Osi model

Understanding the OSI ModelUnderstanding the OSI Model

Understanding the OSI ModelUnderstanding the OSI Model

• International Organization for Standardization (ISO)– Multi-industry association that attempts to standardize

and define items that increase communication and compatibility in many different countries

• Open Systems Interconnection (OSI)– Seven layer reference model created by the ISO

– Defines and separates networking hardware and software into distinct layers and functions

The OSI (Open Systems Interconnection)

Reference Model is a seven-layer modelto describe how to connect any combination of devices for purposes of communications.

This model describes the task in termsof seven functional layers, and specifies thefunctions that must be available at eachlayer.

The focus in this model is on the “interconnection”and on the information thatcan be passed over this connection.

The OSI model does not concern itself with the internal operations of the systems involved.

Reasons for LayeringReasons for Layering

• Simplifies the network model• Enables programmers to specialize in a

particular level or layer of the networking model

• Provides design modularity• Encourages interoperability• Allows for standardized interfaces to be

produced by networking vendors


• Protocol– Defined method for communicating between

systems

• Computers must use a common protocol to communicate properly– Examples: TCP/IP and IPX/SPX


• As a group, these layers form the OSI protocol stack

Figure 1-1:OSI Reference Model

Peer OSI CommunicationPeer OSI Communication

• Peer communication– Each layer will only talk to its peer on the opposite side of

the communications process– Each layer is unaware of the activities of all other layers of

the model– Allows error checking to occur on two separate layers

simultaneously

• Each layer does provide services to the layer above it and receives services from the layer below it– Layers do not acknowledge these services in any way

Seven Layers of the OSI ModelSeven Layers of the OSI Model

• Physical layer

• Data Link layer

• Network layer

• Transport layer

• Sessions layer

• Presentation layer

• Application layer

Peer OSI CommunicationPeer OSI Communication

• The seven layers of the OSI reference model communicate with each other via peer communication

Figure 1-2:Peer communication

Layer Functions:Layer Functions:Physical (Layer 1)Physical (Layer 1)

• Layer 1 in the OSI model

• Has the following responsibilities:– Definition of the physical characteristics of the

network hardware– Representation of binary digits as voltages

(encoding)– Transmission of signals on the wire


Figure 1-3:568B twisted pair wiring scheme


• Connectors, cables, and devices like repeaters and hubs can be associated with the physical layer

• When choosing cable, consider the following:– Expense

– Physical location

– Distance

– Security requirements

– Transmission speed required

Layer Functions:Layer Functions:Data Link (Layer 2)Data Link (Layer 2)

• Has several responsibilities:– NIC software functions, including identification of

source and destination nodes via physical addresses (Media Access Control (MAC) Address )

– Definition of how data is packaged for transport in smaller units known as frames

– Error detection / notification

– Flow control of information sent across the link


• Data Link layer has two sublayers that further articulate its function:– Logical Link Control (LLC) layer

– Media Access Control (MAC) layer

• The Institute of Electrical and Electronics Engineers (IEEE) created these sublayers to identify and isolate the separate responsibilities required at this level of the protocol stack


Figure 1-4:Data Link layer subdivision


• MAC layer defines the media access method and provides a unique identifier for the network card

• The unique identifier is a 48-bit address represented as 12-digit hexadecimal number given to each network card during production

• Every network interface card must have a unique physical address (also called the MAC address)


Figure 1-5:MAC address


• Ethernet– A standard networking architecture that defines the

physical layout, lengths, and types of media that can be used

• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)– Network access method used by Ethernet networks

Layer Functions:Layer Functions:Network (Layer 3)Network (Layer 3)

• Has the following function:– Error handling– Software addressing for data packets– Data routing and connectivity– Best path selection

• The Network layer contains the logical address for the computer– Software/logical addressing for data packets, such as IP, IPX, and

AppleTalk

• Protocols at the Network layer allow computers to route packets to remote networks using a logical address

Layer Functions:Layer Functions:Transport (Layer 4)Transport (Layer 4)

• Provides point-to-point data transportation• Responsibilities include:

– End-to-end error-free transmission and delivery between the ultimate sender and ultimate receiver

– Flow control– Data segmentation into maximum transmission unit

(MTU) size– Messaging service for the Session layer– Protocols that reside at the Transport layer can be

connection-oriented or connectionless

Layer Functions:Layer Functions:Transport (Layer 4)Transport (Layer 4)

• Protocols that reside at the Transport layer can be:– Connection-oriented– Connectionless• Data sent by a connectionless transport is also

called a datagram

Layer Functions:Layer Functions:Session (Layer 5)Session (Layer 5)

