Osi model

1. OSI MODEL AND DATA LINK LAYER 2 ND UNIT COMPUTER NETWORK AND SECURITY

2. Chapter……. Network Models Error Detection and Correction Flow and Error Control

3. Network Models Protocol Set of rules that enable two device to connect and transmit data to one another. Service Set of opration that layer provides to the layer above or below it.

4. Key Elements of Protocol Syntax Structure or format of the data. Semantics meaning of each Section of bit. Timing When data should be sent and how fast they can be sent.

5. Standards De facto De facto standards are often established originally by manufacturers who request to define the functionality of a new product or technology. De jure Thos standards that have legislated by an officially recognized body are de jure standards.

6. OSI Model Developed by the International standard Organization(ISO). Th e model is called ISO OSI(Open System Interconnection) The OSI model has Seven layers . H- Header Information(Source & Destination address) T- Trailer Information(Error control)

7. OSI Layers Application layer Presentation Layer Session Layer Transport layer Network Layer Data link layer Physical layer

8. OSI Reference model Session Transport Network Physical Presentation Application Application Presentation Session Transport Network Physical Data Unit(Bits) DH Data Unit DT TH Data Unit SH Data Unit PH Data Unit AH Application data Application Data Receiving Device Sending Device

9. Physical Layer The physical layer is responsible for transmitting individual bits from one node to the next.

10. Data Link Layer The data link layer is responsible for transmitting frames from one node to the next.

11. Functions of Data link layer Framing Divides the data in to smaller unit. Physical addressing Hardware address is added in Header. Flow control Control the flow of data between sender and receiver Error control generally error control mechanisms are added in Data link Trailer(DT)

12. Network Layer T he network layer is responsible for the delivery of packets from the original source to the final destination.

13. Functions of Network Layer Logical addressing The physical addressing implemented by the data link layer handles the addressing problem locally. Routing Finding the shortest path between source and destination.

14. Transport Layer The transport layer is responsible for delivery of a message from one process to another.

15. Functions of Transport Layer Service point addressing Used to identify the process Segmentation and reassembly A message is divided into transmittable segments, each having a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arrival at the destination. Flow control. Error control.

16. Session Layer Allow a user to log into a remote system or to transfer a file between two machines. Functions of Session Layer: Dialogue control Control the conversation between two system Synchronization Coortinate the interaction among communication system.

17. Presentation Layer It concerned with the syntax and semantics of the information transmitted. Functions of Session Layer: Translation Own format information is exchange between two different system Encryption Original message into secret form Compression Reduce the number of bit s contained in the information

18. Application layer The application layer is responsible for providing services to the user.

19. Functions of Application Layer Network Virtual Terminal It allow a user to log on to the remote computer. File Transfer, Access and Management Allow user to access file from remote computer. Mail Service It provide the basic for E-Mail forwarding and Storage.

20. Error detection and Correction Error : unpredictable changes of bits from 1->0 or 0-> 1 Types : Single bit error Burst error(Multiple)

21. Single bit error A 0 is changed to 1 or a 1 is changed to 0 1 1 1 1 1 1 1 1 1 1 1 Error Received Sent

22. Burst Error(Multiple) 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Corrupted Bits Length of burst error (8 bits)

23. CRC Cyclic codes are special block codes with one extra property. A codeword is cyclically shifted (rotated) the result is another codeword. MSB LSB MSB LSB 1 1 1 1 1 1

24. CRC Encoder & Decoder Data word - 1001(data to be sent) Divisor - 1011(predetermined constant) Code word- Data word + Remainder Code word-100110 1 0 0 1 0 0 0 1 0 1 0 1 0 1 1 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 1 1 0 0 0 0 0 1 1 0 Quotient 1 0 1 1 Remainder

25. CRC Encoder & Decoder Data word - 1001(data to be sent) Divisor - 1011(predetermined constant) Code word- Data word + Remainder-1001110 Code word-100110 1 0 0 1 1 1 0 1 0 1 0 1 0 1 1 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 0 0 Quotient 1 0 1 1 Remainder

26. Parity Check Parity is a system in which each transmitted character contain one additional bit. Two system of parity are normally used: odd parity even parity Odd parity means the total number of binary 1’s is in the character,including the parity bit is odd. Even parity means that the number of binary 1’s bit in the character,including the parity bit is even. This technique only detects the single bit error.

27. Checksum Several protocols still use the checksum for error detection. For example, if the set of number is(7,10,3,11,5),we send(7,10,3,11,5,36),where 36 is the sum of (7+10+3+11+5) the original number. The receiver adds the five numbers and compares the result with the sum. If the two are the same,the receiver assumes no error,accepts the five numbers and omits the sum.

28. Check sum There is an error somewhere and the data are not accepted The traditional checksum uses a small number of bits(16) to detect error in a massage of any size. It is not strong as CRC in error checking capability. For example,If the value of one word is increamented and the value of another word is decreamented by the same amount,the two error cannot be detected because the sum remains the same.

29. Flow control Flow control The management of data flow between computers or device or between nodes in a network so that the data can be handled at an efficient pace Types of flow control Stop – and – wait flow control Sliding window flow control

30. Stop and Wait Flow control Sender sends a frame and wait for acknowledgement from the receiver. After receiving the acknowledgement from the receiver,sender sends next frame.

31. Stop and wait flow control Advantage Gurantee for the delivery of every frame.Because sender sends the next frame only after receiving the acknowledgement of previous frame. Disadvantage It is time consuming.

32. Sliding window flow control The flow control technique that allows multiple frames to be in transit on the line at one time is called sliding window .

33. Error control  Error control is a method that can be used to recover the  corrupted data whenever possible. Types of  error control backward error control forward error control

34. Damaged frame A recognizable frame does arrive,but some of the bit are in error.

35. Lost frame A frame fail to arrive at the other side.

36. Lost acknowledgement An acknowledgement fails at the source.the sender is not aware that acknowledgement has been transmitted from the receiver.

37. ARQ Full form --Automatic Repeat Request The purpose of ARQ is to change an unreliable data link into a reliable one. Version of ARQ Stop-and –wait ARQ Go-Back-N ARQ Selective-repeat ARQ

38. Stop-and-wait ARQ Sender transmits a single frame and then twaits for acknowledgement(ACK).

39. Go back N ARQ A station may send a series of frames sequentially numbered with some maximum value. It give a NACK from the receiver.

40. Selective Repeat ARQ