The network layer offers its functions to the transport layer, and for that, it takes services of the data link layer. Its functions are carried out by adding a header to every Network Service data unit (N-SDU). This header is in the form of Protocol Control Information (PCI). Thus a formed Network protocol data unit is transported over the existing data-link connection.
The functions of the network layer are as follows −
When the Network layer receives a call request from the transport layer, it establishes a network connection across the subnetwork. For this, it makes use of data link connections. The end system's network layer interacts with the network layer of the subnetwork access point for this purpose.
The Network Layer selects an appropriate route between source and destination device. This route is either decided in the beginning or for each N-SDU, depending on the type of services. The routing functions are facilitated by sub layering of the network layer.
For optimum use of data link connections, many network connections are sometimes required to be multiplexed on a single data-link connection. This is done by a network entity that can multiplex several network connections on a data link connection, as shown in the figure.
The network layer uses error detection functions to ensure that the quality of service provided over the network connection is maintained. Most of the errors are detected and corrected at the data link layer. The residual errors, Network Layer, notifies any. It relies on the quality of service provided, and the network layer incorporates the mechanism for error recovery.
The network service data unit is segmented at the network layer, and blocking is done to produce a network protocol data unit (N-PDU). The delimiter of NSDU is preserved during segmenting and blocking.
The process is shown in the figure. The network entity also carries out sequencing and flow control of network service data units on request of the transport layer. At the request of the transport layer, it can reset the network connection too.