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.