Chapter 1 _OSI Model

832019 Chapter 1 _OSI Model

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THE PURPOSE OF THE MODEL

bull It breaks communication into smaller simpler parts that are easier

to develop

bull It facilitates standardization of network components to allow

multiple vendor development and support

bull It allows different types of network hardware and software to

communicate with each other

bull It prevents changes in one layer from affecting the other layer so

that they can develop more quickly



Tasks involved in sending letter



OSI Reference Model 7 Layers





WORKING OF OSI MODEL



bullProvides physical interface for transmission of information

bull Defines rules by which bits are passed from one system to

another on a physical communication medium

bull Covers all - mechanical electrical functional and procedural -

aspects for physical communication

bull Such characteristics as voltage levels timing of voltage

changes physical data rates maximum transmission

distances physical connectors and other similar attributes are

defined by physical layer specifications

Physical Layer



Data Link Layer

bull Data link layer attempts to provide reliable communication over

the physical layer interface

bull Breaks the outgoing data into frames and reassemble the

received frames

bull Create and detect frame boundaries

bull Handle errors by implementing an acknowledgement and

retransmission scheme

bull Implement flow control

bull Supports points-to-point as well as broadcast communication

bull Supports simplex half-duplex or full-duplex communication

OSI Model



Data Link Layer



bull Implements routing of frames (packets) through the network

bull Defines the most optimum path the packet should take from

the source to the destination

bull Defines logical addressing so that any endpoint can be

identified

bull Handles congestion in the network

bull Facilitates interconnection between heterogeneous networks

(Internetworking)

bull The network layer also defines how to fragment a packet into

smaller packets to accommodate different media

Network Layer



Transport Layer

bull Purpose of this layer is to provide a reliable mechanism for the

exchange of data between two processes in different computers

bull Ensures that the data units are delivered error free

bull

Ensures that data units are delivered in sequence

bull Ensures that there is no loss or duplication of data units

bull Provides connectionless or connection oriented service

bull Provides for the connection management

bull Multiplex multiple connection over a single channel





Session Layerbull Session layer provides mechanism for controlling the dialogue between the

two end systems It defines how to start control and end conversations (called

sessions) between applications

bull This layer requests for a logical connection to be established on an end-userrsquos

request

bull Any necessary log-on or password validation is also handled by this layer

bull Session layer is also responsible for terminating the connection

bull This layer provides services like dialogue discipline which can be full duplex or

half duplex

bull Session layer can also provide check-pointing mechanism such that if a failure

of some sort occurs between checkpoints all data can be retransmitted from

the last checkpoint



Presentation Layer



Application Layer

bull Application layer interacts with application programs and is the

highest level of OSI model

bull Application layer contains management functions to support

distributed applications

bull Examples of application layer are applications such as file

transfer electronic mail remote login etc







Shortcomings of OSI Model

Just because someone says it is a modelstandard does not mean

you have to follow

bull All layers do not have the same size and importance

-session and presentation layers seldom present

-data link network and transport layers often very full

bull Little agreement on where to place various features ndashEncryption

network management

bull Large number of layers increases overheads



PDUs are relevant in relation to each of the first 4 layers of the OSI model as follows

The Layer 1 (Physical Layer) PDU is the bit

The Layer 2 (Data Link Layer) PDU is the frame

The Layer 3 (Network Layer) PDU is the packet

The Layer 4 (Transport Layer) PDU is the segment (eg TCP segment)



UNIFIED VIEW OF LAYERS PROTOCOLS SERVICES







A layer may impose a limit on size of data block that can be

transferred due to implementation or due to other reasons

The layer n SDU may be too large to be handled as a single

unit by layer (n-1)

Sender side SDU is segmented into multiple PDUs

Receiver side SDU is reassembled from sequence of PDUs





bull Multiplexing sharing of n service by a multiple layer n+1 users

bull Multiplexing a tag or ID required in each PDU to determine which

user an SDU belongs to

bull Eg port number in TCPIP



TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer

the services that user applications use to communicate over the

network and it is the layer in which user-access network processes

reside

process user data such as data encryption and decryption and

compression and decompression

manages the sessions (connections) between cooperating

applications

responsible for standardizing the presentation of data



Transport Layer

It is responsible for end-to-end data integrity

two most important protocols employed at this layer are the

Transmission Control Protocol (TCP) and User Datagram Protocol

(UDP)

TCP provides reliable full-duplex connections and reliable service by

ensuring that data is resubmitted when transmission results in an

error (end-to-end error detection and correction) Also TCP enables

hosts to maintain multiple simultaneous connections

When error correction is not required UDP provides unreliable

datagram service (connectionless) that enhances network throughput

at the host-to-host transport layer



Internetwork Layer

the basic packet delivery service for all TCPIP networks

Define the datagram which is the basic unit of transmission in

the Internet

Move data between the Network Access Layer and the Host-to-

Host Transport Layer

Fragment and reassemble datagram



N t k A L



Network Access Layer

bull it is concerned with getting data across a specific type of

physical network

bull encapsulating the IP datagram into frames that are

transmitted by the network

bull maps the IP addresses to the physical addresses used by the

network

bull responsible for routing and must add its routing information

to the data









THE PURPOSE OF THE MODEL

bull It breaks communication into smaller simpler parts that are easier

to develop

bull It facilitates standardization of network components to allow

multiple vendor development and support

bull It allows different types of network hardware and software to

communicate with each other

bull It prevents changes in one layer from affecting the other layer so

that they can develop more quickly























Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data





























Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data


























Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data























Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data





















Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data


















Physical Layer



Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Data Link Layer




received frames







OSI Model



Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Data Link Layer







identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data













identified



(Internetworking)



Network Layer



Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Transport Layer




bull














request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data















request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data













request




half duplex



the last checkpoint



Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Presentation Layer



Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Application Layer















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data















you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data













you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data











you have to follow





network management





















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data


























unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data



















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data
















unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data














unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data












unit by layer (n-1)













TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data

















TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data















TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









TCP IP ARCHITECTURE



TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









TCP IP ARCHITECTURE



ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









ADDRESSING



A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









A li ti L



Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Application Layer



reside




applications




Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Transport Layer




(UDP)










Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









Internetwork Layer



the Internet






N t k A L





physical network




network


to the data









N t k A L





physical network




network


to the data











physical network




network


to the data



















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Chapter 1 _OSI Model