John Walmsley

The OSI Model

The preceding chapter has discussed the philosophy of layered communications architectures, and the functions which should be performed within such an architecture. It is appropriate now to focus on the seven layer OSI basic reference model as defined by the International Organization for Standardization (ISO).

Open systems interconnection

Philosophy of layered architectures the OSI model physical layer data link layer network layer transport layer session layer presentation layer application layer applications of OSI definition of OSI terms.

Physical Layer — Layer 1

Open image in new window The Physical Layer is the lowest layer in the OSI model, and is the only layer where a subsystem in one open system communicates directly with the corresponding subsystem in another. Its function is to transfer information transparently between physical service users (data link entities). This transfer is implemented by the transmission of bit streams over a physical medium.

Applications of OSI

The reader who has come this far might be forgiven for thinking that for so powerful and flexible a concept as OSI, its universal application can only be a matter of time. The unremitting and sometimes misleading publicity which OSI has received in the computer trade press over recent years has tended to purvey an image of a universal solution to all problems of data communications and system incompatibility. However, like all technologies it has strengths and weaknesses which limit the range of problems to which it can be successfully applied. The purpose of this chapter is to identify these limitations, and also describe some of the successful applications which are already being realised.

Philosophy of Layered Architectures

The design of a communications network which allows interconnection between many systems, manufactured by different companies, in many different locations, for a multitude of purposes, is a complex undertaking. As with any large task, the most effective way of defining and completing it successfully is to break it down into a number of smaller tasks. If the resulting tasks are still too large to be easily managed, then they too should be broken down. This process can be repeated as often as required until each portion is manageable.

Application Layer — Layer 7

Open image in new window We have now arrived at the highest layer of the OSI model, where the model and the world of application processes meet and interact. This layer is different from all the other layers in that it does not have a strictly defined interface with a higher layer: instead, the Application Layer and application processes overlap. The application process consists of two parts, one which performs processing outside the OSI environment and one which resides within the Application Layer.

Presentation Layer — Layer 6

Open image in new window The layers described in the preceding chapters provide the services necessary for one system to transfer data to another. What these layers cannot guarantee is that the receiver understands the sender’s data. Telephones may allow you to speak to your Chinese counterpart in Peking, but unless you can agree to conduct your communication in a commonly understood language, your telephone conversation will be merely a meaningless exchange of sounds.

Transport Layer — Layer 4

Open image in new window The Transport Layer is the highest layer in the OSI model which is directly involved with data communications. The network service provides routeing and relaying across real subnetworks, possibly involving the use of intermediate open systems; the transport service, in contrast, is concerned only with communication between end open systems and has no interest in the route actually taken by data travelling between those end systems. Its responsibility is to provide session entities with a reliable, cost-effective means of transferring data, protecting them from needing to know anything about the underlying real data communications system. The Transport Layer thus links the layers below, which are responsible for transmitting data across real subnetworks, with the layers above, which are responsible for maintaining an orderly dialogue between application processes in end open systems.

Network Layer — Layer 3

Open image in new window The Network Layer provides transparent transfer of data between two corresponding network service users (transport entities). The main purpose of the Network Layer is to provide routeing of network service data units (NSDUs) between network service users in communicating end open systems. Routeing may involve the use of intermediate open systems as relays, but this relaying function is invisible, both to the Transport Layer and to the layers above.

Data Link Layer — Layer 2

Open image in new window The Data Link Layer (sometimes abbreviated to Link Layer) provides transparent transfer of data link service data units between network entities. The Physical Layer described in the previous chapter provides transparent transmission of physical service data units between data link entities. Physical service data units consist of one bit in serial transmission or n bits in parallel transmission. In the Data Link Layer the service data unit is an arbitrary but variable number of bits, containing user data and protocol information from the layers above.