Multimedia communication

Multimedia communication deals with the transfer, the protocols, services and mechanisms of discrete media data (such as text and graphics) and continuous media data (like audio and video) in/over digital networks.

Such a communication requires all involved components to be capable of handling a well-defined quality of service. The most important quality of service parameters are used to request (1) the required capacities of the involved resources, (2) compliance to end-to-end delay and jitter as timing restrictions, and (3) restriction of the loss characteristics.

Multimedia systems have attracted much attention during the last few years in the society as a whole and in the information technology field in particular. Multimedia communication comprises the techniques needed for distributed multimedia systems. To enable the access to information such as audio and video data, techniques must be developed which allow for the handling of audiovisual information in computer and communication systems.

We first describe in a more precise way what we mean by the term “multimedia”. Unfortunately,

“multimedia” has become a buzzword used to denote any kind of “new digital media” being manipulated or displayed by machines. This very imprecise (and very often employed) notion leads to a labelling of all types of media data computation, transmission, storage, manipulation and presentation with the term “multimedia”. Since the mid eighties we have proposed (and even from time to time we imposed) a much more crisp and restricted specification.

Multimedia itself denotes the integrated manipulation of at least some information represented

as continuous media data as well as some information encoded as discrete media data (such as text and graphics). The “manipulation” refers to the act of capturing, processing, communication, presentation and/or storage.

Hence “multimedia communications” deals with the transfer, the protocols, the services and the mechanisms of/for discrete and continuous media in/over digital networks. The transmission of digital video data over a dedicated TV distribution network is not multimedia as long as it does not allow the transfer of some type of discrete media data as well. A protocol designed to reserve capacity for continuous media data transmitted in conjunction with discrete media data over, e.g., an ATM-LAN, is certainly a multimedia communication issue.

Information processing in a time-sharing environment is performed without any hard time constraints. The system responds to a user interaction as soon as possible but often lacks support for realtime data. The use of discrete media still governs traditional computing, while the integration of continuous media into existing computer environments creates the new complexity of time-dependent data processing. ‘Correctness’ in multimedia communications is – in addition to the traditional computer communications error handling – determined by whether deadlines are met or not.

In networked multimedia applications various entities typically cooperate in order to provide the

mentioned real-time guarantees to allow data to be presented at the user interface. These requirements are most often defined in terms of Quality of Service (QoS). QoS is defined as the set of parameters which defines the properties of media streams. In accordance with [StNa95] we distinguish four layers of QoS: User QoS, Application QoS, System QoS and Network QoS. The user QoS parameters describe requirements for the perception of multimedia data at the user interface. The application QoS parameters describe requirements for the application services possibly specified in terms of media quality (like end-to-end delay) and media relations (like inter/intra-stream synchronization). The system QoS parameters describe requirements on the communication services resulting from the application QoS. These may be specified in terms of both quantitative (like bits per second or task processing time) and qualitative (like multicast, interstream synchronization, error recovery or ordered delivery of data) criteria. The network QoS parameters describe requirements on network services (like network load or network performance).