25-07-2012, 02:29 PM
Network Layer
A Network Layer.pptx (Size: 1.16 MB / Downloads: 33)
Concerned with getting packets from source to destination.
End-to-end transmission.
The network layer must know the topology of the communication subnet subnet (i.e., the set of all routers) and choose appropriate paths through it.
When source and destination are in different networks, the network layer (IP) must deal with these differences.
Key issue: what service does the network layer provide to the transport layer (connection-oriented or connectionless).
when the source and destination are in different networks, new problems occur. It is up to the network layer to deal with them.
Network Layer Design Goals
The services provided by the network layer should be independent of the subnet topology.
The Transport Layer should be shielded from the number, type and topology of the subnets present.
The network addresses available to the Transport Layer should use a uniform numbering plan (even across LANs and WANs).
Network Layer Design Issues
Choice of the Services to be provided by the Network Layer to the Transport Layer and their nature:
The primary issue:
Choice of any one of the Connection-oriented or Connectionless types of service to be provided by the Network Layer.
Services to be provided by the Network Layer to the Data Link Layer and their nature:
The primary issue:
The format and size of the Network Layer Data Unit (e.g. Packet or Cell) in which the data is to be passed to the Data Link Layer (if it does exist) with or without encapsulation and additional information.
It needs to be noted here that in practice, the Network Layer and the lower layers (e.g. DLL and PL) are not really totally transparent to one-another.
Degree of transparency is, therefore, another major issue, which is seldom discussed in literature.
Implementation of Connectionless Service
If connectionless service is offered, packets are injected into the subnet individually and routed independently of each other. No advance setup is needed. In this context, the packets are frequently called datagram's (in analogy with telegrams) and the subnet is called a datagram subnet.
If connection-oriented service is used, a path from the source router to the destination router must be established before any data packets can be sent. This connection is called a VC (virtual circuit), in analogy with the physical circuits set up by the telephone system, and the subnet is called a virtual-circuit subnet.
Routing
Routing algorithm:: that part of the Network Layer responsible for deciding on which output line to transmit an incoming packet.
Remember: For virtual circuit subnets the routing decision is made ONLY at set up.
Algorithm properties:: correctness, simplicity, robustness, stability, fairness, optimality, and scalability.