Basic functions of network layer is routing and congestion control.
Routing: This layer deals with determining how packets will be transferred from source to destination. It can be of three types
- Static: Routes are depends on static tables and these are "wired into" the network and are rarely changed.
- Dynamic: All packets of single application can follow different routes depending upon the topology of the network, the shortest path and the current network load.
- Semi-Dynamic: A route is selected at the start of each conversation and then all the packets of the application follow the same route.
The services provided by the network are of the two types :
- Connection less service: Each and every packet of an application is treated as an independent unit. On each packet of the application the destination address is given and the packet is routed.
- Connection oriented service: Here, initially a connection is established and then all packets of the application follow the same path. To understand the above idea, we can also draw an analogy from the our real life. "Connection oriented" service is modelled after the telephone system. All voice packets go on the same route after the connection is established till the connection is disturbed. It acts like a tube ; the sender pushes the objects in at one end and the receiver takes them out in the same order at the other receiving end. "Connection less service" is modelled after the postal system. Each letter carries the destination end address and is routed independent of all the others. Here, it may be possible that the letter sent first is delayed so that the second letter reaches at the destination before the first letter.
Congestion Control: A router can be connected to many networks. If all the networks send packet at the same time with maximum rate possible then the router may not be able to handle all the packets and may drop some or all packets. In this situation the dropping of the packets should be minimized and the source whose packet was dropped should be informed. The management of such congestion is also a important function of the network layer. Other issues that are related with this layer are transmitting time,delays and jittering.
Internetworking: Internetworks are many networks that are connected in such a way that they act as a big network, connecting several office or department networks. Internetworks are linked by networking machines such as routers, switches, and hubs. Internetworking is a solution born of three networking problems: isolated LANs, duplication of resources, and the not have a centralized network management system. With linked LANs,companies no longer have to copy programs or resources on each network. This in turn gives technique to managing the network from one central location instead of trying to manage each single LAN. Now we are able to transmit any packet from one network to any other network even if they follow different protocols or they use different addressing modes.
Network Layer does not guarantee that the packet will reach its intended end. There are no reliability guarantees.
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