Network Layer and Routing
As discussed in previous chapters the physical layer provides connection sand services to the data link layer while the data link layer is connected with the construction of frames error control flow control and the transmission of frames over the physical circuit connection between computers or even between different types of networks. The frames may have to go through a number of nodes ( computer networks ) which may be connected in different topologies. The network layer entity uses the services of the data link layer and provides services( routing of packets relaying congestion control etc) to its higher layer ( transport) and also to different types of intermediate networks of sub networks. The primary function of the network layer are routing selection of appropriate faults controlling of overloading or congestion of traffic on communication lines. Efficient use of various communication liens, etc. This layer manages network layer connections and provides an end to end reliable packet delivery mechanism. This provides user to user connection for exchange of data between them.
The network layer in the OSI model is also known as the subnet which may run on internet where the network layer runs on interface message processors. The transport layer of the OSI model runs on the horsts. The control and management of the network layer are provided by different organization in different countries such as PTT. At and t etc PTT is very poplar in European countries and undeveloped countries . thus the network layer not only provides services to the transport layer an different sub networks but also defines an interface and connection different hosts and different type of networks.
shows a simple network with two hosts. H1 and H2. And several routers on the path between H1 and H2. Suppose that H1. Is sending information to H2.and consider the role of the network layer in these hosts and in the intervening grouters. The network layer in H1 takes segments from the transport layer inH1. Encapsulated each segment into a datagram ( that is a network layer packet) and then starts the datagram's on their journey to their destination that is it sends the datagram's to its nearby router R1. At the receiving host H2. The network receiver the datagram's from its nearly router R2 extracts the transport layer segments and delivers the segments up to the transport layer atH2 the primary role of the routers is to forward datagram's from input links to output links. Note that the routers in figure are show with a truncated protocols stack that is with no upper layers above the network because router do not run application and transport layer protocols.