Data Communication Chapter # 1: By: Introduction William Stalling
Data Communication The exchange of data between two devices via some form of transmission medium such as cable wire. For data communications to occur, the communicating devices must be part of a communication system made up of combination of hardware (physical equipments) and software (programs).
Characteristics Depends upon following fundamental characteristics. 1. Delivery 2. Accuracy 3. Timeline 4. Jitter
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 and user. Accuracy The system must be deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. Timeline The system must deliver data in a timely manner. Data delivered late are useless. In 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.
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 30-s. If some of the packets arrive with 30- s delay and others with 40-s delay, an uneven quality in the video is the result.
Communication Process
Components Message The message is the information(data) to be communicated. Popular forms of information include text, numbers, and video. Sender The sender is the device that sends the data message. It can be a computer, workstation, telephone, video camera and so on. Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone, and so on.
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 wire and radio waves. Protocols 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 Japenese.
Simplified Communications Model - Diagram
Signal Conversion
Data Flow
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.
Half-Duplex In half-duplex mode, each station can both transmit and receive, and vice versa. The half-duplex mode is like a one-lane road with traffic allowed in both directions.
Full-Duplex In full-duplex mode(also called duplex), both stations can transmit and receive simultaneously. The full-duplex mode is like a two way street with traffic flowing in both directions share with 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 on the network. Distributed Processing Most networks use distributed processing, in which a task is divided among multiple computers. Instead of one single large machine being responsible for all aspects of a process, separate computers(usually personal computer or workstation) handle a subset.
Network Criteria A network must be able to meet a certain number of criteria. The most important of these are. Performance Reliability Security
Performance Performance can be measured in many ways. Including transit time and response time. The performance of a network depends on a number of factors, Including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software. o Transit Time Transit time is the amount of time required for a message to travel from one device to another. o Response time Response time is the elapsed time between an inquiry and a response.
Reliability In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network s robustness in a catastrophe. Security Network security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from branches and data losses.
Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Error detection and correction Security Network management
Transmission System Utilization Refers to the need to make efficient use of transmission facilities that are typically shared among a number of communicating devices. Various techniques (referred to as multiplexing) are used to allocate the total capacity of a transmission medium among a number of users.
Interface An interface is a shared boundary or connection between two dissimilar objects, devices or systems through which information is passed. The connection can be either physical or logical. They use of electromagnetic signals propagated over a transmission medium.
Signal Generation Signal Generation is required for communication. The properties of the signal, such as form and intensity, must be such that they are 1) Capable of being propagated through the transmission system. 2) Interpretable as data at the receiver.
Synchronization Synchronization between transmitter and receiver. The receiver must be able to determine when a signal begins to arrive and when it ends. It must also know the duration of each signal element. Exchange Management There are a variety of requirements for communication between two parties that might be collected under the term exchange management. If data are to be exchanged in both directions over a period of time, the two parties must cooperate.
Error Detection and Correction In all communications systems, there is a potential for error; transmitted signals are distorted to some extent before reaching their destination. Error detection and correction are required in circumstances where errors cannot be tolerated; this is usually the case with data processing system.
Physical Structures Before discussing networks, we need to define some network attributes. Type of Connection A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For visualization purposes, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time.
Point to Point A point to point connection provides a dedicated links between two devices. The entire capacity of the link is reserved between those two devices. Most point to point connections use an actual length of wire or cable to connect the two ends, but other options microwave or satellite links, are also possible. Multipoint A multipoint connections is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection.
Physical Topology The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of all the links and linking devices. There are four basic topologies. Bus Topology Star Topology Ring Topology Mesh Topology
Bus Topology A bus topology, is multipoint. One long cable acts as a backbone to link all the devices in network. Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and main cable. A tap is a connector that either specifies into the main cable.
