Data Communications Concepts PDF

1.1 4 1-1 DATA COMMUNICATIONS  Telecommunication: means communication at a distance. The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance (tele is Greek...

1.1 4 1-1 DATA COMMUNICATIONS  Telecommunication: means communication at a distance. The term telecommunication, which includes telephony, telegraphy, and television, means communication at a distance (tele is Greek for "far").  Data: refers to information presented in whatever form is agreed upon by the parties creating and using the data.  Data communications: are the exchange of data between two devices via some form of transmission medium such as a wire cable.  Topics discussed in this section  Components  Data Representation  Data Flow COMMUNICATION SYSTEM  For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs).  The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter. COMMUNICATION SYSTEM CHARACTERISTICS  Delivery: System must deliver data to the correct destination.  Accuracy: The system must deliver the data accurately  Timeliness: The system must deliver data in a timely manner. 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 (real-time transmission).  Jitter: The variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. COMMUNICATION SYSTEM CHARACTERISTICS Figure 1.1 Five components of data communication COMPONENTS  Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.  Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.  Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. COMPONENTS  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.  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  Text  Numbers  Images  Audio  Video DATA FLOW Figure 1.2 Data flow (simplex, half-duplex, and full-duplex) DATA FLOW  SIMPLEX:  The communication is unidirectional, as on a one-way street.  Ex: Keyboards and traditional monitors are examples of simplex devices  HALF-DUPLEX:  Each station can both transmit and receive, but not at the same time.  Like a one-lane road with traffic allowed in both directions.  Ex: Walkie-talkies DATA FLOW  FULL-DUPLEX:  Both stations can transmit and receive simultaneously  Like a two-way street with traffic flowing in both directions at the same time  Ex: telephone network 1-2 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.  Topics discussed in this Section:  Distributed Processing  Network Criteria  Physical Structures  Network Models  Categories of Networks  Interconnection of Networks: Internetwork DISTRIBUTED PROCESSING  Most networks use distributed processing, in which a task is divided among multiple computers.  Instead of using one large computer to perform a task, several smaller computers are used to perform the task. NETWORK CRITERIA  Performance:  Performance can be measured in many ways  Transit time  Response time  Throughput  Delay  Reliability:  Measured by accuracy of delivery, the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe.  Security:  Protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses. TYPES OF CONNECTION Figure 1.3 Types of connections: point-to-point and multipoint TYPE OF CONNECTION  Point-to-Point:  A point-to-point connection provides a dedicated link between two devices.  The entire capacity of the link is reserved for transmission between those two devices.  Example: Point-to-point connection between the remote control and the television's control system. TYPE OF CONNECTION  Multipoint:  A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link  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.  If users must take turns, it is a timeshare connection. PHYSICAL TOPOLOGY  Refers to how a network is laid out physically.  One or more devices connect to a link; two or more links form a topology.  The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. Figure 1.4 Categories of topology  MESH TOPOLOGY  Here, every device has a dedicated point-to- point link to every other device.  The term dedicated means that the link carries traffic only between the two devices it connects. Figure 1.5 A fully connected mesh topology (five devices) MESH TOPOLOGY Number of physical links in a fully connected mesh network Let us assume we have n nodes; each node must be connected to every other node. Node 1 must be connected to (n-1) nodes Node 2 must be connected to (n-1) nodes … Node n must be connected to (n-1) nodes. Therefore, we need n(n - 1) physical links. Duplex-mode links: n(n -1)/2 Every device on the network must have (n-1) input/output ports to be connected to the other (n-1) stations. MESH TOPOLOGY  A mesh offers several advantages over other network topologies.  Advantages:  Eliminating the traffic problems: the use of dedicated links guarantees that each connection can carry its own data load  Robustness: If one link becomes unusable, it does not incapacitate entire system.  Privacy or security: When every message travels along a dedicated line, only the intended recipient sees it.  Easier fault identification and fault isolation MESH TOPOLOGY  Disadvantages:  High amount of cabling and the number of I/O ports  Installation and reconnection difficulty: n(n-1)/2 links  Hardware expensive can be highly: Too many links  Exp: Connection of telephone regional offices in which each regional office needs to be connected to every other regional office.  STAR TOPOLOGY  Here, each device has a dedicated point-to-point link to a central controller, usually called a hub.  The devices are not directly linked to one another.  Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then Figure 1.6 A star topology connecting four relays the data to the other stations connected device STAR TOPOLOGY  Advantages:  Less expensive than Mesh topology: each device needs only one link and one I/O port  Robustness: If one link fails, only that link is affected. All other links remain active.  Easier fault identification and fault isolation STAR TOPOLOGY  Disadvantages:  If the hub goes down, the whole system is dead.  Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).  Exp: The star topology is used in local-area networks (LANs) BUS TOPOLOGY  A bus topology is multipoint.  One long cable acts as a backbone to link all the devices in a network  Bus topology was the one of the first topologies used in the design of early LANs.  Relatively less used BUS TOPOLOGY Figure 1.7 A bus topology connecting three stations BUS TOPOLOGY  Nodes are connected to the bus cable by drop lines and taps.  Drop line: Connection running between the device and the main cable.  Tap: Connector between drop line and main cable.  As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. BUS TOPLOGY  Advantages:  Ease of installation  Less cabling than mesh or star topologies  Disadvantages:  Difficult reconnection and fault isolation  Difficult to add new devices  Degradation in quality  A fault or break in the bus cable stops all transmission RING TOPLOGY Figure 1.8 A ring topology connecting six stations RING TOPOLOGY  In a ring topology, each device has a dedicated point-to- point connection with only the two devices on either side of it.  A signal is passed along the ring in one direction, from device to device, until it reaches its destination.  Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. RING TOPOLOGY  Easy to install and reconfigure.  Each device is linked to only its immediate neighbors (either physically or logically).  To add or delete a device requires changing only two connections.  Fault isolation is simplified. HYBRID TOPOLOGY Figure 1.9 A hybrid topology: a star backbone with three bus networks NETWORK MODELS  The OSI standard  The TCP/IP model  1.58 READING  Chapter 1 in textbook  1.59 EXERCISES  Identify the five components of a data communications system.  A color image uses 16 bits to represent a pixel. What is the maximum number of different colors that can be represented?  Assume six devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device?  You have two computers connected by an Ethernet hub at home. Is this a LAN, a MAN, or a WAN? Explain your reason.  Discuss the consequences if a connection fails in each of the following networks:  Five devices arranged in a mesh topology  Five devices arranged in a star topology (not counting the hub)  Five devices arranged in a bus topology  Five devices arranged in a ring topology REFERENCES  These lecture notes are based on contributions from  Dr. Ahmed Al-Gindy  Dr. Kuljeet Kaur THANKS!