The Seven Layers of the OSI Reference Model

 

The Open System Interconnection (OSI) reference model is composed of seven layers:

 

Layer 1 -- Physical layer
Layer 2 -- Data link layer
Layer 3 -- Network layer
Layer 4 -- Transport layer
Layer 5 -- Session layer
Layer 6 -- Presentation layer
Layer 7 -- Application layer

 

Top Layer 1 - OSI Model Physical Layer

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.


Common Physical Layer Implementations

Physical layer implementations can be categorized as either LAN or WAN specifications.The following figure shows some common LAN and WAN physical layer implementations:

 

LAN Implementations

The most common LAN physical layer implementations follow:
Ethernet/IEEE 802.3
Baseband LAN specifications that operate at 10 Mbps over coaxial cable.
100-Mbps Ethernet (Fast Ethernet)
High-speed LAN technology that offers increased bandwidth to desktop users, servers, and server clusters.
Fiber Distributed Digital Interface (FDDI)
100-Mbps token-passing, dual-ring LAN using fiber-optic cable.
Token Ring/IEEE 802.5
Token passing LANs that run at 4 or 16 Mbps over a star topology.


WAN Implementations

Some common WAN physical layer implementations follow:
High-Speed Serial Interface (HSSI)
Network standard for high-speed serial communications over WAN links.
SMDS Interface Protocol (SIP)
SMDS physical layer implementation used in SMDS WANs.
X.21bis
Physical layer protocol used in X.25 networks.

 

Top Layer 2 - OSI Model Data Link Layer

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.


IEEE Data Link Sublayers

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). The following figure shows the IEEE sublayers of the data link layer:

 

Logical Link Control Sublayer

The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification.

IEEE 802.2 defines a number of fields in data link layer frames that allow multiple higher-layer protocols to share a single physical data link.

LLC supports both connectionless and connection-oriented services used by higher-layer protocols.


Media Access Control Sublayer

The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium.
The IEEE MAC specification defines
MAC addresses , which allow multiple devices to uniquely identify one another at the data link layer.


Common Data Link Layer Implementations

Data link layer implementations can be categorized as either LAN or WAN specifications.The following figure shows some common LAN and WAN data link layer implementations:



LAN Implementations

The most common LAN data link layer implementations follow:
Ethernet/IEEE 802.3
Baseband LAN specifications that operate at 10 Mbps over coaxial cable.
100-Mbps Ethernet (Fast Ethernet)
High-speed LAN technology that offers increased bandwidth to desktop users, servers, and server clusters.
Fiber Distributed Digital Interface (FDDI)
100-Mbps token-passing, dual-ring LAN using fiber-optic cable.
Token Ring/IEEE 802.5
Token passing LANs that run at 4 or 16 Mbps over a star topology.


WAN Implementations

Some common WAN data link layer implementations follow:
Frame Relay
High-performance packet-switched WAN protocol.
Link Access Procedure, Balanced (LAPB)
Protocol used in X.25 that manages communication and packet framing between DTE and DCE.
Synchronous Data Link Control (SDLC)
IBM bit-synchronous data link layer protocol used in SNA networks.
Point-to-Point Protocol (PPP)
Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.
SMDS Interface Protocol (SIP)
SMDS data link layer protocol used in SMDS WAN´s.


Top Layer 3 - OSI Model Network Layer

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.


Common Network Layer Implementations

Network layer protocols are typically routing protocols . However, other types of protocols, such as the Internet Protocol (IP) , are implemented at the network layer as well. Some common routing protocols follow:

Border Gateway Protocol (BGP)
Internet interdomain routing protocol.
Open Shortest Path First (OSPF)
Link-state, interior gateway protocol developed for use in TCP/IP networks.
Routing Information Protocol (RIP)
Internet routing protocol that uses hop count as its metric.

 

Top Layer 4 - OSI Model Transport Layer

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.


Common Transport Layer Implementations

Some examples of transport layer implementations follow:

Transmission Control Protocol (TCP)
Protocol in the TCP/IP suite that provides reliable transmission of data.
Name Binding Protocol (NBP)
AppleTalk protocol that associates AppleTalk names with addresses.
OSI transport protocols
A series of transport protocols in the OSI protocol suite.

 

Top Layer 5 - OSI Model Session Layer

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.


Common Session Layer Implementations

Some examples of session layer implementations follow:

Zone Information Protocol (ZIP)
AppleTalk protocol that coordinates the name binding process.
Session Control Protocol (SCP)
The DECnet Phase IV session layer protocol.


Top Layer 6 - OSI Model Presentation 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.


Common Presentation Layer Implementations

Presentation layer implementations are not typically associated with a particular protocol stack.

Some well known standards for video follow:
QuickTime -- QuickTime is an Apple Computer specification for video and audio.
Motion Picture Experts Group (MPEG) -- MPEG is a standard for video compression and coding.

Some well known graphic image formats follow:
Graphics Interchange Format (GIF) -- GIF is a standard for compression and coding of graphic images.
Joint Photographic Experts Group (JPEG) -- JPEG is another compression and coding standard for graphic images.
Tagged Image File Format (TIFF) -- TIFF is a standard coding format for graphic images.


Top Layer 7 - OSI Model Application Layer

The application layer interacts with software applications that implement a communicating component. Such application programs are outside of the scope of the OSI model. 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.


Common Application Layer Implementations

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) .