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The OSI Model
by
Matthew Martin
The Open Systems Interconnection (OSI) model is the main model used for understanding network protocols. Whilst the OSI model cannot be strictly applied to all protocols, it is a great assistance in the understanding of how network protocols work and how they work together at different levels of the model.
The OSI Model Has Seven Layers
Layer 1: Physical Layer
This
layer is concerned with the transmission of data over the physical wires and
cables of the network. Specialised software (usually firmware) and hardware
deals with the encoding at the physical layer, being concerned with electrical
currents and/or pulses of light (for optical systems).
Standards
for the physical layer determine the correct means of communication using
electricity or light through fibre optic cabling. e.g. which wire at one
end connects to which pin at the other end. This layer is the fundamental layer
upon which all the other layers rely.
Layer 2: Data-Link Layer
Provides
error-checking and control for the physical layer, ensuring that communication
at the physical layer is reliable.
A
number of different protocols are available for this layer. These include
bisynchronous communications and Advanced Data Communications Control
Procedures (ADCCP), two name only two.
Layer 3: Network Layer
The
software of this layer provides a link between the lower two layers (data-link
& physical layers) and the higher level software operating at a higher
layer. The network layer determines the path that data will take on larger
networks. The determination of the route taken will depend upon network traffic
levels, priority of service and other factors.
The
network layer software is usually located in switches on the network. Today,
the most common network layer protocol is IP (Internet Protocol).
Layer 4: Transport Layer
The
transport layer software provides reliable communication, through the provision
of error-checking for packets of data travelling across the network. The
transport layer also provides routing information, similar to that provided by
the network layer. Often software working at the transport layer works
co-operatively with the software of the network layer to correct errors in the
transport of data packets across the network. The transport layer software detects
lost packets and resends them, requesting alternative routes from the network
layer software. The transport layer software packages the data, places an
envelope of routing and error correction data in the envelope, also referred to
as the header and footer of the packets of data.
The
most common transport layer protocol used today is TCP (Transport Control
Protocol, it is the one used on the Internet). Other transport layer protocols
include NetBIOS (by Microsoft) and Internetwork Packet Exchange (IPX, by
Novell). Transport layer software is found in ever station on the network.
Layer 5: Session Layer
This provides a means of moving data between
software applications. The session layer software provides functions concerned
with logging on, security, logging, administration, etc. Session layer
software allows applications to work together across a network.
Some software straddles the layers of the OSI model,
operating on more than one layer. e.g. NetBIOS works on both the
transport layer and the session layer.
Layer 6: Presentation Layer
Data
is formatted in this layer for viewing within a specific type of application.
All the text effects (font, reverse video, etc.) are handled at this
level. Encryption may also be handled in the layer.
One
of the most common protocols at the presentation layer is HTTP (Hyper-Text
Transfer Protocol), used for the World Wide Web (WWW).
Layer 7: Application Layer
The
standards set for look and feel are implemented in this layer. This is where
the network Operating Systems (OS) and applications reside. Tasks such as print
job spooling and file sharing are performed in the application layer. Also
e-mail is implemented in the application layer.
An
examples of an application layer protocol is Systems Application Architecture
(SAA) by IBM.
Matthew Martin