The network layer provides the means to establish, maintain and terminate network-connections between open-systems containing communicating application-entities and the functional and procedural means to exchange network service-data-units between transport-entities over network-connections.
Functions of the network layer
- Routing - There are many routing algorithms but they all have the same basic function - to minimise overall network delay.
- Relaying
- Multiplexing
- Error detection and recovery - Replicates X.25 Link Layer
- Sequencing
- Flow control
- Management
Within a local area network, there is often no routine function to perform
(eg. in a CSMA/CD network all data is broadcast).
The network layer is not confined to a single network and indeed, much of its complexity stems from the fact that different networks may be connected at the network layer (hence the relevant protocols are often referred to as "inter-network protocols") in such cases the network layer must provide relaying functions between (sub-)networks.
The Network Layer - Internal Arcitecture
The network layer has a defined internal architecture (DP 8648).
It is split into 4 sublayers.
| Transport Layer | ||||
Network Layer
|
||||
| Link Layer | ||||
| Physical Layer |
The lower 2 sublayers are dependent on the subnetwork(s) being used while
the upper 2 are independent.
22.3 - Connectionless Mode Data Transfer
In the first years of the OSI Model, there was an implicit assumption that
communication took place through logical connections. That is, the entity requiring
communication would initiate the connection (and, during this phase, would discuss
the parameters of the interaction with the target entity); it would then exchange
data; finally, it would terminate the connection.
In many cases this was an appropriate mode to use (and had been used previously
in many networks for "virtual circuits").
However, there is another mode of communication ("datagrams") in
which a data packet is transmitted completely independently of any other packets
being transmitted between the two entities (at least at this level - there must
be a higher level that imposes ordering and error recovery).
The need for such a mode arises especially in local area networks and interworking.
22.4 - NETWORK LAYER STANDARDS
An early protocol that is often quoted as conforming to the network layer
X.25 level 3 (or X.25 level 3). There is still some dispute about this. Although
the 1984 revision of X.25 is very much closer than earlier versions.
DP 8648 Internal organisation of the network later.
X.213=DIS 8348 Connection-mode network service definition
DIS 8473 Protocol for providing the connectionless-mode network service.
DP 8878 Use of X.25 to provide the OSI connection-oriented network service.
DP 8880 Provision of connection-oriented network service over local area networks operating logical link control type 2.
22.5 Services provided to the Transport Layer
- network-addresses;
- network-connections;
- network-connection-endpoint-identifiers;
- network-service-data unit transfer;
- quality of service parameters;
- error notification;
- sequencing;
- flow control;
- expedited network-service-data-unit transfer;
- reset;
- release;
- receipt of confirmation.
Functions within the network layer
- routing and relaying;
- network-connections;
- network-connection multiplexing;
- segmentation and blocking;
- error detection;
- error recovery;
- sequencing;
- Flow control;
- expedited data transfer;
- reset;
- service selection;
- network layer management.
22.6 Queue model concepts
-------------- --------------
! NS user A ! ! NS user B !
-------------- --------------
! !
-------------- --------------
!--------------! NSAP !---------------------! NSAP !-----!
! -------------- -------------- !
! ^ . ^ . !
! . ......queue from A to B .... . !
! . . !
! ..............queue from B to A ........... !
! !
! !
! NS provider !
! !
!--------------------------------------------------------------------!
Objects which can be placed on the queue by a NS user,
1 - Connect objects
2 - Normal data
3 - indications of end of NSDU's
4 - expedited NSDU's
5 - data acknowledge objects
6 - reset objects
7 - disconnect objects
Objects which can be placed on the queue by NS provider,
1 - Reset objects
2 - Synchronization mark objects
3 - Disconnect objects
THE NETWORK LAYER
