The
Open Systems Interconnection model (
OSI model) is a product of the
Open Systems InterconnectionOpen Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
effort at the
International Organization for StandardizationThe International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
. It is a prescription of characterizing and standardizing the functions of a
communications systemIn telecommunication, a communications system is a collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment usually capable of interconnection and interoperation to form an integrated whole...
in terms of
abstraction layerAn abstraction layer is a way of hiding the implementation details of a particular set of functionality...
s. Similar communication functions are grouped into logical layers. An instance of a layer provides services to its upper layer instances while receiving services from the layer below.
For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.
History
Work on a layered model of network architecture was started and the
International Organization for StandardizationThe International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
(ISO) began to develop its OSI framework architecture. OSI had two major components: an
abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.
The concept of a seven-layer model was provided by the work of
Charles BachmanCharles William "Charlie" Bachman is an American computer scientist, who spent his entire career as an industrial researcher rather than in academia...
, Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN,
CYCLADESThe CYCLADES packet switching network was a French research network created in the early 1970s. It was developed to explore alternatives to the ARPANET design and to support network research generally...
network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it, and provided facilities for use by the layer above it.
Protocols enabled an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host.
The OSI standards documents are available from the ITU-T as the X.200-series of recommendations. Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO, but only some of them without fees.
Description of OSI layers
According to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the layer-N entity.
At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting
protocol data unitIn telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
s (PDU).
A
Service Data UnitIn Open Systems Interconnection terminology, a service data unit is a unit of data that has been passed down from an OSI layer to a lower layer and that has not yet been encapsulated into a protocol data unit by the lower layer...
(SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
The PDU at any given layer, layer N, is the SDU of the layer below, layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.
| OSI Model |
|
Data unit |
Layer |
Function |
Host
layers |
Data |
7. Application The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
|
Network process to application |
| 6. Presentation The presentation layer is layer 6 of the seven-layer OSI model of computer networking and serves as the data translator for the network. It is sometimes called the syntax layer.-Description:...
|
Data representation, encryption and decryption, convert machine dependent data to machine independent data |
| 5. Session The session layer is layer 5 of the seven-layer OSI model of computer networking.The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses...
|
Interhost communication, managing sessions between applications |
| Segments |
4. Transport In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
|
End-to-end connections, reliability and flow control In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
|
Media
layers |
Packet/Datagram |
3. Network The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
|
Path determination and logical address In computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program....
ing |
| Frame |
2. Data link The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
|
Physical addressing |
| Bit |
1. Physical The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
|
Media, signal and binary transmission |
Some orthogonal aspects, such as management and security, involve every layer.
Security servicesSecurity service is a service, provided by a layer of communicating open systems, which ensures adequate security of the systems or of data transfers as defined by ITU-T X.800 Recommendation....
are not related to a specific layer: they can be related by a number of layers, as defined by
ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
X.800 Recommendation.
These services are aimed to improve the
CIA triadInformation security means protecting information and information systems from unauthorized access, use, disclosure, disruption, modification, perusal, inspection, recording or destruction....
(
confidentialityConfidentiality is an ethical principle associated with several professions . In ethics, and in law and alternative forms of legal resolution such as mediation, some types of communication between a person and one of these professionals are "privileged" and may not be discussed or divulged to...
,
integrityIntegrity is a concept of consistency of actions, values, methods, measures, principles, expectations, and outcomes. In ethics, integrity is regarded as the honesty and truthfulness or accuracy of one's actions...
, and
availabilityIn telecommunications and reliability theory, the term availability has the following meanings:* The degree to which a system, subsystem, or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time...
) of transmitted data. Actually the availability of communication service is determined by network design and/or
network managementNetwork management refers to the activities, methods, procedures, and tools that pertain to the operation, administration, maintenance, and provisioning of networked systems....
protocols. Appropriate choices for these are needed to protect against denial of service.
Layer 1: physical layer
The
physical layerThe physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a
transmission mediumA transmission medium is a material substance that can propagate energy waves...
, such as a copper or optical cable. This includes the layout of
pinsIn electronics, a lead is an electrical connection consisting of a length of wire or soldering pad that comes from a device. Leads are used for physical support, to transfer power, to probe circuits , and to transmit information...
,
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s,
cableA cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
specifications, hubs,
repeaterA repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
s,
network adapterA network interface controller is a computer hardware component that connects a computer to a computer network....
s,
host bus adapterIn computer hardware, a host controller, host adapter, or host bus adapter connects a host system to other network and storage devices...
s (HBA used in
storage area networkA storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
s) and more.
