The OSI Model

 APPLIATION LAYER

It is the seventh layer in the OSI Model. The application layer consists of what
most users think of as programs. The application does the actual work at hand.

Layer 6: Presentation Layer

The Presentation Layer establishes context between Application Layer entities, in

which the higher-layer entities may use different syntax and semantics if the
presentation service provides a mapping between them. If a mapping is available,
presentation service data units are encapsulated into session protocol data units,
and passed down the stack.
This layer provides independence from data representation (e.g., encryption) by
translating between application and network formats. The presentation layer
transforms data into the form that the application accepts. This layer formats and
encrypts data to be sent across a network. It is sometimes called the syntax layer.
[citation needed]

The original presentation structure used the basic encoding rules of Abstract
Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-
coded text file to an ASCII-coded file, or serialization of objects and other data
structures from and to XML.

It is the sixth layer of the OSI Model. This layer is concerned with Syntax and
Semantics of the information transmitted, unlike other layers, which are interested
in moving data reliably from one machine to other. Few of the services that
Presentation layer provides are:

1. Encoding data in a standard agreed upon way.

2. It manages the abstract data structures and converts from representation used
inside computer to network standard representation and back.

 SESSION LAYER

The Session Layer controls the dialogues (connections) between computers. It

establishes, manages and terminates the connections between the local and
remote application. It provides for full-duplex, half-duplex, or simplex operation,
and establishes checkpointing, adjournment, termination, and restart procedures.
The OSI model made this layer responsible for graceful close of sessions, which is
a property of the Transmission Control Protocol, and also for session
checkpointing and recovery, which is not usually used in the Internet Protocol
Suite. The Session Layer is commonly implemented explicitly in application
environments that use remote procedure calls

It is the fifth layer of the OSI Model. This layer allows users on different machines
to establish session between them. A session allows ordinary data transport but it
also provides enhanced services useful in some applications.

Some of the session related services are:

1. This layer manages Dialogue Control -- Session can allow traffic to go in

both

direction at the same time, or in only one direction at one time.

2. Token management -- For some protocols, it is required that both sides don't
attempt

same operation at the same time. To manage these activities, the session layer
provides

tokens that can be exchanged. Only one side that is holding token can perform the
critical

operation. This concept can be seen as entering into a critical section in operating
system

using semaphores.

3. Synchronization -- Consider the problem that might occur when trying to

transfer a 4-

hour file transfer with a 2-hour mean time between crashes. After each transfer was

aborted, the whole transfer has to start again and again would probably fail. To
Eliminate

this problem, Session layer provides a way to insert checkpoints into data streams,
so that

after a crash, only the data transferred after the last checkpoint have to be repeated.

 TRANSPORT LAYER

It is the fourth layer of the OSI Model. The transport level provides end-to-end
communication between processes executing on different machines.

The Transport Layer provides transparent transfer of data between end users,

providing reliable data transfer services to the upper layers. The Transport Layer
controls the reliability of a given link through flow control,
segmentation/desegmentation, and error control. Some protocols are state and
connection oriented. This means that the Transport Layer can keep track of the
segments and retransmit those that fail. The Transport layer also provides the
acknowledgement of the successful data transmission and sends the next data if no
errors occurred.
Although not developed under the OSI Reference Model and not strictly
conforming to the OSI definition of the Transport Layer, typical examples of Layer
4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

Link -- http://en.wikipedia.org/wiki/OSI_Model#Layer_4:_Transport_Layer

Layer 3: Network Layer

The Network Layer provides the functional and procedural means of transferring variable
length data sequences from a source host on one network to a destination host on a different network,
while maintaining the quality of service requested by the Transport Layer (in contrast to the data link layer
which connects hosts within the same network). The Network Layer performs network routing functions,
and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this
layer—sending data throughout the extended network and making the Internet possible. This is a logical
addressing scheme – values are chosen by the network engineer. The addressing scheme is not
hierarchical.

Careful analysis of the Network Layer indicated that the Network Layer could have at least three
sublayers:

1. Subnetwork Access - that considers protocols that deal with the interface to networks, such as
X.25;
2. Subnetwork Dependent Convergence - when it is necessary to bring the level of a transit network
up to the level of networks on either side;
3. Subnetwork Independent Convergence - which handles transfer across multiple networks.
The basic purpose of the network layer is to provide an end-to-end communication

capability in contrast to machine-to-machine communication provided by the data

link

layer. This end-to-end is performed using two basic approaches known as

connection-

oriented or connectionless network-layer services

Layer 2: Data Link Layer

The Data Link Layer provides the functional and procedural means to transfer data between network
entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this
layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in
the telephone system. Local area network architecture, which included broadcast-capable multiaccess
media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed
sublayering and management functions not required for WAN use. In modern practice, only error
detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point
Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on
the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are
rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols
such as TCP, but is still used in niches where X.25 offers performance advantages.

The goal of the data link layer is to provide reliable, efficient communication
between
adjacent machines connected by a single communication channel.

Layer 1: Physical Layer

The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines
the relationship between a device and a transmission medium, such as a copper or optical cable. This
includes the layout of pins, voltages, cablespecifications, hubs, repeaters, network adapters, host bus
adapters (HBA used in storage area networks) and more.

To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer.
Think of the Physical Layer as concerned primarily with the interaction of a single device with a medium,
whereas the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two)
with a shared medium. Standards such as RS-232 do use physical wires to control access to the medium.

The major functions and services performed by the Physical Layer are:

 Establishment and termination of a connection to a communications medium.

 Participation in the process whereby the communication resources are effectively shared among
multiple users. For example, contention resolution and flow control.
 Modulation, or conversion between the representation of digital data in user equipment and the
corresponding signals transmitted over a communications channel. These are signals operating over
the physical cabling (such as copper and optical fiber) or over a radio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol
is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also
in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other
local-area networks, such as token ring, FDDI, ITU-TG.hn and IEEE 802.11, as well as personal area
networks such as Bluetooth and IEEE 802.15.4.

The physical layer is concerned with transmission of raw bits over a

communication

channel. It specifies the mechanical, electrical and procedural network interface

specifications and the physical transmission of bit streams over a transmission
medium

connecting two pieces of communication equipment. In simple terns, the physical

layer

decides the following:

• Number of pins and functions of each pin of the network connector (Mechanical)

• Signal Level, Data rate (Electrical)

• Whether simultaneous transmission in both directions

• Establishing and breaking of connection

• Deals with physical transmission