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Osi Model

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Letter of Transmittal


Table of Contents

Written Presentation



The Open Systems Interconnection (OSI) model is a reference tool for understanding data

communications between any two networked systems. It divides the communications processes into

seven layers. Each layer both performs specific functions to support the layers above it and offers

services to the layers below it. The three lowest layers focus on passing traffic through the network to

an end system. The top four layers come into play in the end system to complete the process.

This presentation will provide you with an understanding of each of the seven layers, including their

functions and their relationships to each other. This will provide you with an overview of the network

process, which can then act as a framework for understanding the details of computer networking. Also

this paper will explain how the 802 specifications expanded the OSI reference model by dividing the

data link layer into two layers.

Finally, this paper will draw comparisons between the theoretical OSI model and the functional

TCP/IP model. Although TCP/IP has been used for network communications before the adoption of the

OSI model, it supports the same functions and features in a differently layered arrangement.

The history of the development of the OSI model is, for some reason, a little-known story. Much of

the work on the design of OSI was actually done by a group at Honeywell Information Systems, headed

by Mike Canepa, with Charlie Bachman as the principal technical member. This group was chartered,

within Honeywell, with advanced product planning and with the design and development of prototype


In the early and middle '70s, the interest of Canepa's group was primarily on database design and

then on 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

required. 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 internally as the distributed systems

architecture (DSA).

Meanwhile, in 1977 the British Standards Institute proposed to the International Organization for

Standardization (ISO) that a standard architecture was needed to define the communications

infrastructure for distributed processing. As a result of this proposal, ISO formed a subcommittee on

Open Systems Interconnection (Technical Committee 97, Subcommittee 16). The American National

Standards Institute (ANSI) was charged to develop proposals in advance of the first formal meeting of

the subcommittee.

Bachman and Canepa participated in these early 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 1978, the Honeywell team presented their solution.

A consensus was reached at that meeting that this layered architecture would satisfy most requirements

of Open Systems Interconnection, and had the capacity of being expanded later to meet new

requirements. A provisional version of the model was published in March of 1978. The next version,

with some minor refinements, was published in June of 1979 and eventually standardized in 1984. The

resulting OSI model is essentially the same as the DSA model developed in 1977.

OSI Model Physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for

activating, maintaining, and deactivating the physical link between communicating network systems.

Physical layer specifications define characteristics such as voltage levels, timing of voltage changes,

physical data rates, maximum transmission distances, and physical connectors. Physical layer

implementations can be categorized as either LAN or WAN specifications.

OSI Model Data Link Layer

The data link layer provides reliable transit of data across a physical network link. Different data

link layer specifications define different network and protocol characteristics, including physical

addressing, network topology, error notification, sequencing of frames, and flow control. Physical

addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.

Network topology consists of the data link layer specifications that often define



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