Chapter 5


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Chapter 5

  1. 1. Reduces the number of bits contained in the information. Sometimes programs need to send more data in a timely fashion than the bandwidth of the network supports. Need to compress the data at the sender and decompress it at the receiver.
  2. 2. In terms of storage, the capacity of a storage device can be effectively increased with methods that compresses a body of data. The bandwidth of a digital communication link can be effectively increased by compressing data at the sending end and decompressing data at the receiving end.
  3. 3. Lossless Compression – data is compressed and can be uncompressed without loss of information. These are referred to as bit-preserving or reversible compression systems. Lossy Compression – aim to obtain the best possible fidelity for a given bit-rate or minimizing the bit-rate to achieve a given fidelity measure. Most suited to video and audio compression techniques
  4. 4. Lossless - Image quality is not reduced. Use in: artificial images that contain sharp- edged lines such as technical drawings, textual graphics, comics, maps or logos.
  5. 5.  Lossy - reduces image quality. Cannot get the original image back & lose some information. Use in: natural images such as photos of landscapes
  6. 6.  Lossless - allows one to preserve an exact copy of one's audio files Usage: For archival purposes, editing, audio quality.
  7. 7.  Lossy - irreversible changes , achieves far greater compression, use psychoacoustics to recognize that not all data in an audio stream can be perceived by the human auditory system. Usage: distribution of streaming audio, or interactive applications
  8. 8. • Start by encoding the first frame using a still image compression method. • It should then encode each successive frame by identifying the differences between the frame and its predecessor, and encoding these differences. If the frame is very different from its predecessor it should be coded independently of any other frame.
  9. 9. In the video compression literature, a frame that is coded using its predecessor is called inter frame (or just inter), while a frame that is coded independently is called intra frame (or just intra).
  10. 10. • To carry sensitive information, a system must be able to assure privacy. • One way to safeguard data from attacks is encrypting the data.
  11. 11. • Encryption – sender transform original information (plaintext) to another form (ciphertext) by a function that is parameterized by a key. • Decryption – reverses the original process to transform the message (ciphertext) back to its original form (plaintext).
  12. 12. Plaintext Plaintext Ciphertext
  13. 13. Symmetric Keys – use same key to encrypt and decrypt a message.
  14. 14. Asymmetric Keys -2 keys are needed (public key and private key); 1 key to encrypt, another key to decrypt and vice versa.
  15. 15. Protection Description Confidentiality Allow only authorized users to access information. Authentication Verify who the sender was and trust the sender is who they claim to be. Integrity Trust the information has not been altered No repudiation Ensure that the sender or receiver cannot deny that a message was sent or received. Access Control Restrict availability to information.