Introduction to the OSI Reference Model

The OSI model (minus the physical medium) is showed below. The model is called the OSI (Open Sytems Interconnection) reference model because it deals with connecting open systems—that is, systems that are open for communication with other systems.

                  

      Note that the OSI model itself is not a network architecture because it does not specify the exact services and protocols to used in each layer. It just tells what each layer should do. The OSI model has seven layers and each layer is to be discussed below.

The Physical Layer

      The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit it received by the other side as a 1 bit, not as a 0 bit. Typical questions here are what electrical signals should be used to represented a 1 and a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established, how it is torn down when both sides are finished, how many pins the network connector has, and what each pin is used for.

The Data Link Layer

      The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors. It does so by masking the real errors so the network layer does not see them. It accomplishes this task by having the sender break up the input data into data frames (typically a few hundred or a few thousand bytes) and transmit the frames sequentially. If the service is reliable, the receiver confirms correct receipt of each frame by sending back an acknowledgement frame.

     Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast transmitter from drowning a slow receiver in data.

      Broadcast networks have an additional issue in the data link layer: how to control access to the shared channel. A special sublayer of the data link layer, the medium access control sublayer, deal with this problem.

The Network Layer

      The network layer controls the operation of the subset. A key design issue is determining how packets are routed from source to destination. If too many packets are present in the subnet at the same time, they will get in one another's way, forming bottlenecks. Handling congestion is also a responsibility of the network layer, in conjunction with higher layers that adapt the load they place on the network.

      It is up to the network layer to overcome the problems, such as using different addressing, accepting too large packets and using different protocols, to allow heterogeneous networks to be interconnected.

      In broadcast networks, the routine problem is simple, so the network layer is often thin or even nonexistent.

The Transport Layer

      The basic function of the transport layer is to accept data from above it, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.

      The transport layer also determine what type of service to provide to the session layer, and, ultimately, to the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent.

      The transport layer is a true end-to-end layer; it carries data all the way from the source to the destination. In the lower layers, each protocol is between a machine and its immediate neighbors, and not between the ultimate source and destination machines, which may be separated by many routers. The difference between layer 1 through 3, which are chained, and layers 4 through 7, which are end-to-end, is illustrated in figure above.

The Session Layer

      The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control (keeping track of whose turn it is to transmit), token management ( preventing two parties from attempting the same critical operation simultaneously), and synchronization (checkpointing long transmissions to allow them to pick up from where they left off in the event of a crash and subsequent recovery).

The Presentation Layer

      Unlike the lower layers, which are mostly concerned with moving bits around, the presentation layer is concerned with the syntax and semantics of the information transmitted. The presentation layer manages abstract data structures and allows higher-level data structures (e.g., banking records) to be defined and exchanged, thus making it possible for computers with different internal data representation to communicate.

The Application Layer

      The application layer contains a variety of protocols that are commonly needed by users. One widely used application protocol is HTTP (HyperText Transfer Protocol), which is the basis for the World Wide Web. Other application protocols are used for file transfer, electronic mail, and network news.