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 model
to describe how to connect any combination of
devices for purposes of communications.
This model describes the task in terms
of seven functional layers, and specifies the
functions that must be available at each
layer.

The focus in this model is on the “interconnection”
and on the information that
can 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

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

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

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

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

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

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

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

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

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

Application (Layer 7)Application (Layer 7)
Programs and protocols that provide application-layer services include the
following:
• 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 referenceOpen System Interconnection reference
model. Network architectural modelmodel. Network architectural model
developeddeveloped
by ISO and ITU-T. The model consists ofby ISO and ITU-T. The model consists of
sevenseven layers, each of which specifieslayers, each of which specifies
particular network functions such asparticular network functions such as
addressing, flow control, error control,addressing, flow control, error control,
encapsulation, and reliable messageencapsulation, and reliable message
transfer.transfer.

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

Osi model

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Osi model