10-12-2012, 01:07 PM
The OSI Model: Understanding the Seven Layers of Computer Networks
The OSI Model Understanding.pdf (Size: 601.35 KB / Downloads: 17)
Introduction
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 white paper 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.
Since the discussion of networking often includes talk of “extra layers”, this paper will address these unofficial
layers as well.
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.
An Overview of the OSI Model
A networking model offers a generic means to separate computer networking functions into multiple layers.
Each of these layers relies on the layers below it to provide supporting capabilities and performs support to
the layers above it. Such a model of layered functionality is also called a “protocol stack” or “protocol suite”.
Protocols, or rules, can do their work in either hardware or software or, as with most protocol stacks, in a combination
of the two. The nature of these stacks is that the lower layers do their work in hardware or firmware
(software that runs on specific hardware chips) while the higher layers work in software.
The Open System Interconnection model is a seven-layer structure that specifies the requirements for communications
between two computers. The ISO (International Organization for Standardization) standard 7498-1
defined this model. This model allows all network elements to operate together, no matter who created the
protocols and what computer vendor supports them.
Layer 3 – The Network Layer
Layer 3, the network layer of the OSI model, provides an end-to-end logical addressing system so that a packet
of data can be routed across several layer 2 networks (Ethernet, Token Ring, Frame Relay, etc.). Note that network
layer addresses can also be referred to as logical addresses.
Initially, software manufacturers, such as Novell, developed proprietary layer 3 addressing. However, the networking
industry has evolved to the point that it requires a common layer 3 addressing system. The Internet
Protocol (IP) addresses make networks easier to both set up and connect with one another. The Internet uses
IP addressing to provide connectivity to millions of networks around the world.
To make it easier to manage the network and control the flow of packets, many organizations separate their
network layer addressing into smaller parts known as subnets. Routers use the network or subnet portion of
the IP addressing to route traffic between different networks. Each router must be configured specifically for
the networks or subnets that will be connected to its interfaces.
Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP)
and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the
best way to reach each network based on a variety of criteria (such as the path with the fewest routers).
Routers and other networked systems make these routing decisions at the network layer.
Summary
The most identified benefit of the OSI model is that it organizes thinking about networks and give novices,
journeymen, and masters a common, computer networking language. Human communication, discussions, and
collaboration can use this language to remove ambiguity and clarify intent.