The presentation layer is responsible for data representation, compression, encryption and formatting for transmission between applications. It encodes application data into messages and decodes received messages. Common data representations include ASN.1 and XDR. Lossy and lossless compression techniques are used to reduce file sizes. Encryption transforms plaintext into ciphertext using keys to protect confidentiality during transmission.

1. BITS 2513 – Internet Technology Lecture 6: Presentation Layer

4. Presentation Layer From application layer To application layer From session layer To session layer Presentation layer Presentation layer Data Data

7. Presentation formatting Presentation formatting involves encoding and decoding application data. Application data Presentation encoding Application data Presentation decoding Message Message Message ■ ■ ■

9. DATA REPRESENTATION

18. XDR 4 bytes of XDR message:

20. DATA COMPRESSION

25. Steps of Data Compression Major steps of image compression, can also be applied to audio and video data Uncompressed Picture Picture Preparation Picture Processing Quantization Entropy Encoding Compressed Picture

30. Comparison of graphics file formats FORMAT FILE EXTENTION TYPE OF COMPRESSION METHODS USAGE BMP (bitmap) .bmp Cosiderably compressed with lossless ZIP used to store bitmap digital images JPEG (Joint Photographic Experts Group) .jpg , .jpeg , .jpe Lossy Lossless - Discrete Cosine Transform (DCT) & Chroma Subsampling - Run-Length Encoding (RLE) For natural images GIF (Graphics Interchange Format .gif , .giff , .gfa Lossless LZW (Lempel-Ziv-Welch) For artificial images (sharp-edge lines and few colors) & support animation PNG (Portable Network Graphics) .png Lossless DEFLATE Better compression & features than GIF, but don’t support animation TIFF (Tagged Image File Format) .tiff , .tif Lossless RLE / LZW / DEFLATE / ZIP Flexible file format, can store multiple images in a single file JPEG2000 jp2, .j2c, jpc, j2k, jpx Lossy & Lossless Discrete Wavelet Transform (DWT) Better image quality than JPEG (up to 20%), not widely used because of some patent issues.

31. Block Diagram of JPEG Compression Transformation coding performed using the Discrete Cosine Transform (DCT) Quantization of all DCT coefficients ( a lossy process) Huffman coding and arithmetic coding as entropy encoding methods

38. MPEG Audio Encoding Uncompressed Audio Signal Division in 32 Frequency Bands Psychoacoustic Model Quantization (if applicable) Entropy Encoding Compressed Audi o Data controls

48. ENCRYPTION

51. Encryption Example of Encryption / Decryption Process Plaintext Plaintext Ciphertext