The Seven Layers

The seven layers of the OSI model may be divided into two groups (Figure 6.5). The upper three layers are concerned with applications - the synchronisation of activity within a set of distributed applications, the representation of data, the management of associations, concurrency etc.. These are called the 'upper' or 'application oriented' layers. The lower four layers are concerned with the technology in use - error and flow control, routing etc.. These are the 'transport oriented' layers. Note that the Transport Service interface forms the boundary between these two groups. The idea is that the application oriented layers should not have to worry about the vagaries of the technological differences in the lower layers - the Transport layer should mask all these. Thus the functionality provided by the Transport layer has a great significance in determining the nature of the communications service available to the applications. Generally speaking, the boundaries between the lower three layers reflect boundaries that already existed when the OSI effort began. The upper layers are more interesting as they attempt to separate out common functionality required by applications which had not previously been the subject of standardization. This should be a 'good thing' as it should prevent new applications from re-inventing the wheel. It is these upper layers that are of most interest from the point of view of distributed system designers. Notice that you can separate out two aspects of a protocol layer:

Signaling
Data

In the telecom world, this separation is made obvious, since almost all signaling is done at the beginning' and end of a communications session. In the data communications world, it is less obvious, since signaling type activities may be required almost every time some data is send or received. In summary, the functions of the layers are:

7) Application Layer Definition of services for specific applications, File Transfer and Electronic Mail for example. Both the syntax of the Service Elements to be exchanged between Application Entities, and the actions they should perform are defined. Often, service elements from different applications will have broadly similar semantics, for example, all OSI applications have an initialization and termination phase.
6) Presentation Layer Before communication can take place between application entities, there must be agreement both on the 'abstract' syntax of the messages which may be exchanged and the way in which this abstract syntax should be represented as a sequence of bits. This latter form is called a 'Concrete' or 'Transfer' syntax. The Presentation layer handles the negotiation of abstract and transfer syntaxes and translates between native data representations and the transfer syntax.
5) Session Layer The session layer manages the duplex communication channel provided by the layers below. It provides service elements for initialising the channel, for synchronising the two ends, for determining which end has the right to transmit next, and for re-synchronising in the event of errors.
4) Transport Layer The Transport Layer provides an 'end to end', network independent communication service with known reliability and performance characteristics. It is up to the Transport layer to provide this service irrespective of the service provided by the layer below. A range of transport protocols of increasing complexity has been defined in order to cope with the different qualities of service (QoS) which might be provided by the layers below.
3) Network Layer The network layer takes account of the fact that communication takes place across real networks such as Ethernets and Public X.25 networks. Each of these (called real subnetworks by ISO) is likely to provide a slightly different 'subnetwork service'. The Network layer builds on these to provide a common OSI Network Service, though great differences in the QoS may remain. Computers are attached to subnetworks and are identified by their points of attachment to subnetworks. An important function of the Network layer is the provision of addresses for computers which are global throughout the OSI world. Communication often takes place across a series of interconnected subnetworks. the routing of traffic between subnetworks is also a network layer responsibility.
2) Datalink Layer This provides framing, error and flow control on a single physical link - for example a piece of wire. This definition is appropriate mainly to mesh-style WANs which consist of a collection of packet switches connected together by links.
1) Physical Layer This provides mechanical and electrical interface definitions.