【数据通信与网络】Lecture8-DataReliabilityAndSecurity.ppt

Ciphers Encryption and decryption algorithms Controlled by keys Secret At sender and receiver Symmetric-key (secret-key) and public-key (asymmetric-key) key cryptography Crypthography ”Secret writing” Data communications and networking Data Communications and Networking 数据通信与计算机网络 Lecture 8. Data reliability and Security Lecture 8. Data reliability and Security 8.2 Error encoding: CRC、Hamming Code 8.1 Error and Error Correction 8.3 Data Encryption 8.4 DES and RSA 8.5 Digital Signature 8.1 Error and Error Correction Data may be corrupted (alter or wipe away) during transmission. Error —— Bit is changed from 1 to 0 or from 0 to 1. For reliable system, Errors must be detected and corrected. Error Types of Error Single-bit error (random error): Only one bit in the data unit has changed. Burst error (突发错误) Two or more bits in the data unit have changed The length of burst error depends on the data rate and duration of noise. Scheme of Correction Detection mechanism —— redundancy technique , 冗余技术 Adding extra bits(error codes) into information for detecting errors at the destination. Redundancy As soon as detection is completed,the extra bits are discarded（丢弃）. (1) Detection: （检错） We recognize immediately it when error occurs. It make the sender re-transmit the whole data unit while finding out errors (ARQ). OR Using correction encoding (纠错编码) to auto correct some errors (FEC). (2) Correction: （纠错） Error Encoding —— Including detection encoding and correction encoding （检错编码, 纠错编码） Parity checking （奇偶校验） It counts all “1” in the data and adds one extra bit to make the total number of 1 bits even (even parity) or odd (odd parity). The extra bit is called the parity bit and is transmitted with the data. Example: -------0