Anamarija Pogorelec

ped-inf

3. g.

Maja Brkić

eng-inf

3. g.
 

The network layer determines how messages are routed within the network. It provides transport entities independence from routing and relay considerations, including the case where subnetworks (relay-only) nodes are used. All relay functions are operated within or below this layer.

The basic service of the network layer is to provide the transparent transfer of data between transport-entities. The layer contains functions to mask the differences in characteristics of different transmission and subnetwork technologies into a consistent network service. This means that even if the end-to-end service spans a number of different types of subnetwork, each providing different service (e.g Ethernet, Token Ring, X25..) this is made transparent to the layer above.

A feature of the route a message will take is the quality of service. This can be measured in terms of error rate, service availability, reliability, throughput, transit delay and connection establishment delay. This is negotiated between transport-entities.

The services which may be provided by the network layer are:

Provision of network addresses for use by the transport layer.

The Network layer uniquely identifies each end open system by means of a network address this may be independent of the addressing used by the lower layers.

Establishment and release of a connection between two users.

A network connection is a means of transferring data between network addresses (transport-entities) A point-to-point connection

NB - More than one network connection may exist between the same pair of network-addresses.

A means of establishing a required quality of service.

See above

NSDU Transfer

A means of transferring messages of a specified fixed size (network-service-data-units) so that the packets are received as they were sent. Content integrity is maintained by the network layer.

Flow control.

The flow control at this level is different from that at the lower levels in that a transport layer.

Sequencing

This is provided when requested

Optional delivery confirmation.

Only included to support existing features of CCITT X25 - needs to be requested at connection establishment

Optional Reset

Causes all NSDUs in transit to be discarded and reset the connection. (Needs to be requested)

 

Notes - Network connections may be multiplexed The network layer may segment and/or block data for transmission Detected errors may recovered if the quality of service allows