data communication

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Overview of Data Communications and Computer Networks

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1.1 Data Communication Data Communication is the exchange of data between two devices via some form of transmission medium such as a wire cable

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Components of data communication Figure 1.1 Five components of data communication

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Components of a data communications system l. 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. 3. Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television. 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

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Communication between two devices can be - Simplex half-duplex full-duplex Data Flow

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Simplex In simplex mode, the communication is unidirectional. 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 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

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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 ln a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems

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Full-duplex In full-duplex mode both stations can transmit and receive simultaneously In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. 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

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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.

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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 communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections: point-to-point and multipoint

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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. When we change television channels by infrared remote control, we establish a point-to-point connection between the remote control and the television's control system Point-to-Point Connection

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A multipoint connection is one in which more than two specific devices share a single link 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 timeshared connection. Multipoint Connection

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Categories of topology The term physical topology refers to the way in which a network is laid out physically. It is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, & ring

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Fully connected mesh topology (for five devices) In a mesh topology, 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

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A mesh offers several advantages over other network topologies. Use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. A mesh topology is robust. If one link becomes unusable, it does not harm the entire system. There is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent unauthorized users from gaining access to messages. Finally point-to-point links make fault identification and fault isolation easy. The main disadvantage of a mesh are related to the amount of cabling and the number of I/O ports required. Every device must be connected to every other device, installation and reconnection are difficult The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office

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Star topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a HUB. The devices are not directly linked to one another. 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 relays the data to the other connected device. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to he housed Robustness If one link fails, only that link is affected. All other links remain active. One big disadvantage is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead

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Bus topology The bus topology is multipoint. One long cable acts as a backbone to link all the devices in a network Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core

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Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh Disadvantages include difficult reconnection and fault isolation a fault or break in the bus cable stops all transmission

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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

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Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology

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