Open Systems Interconnection model (OSI model) is a conceptual
model that characterizes and standardizes the communication functions
of a telecommunication or computing system without regard to its
underlying internal structure and technology. Its goal is the
interoperability of diverse communication systems with standard
protocols. The model partitions a communication system into
abstraction layers. The original version of the model defined seven
A layer serves the layer above it and is served by the layer below it.
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 comprise
the contents of that path. Two instances at the same layer are
visualized as connected by a horizontal connection in that layer.
The model is a product of the
Open Systems Interconnection project at
International Organization for Standardization
International Organization for Standardization (ISO), maintained
by the identification ISO/IEC 7498-1.
Communication in the OSI-Model (example with layers 3 to 5)
2 Description of OSI layers
2.1 Layer 1: Physical Layer
2.2 Layer 2:
Data Link Layer
2.3 Layer 3: Network Layer
2.4 Layer 4: Transport Layer
2.5 Layer 5: Session Layer
2.6 Layer 6: Presentation Layer
2.7 Layer 7: Application Layer
3 Cross-layer functions
6 Comparison with TCP/IP model
7 See also
9 External links
In the late 1970s, one project was administered by the International
Organization for Standardization (ISO), while another was undertaken
by the International Telegraph and Telephone Consultative Committee
(CCITT, from French: Comité Consultatif International Téléphonique
et Télégraphique). These two international standards bodies each
developed a document that defined similar networking models.
In 1983, these two documents were merged to form a standard called The
Basic Reference Model for Open Systems Interconnection. The standard
is usually referred to as the
Open Systems Interconnection Reference
Model, the OSI Reference Model, or simply the OSI model. It was
published in 1984 by both the ISO, as standard ISO 7498, and the
renamed CCITT (now called the Telecommunications Standardization
Sector of the International
Telecommunication Union or ITU-T) as
OSI had two major components, an abstract model of networking, called
the Basic Reference Model or seven-layer model, and a set of specific
The concept of a seven-layer model was provided by the work of Charles
Bachman at Honeywell Information Services. Various aspects of OSI
design evolved from experiences with the ARPANET, NPLNET, EIN,
CYCLADES 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 enable 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. Not all are free of charge.
Description of OSI layers
The recommendation X.200 describes seven layers, labeled 1 to 7. Layer
1 is the lowest layer in this model.
Protocol data unit
Protocol data unit (PDU)
High-level APIs, including resource sharing, remote file access
Translation of data between a networking service and an application;
including character encoding, data compression and
Managing communication sessions, i.e. continuous exchange of
information in the form of multiple back-and-forth transmissions
between two nodes
Segment (TCP) /
Reliable transmission of data segments between points on a network,
including segmentation, acknowledgement and multiplexing
Structuring and managing a multi-node network, including addressing,
routing and traffic control
Reliable transmission of data frames between two nodes connected by a
Transmission and reception of raw bit streams over a physical medium
At each level N, two entities at the communicating devices (layer N
peers) exchange protocol data units (PDUs) by means of a layer N
protocol. Each PDU contains a payload, called the service data unit
(SDU), along with protocol-related headers or footers.
Data processing by two communicating OSI-compatible devices is done as
The data to be transmitted is composed at the topmost layer of the
transmitting device (layer N) into a protocol data unit (PDU).
The PDU is passed to layer N-1, where it is known as the service data
At layer N-1 the SDU is concatenated with a header, a footer, or both,
producing a layer N-1 PDU. It is then passed to layer N-2.
The process continues until reaching the lowermost level, from which
the data is transmitted to the receiving device.
At the receiving device the data is passed from the lowest to the
highest layer as a series of SDUs while being successively stripped
from each layer's header or footer, until reaching the topmost layer,
where the last of the data is consumed.
