OSI Model

- The OSI Reference Model 

 The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model. It divides network communication into seven layers of network protocols: physical, data link, network, transport, session, presentation, and application. 

A protocol is a set of rules that governs the communications between computers on a network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer. Data networking is generally implemented as a stack of several protocols - each responsible for a specific aspect of networking. 

- The OSI Layers

The OSI reference model organizes a network into seven layers (a protocol stack).  These layers define how networking hardware and software are to hand data and transfer it across a network.  Interoperability, the purpose for defining a standard protocol model, exists when there is compatibility between the protocol stack of one workstation or peripheral device and that of another.  Each layer is able to communicate with the corresponding layer of a receiving station.

In the OSI model, layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7, called the the upper layers, contain application-level data. Networks operate on one basic principle: "pass it on." Each layer takes care of a very specific job, and then passes the data onto the next layer. Please refer to "The OSI Reference Model" for more details.

In the OSI model, control is passed from one layer to the next, starting at the application layer (Layer 7) in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The OSI model takes the task of inter-networking and divides that up into what is referred to as a vertical stack that consists of the 7 layers.

- Layered Software and The OSI Model

Layers provide a division of the work done by a network. Networks are set up with a protocol hierarchy that divides the communication task into several layers. A protocol is a set of rules for communication within a layer. A service is what the layer provides to the layer above it through an interface.  Protocols at one layer are unaware of issues at another layer. 

The OSI reference model gives us a terminology to discuss and compare different networks. Any specific network may not necessarily implement all these layers. The Internet protocol stack relies on layers 1 through 4 (physical through transport) but it is up to applications to implement and use session, presentation, and, of course, application layers. 

A key aspect of this layering approach is that each layer only has to interact with the corresponding layer on the other side. For example, an application talks to another application. TCP on one system deals with issues of retransmission and message acknowledgement by talking to the TCP layer on the remote system. A layer also does not need to be aware of the implementation of layers above it or below it: they are just data sources and data sinks.

Thie OSI model that was proposed as a set of standard layers and protocols for communication between different computers around the world and has been in use since 1983.  Although not universally adopted, much of this model is considered the standard and in heavy use.  This protocol is different from TCP/IP which will discussed below. 

[More to come ...]