The OSI model describes the method for transmitting data between two systems. The OSI model allows you to selectmultiple vendors in your organization. Thus, you can have Cisco routers and 3Com hubs connecting IBM mainframes and Compaq computers. Manufacturers
that develop products that meet the standard can, in theory, connect their products to any other manufacturer whose products meet the standard. While most of you are probably familiar with the model, different people de.ne the seven layers differently. Thus, the OSI model is offered here in review.
The Open Systems Interconnection (OSI) model consists of seven layers. Each of
these layers has a distinct function and interacts and communicates with the layers
directly above and below it.
Application Layer
The application layer is the highest layer of the OSI model. This is responsible for supporting the communication components of an application. This is an important concept. Programs that use the application layer are known as application processes.
It is possible for a user program to interface directly with the presentation layer; however, to do this, the program must initialize communication with peer application processes, establish appropriate presentation contexts, and transfer .les or messages itself. Alternately, user-level processes may include available modules that support commonly required application-related services, such as .le transfer, e-mail, or and print services. User-level processes are actually gateways to the presentation layer.
Presentation Layer
Different machines have different ways of representing data internally, so conversions are necessary to ensure that different computers can understand each other. The job of the presentation layer is to take the internally formatted data from the sending machine, convert it into a suitable bitstream for transmission, and then decode it to a format the receiving machine can understand at the other end. That is, the presentation layer acts as an interpreter that understands both formats and is responsible for making sure both computers get information in a format each can understand. This is true whether one is talking about compression, conversion, or encryption.
Session Layer
The primary function of the session layer is to allow users to establish connections or sessions, and to transfer data over those connections (or sessions) in a controlled manner. There are two types of services provided by the session layer: administrative and dialog. The administrative service handles the establishment and teardown of a connection between two presentation entities. The administrative service also determines the type of connection established. For example, the connection might be full-duplex or half-duplex. Sessions are established when one application process requests access to another application process. After the session is established, dialog services are used to control and supervise the actual data transfer.
Transport Layer
The transport layer is the highest layer directly associated with the transport of data through the network. This layer defines end-to-end connectivity between host applications. The basic functions of the transport layer are to:
_ Establish end-to-end operations: provides end-to-end transport services, which constitute logical connections between the sending and receiving hosts
_ Segment upper-layer applications: allows multiple applications to use the network simultaneously, as it segments data from multiple upper-layer applications into the same data streams for transport on the network
_ Send segments from one host to another: uses checksum calculations and built-in .ow control to ensure the integrity of segmented data
_ Ensure data reliability: can optionally request that the receiving host acknowledge that it is actually receiving the data
Network Layer
The primary concern of the network layer is getting data all the way from the source to the destination. The layers above the network layer (i.e., transport, session, presentation, and application) typically run on the user’s machine. The most important function of the network layer is route determination.The network layer determines the path that data will take to travel between a node on one network to a node on another network. What path is actually used depends on what routing protocols are used within the network. The functions implemented at the network layer include routing, switching, .ow control, data sequencing, and error recovery. Some of these functions might appear to duplicate those of the transport layer, but, in fact, they do not. The network layer’s functions are concerned with end-to-end connections, possibly spanning multiple network links. The transport layer does not concern itself with the intermediary links and devices as does the network layer. Flow control at the network layer is concerned not with the two end stations, but instead with the links and devices that the conversation crosses. If too many packets are present at any one link at any given time, those packets will interfere with each other, causing congestion errors and bottlenecks. Communicating systems use flow control at the network layer to prevent those types of conditions from occurring by trying to provide fair, orderly, and ef.cient access to network
links.
Data-Link Layer
The data-link layer is the layer responsible for moving data in and out across the physical network. There are two sublayers in the data-link layer:
_ The Logical Link Control (LLC) sublayer
_ The Media Access Control (MAC) sublayer
The MAC sublayer deals with interfacing the physical media, while the LLC sublayer handles the interface to the network layer. The LLC sublayer is responsible for assembly and disassembly of frames, addressing, address recognition, and cyclical redundancy check (CRC)
calculations and validation. The MAC sublayer is responsible for de.ning how access is gained to the shared network media. This is dependent on the type of network, while the LLC is media independent. Together, the two sublayers divide output data into frames for transmission
on the physical link. The data-link layer provides framing, .ow control, and error detection and correction.
Physical Layer
The bottom layer of the OSI model is the physical layer. At the physical layer, bits are transformed into signals (for analog lines) on the transmission medium. It is the physical layer that defines what signals on the line constitute 1s and 0s, and which are just noise. Physical layer speci.cations de.ne things such as allowable cable lengths, maximum capacities, and the physical wiring of the network. It is worth noting that the application, presentation, and session layers provide communication. The communication layers set up the interactions for user-level applications.