Network Layer

by : Jesse Miller

The network layer provides for the transfer of information between computers across some sort of communication network. It relieves higher layers of the need to know anything about the underlying data transmission and switching technologies used to connect systems. The network service is responsible for establishing, maintaining, and terminating connections across the intervening network. At this layer, the computer system engages in a dialogue with the network to specify the destination address and to request certain network facilities, such as priority.

There is a spectrum of possibilities for intervening communications facilities to be managed by the network layer. At one extreme, there is a direct point to point link between stations. In this case, there may be no need for a network layer because the data link layer can perform the necessary function of managing the link.

The systems could be connected across a single network, such as circuit switching or packet switching network. Lower three layers are concerned with attaching to and communicating with the network. The packets that are created by the end system pass through one or more network nodes that act as relays between the two end systems. The network nodes implement layers 1 to 3 of the architecture. Within the node there are two data link layers and two physical layers, corresponding to the links to the two end systems. Each data link layer operates independently to provide service to the network layer over its respective link. The upper four layers are"end to end" protocols between the attached computers.

At the other extreme, two stations may communicate but are not even connected to the same network. Rather, they are connected to networks that, directly or indirectly, are connected to each other. Logic involved includes functions for between networks as well as routing within networks.

The transport layer provides a reliable mechanism for the exchange of data between computers. It ensures that data are delivered error free, in sequence, and with no losses or duplications. The transport layer may also be concerned with optimizing the use of network services and providing a requested quality of service. The session layer may specify delay, priority, and security features.

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The mechanism used by the transport protocol to provide reliability is similar to those used by data link protocols such as HDLC. The reason for this apparent, duplication of effort is that the data link layer deals with only a single.