What is Function of the ISO-OSI layers ?
- All the application need not use all the seven layers show in table.
- Each layer is supposed to handle message or data from the layers which are immediatlely above or below it.
- This is done by following following the protocols rules.Thus each layers takes data from the adjacent layer,handles it according to these rules and then passes the processed data to the next layer on the other side.
Level | Name of the Layer | Functions |
1 | Physical Layer | Make and break connections,define voltage and data rates data bits into electrical signal.Decide whether transmission is simple,half duplex or full duplex. |
2 | Data Link Layer | Synchronization,eroor detection and correction.To assemble outgoing message into frames. |
3 | Network Layer | Routing of the signals,divide the outgoing message into packets to act as network controller for routing data. |
4 | Transport Layer | Decide whether transmission should be parallel or single path Multiplexing,splitting or segmenting the data,to break data into smaller units for efficient handling. |
5 | Session Layer | To manage and synchronize conversation between two systems. it controls logging on and off user indentification billing and session management. |
6 | Presentation Layer | It works as a translating layer. |
7 | Application Layer | Retransferring files of information,LOGIN,password checking etc |
Functions of Different Layers :
- To activate,maintain and deactivate the physical connection.
- To define voltage and data deactive the physical connection.
- To convert the digital bits into electrical signal.
- To decide whether the transmission is simplex,half duplex or full duplex.
- A physical layer does not perform the following operations:
- It does not detect or correct erros.
- it does not decide the medium and modulation.
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| physical layer |
- Function of the data link layer are synchronization and error control for the information which is to e transmistted over the physical link.
- To enable the error detection,it adds error detection bits to the data which is to be transmitted
- The encoded data is then passed to the physical layer.
- These error detection bits are used by the data link layer on the other side to detect and correct the errors.
- At this level the outgoing message are assembled into frames,and the system wait for the acknowledge to be received after every frame transmitted.
- Correct opertion of the data link layer ensures reliable transmission of each message.Example of data link layer protocols are HDLC,SDLC and X.25 protocols.
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| data-link layer |
- To route the signal through various channels to the other end.
- To act as the network controller by deciding which route data should take.
- To devide the outgoing message into packets and to assemble incoming packets into message for the higher level.
- In the short network layer acts as the network controller for routing data.
- It decides if the data transmission should take place on parallel paths or single path.
- It does the functions such as multiplexing,splitting or segmenting on the data.
- Transport layer gurantees transmission of data from one system to other.
- It break the data group into smaller units so that they are handled more efficiently by the network layer.
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| transport layer |
- This layer manages and synchronizes conversations betweenn two different application.this is the level at which the user will establish system to system connection.
- It controls logging on and off,user indentification,billing and session management.
- In the transmission of data from one system to the other,at session layer streams of data are marked and resynchronised properly so that the ends of the message are not cut prematurely and data loss is avoided.
1. Dialog control 2. Dialog separation
Dialog Control : Dialog Control is the means by which a sending and receiving system initiate a dialog,exchange message and finally end the dialog.
Dialog Separation : It is a process of inserting a reference marker called as checkpoint into the data stream travelling between the sending andd receiving system.this allow the checking the status of both the machine at the same point in time.This will avoid any possiblr cofusion and collision situation.
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| session layer |
- The presentation layer makes it sure that the information is delivered is such a form that the receiving system will understand and use it.
- The form and syntax of the two communication system can be different examle,one system is using the ASCII code for file transfer and the other uses IBM EBCDIC .
- Under such condition the presentation layer provides the "translation" from ASCII to EBCDIC and vice versa.
- It carries out data compression to reduce the bandwidth of the data to be transmitted.
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| presentatio layer |
- Application layer is the top of the all layrs. it provides different services such as manipulation of information in various ways,retransferring the files of information ,distributing the result etc to the user who is sitting above this layer.
- The function such as LOGIN ,or password checking are also performed by the application layer.
- It provides distributed database sources and access to the worldwide information about various object and services.
- It provides a basic for e-mail forwardig and string.
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| application layer |
- At the physical layer, communication is direct machine X send a stream of bits to machine Y.
- At higher layer,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 packge to the layer just below it as.
- The information added by each layer is in the form of header pr trailers.Headers are added to the message at the layers 6,5,4,3, and 2 .A trailer is added at layer 2.
- 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 with each process receiving and removing the data meant for it.
- The upper OSI layer are always implemented in software(4,5,6 and 7) and lower layer are a combination of hardware and software(2,3) except for the physical layer which is mostly hardware.
- Layer 1,2 and 3 are the nnetwork suppot layers.They deal with the physical aspect of moving data from one device to another such as electrical specifications,physical connections,physical addressing and transport timing and reliability.
- Layer 4 the transport layer ensure end to end reliable data transmission.
- Layer 5,6 and 7 they allow interoperability among unrelated software systems.
difference between tcp/ip and osi model
OSI | TCP/IP |
Has 7 layers | Has 4 layers |
Transport layer gurantees delivery of pakckets. | Transport layer does not gurantee delivery of packets. |
Separate presentation layer | No presentation layer,characteristic are provided by application layer. |
Seprate session layer | No session layer ,characteristic are pro -vided by application layer. |
Network layer provides both connection -less and connection oriented services. | Network layer provides only connectio -nless services. |
It defines the services,interface and pro -tocols very clearly and makes a clear distinction between them. | It does not clearly distinguish between services interface and protocols. |
The protocols are better hidden and can be easily replaced as the technology cha -ges. | It is not easy to replace the protocols. |
OSI is truly a general model | TCP/IP can not be used for any other application. |
Horizontal approach | Vertical approach |
It has a problem of protocol fitting into a model | The model does not fit any other proto -cols stack |
