Network designers have developed general blueprints-usually called a network picture-that guide the design and implementation of network. Set of events for network design-a computer network must provide general, costrel, fair, and robust connectivity among a large number of computers. As if this rot enough, networks do not remain fixed at any single point in time, but must to accommodate changes in both the underlying technologies upon which they send and changes in the demands placed on them by application programming a network to meet these requirements is no small task.
The essential elements of a network architecture are as:
Digital transmission lines for the transfer of streams of binary information between equipment.
Exchange of frames of information between adjacent equipment; these frames contain delineation information as well as check bits for error control. Medium access control procedures to regulate the transmission of frames from the multiple user's to a shared broadcast medium.
Addresses to identify points of attachment to a network or internet work. Exchange of packets of information between packet switches in a network: Routing tables in packet switches are used to select the path of each packet as it traverses the network.
Dynamic calculation of routing tables at the packet switches in changes in network traffic and topology.
• Congestion control mechanisms may be used to prevent congestion network.
• Internetworking provides connectivity across multiple, possibly networks by using gateways or routers.
• Segmentation and reassembly of messages into packets at the in egress from the network. End-to-end error recovery mechanism reliable transfer across a network or internetwork.
• A multiplicity of applications that build on the transfer of message computers.
The above network elements can be organized in a layered fashion, builds on top of another. In next section, we will discuss open systems inter (OSI) architecture.
OPEN SYSTEMS AND OSI MODEL
A set of protocols that would allow any two different systems to c regardless of their underlying architecture is called an open system. I Organization for Standardization (ISO) has addressed the problem of al: systems to communicate by developing the Open Systems Interconnection model as a computer protocol architecture and as a framework or eve standards.
OSI guidelines specify:
How network devices contact each other, and how devices us: protocols s commumcate
How work device knows when to transmit or not to transfer to the physical network devices are arranged and how t methods to ensure that network transmissions are received core network devices maintain a consistent rate of data flow?
Layers of OSI Model
The OSI model consists of the following 7 layers:
1.Application layer
2. Presentation layer
3. Session layer
4. Transport layer
5. Network layer
6. Data link layer
7 . Physical layer
The OSI model divides the basic communication functions into s shown in Figure Here it is assumed that application A in on communicating with application B in another computer. Applicat communicate using an application layer (Layer 7) protocol.
The purpose of the application layer is to provide user services such as electronic mail, file transfers, remote job entry, and resource allocation. Application layer protocols have been developed for file transfer, e-mail, network management and other applications.
The presentation layer is responsible for presenting data in the format the user can understand. For example, suppose two different computers use different numeric and character formats. The presentation layer translates data from one representation to another and insulates the user from the differences so that the user can exchange information, not raw bit steams. measures. It may encrypt data before handing it to the lower layers for transfer. presentation layer at the other end would decrypt the data after receiving it. The session layer allows applications on two different computers to establish session, or logical connection. For example, a user may log on to a remote system may communicate by alternately sending and receiving messages. The session 1 concerns file management and overall networking functions. Access availability system time allocations are included at this layer. Session layer protocols provide method by which presentation sessions can organize, synchronize, and manage transfers of information between themselves.
The transport layer is responsible to ensure that data is sent reliably from sending node to the destination node (computer). Transport layer provides end-to-end error recovery and flow control.
The network layer deals with routing strategies, which are responsible delivery of a packet from source to destination. For example, if two systems connected in a bidirectional ring network, there are two paths between any two points Should an entire message follow the same route or should part of it be transfer independently?
If two systems are connected to the same link, there is usually no need for network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for network layer to accomplish source-to-destination delivery. The network layer controls the communication subset, the collection of transmission media and switch elements required for routing and data transmission.
The data link layer provides for the transfer of frames (blocks of informal across a transmission link that directly connects two nodes. The data link layer recognizes a format. Data are often transmitted in the frames, which consist of a group of bytes organized according to a specified format. Data link layer uses error detection and correction techniques to ensure that a transmission contains no errors. It also flow control techniques, which control how much information is sent at a time.
The physical layer deals with the transfer of bits over a communication char for example, copper wire pairs, coaxial cable, radio or optical fibre. It defines mechanical, electrical, functional and procedural specifications that identify items the connectors and cable types used as the medium of communications link. Elect: specifications cover aspects that affect the impedance operating power, attenuation frequency and bandwidth limitations etc. that affect the transfer of data. Functinal aspects define the use of any interconnecting lines such as data and control function Procedural concerns define the lowest type of protocol rules that are required to as a good communication link. The functions of the seven OSI layers are given Table 1.1. The U.S. Department of Defence (DOD) wanted to interconnect computers and network it had acquired from different vendors. Because progress on standards for the C model was slow, the government's Advanced Research Project Agency (AR F developed a set of protocols called the Transmission Control Protocol/Internet Protocol (TCP/IP) to enable the interconnection. The Original use of these protocols was in the ARPANET, a network that connected various government and university research laboratories, which eventually evolved into the Internet. The DOD eventually mandated that TCP/IP be used in all of its computers and networks, which automatically provided a huge base of equipment with protocols installed, and a large market opportunity ,for software and equipment vendors. Because of this and the slowness of the OSI standards development on which products could be based. TCP/IP became the architecture and protocol on which the Internet was based. The rapid expansion of Internet usage in all facts of society has subsequently made TCP/IP the defacto standard architecture for a growing majority of public and private network.
When computers talk over the Internet, the language they speak is the Transmission Control Protocol/Internet Protocol (TCP/IP). TCP/IP is also the protocol of choice for most medium and large-sized networks. Novell Netware, UNIX, a Window NT networks can all implement TCP/IP, particularly on growing networks a on ones that use client/server or web-based applications. TCP/IP is one of the old, protocols and is a proven technology that is used by millions of computer users around the globe. Its broad acceptance, reliable history, and extensive capabilities make it a good choice for most LAN-to-WAN installations.
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