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OSI model
The 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 their 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 layers.
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 standard X.200.
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.
A layer serves the layer above it and is served by the
layer below it. For example, a layer that provides errorfree 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 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
The model is a product of the Open Systems Inter- addenda. In this model, a networking system was divided
connection project at the International Organization for into layers. Within each layer, one or more entities imStandardization (ISO), maintained by the identification plement its functionality. Each entity interacted directly
only with the layer immediately beneath it, and provided
ISO/IEC 7498-1.
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 ITUT as the X.200-series of recommendations.[1] 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.[2]
Communication in the OSI-Model (example with layers 3 to 5)
1
2 Description of OSI layers
History
The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model.
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, or CCITT (the
abbreviation is from the French version of the name).
These two international standards bodies each developed
a document that defined similar networking models.
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 and/or footers.
In 1983, these two documents were merged to form a Data processing by two communicating OSI-compatible
standard called The Basic Reference Model for Open devices is done as such:
1
2
2 DESCRIPTION OF OSI LAYERS
1. The data to be transmitted is composed at the top- 2.2 Layer 2: Data Link Layer
most layer of the transmitting device (layer N) into
The data link layer provides node-to-node data transfer—
a protocol data unit (PDU).
a link between two directly connected nodes. It detects
2. The PDU is passed to layer N-1, where it is known and possibly corrects errors that may occur in the physias the service data unit (SDU).
cal layer. It, among other things, defines the protocol to
establish and terminate a connection between two phys3. At layer N-1 the SDU is concatenated with a header, ically connected devices. It also defines the protocol for
a footer, or both, producing a layer N-1 PDU. It is flow control between them.
then passed to layer N-2.
IEEE 802 divides the data link layer into two sublayers:[6]
4. The process continues until reaching the lowermost
level, from which the data is transmitted to the re• Media Access Control (MAC) layer - responsible for
ceiving device.
controlling how devices in a network gain access to
medium and permission to transmit it.
5. At the receiving device the data is passed from
• Logical Link Control (LLC) layer - responsible for
the lowest to the highest layer as a series of SDUs
identifying Network layer protocols and then encapwhile being successively stripped from each layer’s
sulating them and controls error checking and frame
header and/or footer, until reaching the topmost
synchronization.
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[5] ). 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 service.
2.1
Layer 1: Physical Layer
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 (e.g., a copper or fiber optical cable, radio frequency). 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. 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 of Parallel SCSI operates in this layer,
as do the physical layers of Ethernet and other local-area
networks, such as Token Ring, FDDI, ITU-T G.hn, and
IEEE 802.11 (Wi-Fi), as well as personal area networks
such as Bluetooth and IEEE 802.15.4.
The MAC and LLC layers of IEEE 802 networks such
as 802.3 Ethernet, 802.11 Wi-Fi, and 802.15.4 ZigBee,
operate at the data link layer.
The Point-to-Point Protocol (PPP) is a data link layer that
can operate over several different physical layers, such as
synchronous and asynchronous serial lines.
The ITU-T 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 sliding-window
protocol.
2.3 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
to the same network. It translates logical network address
into physical machine address. 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 need not, report delivery errors.
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 Message delivery at the network layer is not necessarily
this layer are network adapters, repeaters, network hubs, guaranteed to be reliable; a network layer protocol may
modems, and fiber media converters.
provide reliable message delivery, but it need not do so.
2.5
Layer 5: Session Layer
3
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.[7]
transport packet.
2.4
2.5 Layer 5: Session Layer
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 via one or more networks, while maintaining the quality of service functions.
An example of a transport-layer protocol in the standard
Internet stack is Transmission Control Protocol (TCP),
usually built on top of the Internet Protocol (IP).
The transport layer controls the reliability of a given link
through flow control, segmentation/desegmentation, and
error control. Some protocols are state- and connectionoriented. 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. The transport layer creates packets
out of the message received from the application layer.
