The OSI Model

The OSI Model
tncapie
Topics
What are Protocols ?
Why we need Protocols and Standards
The OSI Reference Model
The Seven Layers of the Model
Responsibilities of Each Layer
The Application Layer
The Presentation Layer
The Session Layer
The Transport Layer
The Network Layer
The Data Link Layer
The Physical Layer
What are Protocols ?
Let’s look at what happen:
Oh my God, what did he say ?
Không biết thằng này đang nói cái gì nữa ?
They do not understand each other because they are using different languages.
What are Protocols ? (cont)
Now, Let’s look at what happens next:
They can understand each other because they are using the same language.
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)
What are Protocols ? (cont)
A group of protocol is called a protocol suite or a protocol stack.
A single protocol addresses one particular issue that helps to enable communication – for example, defining what an IP address looks like.
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 Standards
Rules – or protocols and standards – are important to ensure compatibility between different kinds of things
Developing protocols is an ongoing, ever changing science. As the industry is increasing so dynamically and rapidly, more protocols are unleashed to handle the boom.
However, 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 Model
For 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, more manageable groups of questions. 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.
The OSI Reference Model (cont)
The OSI reference model functions as a baseline for comparison to any protocol suite.
This baseline function of the OSI model is similar to a model home.
When designing your new home, a model can be used as a baseline. Everyone in the neighborhood also used the model home as reference to help make the choices in the new homes that they are building.
The OSI Reference Model (cont)
All the homes will vary slightly from the model, but the model provides a means for comparison.
In the same way, you can compare any protocol suite to the OSI reference model because protocols are designed from this model.
For example, IPX/SPX, AppleTalk, TCP/IP are protocol suites that made based on the OSI model.
The Seven Layers of the OSI Model
The 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 Layer
The 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 Header
1
Frame Header
I
Frame Header
I
IP Header
1
The Application Layer
Let’s look at what is happening:
You are surfing on the Internet…
You type an address of a Website…
The Website is delivered to you…
The Application Layer
The Application layer is the top layer of the OSI model.
The purpose of the Application layer is to manage communications between applications.
The Application Layer
Example of the Application layer:
File transfer
Electronic mail
Terminal access
Word processing
Web Browser
The Presentation Layer
Let’s look at what is happening:
The Presentation Layer
The 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 Layer
Actions of the Presentation layer:
Format of data
Data structure
Data conversion
Data compression
Data encryption
Examples of the Presentation layer:
PICT – This is picture format used by Macintosh
MIDI – The Musical Instrument Digital Interface is used for digitized music.
MPEG – The Moving Picture Experts Group’s 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 Layer
Let’s look at what is happening:
The Session Layer
The Session is below the Presentation layer.
The Session layer establishes, manages, and terminates sessions between two communicating hosts.
The session layer
Actions of the Session layer:
Sessions
Dialog
Conversations
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 Layer
The Transport layer is below the Session layer.
The Transport layer can guarantee that packets are received.
The Transport Layer
The Transport layers segments and reassembles data into a data stream
The Transport Layer
Connectionless transmission
Connection-oriented transmission
The three-way handshake
Flow control
Acknowledgement
Windowing
The Transport Layer
The 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 letter then delivered from the local Post Office through the Post Office system arrive at the Destination Post Office then delivered to Wilson
That’s 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.
The Transport Layer
Connectionless transmissions
Let’s look at what happens:
Harry want to send mail to Wilson
So he goes to the local Post Office and left the letter there
The Transport Layer
Connectionless transmissions
(cont)
The first reason is that’s the data is not important
The advantage of connectionless transmission using connectionless protocol is that it is very fast.
The second reason is that it’s 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 that’s OK.
The Transport Layer
Connectionless transmissions
(cont)
User Datagram Protocol (UDP) is an example of connectionless protocol
Applications that use UDP to send their data information is DNS, TFTP….
The Transport Layer
Connection-oriented transmissions
Let’s 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 that he had received the letter.
The Transport Layer
Connection-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 Layer
Connectionless transmissions
(cont)
Transmission Control Protocol (TCP) is an example of connection-oriented protocol
Applications that use TCP to send their data information is DNS, FTP, HTTP, SMTP,…
The Transport Layer
the three-way handshake
In 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 Layer
the three-way handshake (cont)
Let’s 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 Established
Data Transfer
(send segments)
The Transport layer
Flow Control
Once data transfer is in progress, congestion can occur for two reasons.
The Transport layer
Flow Control (cont)
First, the sending device might be able to generate traffic faster than the network can transfer it.
The Transport layer
Flow Control (cont)
The second reason is that multiple devices need to send data to the same destination.
The Transport layer
Flow Control (cont)
When datagram arrive too quickly for a device to process, it temporarily stores them in memory.
The Transport layer
Flow Control (cont)
If the datagrams are part of a small burst, this buffering solves the problem.
The Transport layer
Flow 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 layer
Flow Control (cont)
Instead of losing the data, the transport function can issue a “not ready” indicator to the sender.
The Transport layer
Flow Control (cont)
This acts like a stop sign and signal the sender to discontinue sending segment traffic to the receiver.
The Transport layer
Flow 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.
The Transport layer
Flow Control (cont)
When they receives this indicator, the senders can resume segment transmission.
The Transport Layer
Acknowledgement
The transport layer provide a reliable service regardless of the quality of the underlying network
The Transport Layer
Acknowledgement (cont)
One technique that is used to guarantee reliable delivery is called positive acknowledgement with retransmission.
This requires the receiver to issue an acknowledgement message to the sender when it receivers data.
The Transport Layer
Acknowledgement (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.
The Transport Layer
Windowing
Acknowledging every data segment, however, has its drawback.
If the sender has to wait for an acknowledgement of each data segment, the throughput will be very low.
The Transport Layer
Windowing (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 other
You and I will use window size of 1 ?
I agree
I agree
The Transport Layer
Windowing (cont)
Now let’s examine an example with windows size of 3.
You and I will use window size of 3 ?
The Network Layer
Let’s look at what is happening:
172.16.1.1 is in network attached to E2. So……
The Network Layer
The Network layer, which is below the Transport layer.
It is responsible for routing the packet based on its logical address.
The Network Layer
Actions that the Network layer have to deal with:
Packets
Route, routing table, routing protocol
Logical address
Fragmentation
Examples of the Network layer protocols:
Internet Protocol (IP)
Internetwork Packet Exchange (IPX)
The Data Link Layer
Let’s 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 network
So 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.
The Data Link Layer
The Data Link layer is below the Network layer.
The Data Link layer is concerned with physical addressing.
MAC address
00-06-7B-02-EF-05
The Data Link layer provides the physical transmission of the data.
The Data Link Layer
The Data Link layer is made up of 2 sublayers:
The Logical Link Control (LLC) sublayer.
The Media Access Control (MAC) sublayer
The Data Link Layer
Logical Link Control (LLC) sublayer
Logical Link Control (LLC) sublayer is responsible for identifying Network layer protocols and then encapsulating into frame
The Data Link Layer
Media Access Control (MAC) sublayer
Media 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 3
But if the network interface card (NIC) has BNC port, I will transfer frame in another way.
The physical layer
Transmission of an unstructured bit stream over a physical link between end systems.
Electrical, mechanical, procedural and functional specifications
Physical data rate
Distances
Physical connector
The physical layer
For Example:
UTP, STP, coaxial, Fiber cable
RJ45, RJ11 connector
DB9, DB25 connector
10base2, 10base5, 100baseT, 1000baseTx