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Types of Multimedia Conferencing Systems

There are three main types of multimedia conferencing systems: point-to-point, multipoint, and multicast.

Point-to-Point Systems.
Point-to-point systems involve two persons communicating interactively from the desktop or groups of people communicating from a conference room. Point-to-point desktop conferencing systems are becoming popular because of the availability of inexpensive, low-overhead digital cameras such as the Quickcam and associated videoconferencing software. These systems let users share screens of data, text, or images.
Multipoint Systems.
Multipoint conferencing involves three or more locations that are linked either through a local area network or a wide area network and can each send and receive video. Such systems have several unique characteristics, including presentation of multiple media, management and transport of multiple media streams, distributed access to multiple media, high bandwidth requirements, and low-latency bulk data transfer and multipoint communications. Because desktop systems quickly run out of screen space, multipoint conferencing is more effectively conducted in conference rooms with video walls.
Multicasting Systems.
Multicasting involves the transmission of multimedia traffic by one site and its receipt by other sites. Rather than sending a separate stream to each individual user (i.e., unicasting) or transmitting all packets to everyone (i.e., broadcasting), a multicasting system simultaneously transmits traffic to a designated subset of network users. Many existing systems use broadcasting and let the receivers sort out their messages. This inefficient practice fails to maximize use of network bandwidth and poses potential security problems.

Groupware

The notion of conferencing is changing to include such features as shared windows and whiteboards enabled by distributed computing. In addition, the use of computer mediation and integration is increasing.

A shared application or conferencing system permits two or more users at separate workstations to simultaneously view and interact with a common instance of an application and content. With such applications, users working on a report, for example, can collectively edit a shared copy. In general, documents used in groupware are active (i.e., the document displayed on the screen is connected to content in a database or spreadsheet).

Groupware provides such features as support for group protocols and control, including round-robin or on-demand floor-control policy, and both symmetric and asymmetric views of changes. In symmetric view, a change that is made is immediately shown to other users. In asymmetric view, applicable in applications involving teacher-student or physician-patient interactions, the changes made in one window may not be shown in another window. Groupware systems also support such issues as membership control (i.e., latecomers) and media control (i.e., synchronization of media).

TECHNICAL REQUIREMENTS FOR NETWORKED MULTIMEDIA APPLICATIONS

The immediate multimedia applications discussed (i.e., video-on-demand, multimedia conferencing, groupware, and web browsing) have several technical requirements.

Latency

Latency refers to the delay between the time of transmission from the data source to the reception of data at the destination. Associated with delay is the notion of jitter. Jitter is the uncertainty of arrival of data. In the case of multimedia conferencing systems, practical experience has shown that a maximum delay of 150 milliseconds is appropriate.3 Synchronous communications involve a bounded transmission delay.

Exhibit 2. Storage and Communications Requirements for Multimedia Applications
Storage Communication

Text 2k bits per page 1k bps
Graphics 20k bits per page 10k bps
Audio 20k bits per signal 20k bps
Image 300k bits per image 20 kb compressed) 100kbps
Motion Video 150k bps (compressed)for MPEG1 0.42M bps for MPEG227M bytes for NTSE quality 150k bps
Animation 15k bps 15k bps

Synchronization

Existing networks and computing systems treat individual traffic streams (i.e., audio, video, data) as completely independent and unrelated units. When different routes are taken by each of these streams, they must be synchronized at the receiving end through effective and expeditious signaling.

Bandwidth

Bandwidth requirements for multimedia are steep, because high data throughput is essential for meeting the stream demands of audio and video traffic. A minimum of 1.5M bps is needed for MPEG2, the emerging standard for broadcast-quality video from the Moving Picture Experts Group. Exhibit 2 depicts the storage and communications requirements for multimedia traffic streams.

Reliability

The high data-presentation rate associated with uncompressed video means that errors such as a single missed frame are not readily noticeable. Most digital video is compressed, however, and dropped frames are easily noticeable. In addition, the human ear is sensitive to loss of audio data. Hence, error controls (such as check sums) and recovery mechanisms (i.e., retransmission requests) need to be built into the network. Adding such mechanisms raises a new complexity, because retransmitted frames may be too late for real-time processing.

Guaranteeing Quality of Service

Quality-of-service guarantees aim to conserve resources. In a broad sense, quality of service enables an application to state what peak bandwidth it requires, how much variability it can tolerate in the bandwidth, the propagation delay it is sensitive to, and the connection type it requires (i.e., permanent or connectionless, multipoint). The principle of quality of service states that the network must reliably achieve a level of performance that the user/application finds acceptable, but no better than that. Network systems can either guarantee the quality of service, not respond to it, or negotiate a level of service that they can guarantee.


Exhibit 3.  Quality-of-Service Components in Networked Multimedia Applications.

Quality of service has several components, which are depicted in Exhibit 3 and described in the sections that follow.4

Application Parameters

Application quality-of-service parameters describe requirements for applications, such as media quality and media relations. Media quality refers to source/sink characteristics (e.g., media data-unit rate) and transmission characteristics (e.g., end-to-end delay). Media relations specifies media conversion and inter- and intrastream synchronization.

System Parameters

System quality-of-service requirements are specified in qualitative and quantitative terms for communication services and the operating system. Qualitative parameters define the following expected level of services:

  Interstream synchronization, which is defined by an acceptable skew relative to another stream or virtual clock.
  Ordered delivery of data.
  Error-recovery and scheduling mechanisms.

Quantitative parameters are more concrete measures that include specifications such as bits per second, number of errors, job processing time, and data size unit.

Network and Device Parameters

Network quality-of-service parameters describe requirements on the network, such as network load (i.e., ongoing traffic requirements such as interarrival time), and performance or guaranteed requirements in terms of latency and bandwidth. In addition, traffic parameters such as peak data rate, burst length or jitter, and a traffic model are specified. Traffic models describe arrival of connection requests or traffic contract based on calculated expected traffic parameters.

Device quality-of-service parameters typically include timing and throughput demands for media data units.


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