For the ICND exams and in the real world its a good idea to know what layers are in the OSI model, and what functions are performed at each layer.

OSI stands for Open System Interconnection Reference Model. It divides network architecture into seven different layers. A layer is a collection of conceptually similar functions that provide services to the layer above it, and receives services from the layer below. The seven layers are shown in the diagram below.

Layer 7 – The Application Layer

This layer interacts with the end user. It provides network services to the user’s applications. Unlike other layers in the OSI model layer 7 does not provide services to any other OSI layer. Authentication services run at this layer. The application layer establishes the availability of the remote communication partner, and ensures that both ends agree on procedures for error recovery, privacy and control of data integrity.

Common protocols and services that run at layer 7 are SMTP, POP3, Telnet, HTTP, FTP and SNMP.

Firewalls also run at layer 7.

Layer 6 – The Presentation Layer

The presentation layer properly formats the data and ensures that the information the application layer sends out on one system is readable by the application layer of another system. An example of a presentation layer issue is when you open a document in word and there are lots of unrecognisable characters. The presentation layer is concerned with:

• Compatibility with the operating system.
• Proper encapsulation of data for network transmission.
• Data formatting (ASCII, Binary).
• Data encryption, compression, and translation.

Some of the file types that are used at the presentation layer are JPEG, ASCII, GIF etc..

Layer 5 – The Session Layer

The session layer establishes, manages, and terminates sessions between two communicating hosts. It provides its services to the presentation layer. The overall communication between two hosts is referred to as a session. As an example a web server might have a number of users, so many communication processes are open at a given time. Therefore, keeping track of which user communicates on which path is important.

Layer 4 – The Transport Layer

The transport layer segments data from the sending host and ensures that when the data gets to the destination host that it is in the correct order and free of errors. TCP which is connection-oriented and UDP which is connectionless reside at this layer.

Layer 3 – The Network Layer

The network layer provides connectivity and path selection (routing) between two host systems that might be located on geographically separated networks. IP addressing runs at this layer.

Layer 2 – The Data Link Layer

Most Switches, bridges and wireless access points run at this layer. Devices such as cable & DSL modems also run at layer 2. Other specifications that run at layer 2 are:

• Ethernet
• High Data Link Control (HDLC)
• Point-to –point Protocol (PPP)
• Frame Relay

Ethernet MAC addresses are at this layer. Layer 2 performs error detection using FCS (Frame Check Sequence) but this layer does not perform error recovery.

Layer 1 – The Physical Layer

The physical layer handles the actual data transmission. Characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors and cables are defined by physical layer specifications.