The major functions and services performed by the physical layer are:
- Establishment and termination of a connection
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
to a communicationCommunication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
s mediumA transmission medium is a material substance that can propagate energy waves...
.
- Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
.
- Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channelIn telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
. These are signals operating over the physical cabling (such as copper and optical fiberAn optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
) or over a radio link.
Parallel SCSIParallel SCSI is one of the interface implementations in the SCSI family. In addition to being a data bus, SPI is a parallel electrical bus: There is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it...
buses operate in this layer, although it must be remembered that the logical
SCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
protocol is a transport layer protocol that runs over this bus. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data link layer. The same applies to other local-area networks, such as
token ringthumb|Two examples of token ring networks: a) Using a single [[Media Access Unit|MAU]] b) Using several MAUs connected to each otherthumb|Token ring networkthumb|IBM hermaphroditic connector with locking clipthumb|An IBM 8228 MAU...
,
FDDIFiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...
,
ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hnG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
and
IEEE 802.11IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
, as well as personal area networks such as
BluetoothBluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
and IEEE 802.15.4.
Layer 2: data link layer
The
data link layerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in
IEEE Project 802IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...
. IEEE work assumed
sublayerIn telecommunication, the term sublayer has the following meanings:# In a layered open communications system, a specified subset of the services, functions, and protocols included in a given layer....
ing and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as
Point-to-Point ProtocolIn networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
(PPP), and, on local area networks, the IEEE 802.2
LLCThe logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layer by protocols such as
TCPThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, but is still used in niches where
X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
offers performance advantages.
The
ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hnG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete
data link layerThe data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
which provides both error correction and flow control by means of a selective repeat
Sliding Window ProtocolA sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer as well as in the Transmission Control Protocol .Conceptually, each portion of the...
.
Both WAN and LAN service arrange bits, from the physical layer, into logical sequences called frames. Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions. For example, every fifth bit of the FDDI bit stream is not used by the layer.
WAN protocol architecture
Connection-orientedConnection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
WAN data link protocols, in addition to framing, detect and may correct errors. They are also capable of controlling the rate of transmission. A WAN data link layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for
Synchronous Data Link ControlSynchronous Data Link Control is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture . SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header...
(SDLC) and HDLC, and derivatives of HDLC such as
LAPBLink Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
and
LAPDLink Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
.
IEEE 802 LAN architecture
Practical, connectionless LANs began with the pre-IEEE
EthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
specification, which is the ancestor of
IEEE 802.3IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
. This layer manages the interaction of devices with a shared medium, which is the function of a
media access controlThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) sublayer. Above this MAC sublayer is the media-independent
IEEE 802.2IEEE 802.2 is the IEEE 802 standard defining Logical Link Control , which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer...
Logical Link ControlThe logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
(LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.
While IEEE 802.3 is the dominant wired LAN protocol and
IEEE 802.11IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.
Layer 3: network layer
The
network layerThe network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
provides the functional and procedural means of transferring variable length
dataThe term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
sequences from a source host on one network to a destination host on a different network, while maintaining the
quality of serviceThe quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
requested by the transport layer (in contrast to the data link layer which connects hosts within the same network). The network layer performs network
routingRouting is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer, sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical.
The network layer may be divided into three sublayers:
- Subnetwork access – that considers protocols that deal with the interface to networks, such as X.25;
- Subnetwork-dependent convergence – when it is necessary to bring the level of a transit network up to the level of networks on either side
- Subnetwork-independent convergence – handles transfer across multiple networks.
An example of this latter case is CLNP, or IPv7 ISO 8473. It manages the
connectionlessIn telecommunications, connectionless describes communication between two network end points in which a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data addressed to the other, without first ensuring that the...
transfer of data one hop at a time, from end system to
ingress routerAn "ingress router" is a Label Switch Router that is a starting point for a given Label Switched Path . An ingress router may be an egress router or an intermediate router for any other LSP. Hence the role of ingress and egress routers is LSP specific...
, router to router, and from
egress routerAn egress router is a Label Switch Router that is an end point for a given Label Switched Path . An egress router may be an ingress router or an intermediate router for any other LSP. Hence the role of egress and ingress routers is LSP specific...
to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of erroneous packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses.
A number of layer-management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.
