The OSI Reference Model

International Organization for Standards (ISO)

recognized the need to create a network model that would help network builders implement networks that could communicate and work together--OSI Model

The OSI model is a layered communication process with each layer performing a specific task.

The OSI Reference Model

The OSI model describes how information or data moves from one computer through a

network to another computer.

Networking concepts . . . Networking is the interconnection

of workstations, peripherals, and other devices.

In networking, it is possible for different types of computer systems to communicate.

In networking, what is important is that all devices speak the same language or use the same protocol (use same set of rules).

Source, Destination, and Data Packets . . .

Data is sent in bits, 1s and 0s. Data is not the information itself

but an encoded form of information which is a series of electrical impulses into which information is transmitted for sending.

Small, easily transmitted units into which computer data is broken for transmission across a network are called packets.

Source, Destination, and Data Packets . . .

Using the OSI model, data can be called packets, or frames or segments.

Source address specifies the identity of the computer sending the packet.

Destination address specifies the identity of the computer designated to receive the packet.

Why Data Packets?

Computers can take turns sending packets.

If packet is lost, only small amount of data must be retransmitted.

Data can take different paths.

Different Media Types

AIRcarries light, radio,

microwave

Media—material through which data packets travel

uses a “layered” model because it breaks network communication

into smaller, simpler parts that are easier to develop.

it facilitates standardization of network components to allow multiple-vendor development and support.

The OSI Reference Model . . .

allows different types of network hardware and software to communicate with each other.

prevents changes in one layer from affecting the other layers so that they can develop more quickly.

breaks network communication into smaller parts to make learning it easier to understand.

The OSI Reference Model

KNOW THE CORRECT ORDER OF THE LAYERS!

The OSI Reference Model

User interface

Data presentation and encryption

Keeping different applications’ data separate

End-to-end connections

Addresses and best path

Access to media

Binary transmission

Each layer has a unique function.

Application

Presentation

Session

Transport

Network

Data Link

Physical

7

6

5

4

3

2

1

The OSI Reference Model

The OSI Reference Model

The top 3 layers are known as the appli-cation layers because they deal with the user interface, data formatting, and the application access.

Application

Presentation

Session

7

6

5

4

3

2

1

The OSI Reference Model

Layers 1-4 are known as the data flow layers because they control the physical delivery of messages over the network.

Application

Presentation

Session

Transport

Network

Data Link

Physical

7

6

5

4

3

2

1

The OSI Reference ModelLAYER 7: THE APPLICATION LAYER 7: THE APPLICATION

LAYER LAYER closest to the user provides network services to user

applications does not provide services to any

other OSI layer but rather to application processes outside the scope of the OSI model

The OSI Reference Model

LAYER 7: THE APPLICATION LAYER 7: THE APPLICATION LAYERLAYER

identifies and establishes the availability of intended communication partners

synchronizes cooperating applications

establishes agreement on procedures for error recovery and control of data integrity

The OSI Reference Model

LAYER 6: THE PRESENTATION LAYER 6: THE PRESENTATION LAYERLAYER

ensures that information sent by the application layer of one system is readable by the application layer of another system

translates between multiple data representation formats by using a common data representation format

concerned with data structures and negotiation of data transfer syntax

responsible for compression and encryption

The OSI Reference Model

LAYER 5: THE SESSION LAYERLAYER 5: THE SESSION LAYER establishes, manages, and terminates

sessions between communicating hosts synchronizes dialog between presentation

layer entities and manages their data exchange

offers provisions for efficient data transfer, class of service, and exception reporting of session, presentation, and application layer problems

manages data exchange between presentation layer entities

The OSI Reference Model

LAYER 4: THE TRANSPORT LAYERLAYER 4: THE TRANSPORT LAYER segments and reassembles data into a data

stream concerned with how reliable transport over

an internetwork is accomplished responsible for reliable network

communication between end nodes and provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control

The OSI Reference Model

LAYER 3: THE NETWORK LAYERLAYER 3: THE NETWORK LAYER provides connectivity and path selection

between two end systems where routing occurs—these may be located on geographically separated networks

The OSI Reference Model

LAYER 2: THE DATA LINK LAYERLAYER 2: THE DATA LINK LAYER provides reliable transit of data across a

physical link is concerned with physical (as opposed

to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control

The OSI Reference ModelLAYER 1: THE PHYSICAL LAYERLAYER 1: THE PHYSICAL LAYER defines the electrical and functional

specifications for the link between end systems (including media)

defines voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes

Encapsulation wraps data with necessary protocol information before transit.

1. Build the data.

2. Package the data for end-to-end transport.

3. Append (add) the network address to the header (includes control information).

4. Append (add) the local address to the data link header.

5. Convert to bits for transmission.

How Data is Encapsulated

The Importance of TCP/IP

The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted a network that could survive any conditions, even a nuclear war.

The TCP/IP model has only four layers.

The TCP/IP Model

The DoD wants its packets to get through every time, under any conditions, from any one point to any other point. It was this very difficult design problem that brought about the creation of the TCP/IP model, and which has since become the standard on which the Internet has grown.

The TCP/IP Model - Application Layer

The designers of TCP/IP felt that the higher level protocols should include the session and presentation layer details. They simply created an application layer that handles high-level protocols, issues of representation, encoding, and dialog control.  The TCP/IP combines all application-related issues into one layer, and assures this data is properly packaged for the next layer.

The TCP/IP Model - Transport Layer This layer deals with the quality-of-service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications. TCP is a connection-oriented protocol. It dialogues between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a circuit exists between the communicating computers (that would be circuit switching). It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period. This is known as packet switching.

The TCP/IP Model - Internet Layer The purpose of the Internet layer is to send source packets from any network on the internetwork and have them arrive at the destination independent of the path and networks they took to get there. The specific protocol that governs this layer is called the Internet protocol (IP). Best path determination and packet switching occur at this layer. Think of it in terms of the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives.

The TCP/IP Model – Network Access Layer

The name of this layer is very broad and somewhat confusing. It is also called the host-to-network layer. It is the layer that is concerned with all of the issues that an IP packet requires to actually make a physical link, and then to make another physical link. It includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers.

Common TCP/IP Protocols

• Application Layer

• Transport Layer

• Internet Layer

• Network Access

• FTP - File Transfer Protocol

• HTTP - Hypertext Transfer Protocol

• SMTP - Simple Mail Transfer protocol

• DNS - Domain Name System

• TFTP - Trivial File Transfer Protocol

Common TCP/IP Protocols

The transport layer involves two protocols - transmission control protocol (TCP) and user datagram protocol (UDP).

Common TCP/IP Protocols

Similarities:• both have layers• both have application layers, though they include very

different services • both have comparable transport and network layers • packet-switched (not circuit-switched) technology is

assumed • networking professionals need to know both

Differences • TCP/IP combines the presentation and session layer issues

into its application layer • TCP/IP combines the OSI data link and physical layers into

one layer • TCP/IP appears simpler because it has fewer layers • TCP/IP protocols are the standards around which the

Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, typically networks aren't built on the OSI protocol, even though the OSI model is used as a guide. 

Although TCP/IP protocols are the standards with which the Internet has grown, this curriculum will use the OSI model for the following reasons:

• It is a worldwide, generic, protocol-independent standard.

• It has more details, which makes it more helpful for teaching and learning.

• It has more details, which can be helpful when troubleshooting.