The OSI Reference Model

The OSI Reference Model. The OSI Reference Model or Open Systems Interconnection Reference Model is used to help make sure that systems all around the world can interconnect with each other. For example, getting an Ethernet LAN to transparently exchange messages with an IBM Token Ring LAN to exchange messages via an ATM backbone. The overall goal of this standard is to promote interoperability. It was used as the basis of the template for the IP or Internet Protocol.

The OSI model divides operating in networks into seven functional layers. Each layer specifies the functions or set of functions to be performed when data is transferred across a network. Regardless of whether the local network operating system is Novell Netware, AppleTalk or IP, if the operating system adheres to the OSI model more or less the same rules are applied at each layer.

A diagram of the OSI model is given in the following image.

Each of the layers is essential a protocol associated with stack of how to handle data at its layer. The model causes the data to add on or peel off the necessary information at each layer depending on whether the data is being sent or received.

The following table gives a succinct presentation of the function of each layer.

Layer	Description
Physical Layer	This layer is responsible for the transmission of the bit stream. When sending, it accepts frames from the Data Link Layer and transmits their structure and content serially, one bit at a time. It is also responsible for the reception of data, one bit at a time. These streams are then passed up to the Data Link Layer for reframing. physical characteristics of interfaces and media representation of bits data rate/transmission rate synchronization of bits This layer doesn't do anything to determine the significance of what is moving. It's solely concerned with the physical characteristics of electrical and/or optical signals. The Physical Layer label can be misleading, it doesn't actually refer to the transmission media itself!
Data Link Layer	When transmitting this layer is responsible for packing instructions, data and so on into frames. The frame must also contain a way to verify the integrity of its contents upon delivery. Appropriate messages must be sent. When receiving it is also responsible for reassembling any binary streams received from the Physical Layer back into frames. Though, it isn't really rebuilding the frame, it's buffering the incoming stream until it has complete frames. framing MAC addressing - physical address for the device flow control - managing something like a sender sending faster than can be received error control access control
Network Layer	This layer establishes the route to be used between the origin and destination computers. It doesn't so any sort of error detection/correction. It is used to establish communications with systems that lie beyond the local LAN segment. It has its own routing addressing architecture which is separate and distinct from Data Link Layer machine addressing. This layer is required only if the computers exist on different network segments separated by a router. logical addressing routing getting individual packets to destination
Transport Layer	This layer is responsible for the end-to-end integrity of transmissions. This layer provides this function beyond the local LAN segment. It can detect packets that are discarded by routers and automatically send a retransmit request. When receiving, this layer also resequences packets that have arrived out of order. This layer is capable of determining the original sequence of packets and put them back into that order before passing them back up to the Session Layer. port addressing to get packets to correct application segmentation and reassembly of entire message connection control error control of entire message flow control
Session Layer	This layer is often unused as a separate layer. Many protocols bundle this layer's functionality into the Transport Layer's. The function of this layer is to manage the flow of communications during a connection between two computer systems. It is a dialog controller which establishes, maintains and synchronizes the interaction for a session between communicating systems.
Presentation Layer	This layer manages the way data is encoded to handle the semantics and syntax of the information being exchanged. It also handles encryption, decryption and compression.
Application Layer	This layer provides the interface between the application and the network's services. This layer can be thought of as the reason for initiating the communications session. For example, FTPing some files to a server.

In order to communicate, two systems must pass data, instructions, addresses and whatever between the layers. Although communications flow vertically through the stack of layers, each layer functions as if it is communicating directly with its counterpart layer on remote computers.

To create this logical adjacency of the layers, each layer of the original computer's protocol stack adds a header. This header can be used only by that layer or its counterparts on other computers. The receiving computer's protocol stack removes headers, one layer at a time as the data is passed up to the application. The following diagram helps represent this.

This starts to give you some sense of how these communications are executed. While there is a lot of jargon and meaning, eventually it coalesces.

The following diagram represents how packets operate on routed wide area networks, frames operate within local area network segments and signals operate across the physical media

In future web pages we will go into much more detail about technologies and specifications for physical transmission media, LANs and WANs. These correspond to the functions three layers of the OSI Reference Model displayed above. Remember, the OSI Reference Model is a protocol stack, even the physical layer is not really all that physical. But rather than dig into the OSI Model further we will focus on the more physical aspects that underlie these layers.