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<title>Layers</title>
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&nbsp;

&nbsp;

Network Layers

&nbsp;

(7) Application

(6) Presentation
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Upper
Layers&nbsp;

(5) Session

&nbsp;

(4) Transport
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Middle
Layers

(3) Network

&nbsp;

(2) Data Link
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Lower
Layers

(1) Physical

&nbsp;

&nbsp;

(1) Physical

The physical layer defines the electrical, mechanical, procedural, and
functional specifications for activating, maintaining, and deactivating the physical link
between communicating network systems. Physical layer specifications define such
characteristics as voltage levels, timing of voltage changes, physical data rates, maximum
transmission distances, and the physical connectors to be used.

&nbsp;

&nbsp;

(2) Data Link )

The data link layer provides reliable transit of data across a
physical network link. Different data link layer specifications define different network
and protocol characteristics, including the following:

Physical addressing -- Physical addressing (as opposed to network addressing) defines
how devices are addressed at the data link layer.

Network topology -- Data link layer specifications often define how devices are to be
physically connected (such as in a bus or a ring topology).

Error notification -- Error notification involves alerting upper layer protocols that a
transmission error has occurred.

Sequencing of frames -- Sequencing of data frames involves the reordering of frames
that are transmitted out of sequence.

Flow control -- Flow control involves moderating the transmission of data so that the
receiving device is not overwhelmed with more traffic than it can handle at one time.

The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data
link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).

Provides hardware address mapping

&nbsp;

(3) Network )

The network layer provides routing and related functions that allow multiple data links
to be combined into an internetwork. This is accomplished by the logical addressing (as
opposed to the physical addressing) of devices. The network layer supports both
connection-oriented and connectionless service from higher-layer protocols.

&nbsp;

Hardware addresses are tranlated to network addresses here

Provides network topology information

Makes routing decisions.

RARP resides here

&nbsp;

(4) Transport )

&nbsp;

The transport layer implements reliable internetwork data
transport services that are transparent to upper layers. Transport layer functions
typically include the following:

Flow control -- Flow control manages data transmission between devices so that the
transmitting device does not send more data than the receiving device can process.

Multiplexing -- Multiplexing allows data from several applications to be transmitted
onto a single physical link.

Virtual circuit management -- Virtual circuits are established, maintained, and
terminated by the transport layer.

Error checking and recovery -- Error checking involves various mechanisms for detecting
transmission errors. Error recovery involves taking an action (such as requesting that
data be retransmitted) to resolve any errors that occur.

Packet fragmentation &amp; reassembly is done here

Some examples of transport layer implementations follow:

Transmission Control Protocol (TCP), Name Binding Protocol (NBP), OSI transport
protocols

&nbsp;

The transport sets up and tears down connections between host

TCP &amp; UDP work here

(5) Session )

The session layer establishes, manages, and terminates
communication sessions between presentation layer entities. Communication sessions consist
of service requests and service responses that occur between applications located in
different network devices. These requests and responses are coordinated by protocols
implemented at the session layer.

Provides application management. It allows remote users to establish multiple
connections to remote devices.

Token management occurs here

Sockets are used here

Netbios resides here

Some examples of session layer implementations follow:

Apple ZIP, DEC SCP, NFS, SQL, RPC, X Windows, ASP

&nbsp;

&nbsp;

(6) Presentation )

The presentation layer presents data to the application layer. The osi has protocol
standards that define how standard data should be formatted. Task Llike data compression,
decompression, encryption, and decryption are associated with this layer.

&nbsp;

The presentation layer provides a variety of coding and conversion functions that are
applied to application layer data. These functions ensure that information sent from the
application layer of one system will be readable by the application layer of another
system. Some examples of presentation layer coding and conversion schemes follow:

Common data representation formats -- The use of standard image, sound, and video
formats allow the interchange of application data between different types of computer
systems.

Conversion of character representation formats -- Conversion schemes are used to
exchange information with systems using different text and data representations (such as
EBCDIC and ASCII).

Common data compression schemes -- The use of standard data compression schemes allows
data that is compressed at the source device to be properly decompressed at the
destination.

Common data encryption schemes -- The use of standard data encryption schemes allows
data encrypted at the source device to be properly unencrypted at the destination.

Presentation layer implementations are not typically associated with a particular
protocol stack. Some well known standards follow:

Data: ASCII, EBCDIC, Encryption

Visual Imaging: PICT, TIFF, GIF, JPEG

Video: MIDI, MPEG, QuickTime

&nbsp;

(7) Application )

Supports the components that deal with communicating aspects of an application. It is
responsible for identifying and establishing the availability of the intended
communication partner

&nbsp;

The application layer interacts with software applications that implement a
communicating component. Application layer functions typically include the following:

Identifying communication partners -- The application layer identifies and determines
the availability of communication partners for an application with data to transmit.

Determining resource availability -- The application layer must determine whether
sufficient network resources for the requested communication are available.

Synchronizing communication -- Communication between applications requires cooperation
that is managed by the application layer.

The application layer is the OSI layer closest to the end user. That is, both the OSI
application layer and the user interact directly with the software application. Some
examples of application layer implementations follow:

TCP/IP applications -- TCP/IP applications are protocols in the Internet Protocol
suite, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol
(SMTP).

OSI applications -- OSI applications are protocols in the OSI suite such as File
Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common
Management Information Protocol (CMIP).

It provides a standard interface for users
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Erik Mortensen
21. december 2025 07:29:00