Application Layer Two - Local and Wide Area Networks - Lecture Slides, Slides of Social Work

Download Application Layer Two - Local and Wide Area Networks - Lecture Slides and more Slides Social Work in PDF only on Docsity!

Introduction to LAN/WAN

Application Layer 4

Multimedia

Multimedia: Audio + video

Human ear: 20Hz – 20kHz, Dogs hear higherfreqs

DAC converts audio waves to digital

E.g PCM uses 8-bit samples 8000 times per sec

Audio Compression

Two types of audio compression

Waveform coding:

fourier transform, then encode

frequency components

• Perceptual coding:

‹

Exploit flaws of human ear ‹

Encoded form is different but

sounds

same to

human ear

MP3 based on perceptual coding

MP

Frequency masking:

Sound in freq A can mask

softer sound in freq B (Suppress freq B sound)

Temporal masking:

Soft sound B is still not heard

for few seconds even after freq A sound stops

Streaming Audio

Link

metafile

(just name) not actual mp3 file

rtsp://joes-audio-server/song-0025.mp

Browser writes name to disk, launches mediaplayer as helper app

Media player receives streamed mp3 file, browsernot involved

RTSP (Real Time Streaming Protocol) used, notHTTP

Media Player

)

Four major functions

– Manage user interface: skins, user choices– Transmission errors: User RTP, interleaving– Decompress music– Eliminate jitter: Playback buffer,

‹

Pre-download 10-15 secs of music ‹

Try to download new blocks at same rate asplayback

H.

H.323: VoIP standard, an architectural overview

References specific protocols for speech coding,call setup, signalling, data transport, etc

Features gateway, terminals, gatekeeper & zones

The H323 architectural model for Internet telephony.

Session Initiation Protocol (SIP)

H.323 created by telcos: large, complex

IETF set up committee to design better VoIPstandard

SIP resulted, RFC 3261

• How to setup Internet phone calls, video conferences– Unlike H.323 single module, not protocol suite– Integrates well with existing Internet applications– Phone numbers are URI, on web page same as

mailto

MPEG-

-^

MPEG (Motion Picture Experts Group) standard for compressingvideo files since 1993

-^

Movies contain sound: MPEG can compress both audio and video

-^

Different generations of MPEG

-^

MPEG-1:–

Goal: video-recorder quality (352 x 240 for NTSC) using a bit rateof 1.2Mbps

-^

Uncompressed at 24 bits per pixel requires 50.7 Mbps

-^

Compression ratio of 40 required to reduce to 1.2 Mbps

-^

Notes:–

NTSC is video standard in US

-^

PAL is standard in Europe

MPEG

-^

MPEG-2:–

designed for compressing broadcast-quality video into 4-6 Mbps(to fit into NTSC and PAL broadcast)

-^

Also forms basis for DVD and digital satellite TV

-^

MPEG-1 and 2 are similar: MPEG-2 almost superset of MPEG-

-^

MPEG-1: audio and video streams encoded separately, uses same 90-KHz clock for synchronization purposes

Audioencoder Videoencoder

System multiplexer

Clock Audio signal Video signal

MPEG-1 output

MPEG

-^

MPEG-1 output consists of four kinds of frames:–

I (Intracoded)

frames:

‹

self-contained JPEG-encoded still pictures ‹

Act as reference, in case packets have errors, are lost or streamfast forwarded, etc

–^

P (Predictive)

frames:

‹

Block-by-block difference with last frame ‹

Encodes differences between this block and last frame

–^

B (Bi-directional)

frames:

‹

Difference between the last or next frame ‹

Similar to P frames, but can use either previous or next frameas reference

–^

D (DC-coded)

frames:

‹

Encodes average values of entire block ‹

Allows low-res image to be displayed on fast-forward

MPEG

-^

MPEG-2:– I, P, B frames supported– D frames NOT supported– Supports both progressive and interlaced images– Encodes smaller blocks to improve output– Also supports multiple resolutions

MPEG

-^

MPEG-4:–

In addition to previous audio, video encoding and multiplexing,also has

‹

coding of text/graphics and synthetic images ‹

Representation of audio-visual scene and composition

–^

Has some wireless features

-^

New features considered important included robustness to errorsand coding efficiency

-^

Example applications:

‹

Internet and Intranet video ‹

Wireless video ‹

Video databases ‹

Interactive home shopping ‹

Video e-mail, home movies ‹

Virtual reality games, simulation and training

MPEG-

-^

MPEG-4 specific wireless-friendly standards requirements:–

Universal access:

“Robustness in error prone environments: The

capability to allow robust access to applications over a variety ofwireless and wired networks and storage media. Sufficientrobustness is required, especially for low bit-rate applicationsunder severe error conditions”

-^

Compression:

“Improved coding efficiency: The ability to provide

subjectively better audio-visual quality at bit-rates compared toexisting or emerging coding standards”

-^

Formal tests to verify these requirements with:–

high random Bit Error Rate (BER) of 10

–^

Multiple burst errors