Layer 4: transport layer
The
transport layerIn computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.
OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:
| Feature Name |
TP0 |
TP1 |
TP2 |
TP3 |
TP4 |
| Connection oriented network |
|
|
|
|
|
| Connectionless network |
|
|
|
|
|
| Concatenation and separation |
|
|
|
|
|
| Segmentation and reassembly |
|
|
|
|
|
| Error Recovery |
|
|
|
|
|
Reinitiate connection (if an excessive number of PDUsIn telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
are unacknowledged) |
|
|
|
|
|
| Multiplexing and demultiplexing over a single virtual circuit In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
|
|
|
|
|
|
| Explicit flow control |
|
|
|
|
|
| Retransmission on timeout |
|
|
|
|
|
| Reliable Transport Service |
|
|
|
|
|
Perhaps an easy way to visualize the transport layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking,
tunneling protocolComputer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
s operate at the transport layer, such as carrying non-IP protocols such as
IBMInternational Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's SNA or
NovellNovell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
's IPX over an IP network, or end-to-end encryption with
IPsecInternet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
. While
Generic Routing EncapsulationGeneric Routing Encapsulation is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol internetwork.-Overview:...
(GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries
PPPIn networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
frames inside transport packet.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the
Transmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
(TCP) and the
User Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
(UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.
Layer 5: session layer
The
session layerThe session layer is layer 5 of the seven-layer OSI model of computer networking.The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses...
controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for
full-duplexA duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
, half-duplex, or
simplexSimplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the
Transmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use
remote procedure callIn computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
s.
Layer 6: presentation layer
The
presentation layerThe presentation layer is layer 6 of the seven-layer OSI model of computer networking and serves as the data translator for the network. It is sometimes called the syntax layer.-Description:...
establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.
This layer provides independence from data representation (e.g.,
encryptionIn cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.
The original presentation structure used the basic encoding rules of
Abstract Syntax Notation OneData generated at various sources of observation need to be transmitted to one or more locations that process it to generate useful results. For example, voluminous signal data collected by a radio telescope from outer space. The system recording the data and the system processing it later may be...
(ASN.1), with capabilities such as converting an
EBCDICExtended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....
-coded text
fileA computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
to an
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
-coded file, or
serializationIn computer science, in the context of data storage and transmission, serialization is the process of converting a data structure or object state into a format that can be stored and "resurrected" later in the same or another computer environment...
of
objectIn computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
s and other
data structureIn computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
s from and to
XMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
.
Layer 7: application layer
The
application layerThe Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application-layer implementations also include:
- On OSI stack:
- FTAM
FTAM, ISO standard 8571, is the OSI Application layer protocol for File Transfer Access and Management.The goal of FTAM is to combine into a single protocol both file transfer, similar in concept to the Internet FTP, as well as remote access to open files, similar to NFS...
File Transfer and Access Management Protocol
- X.400
X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems — more commonly known as "email"....
Mail
- Common management information protocol
The Common Management Information Protocol is the OSI specified network management protocol.Defined in . It provides an implementation for the services defined by the Common Management Information Service specified in , allowing communication between network management applications and...
(CMIP)
- On TCP/IP stack:
- Hypertext Transfer Protocol
The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
(HTTP),
- File Transfer Protocol
File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
(FTP),
- Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
(SMTP)
- Simple Network Management Protocol
Simple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
(SNMP).
Cross-layer functions
There are some functions or services that are not tied to a given layer, but they can affect more than one layer. Examples are
- security service (telecommunication)
Security service is a service, provided by a layer of communicating open systems, which ensures adequate security of the systems or of data transfers as defined by ITU-T X.800 Recommendation....
as defined by ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
X.800 Recommendation.
- management functions, i.e. functions that permit to configure, instantiate, monitor, terminate the communications of two or more entities: there is a specific application layer protocol, common management information protocol
The Common Management Information Protocol is the OSI specified network management protocol.Defined in . It provides an implementation for the services defined by the Common Management Information Service specified in , allowing communication between network management applications and...
(CMIP) and its corresponding service, common management information serviceThe Common Management Information Service is the service interface specified in that is employed by OSI network elements for network management. It defines the service interface that is implemented by the Common Management Information Protocol as specified in...
(CMIS), they need to interact with every layer in order to deal with their instances.
- MPLS
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
operates at an OSI-model layer that is generally considered to lie between traditional definitions of layer 2 (data link layer) and layer 3 (network layer), and thus is often referred to as a "layer-2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.
