| There are many types of networking
protocols. We will focus on the OSI model and some of the more common
networking protocols. When two nodes on a network wish to communicate,
they must use networking protocols.
The OSI model was developed by ISO
(International Organization for Standardization) to describe the
functions that occur as two nodes on a network communicate. OSI is
short for Open Systems Interconnection and is an internationally
recognized, structured, and standardized architecture for network
design. The 7-layer OSI reference model or just OSI model is one of
the most useful tools we have for analyzing and comparing various
technologies and networking products from different vendors. The model
or "protocol stack" itself is rarely adhered to completely
by any one vendor but it serves as a frame of reference for everyone
to compare to.
The OSI model helps to define the
process of communication between two devices or nodes on a network.
Various protocols operate at each layer to pass the data from one
device or node to another. These "nodes" can be user devices
such as workstations and servers or network devices such as routers
and switches. The important thing to realize is that the OSI model
does not really come into play unless we want to communicate over a
network between two devices. If you open a local document file on your
hard disk with a word processor on your standalone PC at home, the OSI
model is not involved. If you open a file from your workstation, which
is actually in your home folder on a network server in another
building, the OSI model is involved. When your word processor attempts
to open the file on the server, network communication must take place
from the users level down through the protocol stack on your
workstation and out the Network Interface Card (NIC) on your computer,
through the network media (cable) to the server and up its protocol
stack to retrieve the file.
By splitting the process of
communicating from one device to another over a network into smaller
digestible chunks we can study it more easily. We also provide a
structured blueprint from which network equipment engineers can create
their products and create modular network designs. Thus, the OSI model
facilitates communication and education relating to networks as well
as the development of networking products and services. Figure
shows the seven layers of the OSI model with a brief description of
the function of each layer. These are divided into two main groups:
the Applications layers and the Data Flow layers as shown in Figures
and .
Application layer (Layer 7) -
Network Processes to applications.
Provides network communication
services to user applications for file transfers, WWW access,
Email and remote terminal access. For example, a word processing
application is serviced by file transfer services at this layer.
Presentation layer (Layer 6) -
Data Representation
Provides translation for character
sets, data compression, encryption and support for various file
types (graphics, video, sound).
Session layer (Layer 5) -
Inter-host Communications
Keeps track of which application is
doing what and where. It establishes, manages and terminates the
relationships between multiple applications such as client and
server. Acts as a funnel to the lower 4 layers. This layer
establishes, maintains, and manages sessions between applications.
Transport layer (Layer 4) -
End-to-End Connections
This is the shipping department of
OSI model. Packages can be shipped via guaranteed services or
non-guaranteed. Segments information into smaller Packets for
processing by the network layer. This layer segments and
reassembles data into a data stream. The transport layer has the
potential to guarantee a connection and offer reliable transport.
Network layer (Layer 3) -
Addressing and Best Path
Packet are addressed and directed
through an internetwork (between networks) at this layer. This
layer determines the best way to move data from one place to
another. The router operates at this layer. This layer uses
logical addressing schemes that can be managed by an
administrator. The Internet Protocol (IP), IPX and AppleTalk are
examples of addressing schemes.
Data link layer (Layer 2) - Access
to Media
These are the shipping methods and
drivers. The drivers of the shipping vehicles (Truck, Train, Boat,
Plane) and how they get on to the pathways. This layer provides
physical transmission across the medium. It handles error
notification, network topology, and flow control. This layer uses
Media Access Control (MAC) addresses, which also are referred to
as physical or hardware addresses.
Physical layer (Layer 1) - Binary
Transmission
The physical cabling and
connections (wired and wireless) that make up the pathways for
electricity, light and radio waves (the highways, railways,
shipping lanes and airways). This layer provides the electrical,
mechanical, procedural, and functional means for activating and
maintaining the physical link between systems. This layer uses
such physical media as twisted-pair, coaxial, and fiber-optic
cable.
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