Some orthogonal aspects, such as management and security, involve all
of the layers (See
ITU-T X.800 Recommendation). These services are
aimed at improving the CIA triad - confidentiality, integrity, and
availability - of the transmitted data. In practice, the availability
of a communication service is determined by the interaction between
network design and network management protocols. Appropriate choices
for both of these are needed to protect against denial of
Layer 1: Physical Layer
This section may need to be rewritten entirely to comply with
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contain suggestions. (August 2017)
The physical layer defines the electrical and physical specifications
of the data connection. It defines the relationship between a device
and a physical transmission medium (for example, an electrical cable,
an optical fiber cable, or a radio frequency link). This includes the
layout of pins, voltages, line impedance, cable specifications, signal
timing and similar characteristics for connected devices and frequency
(5 GHz or 2.4 GHz etc.) for wireless devices. It is
responsible for transmission and reception of unstructured raw data in
a physical medium.
Bit rate control is done at the physical layer. It
may define transmission mode as simplex, half duplex, and full duplex.
It defines the network topology as bus, mesh, or ring being some of
the most common.
The physical layer is the layer of low-level networking equipment,
such as some hubs, cabling, and repeaters. The physical layer is never
concerned with protocols or other such higher-layer items. Examples of
hardware in this layer are network adapters, repeaters, network hubs,
modems, and fiber media converters.
Data Link Layer
The data link layer provides node-to-node data transfer—a link
between two directly connected nodes. It detects and possibly corrects
errors that may occur in the physical layer. It defines the protocol
to establish and terminate a connection between two physically
connected devices. It also defines the protocol for flow control
IEEE 802 divides the data link layer into two sublayers:
Medium access control
Medium access control (MAC) layer – responsible for controlling how
devices in a network gain access to a medium and permission to
Logical link control (LLC) layer – responsible for identifying and
encapsulating network layer protocols, and controls error checking and
The MAC and LLC layers of
IEEE 802 networks such as
802.11 Wi-Fi, and
ZigBee operate at the data link layer.
Point-to-Point Protocol (PPP) is a data link layer protocol that
can operate over several different physical layers, such as
synchronous and asynchronous serial lines.
G.hn standard, which provides high-speed local area
networking over existing wires (power lines, phone lines and coaxial
cables), includes a complete data link layer that provides both error
correction and flow control by means of a selective-repeat
Layer 3: Network Layer
The network layer provides the functional and procedural means of
transferring variable length data sequences (called datagrams) from
one node to another connected in "different networks". A network is a
medium to which many nodes can be connected, on which every node has
an address and which permits nodes connected to it to transfer
messages to other nodes connected to it by merely providing the
content of a message and the address of the destination node and
letting the network find the way to deliver the message to the
destination node, possibly routing it through intermediate nodes. If
the message is too large to be transmitted from one node to another on
the data link layer between those nodes, the network may implement
message delivery by splitting the message into several fragments at
one node, sending the fragments independently, and reassembling the
fragments at another node. It may, but does not need to, report
Message delivery at the network layer is not necessarily guaranteed to
be reliable; a network layer protocol may provide reliable message
delivery, but it need not do so.
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 layer provides the functional and procedural means of
transferring variable-length data sequences from a source to a
destination host, while maintaining the quality of service functions.
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 re-transmit those
that fail delivery. The transport layer also provides the
acknowledgement of the successful data transmission and sends the next
data if no errors occurred. The transport layer creates packets out of
the message received from the application layer. Packetizing is the
process of dividing a long message into smaller messages.
OSI defines five classes of connection-mode transport protocols
ranging from class 0 (which is also known as TP0 and provides the
fewest 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:
Concatenation and separation
Segmentation and reassembly
Multiplexing / demultiplexing over single virtual circuit
Explicit flow control
Retransmission on timeout
Reliable transport service
a If an excessive number of PDUs are unacknowledged.
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. A post office inspects only the outer envelope of
mail to determine its delivery. 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
protocols operate at the transport layer, such as carrying non-IP
protocols such as IBM's SNA or Novell's
IPX over an IP network, or
end-to-end encryption with IPsec. While Generic
(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 PPP frames inside
Although not developed under the OSI Reference Model and not strictly
conforming to the OSI definition of the transport layer, the
Transmission Control Protocol
Transmission Control Protocol (TCP) and the User
(UDP) of the
Internet Protocol Suite are commonly categorized as
layer-4 protocols within OSI.
