The OSI Model

TopicsWhat are Protocols ?Why we need Protocols and StandardsThe OSI Reference ModelThe Seven Layers of the ModelResponsibilities of Each LayerThe Application LayerThe Presentation LayerThe Session LayerThe Transport LayerThe Network LayerThe Data Link LayerThe Physical Layer

What are Protocols ?Lets look at what happen:Oh my God, what did he say ?Khng bit thng ny ang ni ci g na ?They do not understand each other because they are using different languages.

Balloon callout. Select shape and start typing. Resize box to desired dimensions. Move control handle to aim pointer at speaker.

Hi, What is your name ?


Xin cho, bn tn l g vy ?

What are Protocols ? (cont)Now, Lets look at what happens next:They can understand each other because they are using the same language.


Hi, What is your name ?


Hi, my name is Gary, and what is your name ?


My name is Harry, nice to meet you.


Nice to meet you, too.

What are Protocols ? (cont)So in order for two people to speak to each other, the first thing they should to agree on is the language, or protocols, to use.Communication between hosts on the network is the same. When hosts begin communicating with each other, they first must agree on what protocols to use.

What are Protocols ? (cont)

Windows PC running IP protocol

IP

Netware PC running IPX protocol

IPX

Windows PC running IPX protocol

IPX

Netware PC running IPX protocol

IPX

What are Protocols ? (cont)A group of protocol is called a protocol suite or a protocol stack.When combined with other protocols, the protocol group that result is called a protocol suite. TCP/IP, for example, is a protocol suite.

Why We Need Protocols and StandardsRules or protocols and standards are important to ensure compatibility between different kinds of thingsHowever, before a protocol is accepted and widely implemented, it has to pass rigorous testing. So a standard framework is used to help design, compare, test, and evaluate protocols.

The OSI Reference ModelFor network communications to take place, hundreds of questions must be answered by a set of protocol (how, when, what).Evaluating and working with these hundreds of questions would be unmanageable.

The OSI Reference Model (cont)So, in 1977, the International Standards Organization (ISO) adopted the Open Standard Interconnection (OSI) model.The OSI model breaks down the many tasks involved in moving data from one host to another.The hundreds of questions are divided into seven smaller. The seven groups are called layers.

The OSI Reference Model (cont)The OSI reference model is exactly that; it is only a model.If we think of the model as a set of questions that have to be answered, then the protocols are the answers.Any one protocol may answer only a few of the questions, or in other words, address specific layers in the model. By combining multiple protocols into a protocol suite, we can answer all the questions posed by the model.(M hnh OSI th tr li nhng cu hi t ra)

The Seven Layers of the OSI ModelThe goal of the OSI model is to break down the task of data communication into simple steps. These steps are called layers.The OSI model is made up of seven distinct layers. Each layer has certain responsibilities.

The Seven Layers of the OSI Model

Responsibilities of Each LayerThe purpose of each layer in the OSI model is to provide services to the layer above it.The higher layers do not need to what happened at the lower layers.Following is the process of moving data from one host to another:

IP Header1Frame HeaderIFrame HeaderIIP Header1

Data Stream

Data

Data

Data

Data Stream

Data Stream

Data Stream

Data Stream

Data Stream

Data Stream

Data

Data Stream

Data

Data

Data

Data Stream

Data Stream

Data Stream

Data Stream

Data Stream

Data Stream

Data Stream

The Application LayerLets look at what is happening:You are surfing on the InternetYou type an address of a WebsiteThe Website is delivered to you


The Application layer provide a set of tools and services that the Web Browser used to complete it job

The Application LayerThe Application layer is the top layer of the OSI model.The purpose of the Application layer is to manage communications between applications.

The Application LayerExample of the Application layer: File transferElectronic mailTerminal accessWord processingWeb Browser

The Presentation LayerLets look at what is happening:

Sun PC

Windows PC

MAC PC


I have a good file now.


I want that file but I only accept it in DES encryption format


I want that file too but in 3DES encryption format


OK, I will send that file to MAC PC in DES format and in 3DES format to Sun PC


The Presentation provide set of tools for Window PC to encrypt the file in the suitable format before sending it to Mac PC and Sun PC

The Presentation LayerThe Presentation layer is the layer below the Application layer and above the Session layer.The Presentation ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

The Presentation LayerActions of the Presentation layer:Format of dataData structureData conversionData compressionData encryptionExamples of the Presentation layer:PICT This is picture format used by MacintoshMIDI The Musical Instrument Digital Interface is used for digitized music. MPEG The Moving Picture Experts Groups standard for the compression and coding of motion video for CDs.RTF Rich Text Format is a file format that lets you exchange text files between different word processors, even in different operating systems.

The Session LayerLets look at what is happening:


Hey, Web Server, I want to access your web site using my Internet Explorer.


Ok, I will meet you at port 80.


And what about FTP ?


