Data Communications and Networking: Basic Definitions. Data Communication is the exchange of data (in the form of 0’s and 1’s) between two devices via some form of transmission medium (such a cable) Data Communication: Local or Remote A data communication is considered local if the communicating devices are in the same building or in similarly restricted geographical area.

A data communication is considered remote if the devices are farther apart. A communication system is a set of devices (hardware and software) whose purpose is sharing data. The fundamental characteristics of a communication system are 1.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 or user.

Accuracy: The system must deliver data accurately. Data that have been altered in transmission and left uncorrected are unusable. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video, audio, voice data, 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. The following figure shows a basic model of a communication system

Data communication system components

Components of a Communication System Message: The message is the information (or data) to be transmitted. For example, text, numbers, pictures, sound, video, or any combination of these. Sender: The sender is the device that sends the data message.It can be a computer, telephone, video camera, and so on. Receiver: The receiver is the device that receives the message. Medium (Channel): The transmission medium is the physical path by which a message travels from the sender to receiver. For example twisted pair wire, coaxial cable, fiber-optic cable, radio waves (terrestrial or satellite microwaves).

The medium defines the speed (the data rate) at which data can travel through a connection. For example 100 megabits per second. Protocol: A protocol is a set of rules that govern data communication. It represents an agreement between the communicating devices. Network: A network is a set of devices (often referred to as nodes) connected by 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.

Line Configuration Line configuration refers to the way two or more communication devices attach to a link. A link is the physical communication pathway that transfers data from one device to another. There are two line configurations: 1.Point-to-Point 2.Multipoint A point-to-point line configuration provides a dedicated link between two devices. The entire capacity of the channel is reserved for transmission between those two devices.

Point-point connection

A multipoint line configuration 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 line configuration. If users must take turns, it is a time-shared line configuration. Topology The topology of a network is the geometric representation of the relationship of all links and linking devices to one other. Two or more devices connect to a link; two or more links form a topology.

Multipoint line configuration

There are five basic topologies: 1.Mesh 2.Star 3.Tree 4.Bus 5.Ring Note: A consideration when choosing a topology is the relative status of the devices to be linked. Two relations are possible: peer-to-peer, where the devices share the link equally, and primary- secondary, where one device controls traffic and the others must transmit through it.

Topology Purpose Mesh Ring Bus Peer-to-Peer Star Tree Bus Primary-Secondary Mesh Topology 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.

Mesh topology

Advantages The use of a dedicated links guarantees that each connection can carry its own 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 incapacitate the entire system. Privacy. When every message sent travels along a dedicated link, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages. Point-to Point links make fault identification and fault isolation easy.

Disadvantages A big amount of cabling and a big number of I/O ports required, i.e, the hardware required can be very expensive. Because every device must be connected to every other device, installation and reconfiguration are difficult. For these reasons a mesh topology is usually implemented in a limited fashion. For example, as a backbone connecting the main computers to a hybrid network that can include several other topologies.

Star Topology In a star topology, each device has a dedicated point-to- point link only to a central controller, usually called a hub. A star topology does not allow direct traffic between devices. 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. Advantages It is easy to install and reconfigure The star topology is robust. If one link fails, only that link is affected. This factor also lends itself to easy fault identification and fault isolation.

Star topology

Disadvantages A big amount of cabling and a big number of I/O ports required, i.e, the hardware required can be very expensive. Note: Star topology requires far less cable than Mesh topology. Tree Topology A tree topology is a variation of a star. As in the star, nodes in a tree topology are linked to a central hub that controls the traffic to the network. The majority of devices connect to a secondary hub that in turn is connected to a central hub.

The central hub in a tree is an active hub. An active hub contains a repeater, which is a hardware device that regenerates the received bit patterns before sending out. Repeating improves the transmission quality, and increases distance that a signal can travel.

The advantages and disadvantages of a tree topology are generally the same as those of a star. A good example of tree topology can be seen in cable TV technology where the main cable from the main office is divided into main branches and each branch is divided into smaller branches and so on. The hubs are used when a cable is divided. Bus Topology In a bus topology one long cable acts as a backbone to link all the devices in the 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. When a signal travels along the backbone, some of its energy is transformed into heat.Therefore, it becomes weaker and weaker the farther it has to travel. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. Advantages Easy to install Bus topology uses less cabling than mesh, star, or tree topologies.

Bus topology

Disadvantages Difficult reconfiguration and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add a new devices. A fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. Ring Topology In a ring topology, each device has a dedicated point-to- point line configuration only with the two devices on either side of it.

Advantages Easy to install and reconfigure. To add or delete a device require only two connections. Easy fault isolation Disadvantages A break in the ring (such as a disabled station) can disable the entire network.

Hybrid Topologies Often a network combines several topologies as sub networks linked together in a larger topology.

Transmission Mode The term transmission mode is used to define the direction of signal flow between two linked devices. There three types of transmission modes: simplex, half- duplex and full-duplex Simplex:In simplex mode, the communication is unidirectional, as one-way street.

Examples: keyboards and traditional monitors are both examples of simplex devices. 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. Examples: Walkie-talkies and CB (citizen’s band) radios are both half-duplex systems.

Full-Duplex: In a full-duplex mode (also called duplex), both stations can transmit and receive simultaneously. Examples: the telephone network

Categories of Networks

LANs A local area network (LAN) is usually privately owned and links the devices in a single office, building or campus. Currently, LAN size is limited to a few kilometers. LANs are designed to allow resources be shared between personal computers or workstations. The resources to be shared can include hardware (e.g. a printer), software (e.g. an application program), or data.

In general, LANs use only one type of transmission medium. The most common LAN topologies are bus, ring and star. LANs have data rates in the 4 to 16Mbs range. Today, however, speeds are increasing and can reach 100Mbps with gigabit systems in development. Metropolitan Area Networks A metropolitan area network (MAN) is designed to extend over an entire city. A MAN may be wholly owned and operated by a private company, or it may be a service provided by a public company, such as a local telephone company.

MAN

Wide Area Network A Wide Area Network (WAN) provides long-distance transmission of data, voice, image, and video information over large geographical areas that may comprise a country, a continent, or even the whole world.