The Internet Encyclopedia (Volume 3)

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TECHNICALSTANDARDS ANDPROTOCOLS OFWEBCASTING 681

Webcaster,s Server

RSVP defines

minimum

quality

RTP enables

synchronization and

defines encodings

MPEG set

standards

compressing

digital video

and audio files

SMIL allows

easy mixing of

media objects

Protocols

Multicast-Enabled Router

Local Area Network

Figure 5: Webcasting protocols in multicasting.

with the evolution of the Internet. Four webcasting stan-

dards have been adopted by IETF:

IP multicast;

Reservation protocol (RSVP);

Real-time transport protocol (RTP) and real-time control

protocol (RTCP); and

Reliable multicast protocols.

IP Multicast

IETF identified three ways to transmit from a source to

multiple recipients on the Internet:

(1) unicasting—point-to-point transmission;

(2) broadcasting—one-to-all transmission; and

(3) IP multicasting—one copy is sent to a group address.

Unlike unicasting, IP multicasting allows small or large

amounts of digital information to be sent to large audi-

ences. Only group members that should receive the web-

casts will actually receive the programs and only one copy

of the information is needed to first reach a group address,

then routed to individual recipients to allow efficient de-

livery of digital information.

Reservation Protocol (RSVP)

To ensure the quality of the webcast, which includes in-

tegrity, end-to-end predictability, and efficient bandwidth

utilization of data transmission, it is necessary to specify

the minimum quality standard. RSVP, the reservation pro-

tocol, is one kind of quality standards that enhances the

current Internet architecture with support requests for a

specific quality of service from the network for particular

data streams or flows. This protocol is designed to allocate

network resources appropriately for the requirements of

the data being sent. To optimize transmission for partic-

ular types of data such as audio and video, RSVP defines

the network traffic class and is used to control both qual-

ity of service and resource management for unicast and

multicast sessions.

Real-Time Transport Protocol (RTP) and Real-Time

Control Protocol (RTCP)

Because audio and video webcasts transmitted over the

Internet can be lost or experience variable delays, RTP is a

protocol intended to enable synchronization and recovery

from loss or delays. RTP also defines a format for differ-

ent audio and video encodings to promote interoperability

among different computer platforms, operating systems,

and application software products. By having specific data

fields that contain timestamp and sequence information,

the receiving computer can use these fields to reconstruct

the time-specific properties of the RTP data streams. A re-

lated protocol is RTCP, which checks the status of a web-

cast from time to time. Using the RTCP, sender and re-

ceiver reports are transmitted from time to time so that

applications using RTP can get RTCP reports on how well

RTP data are being delivered.

Reliable Multicast Protocols

Reliable multicast protocols aim to offer 100% data in-

tegrity over a network when needed. Sometimes we don’t

need complete data integrity such as when watching a

movie because human eyes and ears can tolerate and com-

pensate for minor loss or interference in sound and pic-

tures. However, for transmitting databases or software,

no loss can be allowed, and it is necessary to use software

that is built in with a reliable multicast protocol.

ISO (International Standards Organization)

Standards

The International Standards Organization (ISO) is a

worldwide organization of national standards organiza-

tions from over 100 countries to promote the development

of standardization and related activities in the world. Its

goals are to facilitate the exchange of goods and services

and to develop international cooperation in intellectual,

scientific, technological, and economic activities. MPEG

is a group of people who met at ISO to generate stan-

dards for digital video and audio compression. MPEG-1

audio compression standards is composed of three coding

levels: Layer-1 (MP1), Layer-2 (MP2), and Layer-3 (MP3).

Each layer level is a higher compression ratio at equal

audio quality. To reproduce CD quality audio, Layer-1 re-

quires 384 Kbps, while Layer-3 only requires 112 Kbps.

MP3 has now become the most popular standard for

digital encoding and transmission of audio and video.

MPEG-4 is the latest standard initiated by the ISO. Dif-

ferent types of multimedia can be combined for different

presentations using MPEG-4 (Mack, 2002).

ITU (International Telecommunications

Union) Standards

ITU (International Telecommunications Union) is an in-

ternational organization where government and the pri-

vate sector work together to coordinate global telecom-

munications network and services. Headquartered in

Geneva, Switzerland, the ITU has played an important

role in standardizing the videoconference industry with