Network Layer
Network Layer
The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are "wired into" the network and rarely changed. They can also be determined at the start of each conversation, for example, a terminal session (e.g., a login to a remote machine). Finally, they can be highly dynamic, being determined a new for each packet, to reflect the current network load.
If too many packets are present in the subnet at the same time, they will get in one another's way, forming bottlenecks. The control of such congestion also belongs to the network layer. More generally, the quality of service provided (delay, transit time, jitter, etc.) is also a network layer issue.
When a packet has to travel from one network to another to get to its destination, many problems can arise. The addressing used by the second network may be different from the first one. The second one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.
In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.
The network layer performs the following functions
» Routing As discussed earlier.
» Congestion Control As discussed before.
» Logical addressing Source and destination logical addresses (e.g. IP addresses)
» Address transformations Interpreting logical addresses to get their physical equivalent (e.g. ARP protocol.)
» Source to destination error – free delivery of a packet.