Network Layers

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(7) Application

(6) Presentation Upper Layers

(5) Session

(4) Transport Middle Layers

(3) Network

(2) Data Link Lower Layers

(1) Physical

(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.

(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

(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.

Hardware addresses are tranlated to network addresses here

Provides network topology information

Makes routing decisions.

RARP resides here

(4) Transport )

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 & reassembly is done here

Some examples of transport layer implementations follow:

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

The transport sets up and tears down connections between host

TCP & 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

(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.

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

(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

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