Information Technology Reference
In-Depth Information
Examples of specifications that operate at the presentation layer are the following:
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Abstract Syntax Notation 1 (ASN.1)
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ASCII
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EBCDIC
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Motion Picture Experts Group (MPEG)
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Graphics Interchange Format (GIF)
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Joint Photographic Experts Group (JPEG)
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Tagged Image File Format (TIFF)
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G.711, G.729a, G.726, G.728
Application Layer (OSI Layer 7)
The application layer provides 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 synchronization
between the peer applications that reside on separate systems.
Examples of application layer specifications are the following:
•
Telnet
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File Transfer Protocol (FTP)
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Simple Mail Transfer Protocol (SMTP)
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Simple Network Management Protocol (SNMP)
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Network File System (NFS)
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Association Control Service Element (ACSE)
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Remote Operations Service Element (ROSE)
Example of Layered Communication
If you use a Telnet application, Telnet maps into the top three layers of the OSI model. Figure
2-3 shows that a user on Host 1 enables the Telnet application to access a remote host (Host 2).
The Telnet application provides a user interface (application layer) to network services. As
defined in Request For Comments (RFC) 854, ASCII is the default code format used (presen-
tation layer). There is no session layer defined for Telnet; it is not an OSI protocol. Per the RFC,
Telnet uses TCP for connectivity (transport layer). The TCP segment gets placed into an IP
packet (network layer) with a destination IP address of Host 2. The IP packet gets placed into
an Ethernet frame (data-link layer), which is converted into bits, and sent onto the wire (phys-
ical layer).
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