Network Design
Layering (ISO/OSI Seven Layer
Network Model)
A vital part of network design
is to use a layered reference model. In other words, processes (like
creating a program), can be more easily managed if they are broken down into
layers or modules, where each of the layers communicate with the layer
directly above and beneath itself. This permits designers to work at any
stage in the development of a project and to divide the design of the
network into more manageable chunks.
In the early/mid 80s the
International Standards Organisation provided such a model. The model,
called the Open Systems Interconnect Seven Layer Model, aided network
designers and vendors in standardising networking protocols and equipment.
The model also provides an invaluable tool used to aid students in the
understanding of networks and how it all fits together. The following is a
very brief description of each layer.
· Application Layer
This layer interacts with the
user to create the message to be sent over the network. It provides the link
between the user's application package and the communications system.
Services that are supported at this layer include:
· File and print services
· E-mail
· Remote program access (Telnet)
· File transfer programs (FTP)
· Presentation Layer
Ensures that machines with the
different data representations can still pass the same meaning from one user
to another. This layer also provides facilities like
compression/decompression encryption/decryption and terminal emulation.
· Session Layer
Controls communications between
applications across the network
· Establish a connection
· Maintain the connection
· Terminate the connection
· Transport Layer
Responsible for the reliable
delivery of the transmitted message.
· Network Layer
Adds unique addressing
information to packets so that they are routed to the correct receiving
station on another network. It is responsible for:
· Determining addresses either
on a local network or an internetwork.
· Finding the best route between
source and destination addresses.
· Data Link Layer
Responsible for creating,
transmitting and receiving data frames. A checksum for error detection is
added to the frame and is sent to layer 1 for transmission
· Physical Layer
Concerned with moving data
between the stations and the medium that connects the stations. This layer
defines the electrical (i.e. voltage) and mechanical (i.e. pin wiring)
requirements for connecting to equipment to the medium
All
networking related concepts and devices, operate at one or more of these
layers, allowing designers to categorise problems and tackle them logi |
