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Chapter: Computer Networks : Fundamentals & Link Layer

Layering and Protocol

The architecture of OSI model is a layered architecture. The seven layers are, 1. Physical layer 2. Datalink layer 3. Network layer 4. Transport layer 5. Session layer 6. Presentation layer 7. Application layer



To reduce the complexity of getting all the functions maintained by one a new technique called layering technology was introduced. In this, the architecture contains several layers and each layer is responsible for certain functions. The general idea is that the services offered by underlying hardware, and then add a sequence of layers, each providing a higher level of service. The services provided at the higher layers are implemented in terms of the services provided by the lower layers. A simple network has two layers of abstraction sandwiched between the application program and the underlying hardware.


The layer immediately above the hardware in this case might provide host to host connectivity, and the layer above it builds on the available host to host communication service and provides support for process to process channels.


Features of layering are: 1. It decomposes the problem of building a network into more manageable components. 2. It provides a more modular design. Addition of new services and modifications are easy to implement.

In process to process channels, they have two types of channels. One for reque st\reply service and the other for messag e stream service.


A protocol provides a communication service that higher level objects use to exchange message. Each protocol defines two different interfaces. First it defines a serv ice interface to other objects on the same syste m that want to use its communication services. This interface defines the operations that local objects can perform on the protocol. Second a protocol defines a peer interface to its counterpart on another machine. It defines the form and meaning of message exchanged between protocol peers to implement the communication service.


There are potentially multiple protocols at any given level, each providing a different communication service. It is known as protocol graph that make up a system.




ISO refers International Standards Organization was established in 1947, it is a multinational body dedicated to worldwide agreement on international standards.


OSI refers to Open System Interconnection that covers all aspects of network communication. It is a standard of ISO.


Here open system is a model that allows any two different systems to communicate regardless of their underlying architecture. Mainly, it is not a protocol it is just a model.




The open system interconnection model is a layered framework. It has seven separate but interrelated layers. Each layer having unique responsibilities.





The architecture of OSI model is a layered architecture. The seven layers are,


1.     Physical layer


2.     Datalink layer


3.     Network layer


4.     Transport layer


5.     Session layer


6.     Presentation layer


7.     Application layer


The figure shown below shows the layers involved when a message sent from A to B pass through some intermediate devices.


Both the devices A and B are formed by the framed architecture. And the intermediate nodes only having the layers are physical, Datalink and network. In every device each layer gets the services from the layer just below to it. When the device is connected to some other device the layer of one device communicates with the corresponding layer of another device. This is known as peer to peer process.


Each layer in the sender adds its own information to the message. This information is known is header and trailers. When the information added at the beginning of the data is known as header. Whereas added at the end then it called as trailer. Headers added at layers 2, 3, 4, 5, 6. Trailer added at layer 2.


Each layer is connected with the next layer by using interfaces. Each interface defines what information and services a layer must provide for the layer above it.




The seven layers are arranged by three sub groups.


1.     Network Support Layers


2.     User Support Layers


3.     Intermediate Layer


Network Support Layers:


Physical, Datalink and Network layers come under the group. They deal with the physical aspects of the data such as electrical specifications, physical connections, physical addressing, and transport timing and reliability.


User Support Layers:


Session, Presentation and Application layers comes under the group. They deal with the interoperability between the software systems.



Intermediate Layer


The transport layer is the intermediate layer between the network support and the user support layers.






The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and the transmission medium.


The functions are,


1.  Physical Characteristics Of Interfaces and Media:



                 It defines the electrical and mechanical characteristics of the interface and the media.

             It defines the types of transmission medium


2.  Representation of Bits


             To transmit the stream of bits they must be encoded into signal.

             It defines the type of encoding weather electrical or optical. 


3.       Data Rate

             It defines the transmission rate i.e. the number of bits sent per second. 


4.       Synchronization of Bits

             The sender and receiver must be synchronized at bit level.


