Information Technology Reference
In-Depth Information
Presentation Layer (OSI Layer 6)
The presentation layer provides application layer entities with services to ensure that in-
for mat ion is pre s er ved dur ing t ran sfer. Knowledge of the s y ntax s elec ted at the applic a -
tion layer allows selection of compatible transfer syntax if a change is required. This layer
provides conversion of character-representation formats, as might be required for reliable
transfer. Voice coding schemes are specified at this layer. Furthermore, compression and
encryption can occur at this layer.
An example of a specification that operates at the presentation layer is Abstract Syntax
Notation 1 (ASN.1).
Application Layer (OSI Layer 7)
The application layer gives the user or operating system access to the network services. It
interacts with software applications by identifying communication resources, determining
network availability, and distributing information services. It also provides synchroniza-
tion between the peer applications residing on separate systems.
Examples of application layer specifications are
Te l n e t
■
File Transfer Protocol (FTP)
■
Simple Mail Transfer Protocol (SMTP)
■
Simple Network Management Protocol (SNMP)
■
Network File System (NFS)
■
Border Gateway Protocol (BGP)
■
TCP/IP Architecture
The suite of TCP/IP protocols was developed for use by the U.S. government and research
universities. The suite is identified by its most widely known protocols: TCP and IP. As
mentioned, the ISO published the OSI model in 1984. However, the TCP/IP protocols had
been developed by the Department of Defense's Advanced Research Projects Agency
(DARPA) since 1969. The TCP/IP uses only four layers (as described in RFC 791) versus
the seven layers used by OSI. The TCP/IP layers are
Application
■
Host-to-host transport
■
Internet
■
Network interface
■
Figure C-3 shows how the TCP/IP layers map to the OSI model.
