The OSI Model is used to describe networks and network application. The OSI model is a technology standard maintained
by the International Standards Organization (ISO). Although today's
technologies do not fully conform to the standard, it remains a useful
introduction to the study of network architecture.
- ISO(International Standards Organization) is a
multinational bogy dedicated to worldwide agreement on international
standards.
- Layers :--
Physical Layer
- Physical characteristics of interfaces and media: The physical layer defines the characteristics of the
interface between devices and the transmission media, including its
type.
- Representation of the bits: the physical layer data consist of a stream of
bits without any interpretation. To be transmitted, bits must be
encoded into signals –electrical or optical-. The physical layer defines
the type of encoding.
- Data rate:
The physical layer defines the transmission rate, the number of bits sent
each second.
- Line configuration: the
physical layer is concerned with the connection of devices to the medium.
- Physical topology
- Transmission Mode
Data Link Layer
- Framing.
The data link layer divides the stream of bits received from the network
layer into data units called frames.
- Physical addressing.
If frames are to be distributed to different systems on the network, the
data link layer adds a header to the frame to define the physical address
of the sender (source address) and/or receiver (destination address) of
the frame.
- Error control.
The data link layer adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. Error
control is normally achieved through a trailer to the end of the frame.
- Access Control.
When two or more devices are connected to the same link, data link layer
protocols are necessary to determine which device has control over the
link at any time.
Network Layer
- Logical addressing.
The physical addressing implemented by the data link layer handles the
addressing problem locally.
- The network layer adds a header to the packet coming
from the upper layer, among other things, includes the logical address
of the sender and receiver.
- Routing. When independent networks or links are
connected together to create an inter network (a network of networks) or a
large network, the connecting devices (called routers or gateways) route
or switch the packets to their final destination.
Transport Layer
- Port addressing: computer often run several
processes (running programs) at the same time.
- Process-to-process delivery means delivery from a
specific process on one computer to a specific process on the other.
- The transport layer header include a type of address
called port address.
- Connection control: The transport layer can be either connectionless
or connection-oriented.
- A connectionless transport layer treats each segment as
an independent packet and delivers it to the transport layer at the
destination machine.
- A connection-oriented transport layer makes a
connection with the transport layer at the destination machine first
before delivering the packets. After all the data are transferred, the
connection is terminated.
- Flow control: the transport layer performs a flow
control end to end. The data link layer performs flow control across a
single link.
- Error control: the transport layer performs error
control end to end. The data link layer performs control across a single
link.
Session Layer
- The services provided by the first three layers are not
sufficient for some processes.
- The session layer is the network DIALOG
CONTROLLER. It establishes, maintains, and synchronizes the
interaction among communication systems.
- Dialog control: The session layer allows two
systems to enter into a dialog. It allows the communication between
two processes to take place in either half-duplex of full-duplex mode.
- Synchronization: The session layer allows a
process to add checkpoints, or synchronization points, to a stream of
data.
Presentation Layer
- The presentation layer is concerned with the syntax and
semantics of the information exchanged between two systems.
- Network process to application
- The presentation layer is responsible for translation,
compression and encryption.
- Translation: The process in two systems are
usually exchanging information in the form of character strings, numbers,
and so on. The information must be changed to bit streams before
being transmitted. Because different computers use different
encoding systems, the presentation layer is responsible for
interoperability between these different encoding methods.
- The presentation layer at the sender changes the
information from its sender-dependent format into a common format.
The presentation layer at the receiving machine changes the common format
into its receiver-dependent format.
- Encryption: To carry sensitive information, a
system must be able to ensure privacy.
- Encryption means that the sender transforms the
original information to another form and sends the resulting message out
over the network. Decryption reverses the original process to
transform the message back to its original form.
- Compression: It reduces the number of bits
contained in the information. In multimedia such as text, audio, and
video.
Application Layer
- It is responsible for providing services to user.
- FTP, mail services, directory services.