- ARP is used to translate IPv4 addresses (OSI layer 3) into Ethernet MAC addresses (OSI layer 2).
Interfaces
Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific.
For example
Microsoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
'
WinsockIn computing, the Windows Sockets API , which was later shortened to Winsock, is a technical specification that defines how Windows network software should access network services, especially TCP/IP. It defines a standard interface between a Windows TCP/IP client application and the underlying...
, and
UnixUnix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
's
Berkeley socketsThe Berkeley sockets application programming interface comprises a library for developing applications in the C programming language that perform inter-process communication, most commonly for communications across a computer network....
and
System VUnix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
Transport Layer InterfaceIn computer networking, the Transport Layer Interface was the networking API provided by AT&T UNIX System V Release 3 in 1987 and continued into Release 4 . TLI was the System V counterpart to the BSD sockets programming interface, which was also provided in UNIX System V Release 4...
, are interfaces between applications (layer 5 and above) and the transport (layer 4).
NDISThe Network Driver Interface Specification is an application programming interface for network interface cards . It was jointly developed by Microsoft and 3Com Corporation, and is mostly used in Microsoft Windows, but the open-source NDISwrapper and Project Evil driver wrapper projects allow...
and
ODIThe Open Data-Link Interface , developed by Apple and Novell, serves the same function as Microsoft and 3COM's Network Driver Interface Specification . Originally, ODI was written for NetWare and Macintosh environments. Like NDIS, ODI provides rules that establish a vendor-neutral interface between...
are interfaces between the media (layer 2) and the network protocol (layer 3).
Interface standards, except for the physical layer to media, are approximate implementations of OSI service specifications.
Examples
| Layer |
OSI Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
protocols |
TCP/IP protocolsThe Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
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Signaling System 7 |
AppleTalk AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
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IPX Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
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SNA Systems Network Architecture is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program...
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UMTS |
Misc. examples |
| # |
Name |
| 7 |
Application |
FTAM FTAM, ISO standard 8571, is the OSI Application layer protocol for File Transfer Access and Management.The goal of FTAM is to combine into a single protocol both file transfer, similar in concept to the Internet FTP, as well as remote access to open files, similar to NFS...
, X.400X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems — more commonly known as "email"....
, X.500X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by ITU-T, formerly known as CCITT, and first approved in 1988. The directory services were developed in order to support the requirements of X.400 electronic mail exchange and...
, DAPDirectory Access Protocol is a computer networking standard promulgated by ITU-T and ISO in 1988 for accessing an X.500 directory service. DAP was intended to be used by client computer systems, but was not popular as there were few implementations of the full OSI protocol stack for desktop...
, ROSEThe Remote Operations Service Element is the OSI service interface, specified in , that provides remote operation capabilities, allows interaction between entities in a distributed application, and upon receiving a remote operations service request, allows the receiving entity to attempt the...
, RTSE, ACSEAssociation Control Service Element is the OSI method for establishing a call between two application programs. ACSE checks the identities and contexts of the application entities, and could apply an authentication security check....
CMIPThe Common Management Information Protocol is the OSI specified network management protocol.Defined in . It provides an implementation for the services defined by the Common Management Information Service specified in , allowing communication between network management applications and...
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NNTP The Network News Transfer Protocol is an Internet application protocol used for transporting Usenet news articles between news servers and for reading and posting articles by end user client applications...
, SIPThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
, SSIThe SSI protocol is a simple communications protocol designed for data transfer between computers or user terminals and smart sensors. The SSI protocol is an Application layer protocol as in the OSI model....
, DNSThe Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
, FTPFile Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
, Gopher, HTTP, NFS, NTPThe Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
, DHCPThe Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
, SMPPThe Short Message Peer-to-Peer protocol is a telecommunications industry protocol for exchanging SMS messages between SMS peer entities such as short message service centers and/or External Short Messaging Entities. It is often used to allow third parties The Short Message Peer-to-Peer (SMPP)...
, SMTPSimple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
, SNMPSimple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
, TelnetTelnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility using a virtual terminal connection...
, RIPThe Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
, BGPThe Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
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INAP The Intelligent Network Application Part is a signalling protocol used in the intelligent network architecture. It is part of the SS7 protocol suite, typically layered on top of TCAP.It can also be termed as logic for controlling telecommunication services migrated from traditional switching...
