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Q103884: The OSI Model’s Seven Layers Defined and Functions Explained

Article: Q103884
Product(s): Windows for Workgroups and Windows NT Networking Issues
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 27-FEB-2002

SUMMARY
=======

The Open Systems Interconnect (OSI) model has seven layers. This article
describes and explains them, beginning with the 'lowest' in the hierarchy (the
physical) and proceeding to the 'highest' (the application). The layers are
stacked this way:

- Application

- Presentation

- Session

- Transport

- Network

- Data Link

- Physical

PHYSICAL LAYER
--------------

The physical layer, the lowest layer of the OSI model, is concerned with the
transmission and reception of the unstructured raw bit stream over a physical
medium. It describes the electrical/optical, mechanical, and functional
interfaces to the physical medium, and carries the signals for all of the higher
layers. It provides:

- Data encoding: modifies the simple digital signal pattern (1s and 0s) used by
  the PC to better accommodate the characteristics of the physical medium, and
  to aid in bit and frame synchronization. It determines:

   - What signal state represents a binary 1

   - How the receiving station knows when a "bit-time" starts

   - How the receiving station delimits a frame

- Physical medium attachment, accommodating various possibilities in the
  medium:

   - Will an external transceiver (MAU) be used to connect to the medium?

   - How many pins do the connectors have and what is each pin used for?

- Transmission technique: determines whether the encoded bits will be
  transmitted by baseband (digital) or broadband (analog) signaling.

- Physical medium transmission: transmits bits as electrical or optical signals
  appropriate for the physical medium, and determines:

   - What physical medium options can be used

   - How many volts/db should be used to represent a given signal state, using
     a given physical medium

DATA LINK LAYER
---------------

The data link layer provides error-free transfer of data frames from one node to
another over the physical layer, allowing layers above it to assume virtually
error-free transmission over the link. To do this, the data link layer
provides:

- Link establishment and termination: establishes and terminates the logical
  link between two nodes.

- Frame traffic control: tells the transmitting node to "back-off" when no
  frame buffers are available.

- Frame sequencing: transmits/receives frames sequentially.

- Frame acknowledgment: provides/expects frame acknowledgments. Detects and
  recovers from errors that occur in the physical layer by retransmitting
  non-acknowledged frames and handling duplicate frame receipt.

- Frame delimiting: creates and recognizes frame boundaries.

- Frame error checking: checks received frames for integrity.

- Media access management: determines when the node "has the right" to use the
  physical medium.

NETWORK LAYER
-------------

The network layer controls the operation of the subnet, deciding which physical
path the data should take based on network conditions, priority of service, and
other factors. It provides:

- Routing: routes frames among networks.

- Subnet traffic control: routers (network layer intermediate systems) can
  instruct a sending station to "throttle back" its frame transmission when the
  router's buffer fills up.

- Frame fragmentation: if it determines that a downstream router's maximum
  transmission unit (MTU) size is less than the frame size, a router can
  fragment a frame for transmission and re-assembly at the destination station.

- Logical-physical address mapping: translates logical addresses, or names,
  into physical addresses.

- Subnet usage accounting: has accounting functions to keep track of frames
  forwarded by subnet intermediate systems, to produce billing information.

Communications Subnet:

The network layer software must build headers so that the network layer software
residing in the subnet intermediate systems can recognize them and use them to
route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data
transmission and intermediate switching technologies used to connect systems. It
establishes, maintains and terminates connections across the intervening
communications facility (one or several intermediate systems in the
communication subnet).

In the network layer and the layers below, peer protocols exist between a node
and its immediate neighbor, but the neighbor may be a node through which data is
routed, not the destination station. The source and destination stations may be
separated by many intermediate systems.

TRANSPORT LAYER
---------------

The transport layer ensures that messages are delivered error-free, in sequence,
and with no losses or duplications. It relieves the higher layer protocols from
any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it
can get from the network layer. For a reliable network layer with virtual
circuit capability, a minimal transport layer is required. If the network layer
is unreliable and/or only supports datagrams, the transport protocol should
include extensive error detection and recovery.

The transport layer provides:

- Message segmentation: accepts a message from the (session) layer above it,
  splits the message into smaller units (if not already small enough), and
  passes the smaller units down to the network layer. The transport layer at
  the destination station reassembles the message.

- Message acknowledgment: provides reliable end-to-end message delivery with
  acknowledgments.

- Message traffic control: tells the transmitting station to "back-off" when no
  message buffers are available.

- Session multiplexing: multiplexes several message streams, or sessions onto
  one logical link and keeps track of which messages belong to which sessions
  (see session layer).

Typically, the transport layer can accept relatively large messages, but there
are strict message size limits imposed by the network (or lower) layer.
Consequently, the transport layer must break up the messages into smaller units,
or frames, prepending a header to each frame.

The transport layer header information must then include control information,
such as message start and message end flags, to enable the transport layer on
the other end to recognize message boundaries. In addition, if the lower layers
do not maintain sequence, the transport header must contain sequence information
to enable the transport layer on the receiving end to get the pieces back
together in the right order before handing the received message up to the layer
above.

End-to-end layers:

Unlike the lower "subnet" layers whose protocol is between immediately adjacent
nodes, the transport layer and the layers above are true "source to destination"
or end-to-end layers, and are not concerned with the details of the underlying
communications facility. Transport layer software (and software above it) on the
source station carries on a conversation with similar software on the
destination station by using message headers and control messages.

SESSION LAYER
-------------

The session layer allows session establishment between processes running on
different stations. It provides:

- Session establishment, maintenance and termination: allows two application
  processes on different machines to establish, use and terminate a connection,
  called a session.

- Session support: performs the functions that allow these processes to
  communicate over the network, performing security, name recognition, logging,
  and so on.

PRESENTATION LAYER
------------------

The presentation layer formats the data to be presented to the application layer.
It can be viewed as the translator for the network. This layer may translate
data from a format used by the application layer into a common format at the
sending station, then translate the common format to a format known to the
application layer at the receiving station.

The presentation layer provides:

- Character code translation: for example, ASCII to EBCDIC.

- Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.

- Data compression: reduces the number of bits that need to be transmitted on
  the network.

- Data encryption: encrypt data for security purposes. For example, password
  encryption.

APPLICATION LAYER
-----------------

The application layer serves as the window for users and application processes to
access network services. This layer contains a variety of commonly needed
functions:

- Resource sharing and device redirection

- Remote file access

- Remote printer access

- Inter-process communication

- Network management

- Directory services

- Electronic messaging (such as mail)

- Network virtual terminals


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