Now, lets turn to the 'bases' on which the whole Internet functions. These are the core concepts of the
Internet and one must know how the Internet functions. I have broken the whole post into
(i) The OSI Model
(ii) The IP Suite
(iii) The DoD Model
(i) A networking model introduced by the International Organization for Standardization (ISO)
to promote multivendor interoperability.
(ii) OSI is a seven-layered conceptual model consisting of the application, presentation,
session, transport, network, datalink, and physical layers.
2. Layers of OSI Model as defined by the International Organisation for Standardisation (ISO)
(i) Application Layer
Layer 7 provides an interface between the communications software and any applications
that need to communicate outside the computer on which the application resides. It
also defines processes for user authentication.
(ii) Presentation Layer
The presentation layer formats data received from the application layer before passing
it to the layers. Encryption also is defined by OSI as a presentation layer service.
(iii) Sessions Layer
The session layer defines how to start, control, and end conversations (called sessions).
This includes the control and management of multiple bidirectional messages so
that the application can be notified if only some of a series of messages are completed.
(iv) Transport Layer
The transport layer works with the network layer to package and transfer data that it
receives from the session layer. Layer 4 focuses on issues related to data delivery to
another computer error recovery and flow control.
(v) Network Layer
The network layer defines three main features: logical addressing, routing (forwarding),
and path determination. The routing concepts define how devices (typically routers)
forward packets to their final destination. Logical addressing defines how each device
can have an address that can be used by the routing process. Path determination
refers to the work done by routing protocols by which all possible routes are learned, but
the best route is chosen for use.
(vi) Data-Link Layer
The data link layer defines the rules (protocols) that determine when a device can send
data over a particular medium. Data link protocols also define the format of a header
and trailer that allows devices attached to the medium to send and receive data
successfully. The data link trailer, which follows the encapsulated data, typically defines
a Frame Check Sequence (FCS) field, which allows the receiving device to detect the transmission errors
(vii) Physical Layer
This layer typically refers to standards from other organizations. These standards deal
with the physical characteristics of the transmission medium, including connectors,
pins, use of pins, electrical currents, encoding, light modulation, and the rules for how to activate and deactivate the use of the physical medium.
I will be again going into the depths of the OSI model, how devices on the network
operate at different layers of this model in the next few weeks. This was only the overview.
I hope you all atleast got the idea as to what is an OSI model. These layers are so crucial
that no communication can be established over the Internet if anyone of these layers are
absent (if we take this model into consideration...about IP Suite and DoD Model, I will
be writing soon).