Ok, We will meet at port 21.


But how the Web Browser and Web Server can negotiate the meeting port ?


The Session layer allows them to meet each other and make agreement.

The Session LayerThe Session is below the Presentation layer.The Session layer establishes, manages, and terminates sessions between two communicating hosts.

Web Browser

The session layerActions of the Session layer: SessionsDialogConversations Data exchange Examples of the Session layer: Network File System (NFS) Developed by Sun Microsystems and used with TCP/IP and Unix workstation to allow transparent access to remote resources.AppleTalk Session Protocol (ASP) client/server mechanism, which both establishes and maintains sessions between AppleTalk client and server machines.

The Transport LayerThe Transport layer is below the Session layer.The Transport layer can guarantee that packets are received.


Hey, Session layer, give your data to me and I guarantee that they will be given to the right destination

The Transport LayerThe Transport layers segments and reassembles data into a data stream

Data Stream

Data Stream

Data Stream

Data

Data

Data

The Transport LayerConnectionless transmissionConnection-oriented transmissionThe three-way handshakeFlow controlAcknowledgementWindowing

The Transport LayerThe primary function of the Transport layer is to ensure that the data packets it receives from the Session layer arrive reliably.The Transport layer does this by using two types of transmissions: connectionless and connection oriented.The Transport layer also has the job of managing the speed of communication between devices. This is known as flow control.

The Transport LayerConnectionless transmissionsThe letter then delivered from the local Post Office through the Post Office system arrive at the Destination Post Office then delivered to WilsonThats OK. The letter is not important, it is just a friendly letter.But the letter may be lost on the way and never come to the destination.Lets look at what happens:Harry want to send mail to WilsonSo he goes to the local Post Office and left the letter there

The Transport LayerConnectionless transmissions(cont)The first reason is thats the data is not importantThe advantage of connectionless transmission using connectionless protocol is that it is very fast.The second reason is that its OK if there is a good and reliable connection between the source and the destination (lease line)Working in the same way, with connectionless transmission using connectionless protocol, when Host A want to send data to Host B, it only put the packet onto the network and hope that it will be arrive at the destination.The packets may be lost on the way (internet). But thats OK.

The Transport LayerConnectionless transmissions(cont)User Datagram Protocol (UDP) is an example of connectionless protocolApplications that use UDP to send their data information is DNS, TFTP.

The Transport LayerConnection-oriented transmissionsLets look at what happens:Harry want to send mail to Wilson again. But this time, it is an important business letter. So he want Wilson phone him to acknowledge when Wilson receive the letter.He then goes to the Local Post Office and send the letter in a certified form. It means that the Post Office guarantee the letter will arrive at the right destination. When Harry receive an acknowledge phone from Wilson, he is now stop worrying about the letter.The letter then sent and when Wilson receives the letter and see that it is an important business letter, so he then phone Harry to acknowledge (bo cho bit) that he had received the letter.

The Transport LayerConnection-oriented transmissions(cont)When Host A receive an acknowledgement packet, it then stop worrying about the packet.The advantage of connection-oriented transmission using connection-oriented protocol is that it is reliable although it has higher latency than connectionless protocol.But after waiting for a predefined time, Host A does not receive an acknowledgement packet from Host B, Host A will resend the packet and wait. The process happens until Host A receive an acknowledgement packet from Host B.Working in the same way, with connection-oriented transmission using connection-oriented protocol, when Host A want to send data to Host B, it put the packet onto the network and wait for the destination to acknowledge of receiving packet.Because Host A and Host B both use connection-oriented protocol to send and receive packets. So when Host B receive packet from A, it will send an acknowledge packet to specify that it has received.

The Transport LayerConnectionless transmissions (cont)Transmission Control Protocol (TCP) is an example of connection-oriented protocolApplications that use TCP to send their data information is DNS, FTP, HTTP, SMTP,

The Transport Layerthe three-way handshakeIn connection-oriented transmission, both the sender and receiver have to join to the session. The receiver must know that the sender is sending packets in order to answer (acknowledgment)So, the sender must first establish a connection-oriented session with the receiver, which is called a call setup, or three-way handshake

The Transport Layerthe three-way handshake (cont)Lets look at what happens:In the three-way handshake process, when Host A (sender) want to establish a session with Host B (receiver), it first send to the receiver a synchronize segment. Host B (receiver) after examine the synchronize packet from Host A, if agree, it will send acknowledgement synchronize segment to Host A.Synchronize (Can I talk to you ?)Ack (Yes), Synchronize ( Can I talk to you ?)Host A after receives the synchronize segment from Host B, it will send an acknowledgement segment to Host B to acknowledge to connection. Ack (Yes)Connection EstablishedData Transfer(send segments)

SenderHost A

TCP

ReceiverHost B

TCP

The Transport layerFlow ControlOnce data transfer is in progress, congestion can occur for two reasons.

