Page 300 - DCAP101_BASIC_COMPUTER_SKILLS

Unit 14: Web Server Applications

conduct transposing of data rates and protocols, and features such as Continuous Presence, Notes
in which multiple parties can be seen on-screen at once. MCUs can be stand-alone hardware
devices, or they can be embedded into dedicated videoconferencing units.
The MCU consists of two logical components:

1. A single multipoint controller (MC), and
2. Multipoint Processors (MP), sometimes referred to as the mixer.
The MC controls the conferencing while it is active on the signaling plane, which is simply
where the system manages conferencing creation, endpoint signaling and in-conferencing
controls. This component negotiates parameters with every endpoint in the network and
controls conferencing resources. While the MC controls resources and signaling negotiations,
the MP operates on the media plane and receives media from each endpoint. The MP
generates output streams from each endpoint and redirects the information to other endpoints
in the conference.
Some systems are capable of multipoint conferencing with no MCU, stand-alone, embedded
or otherwise. These use a standards-based H.323 technique known as “decentralized
multipoint”, where each station in a multipoint call exchanges video and audio directly with
the other stations with no central “manager” or other bottleneck. The advantages of this
technique are that the video and audio will generally be of higher quality because they don’t
have to be relayed through a central point. Also, users can make ad-hoc multipoint calls
without any concern for the availability or control of an MCU. This added convenience and
quality comes at the expense of some increased network bandwidth, because every station
must transmit to every other station directly.

14.5.3 Videoconferencing Modes

Videoconferencing systems have several common operating modes that are used:
1. Voice-Activated Switch (VAS);
2. Continuous Presence.
In VAS mode, the MCU switches which endpoint can be seen by the other endpoints by the
levels of one’s voice. If there are four people in a conference, the only one that will be seen
in the conference is the site which is talking; the location with the loudest voice will be seen
by the other participants.

Continuous Presence mode display multiple participants at the same time. The MP in this
mode puts together the streams from the different endpoints and puts them all together into
a single video image. In this mode, the MCU normally sends the same type of images to
all participants. Typically these types of images are called “layouts” and can vary depending
on the number of participants in a conference.

14.5.4 Echo Cancellation

A fundamental feature of professional videoconferencing systems is Acoustic Echo Cancellation
(AEC). Echo can be defined as the reflected source wave interference with new wave created
by source. AEC is an algorithm which is able to detect when sounds or utterances reenter
the audio input of the videoconferencing codec, which came from the audio output of the
same system, after some time delay. If unchecked, this can lead to several problems including:

1. the remote party hearing their own voice coming back at them (usually significantly
delayed)
2. strong reverberation, rendering the voice channel useless as it becomes hard to
understand and

LOVELY PROFESSIONAL UNIVERSITY 293