In an effort to provide some standard to the way networking should work, a model was developed by the International Standards Organization (ISO). This is a theoretical layered model in which the notion of networking is divided into several layers, each of which has specific functions and features. The model is known as the Open Systems Interconnect (OSI) model and represents only a general guideline for developing usable network interfaces and protocols. Sometimes the distinctions between layers become fuzzy, and some systems do not adhere rigorously to the model. Despite all this, it has earned the honour of being the model upon which all good network protocols are based.

Physical Layer

The physical later is primarily concerned with transmitting raw bits over a communication channel and other physical aspects of the media being used to transmit the data. These characteristics include modulation and encoding of data bits on carrier signals, and ensure bit synchronization. Typical questions asked are:

How many volts should be used to represent a 1 and how many for a 0?
How many microseconds does a bit last?
Can transmission proceed simultaneously in both directions?

Data Link Layer

This layer deals with receiving data across a specific medium and individual links by supplying one or more data link connections between two network entities. The data link layer takes a raw transmission facility and transforms it into a line that appears free of transmission errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver. The Data Link Later is also subdivided into two layers: The Media Access Control Layer (MAC), which concerns itself with the access control method and determines how use of the physical transmission is controlled. The second sub layer is the Logical Link Layer (LLC) which shields the higher level layers from concerns with the specific LAN implementation.

The Data link layer also performs error checking using the Frame Check Sequence (FCS) located at the end of the frame, and discards it if an error is detected. The data between the header and the Frame Check Sequence is then passed to the Network Later.

Network Layer

The Network Layer is responsible for delivery of packets and implements an addressing scheme to accomplish this. Also defined at this layer is the routing of packets through a network, and a method to fragment large packets into smaller ones. The network layer is also concerned with controlling the operation of the subnet. If too many packets are present in the subnet all at once, they will get in each other's way, forming bottlenecks. The control of such congestion also belongs to the network layer. Once the data from layer 2 has been received, the Network Layer examines the destination address and if it is the address of its own host, it passes all the data after the header up to the next layer.

Transport Layer

This layer is responsible for the ordering and reassembly of packets that may have been broken up to travel across certain media. Some protocols in this layer also perform error recovery. The transport layer also decides which type of service to provide to the session layer, as well as establishing and deleting connections across the network. The most popular type of transport connection is an error-free virtual circuit channel that delivers messages in the order in which they were sent. Other possible kinds of transport exist that transport isolated messages with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations. This type of service is determined when the connection is established. This requires some kind of naming mechanism, so that processes have a way to describe with whom it wishes to communicate with. There must also be a method to regulate the flow of information, so that a fast host will not overrun a slower one. This flow control between hosts is distinct from flow control between switches, although similar philosophy applies to both. Furthermore, all this must be done efficiently, and in a way that isolates the session layer from the inevitable changes in the hardware technology. After error recovery and reordering, the data part is passed up to the Sessions Later.

Session Layer

The Session Layer allows users on separate machines to establish sessions. It defines how the data conversations are established, controlled, and terminated.  The sessions may be bidirectional and there might be many of them.  The session layer will manage these conversations and create notifications if anything fails.  A session allows ordinary data transport (as does the transport layer), but it also provides some enhanced services useful in some applications.  Only after a completed conversation will the data be passed up to Presentation Layer.

Presentation Layer

The presentation layer performs functions that are requested enough often to warrant a general solution for them, rather than letting each user solve the problems. The presentation layer is concerned with the syntax and semantics of the information transmitted. The presentation layer is also concerned with other aspects of information representation such as data compression which can be used to reduce the size of information that have to be transmitted, and cryptography which is frequently required for privacy and authentication.

Application Layer

The Application Layer contains a variety of protocols that are commonly needed and is employed in software packages which implement client-server software. The Application Layer is used when an application on one machine starts communicating with another machine. The header contains parameters that are agreed between applications, which is often only sent at the beginning of an application operation.

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