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Wednesday, July 17, 2013

What are network layer design issues?

- The network layer i.e., the third layer of the OSI model is responsible for facilitating the exchange of the individual information or data pieces between hosts over the network. 
- This exchange only takes place between the end devices that are identified. 
For accomplishing this task, 4 processes are used by the network layer and these are:
Ø  Addressing
Ø  Encapsulation
Ø  Routing
Ø  Decapsulation
In this article we focus up on the design issues of the network layer. 

- For accomplishing this task, the network layer also need s to have knowledge about the communication subnet’s topology and select the appropriate routes through it. 
- Another thing that the network layer needs to take care of is to select only those routers that do not overload the other routers and the communication lines while leaving the other lines and router in an idle state.

Below mentioned are some of the major issues with the network layer design:
  1. Services provided to the layer 4 i.e., the transport layer.
  2. Implementation of the services that are connection oriented.
  3. Store – and  - forward packet switching
  4. Implementation of the services that are not connection oriented.
  5. Comparison of the data-gram sub-nets and the virtual circuits.
- The sender host sends the packet to the router that is nearest to it either over a point-to-point carrier link or LAN. 
- The packet is stored until its complete arrival for the verification of the check sum. 
- Once verified, the packet is then transmitted to the next intermediate router. 
- This process continues till the packet has reached its destination. 
- This mechanism is termed as the store and forward packet switching.

The services that are provided to the transport layer are designed based up on the following goals:
  1. They should be independent of the router technology.
  2. Shielding from the type, number and topology of the routers must be provided to the transport layer.
  3. The network addresses that are provided to the transport layer must exhibit a uniform numbering plan irrespective of whether it’s a LAN or a WAN.
Now based up on the type of services that are offered, there is a possibility for two different organizations.

Offered service is Connection-less: 
- The packets are individually introduced in to the sub-net and the routing of the packets is done independently of each other. 
- It does not require any advance set up. 
- The sub-net is referred to as the data gram sub-net and the packets are called data-grams.

Offered service is connection-oriented: 
- In this case the router between the source and the destination must be established prior to the beginning of the transmission of the packets. 
- Here, the connection is termed as the virtual circuit and subnet as the “virtual circuit subnet” or simply VC subnet.

- Choosing a new router every time is a thing to be avoided and this is the basic idea behind the use of the virtual circuits. 
- Whenever we establish a connection, a route has to be selected from source to destination. 
- This is counted as a part of the connection setup only. 
- This route is saved in the routers tables that are managed by the routers and is then used by the flowing traffic. 
- On the release of connection, the VC is automatically terminated. 
- In case of the connection oriented service, an identifier is contained in each packet which tells the virtual circuit to which it belongs.

- In data-gram sub-net circuit setup is not required whereas it is required in the VC circuit. 
- The state info is not held by the routers in the data gram subnet whereas router table space is required for each VC for each connection. 

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