page 1 - © richard l. goldman networking the osi model ©richard l. goldman march 29, 2001

- © Richard L. Goldman

NetworkingThe OSI Model

©Richard L. Goldman

March 29, 2001


The OSI Model• The OSI (Open Systems Interconnection) Model is

a networking architecture developed by the ISO (International Organization for Standardization). Note: ISO in not an acronym – iso means equal in Greek

• The OSI Model is a helpful reference when developing, discussing, or teaching computer-to-computer communications.

• The OSI Model is only a model. It is not an actual networking architecture or protocol that you would find installed on a computer.


OSI ModelSoftw

are

Hardware

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical

The OSI Model is a seven layer set of specifications designed to allow all computers to communicate with each other.

The top six layers contain the software that ensures that data is sent and received in a reliable, error-free manner.

The bottom layer consists of the cables, connectors, and equipment that carry the data from one computer to another.


OSI ModelSoftw

are

SendingComputer Program

Hardware

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

•When an application in one computer wants to send data to another computer it sends it to the Application layer of the OSI Model.

The data is then passed to each succeeding layer until it is finally transmitted on the Physical layer.

Each layer performs specific processing and packaging functions required to transmit the data.


PDU – Packet Data Unit

• As data is passed from layer to layer it is broken down into smaller pieces called:– Packets,

– Data Units, or

– Payloads.

• The official OSI acronym for data placed in packets is a PDU or Packet Data Unit.

• Note: The PDU changes at each layer of the OSI Model as each layer adds its own formatting or addressing to it.


OSI Model

Softw

are

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

ReceivingComputer Program

OSI ModelSoftw

are

Hardware

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

SendingComputer Program

1 - Physical

The Physical layer of the Receiving computer accepts the packaged data.

It is then passed back through each layer.

The unpacked/reprocessed data is finally delivered to the Receiving Application.

Each layer performs a specific re-processing and/or un-packaging function required to receive the transmission.


OSI Model

Computer Program

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

file

file


OSI Model

Computer Program

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

file

Peer Communication

Peer Communication

Peer Communication

Peer Communication

Peer Communication

Peer Communication

Packet orData Frame

Datagram

Segment

Message


OSI Model

Computer Program

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

file


OSI Model

Computer Program

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

e


OSI Model

Computer Program

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

file

file


OSI ModelS

oftware

Computer Program

OSI Model

Hardware

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Softw

are

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

In this manner two computers can communicate with each other over a network.


Physical Layer• The Physical Layer consists of

the actual physical components that make up a network:– Cables and connectors– NICs– Hubs– Repeaters

• The Physical Layer is responsible for transmitting, amplifying, and receiving network traffic signals.

• The operation of the Physical Layer is governed by the RS-232-D and 802 protocol standards.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Data Link Layer - SublayersThe Data Link Layer contains two

sublayers.

• LLC – Logical Link Control– Flow Control

– Error correction

• MAC – Media Access Control– Contains Physical Address

• 12 digit Hex – 1st 6 Mfg, 2nd 6 Unique

– Provides Access Control• Contention

• Token Passing

• Polling

– Controls Logical Network Topology• Ethernet

• Token Ring

• Etc.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Data Link Layer - Send• The sending Data Link Layer

is responsible for creating data frames that are suitable for transmission.

• To accomplish this the Data Link Layer packages the data into the correct size for transmission and then adds the following components:– Destination Address– Source Address– Control Information– Error Checking Information

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Data Link Layer - ReceiveThe receiving Data Link Layer:

– Checks the frames it receives for errors and acknowledges receipt of each frame.

– The sending Data Link Layer will retransmit a frame if it does not receive an acknowledgement.

– Strips away the components added by the transmitting Data Link Layer and passes the PDU to the Network Layer.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Network Layer• The Network Layer is

responsible for three services:– Routing – Provide a path to

the receiving computer through one or more intermediate nodes.

– Congestion Control – Route and reroute data to avoid network congestion.

– Internetworking – Providing the means to communicate between two different types of networks.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Transport Layer• The Transport Layer is

responsible for:– Flow Control – Adjusting rate

of transmission based on network conditions.

– Sequencing – Assigns sequence numbers to PUDs to ensure proper reassembly of the data.

– Acknowledges (ACK) and requests retransmission of corrupt or missing data.

(TCP & SPX work at this layer.)

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Session Layer• The Session Layer, negotiates,

establishes, and maintains the communications session between two nodes.

• Acts as the traffic cop by regulating which node can transmit and for how long.

• Monitor identification of session participants – permits only authorized nodes to communicate.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Presentation Layer

• The Presentation Layer translates data into a format that the network can understand.

• Encrypts and decrypts data to provide secure communications.

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Application Layer

• The Application Layer is a set of instructions that act as an interface between a user application running on a computer and the network.

