) or smart phones (or other handheld devices) where users reside Servers are the computers that that provide some kind of service to other computers connected to them directly or indirectly. Switches are the collection points for the wires that interconnect the workstations Routers are the connecting devices between local area networks and wide area networks such as the Internet Modems are any of a class of electronic devices that convert digital data signals into modulated analog signals suitable for transmission over analog telecommunications circuits. INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS INTRODUCTION TO COMPUTER NETWORKS EXAMPLES OF COMMUNICATION NETWORKS NETWORK ARCHITECTURES Now that you know the different types of networks, we need a framework to understand how all the various components of a network interoperate. When someone uses a computer network to perform a function, many pieces come together to assist in the operation. A network architecture or communications model, places the appropriate network pieces in layers. The layers define a model for the functions or services that need to be performed. Each layer in the model defines what services either the hardware or software (or both) provides. These are frameworks that govern how data is sent and received over a network. The two most common architectures known today are the Transmission Control Protocol / Internet Protocol(TCP/IP) suite and the Open Systems Interconnection (OSI) model. The TCPI/IP protocol suite is a working model (currently used on the Internet), while the OSI model (originally designed to be a working model) has been relegated to a theoretical model. NETWORK ARCHITECTURES THE TCP/IP PROTOCOL SUITE NETWORK ARCHITECTURES - THE TCP/IP PROTOCOL SUITE APPLICATION LAYER The top layer of the TCP/IP protocol suite, the application layer, supports the network applications and might in some cases include additional services such as encryption or compression. The TCP/IP application layer includes several frequently used applications: Hypertext Transfer Protocol (HTTP) to allow Web browsers and servers to send and receive World Wide Web pages Simple Mall Transfer Protocol (SMTP) to allow users to send and receive electronic mail File Transfer Protocol (FTP) to transfer files from one computer system to another Telnet to allow a remote user to log in to another computer system Simple Network Management Protocol (SNMP) to allow the numerous elements within a computer network to be managed from a single point NETWORK ARCHITECTURES - THE TCP/IP PROTOCOL SUITE TRANSPORT LAYER The TCP/IP transport layer commonly uses TCP to maintain an error- free end-to-end connection. To maintain this connection, TCP includes error control information in case one packet from a sequence of packets does not arrive at the final destination, and packet sequencing information so that all the packets stay in the proper order. We say that the transport layer performs end-to-end error control and end-to-end flow control. TCP is not the only possible protocol found at the TCP/IP transport layer. User Datagram Protocol (UDP) is an alternative also in the TCPI/IP protocol suite. NETWORK ARCHITECTURES - THE TCP/IP PROTOCOL SUITE NETWORK LAYER TCP/IP's network layer, sometimes called the Internet layer or IP layer, transfers data within and between networks. The Internet Protocol (IP) is the service that prepares a packet of data to move from one network to another on the Internet or within a set of networks. As this layer sends the packet from node to node, it generates the network addressing necessary for the system to recognize the next intended receiver. To choose a path through the network, the network layer determines routing information and applies it to each packet or group of packets. NETWORK ARCHITECTURES - THE TCP/IP PROTOCOL SUITE NETWORK ACCESS LAYER The next lower layer of the TCP/IP protocol suite is the network access layer. The network access layer prepares a data packet (called a frame at this layer) for transmission from the user workstation to a router sitting between the local area network and the Internet. This frame contains an identifier that signals the beginning and end of the frame, as well as spaces for control information and address information. In addition, the network access layer can incorporate some form of error detection. If an error occurs during transmission, the network access layer is responsible for error control, which it does by informing the sender of the error. The network access layer might also perform flow control. In a large network where the data hops from node to node as it makes its way across the network, flow control ensures that one node does not overwhelm the next node with too much data. Note that these network access operations are quite similar to some of the transport layer operations. The primary difference is that the transport layer performs its operations only at the endpoints, while the network access layer performs its operations at every stop (node) along the path. This is also the last layer before the data is handed off for transmission across the medium. The network access layer is often called the data link layer. Transmission can be over a physical wire, or it can be a radio signal transmitted through the air. To perform this transmission of bits, the network access layer handles voltage levels, plug and connector dimensions, pin configurations, and other electrical and mechanical issues NETWORK ARCHITECTURES – OSI PROTOCOL SUITE NETWORK ARCHITECTURES – OSI PROTOCOL SUITE The OSI model was designed with seven layers. Note further the relationship between the five layers of the TCP/IP protocol suite and the seven layers of the OSI model. The top layer in the OSI model is the application layer, where the application using the network resides. This OSI layer is similar to the application layer in the TCP/IP protocol suite. The next layer in the OSI model, the presentation layer, performs a series of miscellaneous functions necessary for presenting the data package properly to the sender or receiver. For example, the presentation layer might perform ASCII-to-non-ASCII character conversions, encryption and decryption of secure Documents, and the compression of data into smaller units. There is no separate presentation layer in the TCP/IP protocol suite. NETWORK ARCHITECTURES – OSI MODEL The session layer is another layer that does not exist in the TCP/IP protocol suite and is responsible for establishing sessions between users. It also can support token management, a service that controls which user's computer talks during the current session by passing a software token back and forth. Additionally, the session layer establishes synchronization points, which are backup points used in case of errors or failures. For example, while transmitting a large document such as an electronic book, the session layer