OSI Seven(7) Layer Model

The OSI networking model is divided into 7 layers. Each layer has a different responsibility, and all the layers work together to provide network data communication.

* Physical - The Physical layer is the specification for the hardware connection, the electronics, logic circuitry, and wiring that transmit the actual signal. It is only concerned with moving bits of data on and off the network medium. Most network problems occur at the Physical layer.

* Data Link - The Data Link layer is the interface between the upper "software" layers and the lower "hardware" Physical layer. One of its main tasks is to create and interpret different frame types based on the network type in use. The Data Link layer is divided into two sub-layers: the Media Access Control (MAC) sub-layer and the Logical Link Control (LLC) sub-layer.
o LLC sub-layer starts maintains connections between devices (e.g. server - workstation).
o MAC sub-layer enables multiple devices to share the same medium. MAC sub-layer maintains physical device (MAC) addresses for communicating locally (the MAC address of the nearest router is used to send information onto a WAN).
* Network - The Network layer addresses messages and translates logical addresses and names into physical addresses. It also manages data traffic and congestion involved in packet switching and routing. It enables the option of specifying a service address (sockets, ports) to point the data to the correct program on the destination computer.

* Transport - The Transport layer provides flow control, error handling, and is involved in correction of transmission/reception problems. It also breaks up large data files into smaller packets, combines small packets into larger ones for transmission, and reassembles incoming packets into the original sequence.

* Session - The Session layer handles security and name recognition to enable two applications on different computers to communicate over the network. Manages dialogs between computers by using simplex(rare), half-duplex or full-duplex. The phases involved in a session dialog are as follows: establishment, data-transfer and termination.

* Presentation- The Presentation layer determines data exchange formats and translates specific files from the Application layer format into a commonly recognized data format. It provides protocol conversion, data translation, encryption, character-set conversion, and graphics-command expansion.

* Application - The Application layer represents user applications, such as software for file transfers, database access, and e-mail. It handles general network access, flow control, and error recovery. Provides a consistent neutral interface for software to access the network and advertises the computers resources to the network.

Here is an idiotic, yet easy way to remember the 7 layers. Memorize the following sentence: All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of the layers starting with Application and ending with the physical layer.

Layer 1 – The Physical Layer

* Translates data into bits, or bits into data
* Deals only with electrical and mechanical level of data communication, which builds the foundation to allow higher OSI layers to walk through it.
* Physical Layer devices are:
o NIC Cards
o Ethernet Hubs
o Repeaters
o Multistation Access Units (MAUs)
o Transceiver
o Media Filters

Layer 2 – The Data Link Layer

* Provides flow and error control and synchronization for the Physical Layer
* Is made up of the Logical Link control and MAC sublayers
* Data Link Layer Devices are:
o Bridges
o Switches
o Brouters

The Protocols that operate at the Data Link Layer are:

* Point-to-Point Protocol (PPP)
* Serial Line Internet Protocol (SLIP)
* High-Level Data Link Control (HDLC)
* Logical Link Control (LLC)
* Link Access Protocol (LAP)

Layer 3- The Network Layer

* Manages communications routing for packets that are destined for addresses that are not on the LAN
* Uses routable protocols to deliver packets across interconnected networks joined by routers.
* Device which operate at the Network Layer are:
o Routers
o Connectionless Oriented services

The two main protocols that operate at the Network Layer are:

* TCP/IP
* IPX/SPX

Layer 4 – The Transport Layer

o Manages the end-to-end control and error checking by providing an end-to-end connection between the source and the destination node to ensure reliable data delivery
o Also responsible for providing end-to-end recovery, flow control, and releasing the connection

Connectionless-Oriented Protocols that operate at this layer are:

* UDP

Connection-Oriented Protocols that operate at this layer are:

* TCP/IP
* SPX/IPX

Layer 5 – The Session Layer

* The session layer manages the establishment of a continuing series of requests and responses between the applications at each end.
* Mainframes used to use this layer – modern networking does not.

Layer 6 - The Presentation Layer

* Usually part of the Operating System
* It prepares incoming data for the Application layer and formats outgoing data that the Application layer sends and receives.
* Data compression and data encryption.

Layer 7 – The Application Layer

* The top layer of the OSI model
* Provides information services
* FTP and Telnet access the OSI model at this layer.