Telecommunications - OSI Protocols - Presentation Layer

Transfer syntax





Standard representation

OSI Security



Services provided to the application layer

Summary of presentation layer service elements

1.0 - Introduction

The presentation layer provides for the representation of information:

The representation of data to be transferred between application - entities.

The representation of the data structure which application-entities refer to in their communication, along with the representations of the set of actions which may be performed on this data structure.

The presentation layer is concerned only with:

SYNTAX - ie. the representation of the data.

and not with its:

SEMANTICS - if their meaning to the application layer which is known only by the application-entities.

Although it might seem obvious that different syntaxes were needed, depending on the form of data being transferred (files, jobs, terminal streams, etc.), this was not the case and ISO worked on the development of a common protocol which can cope with any transfer syntax (analogous to the concept of a metalanguage in compiler construction).

Transfer syntax

The syntax used for the transfer of data is known as the "transfer syntax" and, although early work on the transfer of data between dissimilar systems tried to define a "network syntax" which the systems had to translate into and out of, there is no need for this and the transfer syntax may be identical to the syntax used in the source or destination machine or both.

Source Syntax

__________________________ __________________________

Name ....................

Name ....................

Address .................

Address .................


__________________________ ____________________________

Ways to determine what transfer syntax:

Syntax is predetermined (ie. when A communicates with B it will always use a certain syntax).

A set of transfer syntaxes which both systems "know about" and they choose one (referred to by name). These syntaxes could be private.

To send a description of the syntax to be used.


Services provided by the presentation layer

A number of the services provided by the presentation layer (eg. synchronisation) map directly onto the corresponding service in the next lower layer-session.

One particular function of the presentation later is the ability to change the transfer syntax during a transfer, using the P-context-define and P-context-select primitives.


The presentation layer requires a kind of metalanguage an example of a metalanguage is BNF.

It was decided to define a rather more flexible metalanguage and this has become known as ASN.1 ("abstract syntax notation one"). This evolved from the xerox "courier" specification (part of XNS).

CCITT has a single standard X.409 - presentation transfer syntax and notation. The X400 series is MHS - message handling systems and is likely to be renumbered in the X.200 series.

DIS 8824 - specifidcation of abstract syntax notation one

DIS 8825 - specification of basic encoding rules for abstract syntax notation one.

code conv table

X409 is based on BNF

OSI Security

It is not clear into what layer of the OSI model encrypton should be done, if it is done below level 3 then in order for intermediate nodes to understand the headers and trailers, the entire message must be decrypted at every intermediate node, this could cause security risks. If encrypton is done only at the highest layer then routing and other information could be gleaned from message headers. Therefore where maximum security is required encrypton must be done in several layers.

Although "security" often implies encrypton (or a related technique) for making the text of a message unreadable, there are many more services required to provide the necessary degree of security in a network


One such service is "traffic flow security". This means that an

unauthorised user cannot obtain information by analysis of the traffic flows.

Another service is authentication. There are a number of varieties of this but they have a common purpose, namely to ensure that messages are received from the correct source and not an unauthorised source. Two related services ("non-repudiation") ensure that a sender or receiver of data cannot deny having sent or received the data (in the case of a receiver, this is analogous to "recorded delivery" in the mail services.

The OSI security Addendum postulates the existance of a Security Management Information Base (SMIB). This specifies the functions required by and from each layer and thus ensures that security is maintained.


IS 7498 - Security appendix to the reference model

DIS 8227 - DES Data encipherment: specification of algorithm DEA1

DIS 8372 - DES Modes of operation for a 64-bit block cipher algorithm

ANSI X3.105 - Data link Encryption.


Services provided to the application layer

1 - Transformation of syntax;

2 - Selection of syntax

Three syntactic versions,

1 - syntax used by the originating application

2 - syntax used by the receiving application

3 - syntax used between presentation-entities


1 - encription

2 - compression

3 - syntax convertion

4 - graphics

Functions within the Presentation Layer

1 - session establishment request;

2 - data transfer;

3 - negotiation and renegotiation of syntax;

4 - transformation of syntax including data transformation and formatting

and special purpose transformations (e.g. compression)

5 - Session termination request.

Added Value Provided by the Presentation Layer

  1. Definitions of contexts, which are sets of catogories of information, each category being a particular syntax.
  2. Transformation of syntax within each context.
  3. Selection of a particular context.

Summary of presentation layer service elements

Capability Service element Type
Establishment P-CONNECT Confirmed
Termination P-RELEASE Confirmed
P-U-ABORT Non-confirmed
P-P-ABORT provider initiated
Context P-CONTEXT-DEFINE confirmed
P-TRANSFER unconfirmed
P-PURGE unconfirmed
Interruption P-SUSPEND confirmed
P-RESUME confirmed
Status P-EXCEPTION-REPORT provider initiated


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