, MAPThe Mobile Application Part is an SS7 protocol which provides an application layer for the various nodes in GSM and UMTS mobile core networks and GPRS core networks to communicate with each other in order to provide services to mobile phone users...
, TCAPTransaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate...
, ISUPThe ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
, TUP |
AFP The Apple Filing Protocol is a network protocol that offers file services for Mac OS X and original Mac OS. In Mac OS X, AFP is one of several file services supported including Server Message Block , Network File System , File Transfer Protocol , and WebDAV...
, ZIP, RTMPRouting Table Maintenance Protocol is a communication protocol used by AppleTalk to ensure that all routers on the network have consistent routing information....
, NBPAppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
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RIP R.I.P. is an abbreviation for rest in peace .R.I.P. or RIP may also refer to:-Biology:*Regulated intramembrane proteolysis, a biochemical process which regulates a semipermeable membrane...
, SAPThe Service Advertising Protocol is included in the Internetwork Packet Exchange protocol. SAP makes the process of adding and removing services on an IPX internetwork dynamic...
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APPC |
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HL7, Modbus Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers . Simple and robust, it has since become one of the de facto standard communications protocols in the industry, and it is now amongst the most commonly available means of...
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| 6 |
Presentation |
ISO/IEC 8823, X.226, ISO/IEC 9576-1, X.236 |
MIME Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
, SSL, TLSTransport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...
, XDRExternal Data Representation is a standard data serialization format, for uses such as computer network protocols. It allows data to be transferred between different kinds of computer systems. Converting from the local representation to XDR is called encoding. Converting from XDR to the local...
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AFP The Apple Filing Protocol is a network protocol that offers file services for Mac OS X and original Mac OS. In Mac OS X, AFP is one of several file services supported including Server Message Block , Network File System , File Transfer Protocol , and WebDAV...
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TDI, ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
, EBCDICExtended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....
, MIDIMIDI is an industry-standard protocol, first defined in 1982 by Gordon Hall, that enables electronic musical instruments , computers and other electronic equipment to communicate and synchronize with each other...
, MPEG |
| 5 |
Session |
ISO/IEC 8327, X.225, ISO/IEC 9548-1, X.235 |
Sockets. Session establishment in TCPThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, RTPThe Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...
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ASP AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
, ADSPAppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
, PAP |
NWLink NWLink is Microsoft's implementation of Novell's IPX/SPX protocols. NWLink includes an implementation of NetBIOS atop IPX/SPX.NWLink packages data to be compatible with client/server services on NetWare Networks. However, NWLink does not provide access to NetWare File and Print Services...
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DLC In the OSI networking model, Data Link Control is the service provided by the data link layer. Network interface cards have a DLC address that identifies each card; for instance, Ethernet and other types of cards have a 48-bit MAC address built into the cards' firmware when they are...
? |
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Named pipes, NetBIOS NetBIOS is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, NetBIOS is not a networking protocol...
, SAPSession Announcement Protocol is a protocol for broadcasting multicast session information.A SAP listening application can listen to the SAP multicast IP address and construct a guide of all advertised multicast sessions...
, half duplex, full duplex, simplexIn geometry, a simplex is a generalization of the notion of a triangle or tetrahedron to arbitrary dimension. Specifically, an n-simplex is an n-dimensional polytope which is the convex hull of its n + 1 vertices. For example, a 2-simplex is a triangle, a 3-simplex is a tetrahedron,...
, RPCIn computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
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| 4 |
Transport |
ISO/IEC 8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234 |
TCPThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, UDPThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
, SCTPIn computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
, DCCPThe Datagram Congestion Control Protocol is a message-oriented Transport Layer protocol. DCCP implements reliable connection setup, teardown, Explicit Congestion Notification , congestion control, and feature negotiation. DCCP was published as RFC 4340, a proposed standard, by the IETF in March,...
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DDP Datagram Delivery Protocol is a member of the AppleTalk networking protocol suite. Its main responsibility is for socket-to-socket delivery of datagrams over an AppleTalk network....
, SPX |
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NBF |
| 3 |
Network |
ISO/IEC 8208, X.25 X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
(PLP), ISO/IEC 8878, X.223CONS, Connection-Oriented Network Service, is one of the two OSI network layer protocols, the other being CLNS...
, ISO/IEC 8473-1, CLNPCLNS is an abbreviation of Connectionless Network Service.It is an OSI Network Layer service that does not require a circuit to be established before data is transmitted...
