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:
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
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.