09-05-2013, 04:07 PM
COMPUTER NETWORK
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Introduction XE
The meaning of computers and communication has had a profound influence on the way computer systems are organized. The concept of the "computer center" as a room with large computer to which users bring their work for processing is now totally obsolete. The old model of a single compute servicing all of the organization’s computational needs has been replaced by one in which large number of separate but interconnected computers do the job. These systems are called "computer Network".
Component of a computer network
Communications functions of computer network are implemented by sing many hardware (physical) and software (logical) components in a computer network.
Hardware component
o Computer peripheral
o Front end processor
o Terminals
o Modems, concentrators, Multiplexers
o Transmission Media
o Data switching Equipment etc.
Bus topology
A one cable LAN in which all workstations are connected to a single cable is called BUS topology. On bus network, all workstations hear all transmission on the cable as shown in figure. Each workstation selects those transmissions addressed to it based on the address information contained in the transmission. A bus network is the simples and most common LAN topology.
The Main features of Bus Topology are as follows:
1. Short cable length and simple wiring layout
2. Bus architecture is simple and more reliable
3. Expandability and reconfigurations of a bus network is easy.
4. Since the control of the network is not centralised, detection of fault is performed at many points in the network. Hence fault diagnosis is difficult.
Ring topology
A LAN topology in which each workstation is connected to tow other workstations, forming a loop or ring. Data is sent from workstation to workstation around the loop in the same direction. Each computer acts as a repeater by resending messages to other computers. Rings have a predictable response time that is determined by the computers. Figure shows the nodes/terminals connected in Ring topology.
Star topology
A LAN topology in which all workstation are wired to a central workstation that establishes, maintains and breaks connections between the workstations. The center of a stat is called the hub. The advantage of a star is that it makes isolating a problem node easy. However if error occurred in central network the entire network collapses. Fig shows a star network.
Mesh topology
A Mesh topology has point to point connections between every device in the network. Each device requires an interface for every other device on the network. In addition unless each station frequently sends signals to all the other station excessive amount of network bandwidth is wasted. However, mesh networks are extremely fault tolerant and each link provides guaranteed capacity.
The Main features of mesh topology are as follows.
1. Ease of installation
2. Ease of trouble shooting
3. Ease of reconfiguration
Network model
OSI Model
In 1970s, International Organization for standardization was created the OSI (Open System Interconnection) model to connect different computers in single group.
It describes how data and network information are communicated form applications on one computer. Through the network media to an application on another computer. The OSI reference model breaks this approach into LAYER. One of the greatest functions of the OSI specification is to assist in data transfer between disparate hosts.
It is not physical, OSI is only a set guideline that application developers can use to create and implement applications that run on a network. OSI model is divided into two groups and it has seven different layers. Here. Top three layers define how the applications within the end stations will communicate with each other & with users. The bottom four layers define how data is transmitted end to end
The Network Layer
The network layer is responsible for routing through an internet work and for network addressing. Router, a physical device is specified at the network layer and provides the routing services in NETWORK.
Any sender computer transmits data to receiver computer into packet format, a packet is received on a router interface, and the destination IP address is checked. If the packet is not destined for the router, then the router will lookup the destination network address in the routing table once interface is chosen, the packet will be sent to the interface. If the entry for the destination network is not found in the routing table, the router drops the packet.
The Data Link Layer
Data Link layer control the messages are delivered to the proper device and translate messages form the network layer into bits for the physical layer to transmit. It convert messages into data frames and adds a customized header contain the hardware destination and source address.
The data link layer transfers raw data between the physical layer and the network layer. The network interface card represents the data-link layer. The data link layers primary function is to prevent data corruption within the physical layer.
The physical layer manages raw data as bits. The data link layer formats or transforms raw binary data into something meaningful to the network layer - normally frames of data. The data link layer also accepts information form the network layer and translates the data into correct binary format for the underlying physical layer. In all cases, the data link layer ensures that binary transmissions between network hosts are free from errors.
The Physical Layer
The physical Layer transmits data through the network’s communication channel. The physical layer includes the physical elements needed to accomplish this function. A, the network’s transmission lines the cables that connect all the computers on the network - are part of the physical layer. Data transmission methods, including control signals and timing are also part of the physical layer. As such the physical layer also includes network technologies that define parameters for data transmission.
The physical layer determines the mechanical and electrical properties of the network communication channel, as well as procedural details related to these characteristics. For example the physical layer determines how may electrical pins or wires network connections uses, what type of cable a transmission cable uses and what characteristics transmission lines have. In a similar fashion, the physical layer contains details about a network's topology.
Local area network
A LAN is a local area network. Within a single building or similar geographical space. A LAN enables you to connect a group of personal computers together. People using the networked computers can share information.
A LAN can do virtually everything a mainframe computer or minicomputer can do, but at a much lower cost. People can share computer resources and information, and they can work together on projects and tasks that require coordination and communication, even though those users may not be physically close. In addition, if the network crashes, a networked user may very well be able to continue working because that person’s personal computer is still functioning.
A network represents a sizable investment and should be one that can grow with computer applications. As we analyze the features of LOCAL AREA NETWORK, we need to consider their ability to handle not only the present need but future needs as well.