Application (Layer 7) - The Application layer integrates network functionality into the host operating system and enables the communication between network clients and services.

• The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network.

• Most Application layer protocols operate at multiple layers down to the Session and even Transport layers. However, these protocols are classified as Application layer protocols because they start at the Application layer (the Application layer is the highest layer they operate in).

• Services typically associated with the Application layer include:

  • Hypertext Transfer Protocol (HTTP)

  • Telnet

  • File Transfer Protocol (FTP)

  • Trivial File Transfer Protocol (TFTP)

  • Simple Network Management Protocol (SNMP)

 

Presentation (Layer 6) - The Presentation layer formats (presents) data in a form compatible for receipt by the Application layer or the destination system. Specifically, the Presentation layer:

• Formats and translates data between systems.

• Negotiates data transfer syntax between systems. It converts character sets to the correct format.

• Encapsulates data into message envelopes. It encrypts and compresses the data.

• Restores data through decryption and decompression.

 

Session (Layer 5) - The Session layer manages the sessions in which we transfer data. A session refers to each client's connection. Session layer functions include:

• Management of multiple sessions. A server can concurrently maintain thousands of sessions.

• Assignment of a session ID number to each session to keep data streams separate.

• The setup, maintenance, and teardown of communication sessions.

 

Transport (Layer 4) - The Transport layer provides a transition between the upper and lower layers of the OSI model. The transition makes the upper and lower layers transparent to each other. Transport layer functions include:

• End-to-end flow control.

• Port and socket number assignment.

• Segmentation, sequencing, and combination.

• Connection services, either reliable (connection-oriented) or unreliable (connectionless) delivery of data.

A data segment is a unit of data at the Transport layer.

 

Network (Layer 3) - The Network layer describes how data is routed across networks and on to the destination. Network layer:

• Identifies hosts and networks by using logical addresses.

• Maintains a list of known networks and neighboring routers.

• Determines the next network point where data should be sent.

To select the optimal path for data, routers use a routing protocol that takes various factors into account, such as the number of hops in the path, link speed, and link reliability.

A packet is a data segment at the Network layer.

 

Data Link (Layer 2) - Logical Link Control (LLC) and Media Access Control (MAC)

The Data Link layer defines the rules and procedures for hosts as they access the Physical layer. These rules and procedures define:

• How physical network devices are identified on the network by defining a unique hardware address (physical or MAC address).

• How and when devices have access to the LAN and can transmit on the network medium (media access control and logical topology).

• How to verify that the data received from the Physical layer is error-free (using parity and cyclic redundancy check (CRC)).

• How devices control the rate of data transmission between hosts (flow control).

The Data Link layer has two sub-layers:

• The LLC is the upper sub-layer. It:

  • Is an interface between the MAC sub-layer and the Network layer.

  • Provides flow control and transmission for analog and/or digital streams of over a shared link for the logical link.

• The MAC layer is the lower sub-layer. The MAC sub-layer:

  • Controls the hardware.

  • Provides flow control and transmission for analog and/or digital streams of over a shared link.

Frames are data segments at the Data Link layer. Switches, bridges and NICs, and WAPs function in Layer 2.

 

Physical (Layer 1) - The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. Protocols at the Physical layer identify:

• Conversion of digital data (bits) to electric pulses, radio waves, or pulses of light.

• Specifications for cables and connectors.

• The physical topology.

Bits are data segments at the Physical layer. NICs, repeaters, hubs, WAPs, and modems function in Layer 1.

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A comparison of the OSI Model and the TCP IP Model: Click here >> to learn about the TCP IP Model