• It is responsible for network:– File transfers– File management– Message handling (e-mail)

OSI Model

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 - Data Link

1 - Physical


Layer 1 DevicesThe Physical Layer

• Layer 1 Devices (Physical Layer)– Cables and other transmission media– NIC– Transceivers– Repeaters– Hub– MAU


Layer 1 Devices (Physical Layer)NICs and Cat. 5 Crossover Cable

Each computer needs a NIC, Network Interface Card, to physically join a network.

This example shows two computers connected with a Cat. 5 crossover cable.


Layer 1 Devices (Physical Layer) NICs and Thinnet

This example shows two computers connected with a Thinnet, coaxial cable.

Terminators are added to complete the circuit.


Layer 1 Devices (Physical Layer) NICs, Thicknet, and Transceivers

This example shows two computers connected with a Thicknet coaxial cable.

Vampire Clamp

Transceiver Drop Cable

AUI connector – Attachment Unit Interface


OSI Model

Softw

are

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

Layer 1 Devices (Physical Layer) OSI Software Layers passing Data through the Hardware

OSI Model

Softw

are

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

Computer Program

Data is passed from the User Application down through each layer of the OSI model until reaches the hardware.

It is then transmitted to the other computer.

Then back up to the receiving User Application.


Layer 1 Devices (Physical Layer) Repeater If the distances between the

computers exceed the maximum specified length, a repeater may be used to extend the length of the run.


Layer 1 Devices (Physical Layer) Hub

Hubs forward data from one computer to all other computer connected to the hub (broadcast). Hubs function as a multiport repeater by amplifying the signal which can also extends the maximum distance between computes.


Layer 1 Devices (Physical Layer) MAU – Multistation Access Unit

MAU forwards data from one computer to another in a ring until it reaches the computer it is addressed to. MAUs are used to connect computers in a Token Ring network.


Layer 2 DevicesThe Data Link Layer

• Switches are capable of directing network traffic only to the computer it is addressed to.– Also Known As

• Switching Hubs

• Layer 1 Switches

• Bridges are used to pass traffic from one subnet to another while restricting local traffic to its own subnet.


Layer 2 Devices (Data Link Layer) Switch

OSI Model

Softw

are

7 - Application

6 - Presentation

5- Session

4 - Transport

3 - Network

2 – Data Link

1 - Physical

A switch uses a MAC Address Table to route traffic to its proper destination. The MAC address table is maintained in the MAC sublayer of the Data Link Layer.


Common Network Devices• Layer 5-7 (Application, Presentation, & Session Layers)

– Gateway

• Layer 3 Devices (Network Layer)– Router– Brouter– Layer 3 Switch

• Layer 2 Devices (Data Link Layer)– Switch (AKA, Switching Hub, or Layer 2 Switch)– Bridge

• Layer 1 Devices (Physical Layer)– Cables and other network media– NIC– Transceivers– Repeaters– Hub– MAU


Layer #

Layer Name Brief Description Devices

7. Application Network services for user applications

1. Gateway

6. Presentation Data conversion, encryption, compression

5. Session Establish, and maintain communications

4. Transport Maintain packet sequenceRequest missing packets

3. Network Logical addressRoutes packets

1. Router2. Brouter

2. Data Link Physical address 1. Switch2. Bridge LLC

MAC

1. Physical Electrical transmission 1. Cabling2. NIC3. Hub4. Repeater


Layer #

Layer Name Brief Description Devices

7.

6.

5.

4.

3.

2.

1.


Ethernet Topology Standards

Standard Name Cable Speed Distance IEEE Spec

10BaseT Ethernet UTP/STP 10Mbs 100m 802.3

10Base2 Thinnet Coax 10Mbs 185m 802.3

10Base5 Thicknet Coax 10Mbs 500m 802.3

10BaseF Ethernet over Fiber

Fiber 10Mbs 500m – 2000m

100BaseTX Fast Ethernet UTP/STP-Cat 5

(uses 2 pair)

100Mbs

(Full Duplex)

100m 802.3u

100BaseT4 Fast Ethernet UTP/STP-Cat 3

(uses 4 pair)

100Mbs

(Half Duplex)

100m 802.3

100BaseFX Fast Ethernet over Fiber

Fiber 100Mbs 2000m

100BaseVG “Any LAN”

(Voice Grade)

UTP/STP-Cat 3 UTP/STP-Cat 5

100Mbs 100Mbs

100m

213m

802.12

1000BaseX Gigabit

Ethernet

UTP/STP-Cat 5

or Fiber

1000Mb/s 802.3z

page 1 - © richard l. goldman networking the osi model ©richard l. goldman march 29, 2001

Documents

osi model richard

data link

application layer

physical layer

data units

packaged data

unpackedreprocessed

receiving computer