งานนำเสนอกำลังจะดาวน์โหลด โปรดรอ

งานนำเสนอกำลังจะดาวน์โหลด โปรดรอ

Data Link Control Error Detection and Correction.

งานนำเสนอที่คล้ายกัน


งานนำเสนอเรื่อง: "Data Link Control Error Detection and Correction."— ใบสำเนางานนำเสนอ:

1 Data Link Control Error Detection and Correction

2 2 Proprietary and Confidential to Accenture Error Detection and Correction Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected.  Single-Bit Error In a single-bit error, only one bit in the data unit has changed.  Burst Error A burst error means that 2 or more bits in the data unit have changed.

3 3 Proprietary and Confidential to Accenture Error Detection and Correction Single-Bit Error In a single-bit error, only one bit in the data unit has changed.

4 4 Proprietary and Confidential to Accenture Error Detection and Correction Burst Error A burst error means that 2 or more bits in the data unit have changed.

5 5 Proprietary and Confidential to Accenture Error Detection and Correction Error Detection  Parity Check  Cyclic Redundancy Check (CRC)  Checksum Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

6 6 Proprietary and Confidential to Accenture Error Detection and Correction Redundancy

7 7 Proprietary and Confidential to Accenture Error Detection and Correction Parity Check Even-Parity check concept In parity check, a parity bit is added to every data unit so that the total number of 1 s is even (or odd for odd-parity).  Even-Parity check  Odd-Parity check Two-Dimensional Parity check

8 8 Proprietary and Confidential to Accenture Error Detection and Correction Even-Parity Check Concept

9 9 Proprietary and Confidential to Accenture Error Detection and Correction Example of Even-Parity check Suppose the sender wants to send the word world. In ASCII the five characters are coded as The following shows the actual bits sent

10 10 Proprietary and Confidential to Accenture Error Detection and Correction Example of Even-Parity check – Receive No error Now suppose the word world in the last sample is received by the receiver without being corrupted in transmission The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

11 11 Proprietary and Confidential to Accenture Error Detection and Correction Example of Even-Parity check – with errors Now suppose the word world in the last sample is corrupted during transmission The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

12 12 Proprietary and Confidential to Accenture Error Detection and Correction Remark: Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors in each data unit is odd.

13 13 Proprietary and Confidential to Accenture Error Detection and Correction Example of Even-Parity check – more than 1 errors The following shows the actual bits errors

14 14 Proprietary and Confidential to Accenture Error Detection and Correction Two-Dimensional parity

15 15 Proprietary and Confidential to Accenture Error Detection and Correction VRC LRC Two-Dimensional parity – single bit error Vertical Redundancy Check Longitudinal Redundancy Check

16 16 Proprietary and Confidential to Accenture Error Detection and Correction VRC LRC √ XX Two-Dimensional parity – double bit error ข้อมูลเดิม คือ พบข้อผิดพลาดจาก การตรวจสอบ LRC

17 17 Proprietary and Confidential to Accenture Error Detection and Correction VRC LRC X X XX Two-Dimensional parity – double bit error ข้อมูลเดิม คือ

18 18 Proprietary and Confidential to Accenture Error Detection and Correction VRC LRC √ √ √ √ Two-Dimensional parity – can not detect err.

19 19 Proprietary and Confidential to Accenture Error Detection and Correction Cyclic Redundancy Check (CRC)  Given a K-Bit Frame  The Transmitter Generate an N-Bits sequence - FCS (Frame Check Sequence) K bitsN bits FCS K + N bits The resulting frame consisting of k+n bits is Exactly divisible by some predetermined number

20 20 Proprietary and Confidential to Accenture Error Detection and Correction Cyclic Redundancy Check (CRC)

21 21 Proprietary and Confidential to Accenture Error Detection and Correction คุณสมบัติตัวหาร (Divisor) ใช้หลักการของ XOR ในการคำนวณ Cyclic Redundancy Check (CRC)  Bit ซ้ายสุดและขวาสุดจะต้องเป็น 1 เสมอ  จำนวนบิตจะต้องน้อยกว่าจำนวนบิตของข้อมูล data1data2XOR output

22 22 Proprietary and Confidential to Accenture Error Detection and Correction Binary division in A CRC generator

23 23 Proprietary and Confidential to Accenture Error Detection and Correction Binary division in CRC Checker

24 24 Proprietary and Confidential to Accenture Error Detection and Correction A polynomial representing a divisor

25 25 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #1  Data to be send is (10 bits)  Divisor is แปลงเป็นโพลีโนเมียลได้ดังนี้ = X 9 +X 7 +X 3 +X = X 5 + X 4 + X 2 + 1

26 26 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #2  Data to be send is (6 bits)  Divisor is 1101  CRC 3 bits แปลงเป็นโพลีโนเมียลได้ดังนี้ = X 5 +X = X 8 +X = X 3 + X 2 + 1

27 27 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #3

28 28 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #4  Data to be send is (6 bits)  Divisor is 1101  CRC 3 bits แปลงเป็นโพลีโนเมียลได้ดังนี้ = X 5 +X = X 8 +X = X 3 + X ฉะนั้น CRC ที่ได้= Remainder of (X 8 +X 5 )/(X 3 + X ) Remainder= 1 CRC= 001 ดังนั้นข้อมูลทั้งหมดที่จะส่งคือ

