THE ISO OSI REFERENCE MODEL

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THE ISO OSI REFERENCE MODEL Powered By Docstoc
					Datacom                                                                     Chapter 4


Chapter 4
THE ISO OSI REFERENCE MODEL

Learning Objectives:
    Understand what is a layered architecture
    Understand what is a peer to peer process
    Understand how the layers of an OSI model are organized
    Understand in detail the functions of each layer

An open system is a set of protocols that allows any two different systems to
communicate regardless of their underlying architecture. Vendor specific protocols
close off communication between unrelated systems. The purpose of the OSI model is
to open communication between different systems without requiring changes to the
logic of the underlying hardware and software. The OSI model is not a protocol; it is a
model for understanding and designing a network architecture that is flexible, robust
an interoperable.

The open systems interconnection model is a layered framework for the design of
network systems that allows for communication across all types of computer systems.
It consists of seven separate but related layers, each of which defines a segment of
process of moving information across a network. Understanding the fundamentals of
the OSI model provides a solid basis for exploration of data communications.

                         APPLICATION LAYER


                         PRESENTATION LAYER

                            SESSION LAYER

                          TRANSPORT LAYER


                             NETWORK LAYER

                             DATA LINK LAYER

                             PHYSICAL LAYER




Layered Architecture

       The OSI model is built of seven ordered layers:
             Layer 1:       Physical layer
             Layer 2:       Data Link Layer
             Layer 3:       Network Layer
             Layer 4:       Transport Layer
             Layer 5:       Session Layer



R.T.T.C., Hyderabad                                                                       1
Datacom                                                                             Chapter 4


               Layer 6:      Presentation Layer
               Layer 7:      Application Layer
The layers involved in the message transfer from A to B can be seen in the figure



       Device A        intermediate node             intermdedate node   Device B




           LAYER 7            Peer to peer protocol layer7               LAYER7


                              Peer to peer protocol layer6
           LAYER6                                                        LAYER6
                              Peer to peer protocol layer5
           LAYER5                                                        LAYER5
                              Peer to peer protocol layer 4

           LAYER4                                                        LAYER4
                                  LAY                    LAY
                                  ER3                    ER3
           LAYER3                                                        LAYER3

                                  LAY                    LAY
                                  ER2                    ER2
           LAYER2                                                        LAYER2


                                  LAY                    LAY
           LAYER1                 ER1                    ER1             LAYER1




As the message travels from A to B, it may pass through many intermediate nodes.
These nodes usually involve only the first three layers of the OSI model. In
developing the model, the designers distilled the process of transmitting data down to
its most fundamental elements. They identified which networking functions had
related uses and collected those functions into discrete groups that became the layers.
Each layer defines a family of functions distinct from those of the other layers. By
defining and localizing functionality in this fashion, the designers created an
architecture that is both comprehensive and flexible. The OSI model allows complete
transparency between otherwise incompatible systems.
The layers of this model can be remembered with the mnemonic
                     Please Do Not Touch Steve’s Pet Alligator.

Peer to Peer Processes

Within a single machine, each layer calls upon the services of the layer just below it.
Layer 3, for example, uses the services provided by layer 2 and provides services for
layer 4. between machines, layer n on one machine communicates with layer n on
another machine. This communication is governed by an agreed upon series of rules
and conventions called protocols. The processes on each machine that communicate at


R.T.T.C., Hyderabad                                                                             2
Datacom                                                                                     Chapter 4


a given layer are called peer to per processes. Communication between machines is
therefore peer-to-peer process using the protocols appropriate to a given layer.

At the physical layer, communications is direct: Machine A sends a stream of bits to
machine B. at the higher layers, however, communication must move down through
the layers on machine A, over to machine B, and then back through the layers. Each
layer in the sending machine adds its own information to the message it receives from
the layer just above it and passes the whole package to the layer just below it. This
information is added in the form of headers or trailers. Headers are added to the
message at layers 6,5,4,3 and 2. A trailer is added at layer2.

At layer 1 the entire package is converted to a form that can be transferred to the
receiving machine. At the receiving machine, the message is unwrapped layer by
layer, with each process receiving and removing the data meant for it. For example,
layer 2removed the data meant for it, then passes the rest to layer 3. Layer 3 removes
the data meant for it and passes the rest to layer 4, and so on.

