The Open System
Interconnection (OSI) reference model released in 1984 was the descriptive
network model.
To address the
problem of network incompatibility, the International Organization for
Standardization (ISO) researched networking models like Digital Equipment
Corporation net (DECnet), Systems Network Architecture (SNA), and TCP/IP in
order to find a generally applicable set of rules for all networks.
The ISO created a
network model that helps vendors create networks that are compatible with other
networks. It provided vendors with a set of standards that ensured greater
compatibilityand interoperability among various network technologies produced
bycompanies around the world.
The OSI model has 7 layers,
each of which has a different level of abstraction and performs a well-defined
function. The principles that were
applied to arrive at the seven layers are as follows:
o
A layer should be
created where a different level of abstraction is needed.
o
Each layer should
perform a well-defined function.
o
The function of each
layer should be chosen with an eye toward defining internationally standardized
protocols.
o
The layer boundaries
should be chosen to minimize the information flow across the interfaces.
o
The number of layers should be large enough that distinct
functions need not be thrown together in the same layer out of necessary, and
small enough that the architecture does not become unwieldy.
The layered approach
offers several advantages. By separating
networking functioninto logical smaller pieces, network problems can more
easily be solved through a divide-and-conquer methodology. OSI layers also allow extensibility. New protocolsand other network services are
generally easier to add to a layered architecture. The seven OSI layers are defines as follows:
7. Application layer: Provides different services to the application
The application
layer is the top layer of the OSI model.
It provides a set of interfaces for applications to obtain access to
networked services as well as access to network services that support
applications directly. This layer also
provides application access security checking and information validation. The Application Layer provides the following
functions
o
File Transfer, Access
and Management (FTAM): Provides handling services in the network. This includes the movement of files between
different systems, reading, writing and deletion of remote files, and
management of remote file storage.
o
Virtual Terminal
(VT): Provides services to access applications in different remote computer
systems through stimulating a real terminal.
o
Electronic Mail and
Messaging Handling (MHS): Facilitates the electronic exchange of documents.
o
Directory Services
(DS): Provides services with the abi lity to match names with addressing
information.
o
Common management
Information Protocol (CMIP): Provides services for network management.
Distributed applications services, whether OSI or TCP/IP
based, have some common characteristics:
o
An end-user interface
that provides a human or another application with the means to enter commands
that direct the application to send fi les to and receive files from a remote
host, list or change directories, rename or delete files, etc.
o
The means of
performing input to and output from mass storage devices.
o
The means of
transferring the files and file-related information between hosts.
6. Presentation layer: Converts the information
The presentation
layer is responsible for the format of the data transferred during network
communications. This layer is concerned with the syntax and semantics of the
information transmitted. For outgoing
messages, it converts data into a generic format for the transmission. For the incoming messages, it converts the
data from the generic form to a format understandable to the receiving
application. Different computers have different codes for representing data. The presentation layer makes it possible for
computers with different representation to communicate. The presentation layer provides common
communication services such as encryption, text compression, and
reformatting.
The presentation
layer is also concerned with other aspects of information representation. Data compression can be used to reduce the
number of bits that have to be transmitted.
Cryptography is frequently required for privacy and authentication.
5. Session layer: Handles problems which are not communication issues
The session layer
permits two parties to hold ongoing communications called a session across a
network. The applications on either end
of the session can exchange data or send packets to another for as long as the
session lasts. The session layer handles
session setup, data or message exchanges, and tear down when the session
ends. It also monitors session
identification so only designated parties can participate and security services
to control access to session information.
A session can be used to allow a user to log into a remote time-sharing
system or transfer a file between two machines.
The session layer has
the option of providing one-or-two-way communication called dialogue
control. Sessions can allow traffic to
go in both directions at the same time, or in only one direction at a
time. Token management may be used to
prevent both sides from attempting the same operation at the same time. To manage these activities, the session layer
provides tokens that can be exchanged.
Only the side holding the token is permitted to perform the critical
operation.
Another session
service is synchronization. Consider the
problems that occur when transferring a file between two machines and the
system crashes not being able to complete the transfer. This process must be restarted from the
beginning. To avoid this problem, the
session layer provides a way to insert checkpoints into the data stream, so
that after a crash, only the data after the last checkpoint has to be
repeated.
4. Transport layer: Provides end to end communication control
The basic function of
the transport layer is to accept data from the session layer, split it up into
smaller units, pass it to the network layer, and ensure that the bits delivered
are the same as the bits transmitted without modification, loss or duplication.