X.233. |
IPThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, IPsecInternet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
, ICMPThe Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
, IGMPThe Internet Group Management Protocol is a communications protocol used by hosts and adjacent routers on IP networks to establish multicast group memberships....
, OSPFOpen Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
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SCCP, MTP The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
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ATP AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
(TokenTalk or EtherTalk) |
IPX Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
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RRC The Radio Resource Control protocol belongs to the UMTS WCDMA protocol stack and handles the control plane signalling of Layer 3 between the UEs and the UTRAN...
(Radio Resource ControlThe Radio Resource Control protocol belongs to the UMTS WCDMA protocol stack and handles the control plane signalling of Layer 3 between the UEs and the UTRAN...
) Packet Data Convergence ProtocolPDCP is an abbreviation for Packet Data Convergence Protocol. It is one of the layers of the Radio Traffic Stack in UMTS and performs IP header compression and decompression, transfer of user data and maintenance of sequence numbers for Radio Bearers which are configured for lossless serving radio...
(PDCPPDCP is an abbreviation for Packet Data Convergence Protocol. It is one of the layers of the Radio Traffic Stack in UMTS and performs IP header compression and decompression, transfer of user data and maintenance of sequence numbers for Radio Bearers which are configured for lossless serving radio...
) and BMCThe Broadcast/Multicast control is a sublayer of layer 2 protocol of Radio Interface Protocol Architecture as per BMC-STD. It exists in the user plane only. It is located above the Radio Link Control , a layer 2 responsible for mapping logical channels. It is similar to 802.11's LLC layer which...
(Broadcast/Multicast ControlThe Broadcast/Multicast control is a sublayer of layer 2 protocol of Radio Interface Protocol Architecture as per BMC-STD. It exists in the user plane only. It is located above the Radio Link Control , a layer 2 responsible for mapping logical channels. It is similar to 802.11's LLC layer which...
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NBF, Q.931 ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
, IS-ISIntermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
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| 2 |
Data Link |
ISO/IEC 7666, X.25 X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
(LAPBLink Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
), Token BusToken bus is a network implementing the token ring protocol over a "virtual ring" on a coaxial cable. A token is passed around the network nodes and only the node possessing the token may transmit. If a node doesn't have anything to send, the token is passed on to the next node on the virtual ring...
, X.222, ISO/IEC 8802-2 LLCIEEE 802.2 is the IEEE 802 standard defining Logical Link Control , which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer...
Type 1 and 2 |
PPP In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
, SBTV SLIP- In science and technology :* Slip , an aqueous suspension of minerals, and frequently deflocculant.* Slip , a positional displacement in a sequence of transmitted symbols...
, PPTP |
MTP The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
, Q.710The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
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LocalTalkLocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate of 230.4 kbit/s...
, AppleTalk Remote AccessAppleTalk Remote Access, or ATRA, was a protocol stack that allowed AppleTalk to be run over modems. It became a fairly major product for Apple Computer in the early to mid-1990s when their first portable and laptop computers were available...
, PPPIn networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
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IEEE 802.3 IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
framing, Ethernet II framing |
SDLC Synchronous Data Link Control is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture . SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header...
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LLC The logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
(Logical Link ControlThe logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
), MACThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(Media Access ControlThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
) |
802.3 (Ethernet)Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, 802.11a/b/g/n MAC/LLCIEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
, 802.1Q (VLAN)IEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
, ATMAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
, HDP, FDDI, Fibre ChannelFibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
, Frame RelayFrame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, HDLC, ISLCisco Inter-Switch Link is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches....
, PPPIn networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
, Q.921, Token Ring, CDPThe Cisco Discovery Protocol is a proprietary Data Link Layer network protocol developed by Cisco Systems. It is used to share information about other directly connected Cisco equipment, such as the operating system version and IP address...
, NDPThe Nortel Discovery Protocol is a Data Link Layer network protocol for discovery of Nortel devices...
ARPAddress Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(maps layer 3 to layer 2 address), ITU-T G.hn DLLG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
CRCA cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
, Bit stuffingIn data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
, ARQ, Data Over Cable Service Interface Specification (DOCSIS)Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
, interface bonding |
| 1 |
Physical |
X.25 X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
(X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530Currently known asTIA-530-A, but often called EIA-530, or RS-530, is a balanced serial interface standard that generally uses a 25-pin connector, originally created by the Telecommunications Industry Association....