The Transport layerFlow Control (cont)First, the sending device might be able to generate traffic faster than the network can transfer it.

The Transport layerFlow Control (cont)The second reason is that multiple devices need to send data to the same destination.

The Transport layerFlow Control (cont)When datagram arrive too quickly for a device to process, it temporarily stores them in memory.

The Transport layerFlow Control (cont)If the datagrams are part of a small burst, this buffering solves the problem.

The Transport layerFlow Control (cont)However, if the traffic continues at this rate, the device eventually exhausts its memory and must discard additional datagrams that arrive.

The Transport layerFlow Control (cont)Instead of losing the data, the transport function can issue a not ready indicator to the sender.

stop

Transport

The Transport layerFlow Control (cont)This acts like a stop sign and signal the sender to discontinue sending segment traffic to the receiver.

Stop

The Transport layerFlow Control (cont)After the receiving device has processed sufficient segments to free space in its buffer, the receiver sends a ready transport indicator which is like a go signal.

data

data

data

data

data

data

go

Transport

Go

The Transport layerFlow Control (cont)When they receives this indicator, the senders can resume segment transmission.

The Transport LayerAcknowledgementThe transport layer provide a reliable service regardless of the quality of the underlying network

The Transport LayerAcknowledgement (cont)One technique that is used to guarantee reliable delivery is called positive acknowledgement (Thng bo xc thc) with retransmission.This requires the receiver to issue an acknowledgement message to the sender when it receivers data.

data

data

The Transport LayerAcknowledgement (cont)The sender also start a timer when it sent a packet.If the timer expires before an acknowledgement is received, it retransmits the packet.

data

The Transport LayerWindowingAcknowledging every data segment, however, has its drawback (iu tr ngi).If the sender has to wait for an acknowledgement of each data segment, the throughput will be very low.

data

data

The Transport LayerWindowing (cont)The quantity of data segments (measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgment for them is called a window.the size of the window controls how much information is transferred from one end to the otherYou and I will use window size of 1 ?I agree

Sender

TCP

Receiver

TCP

The Transport LayerWindowing (cont)Now lets examine an example with windows size of 3.I agreeYou and I will use window size of 3 ?

Sender

TCP

Receiver

TCP

Ack 7

The Network LayerLets look at what is happening:172.16.1.1 is in network attached to E2. So


Packet with Destination IP address of 172.16.1.1


But how router know that the 172.16.1.1 host is in the network attached to E2 interface. And how it can route the packet to the destination.


It is because router is a Network layer device and the Network layer provide a set of tools for router to collect informations about 172.16.1.1 host.


The Network layer also provides a mechanism for router to route packet to 172.16.1.1 host.

The Network LayerThe Network layer, which is below the Transport layer.It is responsible for routing the packet based on its logical address.

The Network LayerActions that the Network layer have to deal : (x l) with: PacketsRoute, routing table, routing protocolLogical addressFragmentationExamples of the Network layer protocols: Internet Protocol (IP)Internetwork Packet Exchange (IPX)

The Data Link LayerLets look at what is happening:Yeah, host 10.0.0.1 is located in network attached to S0.But the network attached to S0 is a Frame Relay networkSo I have to re-encapsulate this packet into Frame Relay frame.This packet is destined for 10.0.0.1 that is in network attached to my E0 interface. And my E0 is an Ethernet interface.

ISDN


This is a packet with a destination IP address of 10.0.0.1

Frame Relay

The Data Link LayerThe Data Link layer is below the Network layer.The Data Link layer is concerned with physical addressing.MAC address00-06-7B-02-EF-05The Data Link layer provides the physical transmission of the data.

The Data Link LayerThe Data Link layer is made up of 2 sublayers:The Logical Link Control (LLC) sublayer.The Media Access Control (MAC) sublayer

The Data Link LayerLogical Link Control (LLC) sublayerLogical Link Control (LLC) sublayer is responsible for identifying Network layer protocols and then encapsulating into frame

The Data Link LayerMedia Access Control (MAC) sublayerMedia Access Control (MAC) sublayer defines how packets are placed on the media.If the network interface card (NIC) has RJ45 port and connected to crossover cable. I will transfer frame on pin 2 and receive frame on pin 3But if the network interface card (NIC) has BNC port, I will transfer frame in another way.

The physical layerTransmission of an unstructured bit stream over a physical link between end systems.Electrical, mechanical, procedural and functional specificationsPhysical data rateDistancesPhysical connector

The physical layerFor Example:UTP, STP, coaxial, Fiber cableRJ45, RJ11 connectorDB9, DB25 connector10base2, 10base5, 100baseT, 1000baseTx

Developing protocols is an ongoings (vic ang pht trin lin lc), ever-changing (lun lun thay i) science. As the industry is increasing so dynamically (si ni) and rapidly.

Vo nm 1977 ISO chp nhn OSI modelAgreement : tha thun

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The OSI Model