[ Computer Network ]
[ Unit 4 ]
[ Network Layer ]
o The Network Layer is the third layer of the OSI model.
o It handles the service requests from the transport layer and further
forwards the service request to the data link layer.
o The network layer translates the logical addresses into physical addresses
o It determines the route from the source to the destination and also
manages the traffic problems such as switching, routing and controls the
congestion of data packets.
o The main role of the network layer is to move the packets from sending host
to the receiving host.
The main functions performed by the network layer are:
o Routing:
When a packet reaches the router's input link, the router will move
the packets to the router's output link. For example, a packet from S1 to R1
must be forwarded to the next router on the path to S2.
o Logical Addressing:
The data link layer implements the physical addressing and network
layer implements the logical addressing. Logical addressing is also used to
distinguish between source and destination system. The network layer adds a
header to the packet which includes the logical addresses of both the
sender and the receiver.
o Internetworking:
This is the main role of the network layer that it provides the logical
connection between different types of networks.
,o Fragmentation:
The fragmentation is a process of breaking the packets into the
smallest individual data units that travel through different networks.
, Forwarding & Routing
In Network layer, a router is used to forward the packets. Every router has a
forwarding table. A router forwards a packet by examining a packet's header field
and then using the header field value to index into the forwarding table. The value
stored in the forwarding table corresponding to the header field value indicates
the router's outgoing interface link to which the packet is to be forwarded.
For example, the router with a header field value of 0111 arrives at a router, and
then router indexes this header value into the forwarding table that determines
the output link interface is 2. The router forwards the packet to the interface 2.
The routing algorithm determines the values that are inserted in the forwarding
table. The routing algorithm can be centralized or decentralized.
[ Unit 4 ]
[ Network Layer ]
o The Network Layer is the third layer of the OSI model.
o It handles the service requests from the transport layer and further
forwards the service request to the data link layer.
o The network layer translates the logical addresses into physical addresses
o It determines the route from the source to the destination and also
manages the traffic problems such as switching, routing and controls the
congestion of data packets.
o The main role of the network layer is to move the packets from sending host
to the receiving host.
The main functions performed by the network layer are:
o Routing:
When a packet reaches the router's input link, the router will move
the packets to the router's output link. For example, a packet from S1 to R1
must be forwarded to the next router on the path to S2.
o Logical Addressing:
The data link layer implements the physical addressing and network
layer implements the logical addressing. Logical addressing is also used to
distinguish between source and destination system. The network layer adds a
header to the packet which includes the logical addresses of both the
sender and the receiver.
o Internetworking:
This is the main role of the network layer that it provides the logical
connection between different types of networks.
,o Fragmentation:
The fragmentation is a process of breaking the packets into the
smallest individual data units that travel through different networks.
, Forwarding & Routing
In Network layer, a router is used to forward the packets. Every router has a
forwarding table. A router forwards a packet by examining a packet's header field
and then using the header field value to index into the forwarding table. The value
stored in the forwarding table corresponding to the header field value indicates
the router's outgoing interface link to which the packet is to be forwarded.
For example, the router with a header field value of 0111 arrives at a router, and
then router indexes this header value into the forwarding table that determines
the output link interface is 2. The router forwards the packet to the interface 2.
The routing algorithm determines the values that are inserted in the forwarding
table. The routing algorithm can be centralized or decentralized.