Interfaces between Layers
The passing of the data and network information down through the layers of the
sending machine and back up through the layers of the receiving machine is made
possible by an interface between each pair of adjacent layers. Each interface defines
what information and services a layer must provide for the layer above it. Well-
defined interfaces and layer functions provide modularity to a network. As long as a
layer still provides the expected services to the layer above it, the specific
implementation of its functions can be modified or replaced without requiring
changes to the surrounding layers.

Organization of the Layers
The seven layers can be thought of as belonging to three subgroups.
Network support Layers:
     Physical layer
     Data link layer
     Network layer
 They deal with the physical aspects of moving data from one device to another, such
 as electrical specification, physical connections, physical addressing and transport
 timing and reliability.
User support Layers
     Session Layer
     Presentation Layer
     Application Layer
 They allow interoperability among unrelated software systems. The transport layer,
 links the two subgroups and ensures that what the lower layers have transmitted is in
 a form that the upper layers can use. The upper OSI layers are almost always
 implemented in software; lower layers are a combination of hardware and software,
 except for the physical layer, which is mostly hardware.
The figure gives the overall view of the OSI layer concept. L stands for the Layer. The process starts
out at layer 7 then moves from layer to layer in descending order sequentially. At each layer (except at
layers 7 and 1), a header is added to the data unit. At layer 2, a trailer is added as well. When the
formatted data unit passes through the physical layer, it is changed into an electromagnetic signal and
transported along a physical link.



R.T.T.C., Hyderabad                                                                                        3
Datacom                                                                                    Chapter 4




      Device A                                                        Device B




        LAYER 7 DATA                                                  LAYER 7 DATA


        LAYER6 DATA     H6                                            LAYER6 DATA     H6


        LAYER5 DATA          H5                                       LAYER5 DATA          H5


        LAYER4 DATA               H4                                  LAYER4 DATA               H4


        LAYER3 DATA                    H3                             LAYER3 DATA                    H3



        LAYER2 DATA                         H2                        LAYER2 DATA                         H2


        LAYER1 DATA                                                   LAYER1 DATA
        01111110101010101010101001111110                              01111110101010101010101001111110




                                            01111110101010101111110




On reaching the destination, the signal passes into layer 1 and is transformed back
into digital form. The data units then move back up through the OSI layers. As each
block of data reaches the next higher layer, the headers and trailers attached to it at the
corresponding sending layer are removed, and appropriate actions are taken at that
layer. By the time the message reaches application layer, it is again in a form
appropriate to the application and is made available to the recipient.




R.T.T.C., Hyderabad                                                                                       4
Datacom                                                                       Chapter 4


FUNCTIONS OF THE LAYERS

Physical Layer
        This layer coordinates the functions required to transmit a bit stream over a
physical medium. It deals with
     Mechanical and electrical specifications of the primary connections such as
        cables and connectors
     Signaling options that physically link two nodes on a network.
The first layer receives a data unit from the second layer and puts it into a format
capable of being carried by a communications link. It oversees the changing of a bit
stream into electromagnetic signals, and their transmission onto and across a medium.

To accomplish this task, the following points are to be considered:

Line configuration: How can two or more devices be linked physically? Are
transmission lines to be shared or limited to use between two devices? Is the line
available or not?
Data transmission mode: it does transmission flow one-way or both ways between
two connected devices? Or does it alternate?
Topology: how are network devices arranged? Do they pass data directly to each
other or through an intermediary? And by what paths?
Signals: What types of signals are useful for transmitting information?
Encoding: how are bit (0s and 1s) to be represented by available signaling systems?
How are data represented by signals?
Interface: what information must be shared between two closely linked devices to
enable and facilitate communication? What is the most efficient way to communicate
that information?
Medium: what is the physical environment for the transmission of data?

Data Link Layer

The data link layer is responsible for delivering data units from one station to the next
without errors. It accepts a data unit from the third layer and adds meaningful bits to
the beginning and end that contain addresses and other control information. A data
unit with these additional informations is called a frame.

To get to its ultimate destination, a transmission may have to be passed along by a
number of intermediate stations, much as a letter from New Delhi to Thane may first
go to Mumbai before finally arriving at Thane. Data link frame headers and trailers
contain the information necessary to move a data unit from one of these stations to the
next.