29 29 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #3

30 30 Proprietary and Confidential to Accenture Error Detection and Correction Sample of a polynomial representing #5 ผู้รับตรวจสอบข้อมูลด้วยตัวหารชุดเดียวกัน  Data received is (9 bits)  Divisor is = X 8 + X = X 3 + X การตรวจสอบ = Result of (X 8 +X 5 +1)/(X 3 + X ) Remainder= 0 เมื่อผลหารลงตัวโดยไม่เหลือเศษ สรุปได้ว่า ข้อมูลที่ส่งมานั้นถูกต้อง

31 31 Proprietary and Confidential to Accenture Error Detection and Correction Standard Cyclic Redundancy Check Polynomials CRC Standard PolynomialBit String Used CRC-12X 12 + X 11 + X 3 + X CRC-16X 16 + X 15 + X CRC-CCITTX 16 + X 12 + X CRC-32X 32 + X 26 + X 23 + X 22 + X 16 + X 12 + X 11 + X 10 + X 8 + X 7 + X 5 + X 4 + X 2 + X ใช้ใน LAN IEEE 802 ใช้ใน LAN IEEE 802

32 32 Proprietary and Confidential to Accenture Error Detection and Correction Checksum The Sender follows these steps:  The unit is divided into k sections, each of n bits.  All sections are added using one’s complement to get the sum.  The sum is complemented and becomes the checksum.  The checksum is sent with the data

33 33 Proprietary and Confidential to Accenture Error Detection and Correction Checksum The Receiver follows these steps:  The unit is divided into k sections, each of n bits.  All sections are added using one’s complement to get the sum.  The sum is complemented.  If the result is zero, the data are accepted: otherwise, rejected.

34 34 Proprietary and Confidential to Accenture Error Detection and Correction Checksum

35 35 Proprietary and Confidential to Accenture Error Detection and Correction Suppose the following block of 16 bits is to be sent using a checksum of 8 bits The numbers are added using one’s complement Sum Checksum The pattern sent is Sample of Checksum - Sender

36 36 Proprietary and Confidential to Accenture Error Detection and Correction Now suppose the receiver receives the pattern sent is no error. When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error Sum Complement means that the pattern is OK. Sample of Checksum - Receiver

37 37 Proprietary and Confidential to Accenture Error Detection and Correction Now suppose there is a burst error of length 5 that affects 4 bits. ( ) When the receiver adds the three sections, it gets Partial Sum Carry 1 Sum Complement the pattern is corrupted. Sample of Checksum – Received with errors

38 38 Proprietary and Confidential to Accenture Error Detection and Correction What is the checksum value for the extended ASCII message “Help!” ? H e l p ! (Sum) (Checksum – one’s complement) Sample of Checksum - Sender

39 39 Proprietary and Confidential to Accenture Error Detection and Correction H e l p ! (Checksum) (sum) (One’s complement) Sample of Checksum - Receiver

40 40 Proprietary and Confidential to Accenture Error Detection and Correction การแก้ไขความผิดเพี้ยนของข้อมูล ประกอบด้วย 2 แนวทางคือ  Forward Error Correction แก้ไขแบบไม่ส่งข้อมูลซ้ำ โดยที่ผู้รับสามารถแก้ไขข้อมูลที่ผิดได้เอง  Retransmission แก้ไขแบบส่งข้อมูลซ้ำ ผู้ส่งจะทำการส่งข้อมูลที่ถูกต้องมาให้ใหม่ อีกครั้ง

41 41 Proprietary and Confidential to Accenture Error Detection and Correction Forward Error Correction วิธีการแก้ไขข้อมูลแบบไม่ส่งซ้ำที่ได้รับความนิยมอย่างสูงได้แก่ การแก้ไขข้อมูลแบบแฮมมิ่ง (Hamming Code) คิดค้นโดย R.W. Hamming ซึ่งนำวิธีการแบบ Parity มาประยุกต์ให้มีความ เที่ยงตรงสูงขึ้น โดยการเพิ่ม bits ข้อมูลสำหรับควบคุมเข้าไปรวม กับตัวข้อมูล และการแก้ไขข้อมูลจะใช้แบบ Even Parity

42 42 Proprietary and Confidential to Accenture Error Detection and Correction Hamming Code: Data and Redundancy bits Number of Data bits (K) Number of Redundancy bits (R) Total Bits (K+R)

43 43 Proprietary and Confidential to Accenture Error Detection and Correction Position of Redundancy bits in Hamming Code:

44 44 Proprietary and Confidential to Accenture Error Detection and Correction Redundancy bits Calculation:

45 45 Proprietary and Confidential to Accenture Error Detection and Correction Sample of Redundancy bits Calculation:

46 46 Proprietary and Confidential to Accenture Error Detection and Correction Error detection using Hamming Code: Normal Case

47 47 Proprietary and Confidential to Accenture Error Detection and Correction Error detection using Hamming Code:

48 48 Proprietary and Confidential to Accenture Questions & Answers Q&A


ดาวน์โหลด ppt Data Link Control Error Detection and Correction.

งานนำเสนอที่คล้ายกัน


Ads by Google