Advantages 1) It is easy to set-up and extend bus network. 2) Cable length required for this topology is the least compared to other networks. 3) Bus topology costs very less. 4) Linear Bus network is mostly used in small networks. Good for LAN. Disadvantages 1) There is a limit on central cable length and number of nodes that can be connected. 2) Security is very low because all the computers receive the sent signal from the source. 3) It is difficult to detect and troubleshoot fault at individual station. 4) Maintenance costs can get higher with time.
Ring Topology Ring topology is a network topology in which the nodes or the computers are connected in a closed loop. Each node is connected to two other nodes and when the data is sent it travels across all nodes in one particular direction. Ring topology is used when there is heavy flow of data as it has greater capability to handle data and doesn t even require any central workstation to handle the data transmission.
Advantages 1) High performance delivered. 2) All nodes have equal opportunity to transmit the data. Disadvantages 1) If one node is disrupted then the whole network goes down. 2) It becomes difficult to add/remove nodes. 3) If more than one token is generated then it may cause ambiguity in the sending of both kinds.
Star Topology In Star topology every node (computer workstation or any other peripheral) is connected to central node called hub or switch. The switch is the server and the peripherals are the clients. Data from the source is first delivered to the hub and is then transferred to the other nodes. It is easy to add or remove nodes or workstations in this topology. Star topology gives better performance as data doesn t pass through every node unlike Bus topology. If a particular workstation or a node gets an error then the entire network is not affected. But if the central workstation or the hub goes down, then the entire network collapses.
Advantages 1) Easy to install and implement 2) Give better performance as messages doesn t pass through various nodes unlike Bus topology 3) Faulty nodes can be easily removed without affecting the other nodes in the loop Disadvantages 1) If the central hub fails then the whole network is disrupted 2) If more nodes are to be added then more cable would be required and this would increase the cost 3) Data transfer and capability depends on capacity of the central hub
Mesh Topology Mesh topology uses one of the two arrangements either Full Mesh topology or Partial Mesh topology. In Full Mesh topology each node is connected to every other node in the network. In Partial Mesh topology every node is not connected to each node in the network. Due to many interconnections much of the cable is required for implementation of mesh topology so it is quite expensive. However, it is reliable because if one node fails, rests of the nodes continue to work with each other. Since lot of cables is involved, mesh topology is quite expensive to implement. So its often coupled with star, ring or any other topology to form hybrid topology. WAN (Wide Area Network) like Internet uses Mesh network structure.
Advantages 1) It is efficient in a sense when one node fails, others continue to work without disruption. 2) Network can be easily expanded. 3) In one particular instance you can send the data from one node to many nodes. Disadvantages 1) It is quite expensive since a higher length of cable is required. 2) Implementation can be a very arduous task.
Network Models Computer networks are created by different entities. Standards are needed so that these heterogeneous networks can communicate with one an another. The two standards are the OSI (Open System Interconnections) model and the Internet model. Categories of Networks Today when we speak of networks, we generally referring to two primary categories: local-areanetworks and wide-area-networks.
Local Area Network A local area network (LAN) is a network that connects computers and other devices in a relatively small area, typically a single building or a group of buildings. Most LANs connect workstations and personal computers and enable users to access data and devices (e.g., printers and modems) anywhere on the network. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.
LANs can be characterized by their topology, protocols and media. Topology is the geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line. Protocols are the rules and encoding specifications for sending data. They also determine whether the network uses a peerto-peer or client/server architecture. The most common type of LAN is Ethernet. Media is what is used to connect the devices, i.e., twisted-pair copper wire, coaxial cables, fiber optic cables or radio waves.
Wide Area Network It is similar to a Local Area Network (LAN), but it's a lot bigger. Unlike LANs, WANs are not limited to a single location. Many wide area networks span long distances via telephone lines, fiber-optic cables, or satellite links. They can also be composed of smaller LANs that are interconnected. The Internet could be described as the biggest WAN in the world.
Metropolitan Area Network A metropolitan area network(man) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example of a MAN is the part of the telephone company network that can provide a high speed DSL line to the customer.