Layer 5: Session Layer
The session layer controls the dialogues (connections) between
computers. It establishes, manages and terminates the connections
between the local and remote application. It provides for full-duplex,
half-duplex, or simplex 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 Protocol, 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 calls.
Layer 6: Presentation Layer
The presentation layer establishes context between application-layer
entities, in which the application-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 protocol stack.
This layer provides independence from data representation by
translating between application and network formats. The presentation
layer transforms data into the form that the application accepts. This
layer formats data to be sent across a network. It is sometimes called
the syntax layer. The presentation layer can include compression
functions. The Presentation Layer negotiates the Transfer Syntax.
The original presentation structure used the
Basic Encoding Rules of
Abstract Syntax Notation One (ASN.1), with capabilities such as
converting an EBCDIC-coded text file to an ASCII-coded file, or
serialization of objects and other data structures from and to XML.
ASN.1 effectively makes an application protocol invariant with respect
Layer 7: Application Layer
The application layer is the OSI layer closest to the end user, which
means 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. The
most important distinction in the application layer is the distinction
between the application-entity and the application. For example, a
reservation website might have two application-entities: one using
HTTP to communicate with its users, and one for a remote database
protocol to record reservations. Neither of these protocols have
anything to do with reservations. That logic is in the application
itself. The application layer per se has no means to determine the
availability of resources in the network.
Cross-layer functions are services that are not tied to a given layer,
but may affect more than one layer. Examples include the following:
Security service (telecommunication) as defined by
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 (CMIP) and its corresponding service,
common management information service (CMIS), they need to interact
with every layer in order to deal with their instances.
Multiprotocol Label Switching (MPLS) MPLS, ATM, and
X.25 are 3a
protocols. OSI divides the Network Layer into 3 roles: 3a) Subnetwork
Access, 3b) Subnetwork Dependent Convergence and 3c) Subnetwork
Independent Convergence. It was designed to provide a unified
data-carrying service for both circuit-based clients and
packet-switching clients which provide a datagram-based 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. Sometimes
one sees reference to a Layer 2.5. This is a fiction create by those
who are unfamiliar with the OSI Model and ISO 8648, Internal
Organization of the Network Layer in particular.
ARP determines the mapping of an
IPv4 address to the underlying MAC
address. This is not a translation function. If it were
IPv4 and the
MAC address would be at the same layer. The implementation of the MAC
protocol decodes the MAC PDU and delivers the User-
Data to the
Ethernet is a multi-access media, a device sending a
PDU on an
Ethernet segment needs to know what IP address maps to what
IPv4 addresses to new systems joining a network.
There is no means to derive or obtain an
IPv4 address from an Ethernet
Domain Name Service
Domain Name Service is an Application Layer service which is used to
look up the IP address of a given domain name. Once a reply is
received from the DNS server, it is then possible to form a Layer 4
connection or flow to the desired host. There are no connections at
Cross MAC and PHY Scheduling is essential in wireless networks because
of the time varying nature of wireless channels. By scheduling packet
transmission only in favorable channel conditions, which requires the
MAC layer to obtain channel state information from the PHY layer,
network throughput can be significantly improved and energy waste can
Neither the OSI Reference Model nor OSI protocols specify any
programming interfaces, other than deliberately abstract service
specifications. Protocol specifications precisely define the
interfaces between different computers, but the software interfaces
inside computers, known as network sockets are
For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets
and System V Transport Layer Interface, are interfaces between
applications (layer 5 and above) and the transport (layer 4). NDIS and
ODI 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.
Sockets (session establishment in TCP / RTP / PPTP)
ATP (TokenTalk / EtherTalk)
RRC / BMC
LLC (type 1 / 2)
IEEE 802.3 framing
Ethernet II framing
IEEE 802.3 (Ethernet)
IEEE 802.11a/b/g/n (
Ethernet MAC and LLC)
IEEE 802.1Q (VLAN)
Linux interface bonding
UMTS air interfaces
Comparison with TCP/IP model
The design of protocols in the
TCP/IP model of the Internet does not
concern itself with strict hierarchical encapsulation and
layering. RFC 3439 contains a section entitled "Layering
considered harmful". TCP/IP does recognize four broad layers of
functionality which are derived from the operating scope of their
contained protocols: the scope of the software application; the
end-to-end transport connection; the internetworking range; and the
scope of the direct links to other nodes on the local network.