If an error occurs
during transmission, the transport layer must correct it. There is a set of rules to follow that detail
the handling of the error and how to correct it. The correction may mean re-sending
just the damaged data or restarting from the beginning.
This can be achieved
because the transport layer protocol includes the capability to acknowledge the
receipt of a packet. “If no
acknowledgement is received, the transport layer can retransmit the packet or
time-out the connection and signal an error.
The transport protocol can also mark packets with sequencing information
so that the destination system can properly order the packets if they are
received out of order.”
If the transport connection
requires a high throughput, the transport layer might create multiple network
connection by dividing the data among the network connections to improve the
throughput. However, the transport layer
might multiplex several transport connections onto the same network to reduce
costs. This multiplexing is transparent
to the session layer.
“Transport protocols provide the capability
for multiple application processes to access the network by using individual
local addresses to determine the destination process for each data stream. These addresses are often referred to as
ports and connection opened to these ports as sockets.”
3. Network layer: Routes the information in the network
The network layer
controls the operation of a sub-net, provides routing, congestion control and
accounting. The network layer provides
both connectionless and connection-oriented services. A key design issue is determining how packets
are routed from source to destination.
Routes can be based on static tables that are within the network and
rarely change. They could also be
determined at the start of each conversion.
Finally, they could be highly dynamic, being newly determined for each packet
to reflect the current network load. It
is up to the network layer to allow heterogeneous networks to be
interconnected. The IP protocol resides
in this layer. All routers in the
network are operating at this level.
If too many packets
are present in the sub-net at the same time, bottlenecks will form. The network
layer helps to control this congestion.
An accounting function is built into the network layer to ensure that
the number of bits sent is the number of bits received.
Controls over network
connections, logical channels, segmenting and sequencing, and data flow can be
placed in this layer.
2. Data Link layer: Provides error control
The main task of the
data link layer is to take a raw transmission and transform it into a line that
appears free of transmission errors in the network layer. It accomplishes this task by having the
sender break the input data up into data frames, transmit the frames
sequentially, and process the acknowledgment frames sent back by the
receiver. The protocol packages the data
into frames that contain source and destination addresses. These frames refer
to the physical hardware address of each network card attached to the network
cable. Ethernet, Token Ring, and ARCnet
are examples of LAN data link protocols.
If communication extends beyond the LAN onto the Internet, the network
might use other data link protocols, such as Point-to-Point Protocol (PPP) or
Serial Line Internet Protocol (SLIP).
The data link layer
sends blocks of data with the necessary synchronization, bit error
detection/correction error control, and flow control. This control of data flow controls
approximately 70 percent of all error handling.
Since the physical layer merely accepts and transmits a stream of bits
without any regard to the meaning of the structure, it is up to the data link
layer to create and recognize frame boundaries.
This can be accomplished by attaching special bit patterns to the
beginning and end of the frame.
Encryption can be
used to protect the message as it flows between each network node. Each node
then decrypts the message received and re-encrypts it for transmission to the
next node.
1. Physical layer: Connects the entity to the transmission media
The physical layer is
concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure
that when one side sends a bit, it is received by the other side as a 1 bit,
not as a 0 bit. Typical questions are
how many volts should be used to represent a 1 and how many for a 0, how many microseconds
a bit lasts, whether transmission may proceed simultaneously in both
directions, how the initial connection is established and how it is torn down
when both sides are finished, and how may pins the network connector has and
what each pin is used for. The design
issues deal largely with mechanical, electrical, functional, and procedural
interface.
The physical layer
describes some type of cabling system as the transmission media. It also describes the network topology and
how the transmission media is to be distributed. Some examples include the bus, star, and ring
topologies.
Summary:
o
The application layer
is the layer at which a user and a computer interface to a network to view a
message, data request, or response. It
contains a variety of commonly used protocols, such as file transfer, virtual
terminal, and email.
o
The presentation
layer converts incoming and outgoing data from one presentationformat to
another. It manages the syntax and
semantics of the information transmitted between two computers.
o
The session layer
manages the establishment of a continuing series of requests anresponses
between the applications at each end. It
establishes and manages sessions, conversions, and dialogues between two
computers.
o
The transport layer
manages the end-to-end control and error checking.
o
The network layer
handles the routing of the data. It
controls the operation of a packet from one network to another.
o
The data link layer
provides error control and synchronization for the physical leve
o
The physical layer
conveys the bit stream through the network at the electrical andmechanical
level. It physically transmits signals
across a communication medium.
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