might insert a synchronization point at the end of each chapter. If an error occurs during transmission, both sender and receiver can back up to the last synchronization point (to the beginning of a previously transmitted chapter) and start retransmission from there. Many network applications do not include a specific session layer and do not use tokens to manage a conversation. If they do, the "token" is inserted by the application layer, or possibly the transport layer, instead of the session layer. Likewise, if network applications use synchronization points, these points often are inserted by the application layer. NETWORK ARCHITECTURES – OSI MODEL The fourth layer in the OSI model, the transport layer, operates in the same way as the transport layer of the TCPIIP protocol suite. It ensures that the data packet that arrives at the final destination is identical to the data packet that left the originating station. The network layer of the OSI model is similar to the network layer of the TCP/ IP protocol suite and is responsible for getting the data packets from router to router through the network. The data link layer, similar to TCP/IP's network access layer, is responsible for taking data from the network layer and transforming it into a frame. The bottom layer in the OSI model-the physical layer-handles the transmission of bits over a communications channel. This layer is essentially identical to the physical layer of the TCP/IP protocol suite. NETWORK ARCHITECTURES – OSI MODEL VERSUS TCP/IP MODEL NETWORK ARCHITECTURES – Logical and physical connections An important concept to understand with regard to The physical connection is the only direct the layers of a communication model is the lines of connection between sender and receiver, and is at communication between a sender and a receiver. the physical layer, where actual ones (1s) and Notice the dashed lines between the sender's and zeroes (0)s-the digital content of the message-are receiver's application layers, transport layers, transmitted over wires or airwaves. network layers, and network access layers. No data flows over these dashed lines. Each dashed line indicates a logical connection. A logical connection is a nonphysical connection between sender and receiver that allows an exchange of commands and responses. The sender's and receiver's transport layers, for example, share a set of commands used to perform transport-type functions, but the actual information or data must be passed through the physical layers of the sender and receiver, as there is no direct connection between the two transport layers. Without a logical connection, the sender and receiver would not be able to coordinate their functions. FUNDAMENTALS OF DATA AND SIGNALS Data and signals are two of the basic building blocks of any computer network. It is important to understand that the terms "data" and "signal" do not mean the same thing and that, in order for a computer network to transmit data, the data must first be converted into the appropriate signals. The one thing data and signals have in common is that both can be in either analog or digital form. FUNDAMENTALS OF DATA AND SIGNALS Data is entities that convey meaning within a computer or computer system. Common examples of data include: A computer file of names and addresses stored on a hard disk drive The binary 1s and 0s of music stored on a CD or hard drive. The dots (pixels) of a photograph that has been digitized by a digital camera and stored on a memory stick The digits 0 through 9, which might represent some kind of sales figures for a business. FUNDAMENTALS OF DATA AND SIGNALS Signals are the electric or electromagnetic impulses used to encode and transmit data. It is the physical form that data takes as it travels over a transmission medium. Common examples of signals include: A transmission of a telephone conversation over a telephone line A live television news interview from Europe transmitted over a satellite system The downloading of a Web page as it is transferred over the cable between your Internet service provider and your home computer FUNDAMENTALS OF DATA AND SIGNALS Analog data and analog signals are represented as continuous waveforms that can be at an infinite number of points between some given minimum and maximum. By convention, these minimum and maximum values are presented as voltages. The most common example of analog data is the human voice. For example, when a person talks into a telephone, the receiver in the mouthpiece converts the airwaves of speech into analog pulses of electrical voltage. FUNDAMENTALS OF DATA AND SIGNALS One of the primary shortcomings of Unfortunately, noise itself occurs as an analog data and analog signals is how analog waveform; and this makes it difficult it is to separate noise from challenging, if not extremely difficult, the original waveform. to separate noise from an analog Noise is unwanted electrical or waveform that represents data. electromagnetic energy that degrades the quality of signals and data. Because noise is found in every type of data and transmission system, and because its effects range from a slight hiss in the background to a complete loss of data or signal, it is especially important that noise be reduced as much as possible.. FUNDAMENTALS OF DATA AND SIGNALS Digital data and digital signals are composed of a discrete or fixed number of values, rather than a continuous or infinite number of values. Digital data takes on the form of binary 1s and 0s. But digital signals are more complex. What happens when you introduce noise into digital signals? As stated earlier,noise has the properties of an analog waveform and, thus, can occupy an infinite range of values; digital waveforms occupy only a finite range of values. When you combine analog noise with a digital waveform, it is fairly easy to separate the original digital waveform from the noise. FUNDAMENTALS OF DATA AND SIGNALS If the amount of noise remains small enough that the original digital waveform can still be interpreted, then the noise can be filtered out, thereby leaving the original waveform. In the simple example in Figure, as long as you can tell a high part of the waveform from a low part, you can still recognize the digital waveform. If, however, the noise becomes so great that it is no longer possible to distinguish a high from a low, as shown in Figure, then the noise has taken over the signal and you can no longer understand this portion of the waveform. FUNDAMENTALS OF DATA AND SIGNALS The ability to separate noise from a digital waveform is one of the great strengths of digital systems. When data is transmitted as a signal, the signal will always incur some level of noise. In the case of digital signals, however, it is relatively simple to pass the noisy digital signal through a filtering device that removes a significant amount of the noise and leaves the original digital signal intact.