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History And Development Of The Osi Model

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steven parks

osi history and development


The history of the development of the OSI model is a little-known story. Much of the work on the design of OSI was done by a group at Honeywell Information Systems, headed by Mike Canepa, with Charlie Bachman as the principal technical member. This group was organized within Honeywell, with advanced product planning and with the design and development of prototype systems.

In the early and mid '70s, the interest of Canepa's group was mainly on database design and then distributed database design. By the mid-70s, it become clear that to support database machines, distributed access, and the like, a structured distributed communications architecture would be needed. The group studied some of the existing solutions, including IBM's system network architecture (SNA), the work on protocols being done for ARPANET, and some of the concepts of presentation services being developed for standardized database systems. The result of this effort was the development by 1977 of a seven-layer architecture known as the distributed systems architecture (DSA).

Bachman and Canepa participated in ANSI meetings and presented their seven-layer model. This model was chosen as the only proposal to be submitted to the ISO subcommittee. When the ISO group met in Washington DC in March of '78, the Honeywell team presented their solution. An agreement was reached at that meeting that this layered architecture would satisfy most requirements of OSI, and had the ability to be expanded later to meet new requirements. A provisional version of the model was published in March of '78. The next version, with some minor adjustments, was published in June of '79 and eventually standardized. The resulting OSI model is essentially the same as the DSA model developed in 1977.

why a layered model?

>reduces complexity

>standardizes interfaces

>gaurantees interoperablity

>accelerates evolution

>simplfies teaching and learning

The layers

1.physical layer

transmission of the bit stream over the physical links including copper wire fiber optic cable and the connectors also includes distances and data rate

2.the data link layer

provides for the reliable transfer across the physical link also deals with;


*physical addressing

*network topology


*error control

*flow control layer

provides connectivity and path selection between two hosts that may or may not be on two diferent physical networks more things associated with it are;


*virtual circuts

*routing tables and routing protocols

*logical addressing (ip addressing)

4.the transport layer

provides reliable data transfer over networks

*end to end flow control

*error detection and recovery

*segmentation and reassembly

*connection orented and connectionless

5.session layer

establishes, manages and terminates sessions between hosts




*data exchange




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