Essay About Osi Model And Data Link Layer

Osi Model
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UNDERSTANDING THE OSI MODEL AND THE RELATIONSHIP WITH TCP/IP
Table Of Contents
Letter of Transmittal
Abstract
Table of Contents
Written Presentation
References
Abstract
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
systems.
In the early and middle 70s, the interest of Canepas 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 IBMs 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