Definition: The OSI model defines internetworking in
terms of a vertical stack of seven layers. The upper layers
of the OSI model represent software that implements network
services like encryption and connection management. The
lower layers of the OSI model implement more primitive,
hardware-oriented functions like routing, addressing, and
flow control.
The OSI model was introduced in 1984. Although it was
designed to be an abstract model, the OSI model remains
a practical framework for today's key network technologies
like Ethernet and protocols like IP.
The OSI model should be used as a guide for how data
is transmitted over the network. It is an abstract representation
of the data pathway and should be treated as such.
The OSI model was specifically made for connecting open
systems. These systems are designed to be open for communication
with almost any other system. The model was made to break
down each functional layer so that overall design complexity
could be lessened. The model was constructed with seven
layers for the flow of information. These are:
Application Layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
1.1 Application layer
Provides a means for the user to access information on
the network through an application. This layer is the main
interface for the user to interact with the application
and therefore the network.
The application layer is the OSI layer closest to the
end user, which means that both the OSI application layer
and the user interact directly with the software application.
This layer interacts with software applications that implement
a communicating component. Such application programs fall
outside the scope of the OSI model. Application layer functions
typically include identifying communication partners, determining
resource availability, and synchronizing communication.
When identifying communication partners, the application
layer determines the identity and availability of communication
partners for an application with data to transmit. When
determining resource availability, the application layer
must decide whether sufficient network resources for the
requested communication exist. In synchronizing communication,
all communication between applications requires cooperation
that is managed by the application layer.
Some examples of application layer implementations include
Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer
Protocol (SMTP).
1.2 Presentation layer
Manages the presentation of the information in an ordered
and meaningful manner. This layer's primary function is the
syntax and semantics of the data transmission. It converts local
host computer data representations into a standard network format
for transmission on the network. On the receiving side, it changes
the network format into the appropriate host computer's format
so that data can be utilized independent of the host computer.
ASCII and EBCDIC conversions, cryptography, and the like are
handled here.
The presentation layer provides a variety of coding and conversion
functions that are applied to application layer data. These
functions ensure that information sent from the application
layer of one system would be readable by the application layer
of another system. Some examples of presentation layer coding
and conversion schemes include common data representation formats,
conversion of character representation formats, common data
compression schemes, and common data encryption schemes.
Common data representation formats, or the use of standard
image, sound, and video formats, enable the interchange of application
data between different types of computer systems. Using different
text and data representations, such as EBCDIC and ASCII, uses
conversion schemes to exchange information with systems. Standard
data compression schemes enable data that is compressed. or
encrypted at the source device to be properly decompressed,
or deciphered at the destination.
Presentation layer implementations are not typically associated
with a particular protocol stack. Some well-known standards
for video include QuickTime and Motion Picture Experts Group
(MPEG). QuickTime is an Apple Computer specification for video
and audio, and MPEG is a standard for video compression and
coding.
Among the well-known graphic image formats are Graphics Interchange
Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged
Image File Format (TIFF). GIF is a standard for compressing
and coding graphic images. JPEG is another compression and coding
standard for graphic images, and TIFF is a standard coding format
for graphic images.
1.3 Session layer
Coordinates dialogue/session/connection between devices over
the network. This layer manages communications between connected
sessions. Examples of this layer are token management (the session
layer manages who has the token) and network time synchronization.
The session layer establishes, manages, and terminates communication
sessions. Communication sessions consist of service requests
and service responses that occur between applications located
in different network devices. These requests and responses are
coordinated by protocols implemented at the session layer. Some
examples of session-layer implementations include Zone Information
Protocol (ZIP), the AppleTalk protocol that coordinates the
name binding process; and Session Control Protocol (SCP), the
Decent Phase IV session layer protocol.
1.4 Transport layer
Responsible for reliable transmission of data and service
specification between hosts. The major responsibility of this
layer is data integrity--that data transmitted between hosts
is reliable and timely. Upper layer data grams are broken down
into network-sized data grams if needed and then implemented
using appropriate transmission control. The transport layer
creates one or more than one network connection, depending on
conditions. This layer also handles what type of connection
will be created. Two major transport protocols are the TCP (Transmission
Control Protocol) and the UDP (User Data gram Protocol).