Despite using a different concept for layering than the OSI model,
these layers are often compared with the OSI layering scheme in the
The Internet application layer includes the OSI application layer,
presentation layer, and most of the session layer.
Its end-to-end transport layer includes the graceful close function of
the OSI session layer as well as the OSI transport layer.
The internetworking layer (Internet layer) is a subset of the OSI
The link layer includes the OSI data link layer and sometimes the
physical layers, as well as some protocols of the 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 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 (for
example OSPF), or in the description of tunneling protocols, which
provide a link layer for an application, although the tunnel host
protocol might well be a transport or even an application-layer
protocol in its own right.
Hierarchical internetworking model
IBM Systems Network Architecture
Internet protocol suite
WAP protocol suite
List of information technology initialisms
ITU-T X-Series Recommendations
^ "Publicly Available Standards". Standards.iso.org. 2010-07-30.
^ "The OSI Model's Seven Layers Defined and Functions Explained".
Microsoft Support. Retrieved 2014-12-28.
^ a b "
ITU-T Recommendataion X.800 (03/91), Security architecture for
Open Systems Interconnection for CCITT applications". ITU. Retrieved
14 August 2015.
^ "5.2 RM description for end stations". IEEE Std 802-2014, IEEE
Standard for Local and Metropolitan Area Networks: Overview and
International Organization for Standardization
International Organization for Standardization (1989-11-15).
"ISO/IEC 7498-4:1989 -- Information technology -- Open Systems
Interconnection -- Basic Reference Model: Naming and addressing". ISO
Standards Maintenance Portal. ISO Central Secretariat. Retrieved
ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073, Open Systems
Interconnection - Protocol for providing the connection-mode transport
^ Grigonis, Richard (2000). Computer telephony- encyclopaedia. CMP.
p. 331. ISBN 9781578200450.
ITU-T X.200 - Information technology – Open Systems
Interconnection – Basic Reference Model: The basic model".
^ Miao, Guowang; Song, Guocong (2014). Energy and spectrum efficient
wireless network design. Cambridge University Press.
ITU-T Recommendation Q.1400 (03/1993)], Architecture framework for
the development of signaling and OA&M protocols using OSI
concepts". ITU. pp. 4, 7.
^ ITU Rec. X.227 (ISO 8650), X.217 (ISO 8649).
^ X.700 series of recommendations from the
ITU-T (in particular X.711)
and ISO 9596.
^ a b "Internetworking Technology Handbook - Internetworking Basics
[Internetworking]". Cisco. 15 January 2014. Retrieved 14 August
^ "3GPP specification: 36.300". 3gpp.org. Retrieved 14 August
^ RFC 3439
^ "RFC 3439 - Some Internet Architectural Guidelines and Philosophy".
ietf.org. Retrieved 14 August 2015.
^ Walter Goralski. The Illustrated Network: How TCP/IP Works in a
Modern Network (PDF). Morgan Kaufmann. p. 26.
Wikimedia Commons has media related to OSI model.
Microsoft Knowledge Base: The OSI Model's Seven Layers Defined and
ISO/IEC standard 7498-1:1994 (PDF document inside ZIP archive)
HTTP cookies in order to accept licence agreement)
ITU-T X.200 (the same contents as from ISO)
"INFormation CHanGe Architectures and Flow Charts powered by Google
App Engine". infchg.appspot.com. The ISO OSI Reference Model, Beluga
graph of data units and groups of layers. Archived from the original
Zimmermann, Hubert (April 1980). "OSI Reference Model — The ISO
Model of Architecture for Open Systems Interconnection". IEEE
Transactions on Communications. 28 (4): 425–432.
CiteSeerX 10.1.1.136.9497 .
Cisco Systems Internetworking Technology Handbook
ISO standards by standard number
List of ISO standards / ISO romanizations / IEC standards