Packetizing is a process of dividing the 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:[8]
Although not developed under the OSI Reference Model
and not strictly conforming to the OSI definition of the
transport layer, the Transmission Control Protocol (TCP)
and the User Datagram Protocol (UDP) of the Internet
Protocol Suite are commonly categorized as layer-4 protocols within OSI.
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.
2.6 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 (e.g., encryption) 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.[9]
The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with
An easy way to visualize the transport layer is to com- capabilities such as converting an EBCDIC-coded text
pare it with a post office, which deals with the dispatch file to an ASCII-coded file, or serialization of objects and
and classification of mail and parcels sent. Do remember, other data structures from and to XML.
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 2.7 Layer 7: Application Layer
that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, The application layer is the OSI layer closest to the end
such as carrying non-IP protocols such as IBM's SNA or user, which means both the OSI application layer and the
Novell's IPX over an IP network, or end-to-end encryp- user interact directly with the software application. This
tion with IPsec. While Generic Routing Encapsulation layer interacts with software applications that implement
(GRE) might seem to be a network-layer protocol, if the a communicating component. Such application programs
encapsulation of the payload takes place only at endpoint, fall outside the scope of the OSI model. ApplicationGRE becomes closer to a transport protocol that uses IP layer functions typically include identifying communicaheaders but contains complete frames or packets to de- tion partners, determining resource availability, and synliver to an endpoint. L2TP carries PPP frames inside chronizing communication. When identifying commu-
4
6
nication 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 resources for the requested
communication exist. In synchronizing communication,
all communication between applications requires cooperation that is managed by the application layer. This layer
supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and
any constraints on data syntax are identified. Everything
at this layer is application-specific.
COMPARISON WITH TCP/IP MODEL
state information from the PHY layer, network
throughput can be significantly improved and energy waste can be avoided.[10]
4 Interfaces
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 implementation-specific.
For example, Microsoft Windows' Winsock, and Unix's
Berkeley sockets and System V Transport Layer Inter3 Cross-layer functions
face, are interfaces between applications (layer 5 and
above) and the transport (layer 4). NDIS and ODI are
Cross-layer functions are services that are not tied to a interfaces between the media (layer 2) and the network
given layer, but may affect more than one layer. Examples protocol (layer 3).
include the following:
Interface standards, except for the physical layer to media, are approximate implementations of OSI service
• Security service (telecommunication)[5] as defined
specifications.
by ITU-T X.800 recommendation.
• Management functions, i.e. functions that permit to
configure, instantiate, monitor, terminate the com- 5 Examples
munications of two or more entities: there is a specific application-layer protocol, common management information protocol (CMIP) and its corre- 6 Comparison with TCP/IP model
sponding service, common management information service (CMIS), they need to interact with every The design of protocols in the TCP/IP model of the Inlayer in order to deal with their instances.
ternet does not concern itself with strict hierarchical encapsulation and layering.[16] RFC 3439 contains a sec• Multiprotocol Label Switching (MPLS) operates at tion entitled “Layering considered harmful".[17] TCP/IP
an OSI-model layer that is generally considered to lie does recognize four broad layers of functionality which
between traditional definitions of layer 2 (data link are derived from the operating scope of their contained
layer) and layer 3 (network layer), and thus is often protocols: the scope of the software application; the endreferred to as a “layer-2.5” protocol. It was designed to-end transport connection; the internetworking range;
to provide a unified data-carrying service for both and the scope of the direct links to other nodes on the
circuit-based clients and packet-switching clients local network.[18]
which provide a datagram-based service model. It
can be used to carry many different kinds of traf- Despite using a different concept for layering than the
fic, including IP packets, as well as native ATM, OSI model, these layers are often compared with the OSI
layering scheme in the following way:
SONET, and Ethernet frames.
• ARP is used to translate IPv4 addresses (OSI layer
3) into Ethernet MAC addresses (OSI layer 2).
• 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
3 connection to the third-party host.
• 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
• 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 network layer.
• 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.