5.       Line Configuration

             It defines the type of connection between the devices. 

             Two types of connection are, 


                   1.       point to point

                   2.       multipoint


6.       Physical Topology

             It defines how devices are connected to make a network.

             Five topologies are,


          1.       mesh

          2.       star   

          3.       tree   

          4.       bus   

          5.       ring

7.       Transmission Mode

          It defines the direction of transmission between devices.


Three types of transmission are,


1.     simplex

2.     half duplex

3.   full duplex





Datalink layer responsible for node-to-node delivery.


The responsibilities of Datalink layer are,


1.       Framing

          It divides the stream of bits received from network layer into manageable data

          units called frames.


2.       Physical Addressing

             It adds a header that defines the physical address of the sender and the receiver.

             If the sender and the receiver are in different networks, then the receiver address is the address of the device which connects the two networks.


3.       Flow Control

             It imposes a flow control mechanism used to ensure the data rate at the sender and the receiver should be same.

4.     Error Control


*       To improve the reliability the Datalink layer adds a trailer which contains the error control mechanism like CRC, Checksum etc.  


5.     Access Control


*       When two or more devices connected at the same link, then the Datalink layer used to determine which device has control over the link at any given time.  





When the sender is in one network and the receiver is in some other network then the network layer has the responsibility for the source to destination delivery.


The responsibilities are,


1.     Logical Addressing


*       If a packet passes the network boundary that is when the sender and receiver are places in different network then the network layer adds a header that defines the logical address of the devices.  



2.  Routing


*   When more than one networks connected and to form an internetwork, the


connecting devices route the packet to its final destination.  


*       Network layer provides this mechanism.  




The network layer is responsible for the end to end delivery of the entire message. It ensures that the whole message arrives in order and intact. It ensures the error control and flow control at source to destination level.


          The responsibilities are,

1.       Service point Addressing

             A single computer can often run several programs at the same time.

             The transport layer gets the entire message to the correct process on that computer.

             It adds a header that defines the port address which used to identify the exact process on the receiver.


2.       Segmentation and Reassembly

             A message is divided into manageable units called as segments.

             Each  segment  is  reassembled  after  received  that  information  at  the receiver end.

             To make this efficient each segment contains a sequence number.



3.       Connection Control

             The transport layer creates a connection between the two end ports.

*   It involves three steps. They are,  

1.     connection establishment


2.     data transmission


3.     connection discard


4.     Flow Control

Flow control is performed at end to end level  

5.     Error Control


*       Error control is performed at end to end level.  





It acts as a dialog controller. It establishes, maintains and synchronizes the interaction between the communication devices.


          The responsibilities are,

1.       Dialog Control

             The session layer allows two systems to enter into a dialog. 

             It allows the communication between the devices. 


2.       Synchronization

          It adds a synchronization points into a stream of bits.




The presentation layer is responsible for the semantics and the syntax of the information exchanged.


The responsibilities are,



1.  Translation


*   Different systems use different encoding systems.  


*   The presentation layer is responsible for interoperability between different





*       The presentation layer t the sender side translates the information from the sender dependent format to a common format. Likewise, at the receiver side presentation layer translate the information from common format to receiver dependent format.  



2.  Encryption


*   To ensure security encryption/decryption is used  


*        Encryption means transforms the original information to another form  

*   Decryption means retrieve the original information from the encrypted



3.     Compression


*       It used to reduce the number of bits to be transmitted.  





The application la yer enables the user to access the network. It provides interfaces between the users to the network.



          The responsibilities are,

1.       Network Virtual Terminal

                 It is a software v ersion of a physical terminal and allows a us er to log on to a remote host.


2.       File Transfer, Access, a nd Management

                 It allows a user to access files in a remote computer, retrieve files, and manage or control files in a remote computer.


3.       Mail Services

                 It provides the ba sis for e-mail forwarding and storage. 


4.       Directory Services

       It provides distri buted database sources and access for global i nformation about various objects an d services.


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