Important features of Transport layer:
Transport layer ensures reliable service.
Breaks the message (from sessions layer) into smaller
packets, assigns sequence number and sends them.
Reliable transport connections are built on top of X.25
or IP.
In case IP, lost packets arriving out of order must
be reordered.
Important features of TCP/UDP:
TCP/IP Widely used for network/transport layer (UNIX).
TCP (Transport Control Protocol): This is a connection
oriented protocol.
UDP (Universal Data gram Protocol): This is a connectionless
transport layer protocol.
Application programs that do not need connection-oriented
protocol generally use UDP.
1.5 Network layer
Responsible for the routing of data (packets) through the
network; handles the addressing and delivery of data. This layer
provides for congestion control, accounting information for
the network, routing, addressing, and several other functions.
IP (Internet Protocol) is a good example of a network layer
protocol. Network layer does not deal with lost messages.
Important features of Network layer protocols:
Concerned with the transmission of packets.
Choose the best path to send a packet (routing).
The routing may be complex in a large network (e.g.
Internet).
Routing packets through a network may be accomplished
by using simple static routes or by using complex dynamic
routing algorithms.
1.6 Data link layer
Provides for the reliable delivery of data across a physical
network. This layer deals with issues such as flow regulation,
error detection and control, and frames. This layer has the
important task of creating and managing what frames are sent
out on the network. The network data frame, or packet, is made
up of checksum, source address, destination address, and the
data itself. The largest packet size that can be sent defines
the maximum transmission Unit (MTU).
Important features of Data link layer:
Handles errors in the physical layer.
Groups bits into frames and ensures their correct delivery.
Adds some bits at the beginning and end of each frame
plus the checksum.
Receiver verifies the checksum.
If the checksum is not correct, it asks for retransmission.
(Send a control message).
Consists of two sub layers:
Logical Link Control (LLC) defines how data is transferred
over the cable and provides data link service to the higher
layers.
Medium Access Control (MAC) defines who can use the network
when multiple computers are trying to access it simultaneously
(i.e. Token passing, Ethernet [CSMA/CD]).
The data link layer provides reliable transit of data across
a physical network link. Different data link layer specifications
define different network and protocol characteristics, including
physical addressing, network topology, error notification, sequencing
of frames, and flow control. Physical addressing (as opposed
to network addressing) defines how devices are addressed at
the data link layer. Network topology consists of the data link
layer specifications that often define how devices are to be
physically connected, such as in a bus or a ring topology. Error
notification alerts upper-layer protocols that a transmission
error has occurred, and the sequencing of data frames reorders
frames that are transmitted out of sequence. Finally, flow control
moderates the transmission of data so that the receiving device
is not overwhelmed with more traffic than it can handle at one
time.
The protocols used in Data link layer are SLIP, PPP, MTU,
and CSLP.
1.7 Physical layer
Handles the bit-level electrical/light communication across
the network channel. The physical layer defines the electrical,
mechanical, procedural, and functional specifications for activating,
maintaining, and deactivating the physical link between communicating
network systems. Physical layer specifications define characteristics
such as media, voltage levels, timing of voltage changes, physical
data rates, maximum transmission distances, and physical connectors.
Basically, this layer ensures that a bit sent on one side
of the Network is received correctly on the other side.
Data travels from the application layer of the sender, down
through the levels, across the nodes of the network service,
and up through the levels of the receiver
To keep track of the transmission, each layer "wraps"
the preceding layer's data and header with its own header. A
small chunk of data will be transmitted with multiple layer
headers attached to it. On the receiving end, each layer strips
off the header that corresponds to its respective level.
Physical layer is concerned with the following:
Physical interface characteristics like electrical,
and mechanical specifications,
Number of bits of second to be transmitted,
Transmission type like duplex or half-duplex etc.
Frequently used Physical layer protocols:
Some of the important standards that deal with physical layer
specifications are:
RS-232(for serial communication lines), X.21, EIA 232, and
G730.
Physical layer and Data link layer implementations can be
categorized as either LAN or WAN specifications.