In addition, the data link layer is responsible for flow control and error detection.
Protocols in this layer regulate the right of device to transit; how to keep
transmissions from overwhelming the receiver; and how to ensure that errors
introduced during transmission are corrected. To this end, headers and trailers also
carry information about synchronization, sequencing and whether or not the last frame
arrived intact.




R.T.T.C., Hyderabad                                                                         5
Datacom                                                                       Chapter 4


Headers and trailers at this level are added by the sending node, then checked and
interpreted by the receiving node. Once a receiving node accepts a frame, it strips off
the header and trailer and passes the examining data unit on to the network layer.

The following are the responsibilities of the data link layer.
Node-to-Node delivery: the data link layer is responsible for node-to-node delivery.
Addressing: headers and trailers added at this layer include the physical addresses of
the most recent node and the next intended node.
Access control: when two or more devices are connected to the same link, the data
link layer protocols are necessary to determine which device has control over the line
at a given time.
Flow control: to avoid overwhelming the receiver, the data link layer regulates the
amount of data that can be transmitted at one time. It adds identifying numbers to
enable the receiving node to control the ordering of the frames.
Error handling: data link layer protocols provide for data recovery by having the
entire frame retransmitted.
Synchronization: headers contain bits to alert the receiving station that a frame is
arriving. In addition, these bits provide a pattern to allow the receiver to synchronize
its timing to that of the transmission. Trailers contain bits for error control and also
bits that indicate the frame has ended, and that anything to follow is either a new
frame or an idle channel.

Network layer
The network layer is responsible for the source to destination delivery of a packet
across multiple network links. Whereas the data link layer oversees station-to-station
delivery (node to node), the network layer ensures that each packet gets from its point
of origin to its final destination successfully and efficiently.
To make such end-to-end delivery possible, the network layer provides two related
services: switching and routing. Switching refers to temporary connections between
physical links, resulting in longer links for network transmission. A telephone
conversation is an example of a switched connection; two lines are temporarily joined
into a single dedicated link for the duration of the conversation. In this case the same
route to the destination sends each packet.
Routing means selecting the best path for sending a packet from one point to another
when more than one path is available. In this case, each packet may take a different
route to the destination, where the packets are collected and reassembled into the
original order. Routing considerations include speed, cost and the ability to change
pathways in midtransmission.
Routing and switching require the addition of a header that includes, among other
information, the source and destination addresses of the packet. These addresses are
different from the physical (node) addresses included in the data link header. Data
link addresses are of the current and next node only. They change as a frame moves
from one node to the next. Network layer addresses are those of the original source
and the final destination. They do not change during transmission and are often called
the logical addresses.

The specific responsibilities of this layer are:

Source to destination delivery: Moving a packet (best effort) from its point of origin
to its intended destination across multiple network links.


R.T.T.C., Hyderabad                                                                        6
Datacom                                                                     Chapter 4


Logical addressing: inclusion of the source and destination addresses in the header.
Routing: deciding which of multiple paths a packet should take.
Address transformation: interpreting logical addresses to find their physical
equivalents.
Multiplexing: using a single physical line to carry data between many devices at the
same time.
The following example may be considered for better understanding.
It is required to send data from a node with network address A and physical address
45, located on one local area network, to a node with a network address D and
physical address 60, located on another local area network. Because the two devices
are located on different networks, we cannot use link addresses only; the link
addresses have only local jurisdiction. What we need here are universal addresses that
can pass thorough the boundaries of local area networks. The network addresses have
this characteristic. The packet at the network layer contains the logical addresses,
which remain the same from the original source to the final destination. They will not
change when we go from network to network. However, the physical addresses will
change when the picket moves from one network to another. The box with R is an
internetworking device called router.
       A                           B




                                             23
     45
                                                           C 20
            T2 data    A G    45 20
                                                                 R

            D                                 Ring   99
           45                                                T2 data A G 99 33


                        E71                               F 33
                                                                     R


                                                                 66
                      T2 data A P 66 60

       G                              H 77



            60




R.T.T.C., Hyderabad                                                                      7
Datacom                                                                        Chapter 4