5
These comparisons are based on the original seven-layer [9] Grigonis, Richard (2000). Computer telephony- encyclopaedia. CMP. p. 331. ISBN 9781578200450.
protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of
[10] G. Miao; G. Song (2014). Energy and spectrum efficient
the network layer document.
wireless network design. Cambridge University Press.
ISBN 1107039886.
The presumably strict layering of the OSI model as it
is usually described does not present contradictions in
[11] “ITU-T Recommendation Q.1400 (03/1993)], ArchitecTCP/IP, as it is permissible that protocol usage does not
ture framework for the development of signaling and
follow the hierarchy implied in a layered model. Such exOA&M protocols using OSI concepts". ITU. pp. 4, 7.
amples exist in some routing protocols (e.g., OSPF), or
in the description of tunneling protocols, which provide [12] ITU Rec. X.227 (ISO 8650), X.217 (ISO 8649).
a link layer for an application, although the tunnel host
[13] X.700 series of recommendations from the ITU-T (in parprotocol might well be a transport or even an applicationticular X.711) and ISO 9596.
layer protocol in its own right.
7
See also
• Hierarchical internetworking model
[15] “3GPP specification: 36.300”. 3gpp.org. Retrieved 14
August 2015.
• Management plane
[16] RFC 3439
• Layer 8
[17] “RFC 3439 - Some Internet Architectural Guidelines and
Philosophy”. ietf.org. Retrieved 14 August 2015.
• Protocol stack
• Service layer
• WAP protocol suite
• List of information technology acronyms
• IBM Systems Network Architecture
• Internet protocol suite
8
[14] “Internetworking Technology Handbook - Internetworking Basics [Internetworking]". Cisco. 15 January 2014.
Retrieved 14 August 2015.
References
[1] ITU-T X-Series Recommendations
[2] “Publicly Available Standards”. Standards.iso.org. 201007-30. Retrieved 2010-09-11.
[3] “The OSI Model’s Seven Layers Defined and Functions
Explained”. Microsoft Support. Retrieved 2014-12-28.
[4] RFC 4253
[5] “ITU-T Recommendataion X.800 (03/91), Security architecture for Open Systems Interconnection for CCITT applications". ITU. Retrieved 14 August 2015.
[6] “5.2 RM description for end stations”. IEEE Std 8022014, IEEE Standard for Local and Metropolitan Area
Networks: Overview and Architecture. ieee.
[7] International Organization for Standardization (1989-1115). “ISO/IEC 7498-4:1989 -- Information technology -Open Systems Interconnection -- Basic Reference Model:
Naming and addressing”. ISO Standards Maintenance
Portal. ISO Central Secretariat. Retrieved 2015-08-17.
[8] “ITU-T Recommendation X.224 (11/1995) ISO/IEC
8073, Open Systems Interconnection - Protocol for providing the connection-mode transport service". ITU.
[18] Walter Goralski. The Illustrated Network: How TCP/IP
Works in a Modern Network (PDF). Morgan Kaufmann.
p. 26. ISBN 978-0123745415.
9 External links
• Microsoft Knowledge Base: The OSI Model’s Seven
Layers Defined and Functions Explained
• ISO/IEC standard 7498-1:1994 (PDF document inside ZIP archive) (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
• Zimmermann, Hubert (April 1980). “OSI Reference Model — The ISO Model of Architecture
for Open Systems Interconnection”. IEEE Transactions on Communications. 28 (4): 425–432.
doi:10.1109/TCOM.1980.1094702.
CiteSeerX:
10.1.1.136.9497.
• Cisco Systems Internetworking Technology Handbook
• Osi Model : 7 Layer Of The Network Communication
6
10
10
10.1
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7
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Satellizer, Lord Chamberlain, the Renowned, Chester Markel, Jjenkins5123, Jakuzem, Bchiap, 336, Widr, Scottonsocks, Suyashparth,
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GAURAV AWASTHI and Anonymous: 2491
10.2
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10.3
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