OSI Model: Open Systems Interconnection Reference Model

Lesson One: OSI Model: open Systems Interconnection Reference Model


7 Layers: All People Seem To need data processing
7. Applications

It sets up communication between Applications and lower layer network services.

The network needs to interpret application request and the application can interpret data to be sent to the network.

　

- In this Layer we see

-HTTP, FTP, DNS, POP3

-this is where all the applications work.

-separates data into PDUs (protocol data units) or discrete amounts of data.

　

　

　

6. Presentation

-Character encoding

-Application encryption and decryption

Often combined with Application layer

-Presentation layer accepts data from the application layer and formats so that I can be readable.

　

　

5. Session

-Communication management between devices


-Half-duplex, full-duplex (way of communication)
-Controls protocols, tunneling protocols

-Session is a connection of a data exchange between two parties.

-Another purpose is that it  establishes and keeps  communication alive between networks And it determines if a connection is cut-off or not.

An example, is an ISP and your computer negotiating a connection. If your physical connections is disconnected that is a fail session.

　

4. Transport

-The Post office layer

-Parcels and letters

-Transport takes info from the session layer and it make sure it reaches its end. If data is being transferred from point A to B it assures reliability and make sure its sent without erros.


TCP(Transmission Control Protocol) and UDP (User Datagram Protocol)

-TCP is connection oriented.

-UDP is not connection oriented. Basically Connectionless.
　

　

3. Network Layer
 The routing layer
 Internet Protocols are in this layer
 Network layer function is to translate network address into the devices and decide how they routed from the sender to the receiver.

IP Fragmentation Process
 -If bytes are too big they are broken and sent through IP header. Split into 3 separate frames. The fragments will always be in multiplea of 8.
　
2. Data Link
 Basic network "language"
 MAC (Media Accesss Control) address on Ethernet
 Every device has a MAC address
 You can find the MAC address by IP address
 Switch
 Data links purpose is to divide data they receive from the network layer into distinct frames so it can be sent to the physical layer.

Data link is divided into 2 sub layers
-Logical Link Control (which provides common setting, reliability and flow of control) (issues requests for the data errors)

-Media Access Control (Takes the MAC address and attaches to the frame)

　

　

1. Physical
 Signaling, cabling, connectors
 This layer isn't about protocols
 Protocols that detect signals

you have a physical layer problem

-Fix your cabling, punch-downs etc.

OSI Layer 1,2,3,4

Physical layer: transporting bits between two machines
Data link layer: MAC addresses and switching
Network layer:Packets, ip addresses and routing
Transport Layer: UDP segments and windowing

Switch

A switch is a device that channels incoming data from any of multiple input ports to a specific output port. It then takes the data and sends it toward its destination. A switch operates at layer 2 or Data Link layer of the OSI model. At layers 2 it looks at each packet, and from the MAC address it determines which device the data unit is intended for. Then with this information the switch sends data it towards the devide's destination.

Layers 2 switching

• Hardware based bridging
• Wire-speed performance
• Collision domain per port
• Traffic containment based on Mac Address

Issues

• No traffic based on Mac address
• Can’t restrict where traffic can go
• No traffic between VLANs
• Unbounded Broadcast domain
• Servers are not centrally located

Bridges

A Bridge is a device that connects two local-area networks (LANs), or two segments of the same LAN. Unlike a router, bridges are protocol independent. They simply forward packets without analyzing and re-routing messages. Bridging occurs at layer 2 of the OSI model which is the data link layer. This means a bridge cannot read IP addresses, but only the outermost hardware address of a packet.

Repeaters

Repeaters are used to increase the signal's strength so it can be transmitted and received over a greater distance without a loss in quality. These devices are used with networks to help the lines running between network devices to reach greater distances. Repeaters operate at the first layer of the OSI model which is the physical layer.