Transport Layer
The transport layer is responsible for source to destination (end to end) delivery of the
entire message. Whereas the network layer oversees end-to-end delivery of individual
packets, it does not recognize any relationship between those packets. It treats each
one independently, as though each piece belonged to a separate message, whether or
not it does. The transport layer, on the other hand, ensures that the whole message
arrives intact and in order; overseeing both errors control and flow control at the
source to destination level.
Computers often run several programs at the same time. For this reason, source to
destination delivery means delivery not only from one computer to the next but also
from a specific application on one computer to a specific application on the other. The
transport layer header must therefore include a type of address called a service point
address (also called a port address or socket address). The network layer gets each
packet to the correct computer; the transport layer gets the entire message to the
correct application on that computer.
The transport layer header also contains sequence, or segmentation, numbers. As the
transport layer receives the message to be sent from the session layer, it divides it into
transmittable segments, indicating the in the header the sequence of the segments so
that they can be reassembled upon receipt at the destination.


 For added security, the transport layer may create a connection between the two end
 ports. A connection is a single logical path between the source and destination that is
 associated with all packets in a message. Creating a connection involves three steps:
     Connection establishment
     Data transfer
     Connection release
By confining transmission of all packets to a single pathway, the transport layer has
more control over sequencing, flow and error detection and correction.
The specific responsibilities of this layer are
     End to end message delivery: overseeing the transmission and arrival of the
        packets of a message at the destination point.
     Service point addressing: Guaranteeing delivery of a message to the
        appropriate application on a computer running multiple applications.
     Segmentation and reassembly: dividing a message into transmittable
        segments, and marking each segment with a sequence number. The numbers
        enable the transport layer to reassemble the message correctly at the
        destination and to identify and replace packets lost in transmission.
     Connection control: deciding whether or not to send all packets by a single
        path.
    Consider the following example where data coming from the upper layers have
    service pint addresses j and k. since the data size is larger than the network
    layer can handle, the data are split into two packets, each packet retaining the
    service point addresses. Then in the network layer, network addresses c and d are
    added to the packets respectively. The packets may travel on different paths and
    arrive at the destination either in order or out of order. The two packets are
    delivered to the destination transport layer, which is responsible for removing the
    network layer headers and combining the two pieces of data for delivery to the
    upper layers.



R.T.T.C., Hyderabad                                                                          8
Datacom                                                                                     Chapter 4




                                                                      Transport layer
    Data          J      K                                         Data          J      K
                              Transport layer

                                                                                Data 1 J K A P
        Data 2 J K A P

                                  Network layer                      Data 2 J K A P
           Data 1 J K A P



                                                                T2 data 1 J K    A P H2
             T2 data 2 J K A P        H2

Data link layer                                                  T2 data 2 J K A P          H2
                      T2 data 1 J J K A P
                      H2


                                                Physical link




Session Layer:
The session layer is the network dialog controller. It establishes, maintains and
synchronizes the interaction between communicating devices. It also ensures that each
session closes appropriately rather than shutting down abruptly and leaving the user
hanging. For example, a user wants to transfer a file of 200 pages. What happens if
the transfer is interrupted after only 52 pages? When the problem is removed and the
connection can be made again, should the session be canceled and started all over
from page 1? Or should the large session be divided into sub sessions so that a
problem after page 52 results in only the last two pages being resent when the session
is restored. These issues are the concerns of the session layer.

The session layer validates and establishes connections between users. The data unit
at this layer may carry the credentials of the host seeking the connection, including
password and login verification. This is essential whenever a system allows remote
access to files. The session layer also controls the exchange of data: whether the
exchange occurs in both directions simultaneously or only one direction at a time. If
one-way at a time, how should turns be taken?

Dividing the session into sub sessions using checkpoints inserted into the stream
creates reliability at the session layer. Checkpoints allow a session to backtrack a
certain distance without completely starting over when problems occur. Depending on
the requirements of the specific transmission, checkpoints can be either extremely
important or ignored altogether.



R.T.T.C., Hyderabad                                                                                     9
Datacom                                                                          Chapter 4


The header of this layer includes control information such as the type of the data unit
being sent and synchronization point information.

The specific responsibilities of the session layer are
Session management: Dividing a session into sub session by the introduction of
checkpoints and separating long messages into shorter units, called dialog units
appropriate for transmission.
Synchronization: Deciding in what order to pass the dialog units to the transport layer
and where in the transmission to require confirmation from the receiver.
Dialog control: Deciding who sends and when.
Graceful close: Ensuring that the exchange has been completed appropriately before
the session closes.

