Identify and describe the functions of each of the seven layers of the OSI reference model

When looking at the OSI model, it should be understood that each layer serves a specific function. Each of the 7 layers functions are defined by OSI and can be applied and used by any network product vendor. By having all network hardware and software vendors referencing the same model, any network can be built in a non-vendor specific multi-vendor environment with an expectation that compatibility will not be an issue. Of course there is a lot to be said for have a primary network vendor for management and customizability. The OSI functions are as follows:

Layer 7
Application
The Application layer provides services to application processes that are at a higher level than the OSI model; examples: FTP, Telnet, E-mail. This layer synchronizes cooperating applications and establishes agreement on procedures for error recovery and control of data integrity. Messages, defined as a logical grouping of information, are processed from applications at this layer.

Layer 6
Presentation
The Presentation layer provides data representation, presentation and code formatting and conversion. It is responsible for making sure that data received from the network can be used by the application and it makes sure information from an application can be sent onto the network. Data management issues include encryption, compression, ASCII, JPEG, EBCDIC.

Layer 5
Session
The Session layer is used for the establishment, maintenance, and management of sessions between applications. It is also responsible for synchronizing communication between application on different hosts.

Layer 4
Transport
The Transport layer Segments, Sequences, and reassembles data to and from data streams. With TCP/IP, this layer is responsible also for communicating with upper layers through Port address. With TCP/IP there are two different categories of Ports. TCP (Transmission Control Protocol) is used for connection-based reliable delivery of data (example: HTTP or FTP services). UDP (User Datagram Protocol) is used for connection-less unreliable transmission delivery of data (example: Real-Audio, QuickTime streaming). The establishment, maintenance, and termination of virtual circuit is also included at this layer along with Informational Flow-Control, Transport Fault Detection and Recovery and Quality of Service are also concepts involved at this layer. In an Novell network, SPX is similar to TCP.

Layer 3
Network
This layer Packets or creates Datagrams and determines the best path including path determination and selection to move data from one location to another. The path information is provided to this layer by the protocol packets network information. With the TCP/IP protocol, IP provides the network information that is extracted from the IP address by the subnet mask. In a Novell network, IPX carries the network information for routing functions. A router will make its decisions by using its routing table located in RAM on the router. Concepts here may include routers, Layer 3 switches, routed protocols, and logical addressing.

Layer 2
Data-Link
This layer Frames and physically addresses the data with a MAC address provides reliable transmission across a PHYSICAL link while also handling error notification, network topology, and flow control. Bridges, Layer 2 Switches, and also items such as Frame-Relay operate at this layer. This layer is comprised of two sub-layers.
LLC sub-layer – This layer deals with framing, flow control, error detection and control, and MAC sub-layer addressing. IEEE 802.2 is an example of an LLC protocol.
MAC sub-layer – This layer manages access to shared media such as CSMA/CD for Ethernet or Token management for Token Ring.

Layer 1
Physical
This layer converts frames to Bits and provides the electrical, mechanical, procedural, and functional means for activating and maintaining the physical link between systems. Binary transmission including the sending and receiving of binary data using various device interfaces occurs at this layer.

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