• Enables two applications on the network to have an ongoing conversation or dialog

• Examples of Session layer protocols include:– SQL– NetBIOS– RPC– X-Windows

Layer Functions:Layer Functions:Session (Layer 5)Session (Layer 5)

• Provides the following services:– Control for data exchange (full or half duplex)– Data synchronization definition– Failure recovery– Communication setup and teardown– Clocking or timing– Initial link setup and link termination when

communications complete

• The Session layer allows the transfer of a large set of data across the network

Layer Functions:Layer Functions:Presentation (Layer 6)Presentation (Layer 6)

• Prepares the data from Application layer for transmission over the network

• Components include extensions and coding schemes such as:– BMP– WAV– EBCDIC– ASCII

Layer Functions:Layer Functions:Presentation (Layer 6)Presentation (Layer 6)

• This layer also provides encryption services when data encryption is used in network communications

• Has these responsibilities:– Data translation

– Data formatting

– Data syntax restructuring

– Data encryption

– Data compression

Layer Functions:Layer Functions:Application (Layer 7)Application (Layer 7)

• Has the following responsibilities:– Initiating the request for network service– Providing network services to applications such as

e-mail and Web browsers

• This layer is concerned with user interaction with the computer and the network– Contains many protocols and utilities, such as

telnet, FTP, HTTP, DNS, SMTP, and SNMP

Layer Functions:Layer Functions:Application (Layer 7)Application (Layer 7)

Programs and protocols that provide application-layer services include thefollowing:• NICE (Network Information and Control Exchange), which provides

network monitoring and management capabilities• FTAM (File Transfer, Access, and Management), which provides capabilities

for remote file handling• FTP (File Transfer Protocol), which provides file transfer capabilities• X.400, which specifies protocols and functions for message handling and

e-mail services• CMIP, which provides network management capabilities based on a

framework formulated by the ISO• SNMP, which provides network management within a non-OSI framework.

This protocol does not conform to the OSI model, but does provide functionality that is specified within the OSI model

• Telnet, which provides terminal emulation and remote login capabilities.Telnet’s capabilities go beyond the application layer

• rlogin, which provides remote login capabilities for UNIX environments

Data EncapsulationData Encapsulation

• Data is sent from one computer to another in a data packet

• The packet contains data from the sending application and additional information added by the protocol stack

• Prior to transmission across the network, the data is organized into a data frame at layer 2


• Protocol data unit (PDU)– Information added to a data packet by the layers

of the protocol stack

• Encapsulation– Process that occurs during transmission through

the protocol stack in which data from the higher layers is wrapped in a protocol header and/or trailer


Figure 1-6: Encapsulation


Table 1-1:Five steps of data encapsulation

OSI reference modelOSI reference model

Open System Interconnection reference Open System Interconnection reference model. Network architectural model model. Network architectural model

developeddevelopedby ISO and ITU-T. The model consists of by ISO and ITU-T. The model consists of

sevenseven layers, each of which specifies layers, each of which specifiesparticular network functions such as particular network functions such as

addressing, flow control, error control,addressing, flow control, error control,encapsulation, and reliable message encapsulation, and reliable message

transfer. transfer.

The The lowest layerlowest layer (the physical layer) is (the physical layer) isclosest to the media technology. The lower closest to the media technology. The lower two layers are implemented in hardware two layers are implemented in hardware and software, while the and software, while the upper five layersupper five layers are are implemented only in software. implemented only in software.

OSI reference modelOSI reference model

The highest layer (the application layer) is closest to the user. The OSI reference model is used universally as a method for teaching and understanding network functionality. Similar in some respects to SNA. See application layer, data-link layer, network layer, physical layer, PQ, session layer, and transport layer.

OSI reference model

Chapter SummaryChapter Summary

• Two or more computers connected by media form a network

• The ISO developed the OSI model in the mid-1980s to standardize networking models

• Data transmission can be connection-oriented or connectionless


• The OSI model has seven layers:– Physical layer– Data Link layer– Network layer– Transport layer– Session layer– Presentation layer– Application layer


• The Presentation layer, the sixth layer, handles data translation, encryption, and formatting for transmission on the network or for interpretation by the Application layer

• The Application layer, the seventh and highest layer, handles the interface between the network and the user

• When the network user sends data to the network, it goes through a five-step data encapsulation process


• When the network user sends data to the network, it goes through a five-step data encapsulation process

• This process takes place as the data packet travels down the OSI stack

Date post:	30-Jul-2015
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Osi model

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