Presentation Layer
The presentation layer ensures interoperability among communicating devices.
Functions at this layer make it possible for two computers to communicate even if
their internal representations of data differ. It provides the necessary translation of
different control codes, character sets, and graphics characters and so on to allow both
devices to understand the same transmission the same way.
The presentation layer is also responsible for the encryption and decryption of data for
security purposes and for the compression and expansion of data when necessary for
transmission efficiency.
Headers added at this layer include information on the type and parameters of the
transmission and the length of transmission. The figure shows the relation ship
between the presentation layer, application layer and the session layers.

           From application layer                                     to application layer

            L7 data                                              L7 data




    presentation layer                                      Presentation layer
          Encoded, encrypted and                                Encoded, encrypted and
          compressed data           H6                          compressed data              H6




                      L6 data                                              L6 data

                to session layer                                      from session layer

The responsibilities of this layer are
    Translation: Changing the format of a message from that used by the sender
       into one mutually acceptable for transmission. The, at the destination,
       changing that format into the one understood by the receiver.
    Encryption: Encryption and decryption of data for security purposes.


R.T.T.C., Hyderabad                                                                               10
Datacom                                                                       Chapter 4


      Compression: Compressing and decompressing data to make transmission
       more efficient.
      Security: Validating passwords and login codes.

Application Layer
The application layer enables the user, whether human or software, to access the
network. It provides user interfaces and support for services such as electronic mail,
remote file transfers, shared database management and other types of distributed
information services.
The relationship of application layer to the user and the presentation layer is shown in
the figure. The information services mentioned in the figure are
X.400 message handling services
X. 500 directory services
FATM file transfer and access management


    Application
    Layer                                               Application
             X.500 x     FATM                           Layer
                                                        X.500 x                 FATM
                                           X.400
                                                                                                X.400




                    L7 data                                     L7 data




The responsibilities of this layer are
Network virtual terminal: A soft ware version of a physical terminal. A virtual
terminal allows the user to log on to a remote host. To do so, the application creates a
software emulation of a terminal at the remote host. The computer talks to the
software terminal, which in turn talks to the host and vice versa. The remote host
believes it is communicating with one of its own terminals and allows the user to log
on.
File access, transfer and management: Allows the user at a remote computer to access
files in another host; to retrieve files from a remote computer for use in the local
computer; and to manage or control files in a remote computer at that computer.
Mail services: provides the basis for electronic mail forwarding and storage.
Directory services: Provides distributed database sources and access for global
information about various objects and services.




R.T.T.C., Hyderabad                                                                        11
Datacom                                                                       Chapter 4




SUMMARY
The summary of the functions of the seven layers is given in the figure.

The OSI model is built of seven ordered layers:
                Layer 1:        Physical layer
                Layer 2:        Data Link Layer
                Layer 3:        Network Layer
                Layer 4:        Transport Layer
                Layer 5:        Session Layer
                Layer 6:        Presentation Layer
                Layer 7:        Application Layer
Organization of the Layers
       The seven layers can be thought of as belonging to three subgroups.
Network support Layers:
     Physical layer
     Data link layer
     Network layer
    They deal with the physical aspects of moving data from one device to another,
 such as electrical specification, physical connections, physical addressing and
 transport timing and reliability.

User support Layers
    Session Layer
    Presentation Layer
    Application Layer

The transport layer, links the two subgroups and ensures that what the lower layers
have transmitted is in a form that the upper layers can use. The upper OSI layers are
almost always implemented in software; lower layers are a combination of hardware
and software, except for the physical layer, which is mostly hardware.

Within a single machine, each layer calls upon the services of the layer just below it.
Layer 3, for example, uses the services provided by layer 2 and provides services for
layer 4. Between machines, layer n on one machine communicates with layer n on
another machine. This communication is governed by an agreed upon series of rules
and. At the receiving machine, the message is unwrapped layer by layer, with each
process receiving and removing the data meant for it. For example, layer 2 removes
the data meant for it, and then passes the rest to layer 3. Layer 3 removes the data
meant for it and passes the rest to layer 4, and so on.

                                         *****




R.T.T.C., Hyderabad                                                                       12

				
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