, G.703G.703 is an ITU-T standard for transmitting voice or data over digital carriers such as T1 and E1. G.703 provides specifications for pulse code modulation .G.703 also specifies E0...
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MTP The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
, Q.710The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
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RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, RS-422, STP, PhoneNetPhoneNet was an implementation of the AppleTalk networking physical layer created by Farallon Computing . Instead of using expensive shielded twisted pair wiring as in Apple's LocalTalk implementation, PhoneNet uses standard four-conductor telephone patch cords and modular connectors, and is...
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Twinax Twinaxial cabling, or "Twinax", is a type of cable similar to coax, but with two inner conductors instead of one. Due to cost efficiency it is becoming common in modern very-short-range high-speed differential signaling applications.-IBM:...
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UMTS Physical layer or L1 Universal Mobile Telecommunications System is a third generation mobile cellular technology for networks based on the GSM standard. Developed by the 3GPP , UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for...
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RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, Full duplex, RJ45, V.35, V.34, I.430The I.431/430 Networking standards are recommendations produced by the ITU. They are Layer 1 specifications for ISDN networks, using either an E1 or T1 circuit. The I.431 standard is known as the 'PRI Physical Layer' whereas the I.430 is known as the 'BRI Physical Layer'.-External links:* from the...
, I.431The I.431/430 Networking standards are recommendations produced by the ITU. They are Layer 1 specifications for ISDN networks, using either an E1 or T1 circuit. The I.431 standard is known as the 'PRI Physical Layer' whereas the I.430 is known as the 'BRI Physical Layer'.-External links:* from the...
, T1In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
, E1In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
, 10BASE-TEthernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
, 100BASE-TX, POTSPlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
, SONETSynchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
, SDH, DSLDigital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
, 802.11a/b/g/n PHYIEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
, ITU-T G.hn PHYG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
, Controller Area NetworkController–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....
, Data Over Cable Service Interface Specification (DOCSIS)Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
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Comparison with TCP/IP
In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as in the OSI model. RFC 3439 contains a section entitled "Layering
considered harmfulIn computer science and related disciplines, considered harmful is a phrase popularly used in the titles of diatribes and other critical essays ....
." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.
Even though the concept is different from the OSI model, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet
application layerThe Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
includes the OSI application layer, presentation layer, and most of the session layer. Its end-to-end
transport layerIn computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...
includes the graceful close function of the OSI session layer as well as the OSI transport layer. The internetworking layer (
Internet layerThe internet layer or IP layer is a group of internetworking methods in the Internet protocol suite, commonly also called TCP/IP, which is the foundation of the Internet...
) is a subset of the OSI network layer (see above), while the
link layerIn computer networking, the link layer is the lowest layer in the Internet Protocol Suite , the networking architecture of the Internet . It is the group of methods or protocols that only operate on a host's link...
includes the OSI data link and physical layers, as well as parts of OSI's network layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the network layer document.
The presumably strict peer layering of the OSI model as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of
tunneling protocolComputer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
s, which provide a link layer for an application, although the tunnel host protocol may well be a transport or even an application layer protocol in its own right.
See also
- Cognitive networks
- Hierarchical internetworking model
The Hierarchical internetworking model, or three-layer model, is a network design model first proposed by Cisco. Thethree-layer model divides enterprise networks into three layers: core, distribution, and access layer. Each layer provides different...
- Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
- Layer 8
Layer 8 is humorous Internet jargon used to refer to a nonexistent "user" or "political" layer on top of the OSI model of computer networking.The OSI model is a 7-layer abstract model that describes an architecture of data communications for networked computers. The layers build upon each other,...
- OSI protocol suite
- Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
- Service layer
In intelligent networks and cellular networks, service layer is a conceptual layer within a network service provider architecture. It aims at providing middleware that serves third-party value-added services and applications at a higher application layer...
- TCP/IP model
- X.25 protocol suite
- WAP protocol suite
- List of information technology acronyms
External links
- ISO/IEC standard 7498-1:1994 (PDF document
Portable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....
inside ZIP archiveZip is a file format used for data compression and archiving. A zip file contains one or more files that have been compressed, to reduce file size, or stored as is...
) (requires HTTP cookies in order to accept licence agreement)
- ITU-T X.200 (the same contents as from ISO)
- The ISO OSI Reference Model , Beluga graph of data units and groups of layers, Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425 – 432.
- Cisco Systems Internetworking Technology Handbook