Special Issue on
          Adaptive Real Time Multimedia Transmission over
                  Packet Switching Networks
                             in
                Journal of Real Time Imaging

        Guest Editorial (Volume 7 number 3, June 2001)

Real-time multimedia transmission over packet switching  networks requires
strict guarantee of network bandwidth and several quality of service (QoS)
parameters, such as  end-to-end delay, delay variation and packet loss.
Unfortunately, most of the current packet switching networks, such as the
Internet, do not provide such guarantees. The available bandwidth of such
networks varies dynamically depending on the congestion level within the
network. However, in the case of adaptive multimedia applications which
can tolerate graceful degradation of audio or video quality in the case of
network congestion, the bandwidth of the media stream can be scaled down
to provide a degraded audio or video quality at the destination.  A number
of schemes to adapt the media rate have been studied in the
literature for both real-time and precoded media.

Adaptation of multimedia applications can be done at several layers of the
network protocol stack. At the physical layer, adaptive power control
techniques can be used to mitigate variations in a wireless environment.
At the data link layer, error control and adaptive reservation techniques
can be used to protect against variation in error and available rate. At
the network layer, dynamic re­routing mechanisms can be used to avoid
congestion and mitigate variations in a mobile environment. At the
transport layer, dynamic re­negotiation of connection parameters can be
used for adaptation. Applications can use protocols such as Real­Time
Streaming Protocol (RTSP) and Real­Time Protocol (RTP). At the
application layer, the application can adapt to changes in network
conditions using several techniques including hierarchical encoding,
efficient compression, bandwidth smoothing, rate shaping, error control,
and adaptive synchronization.  The aim of this special
issue is to bring together survey and original work on the techniques that
have been developed to scale audio or video streams to allow continuous
operation of adaptive multimedia applications.

In the first paper by Vandalore, Feng, Jain and Fahmy, the
authors have carried out a comprehensive survey of the various techniques
to use feedback from lower networking layers by application layer to adapt
to changing networking conditions.  Such applications layer techniques
include compression, application streaming, rate shaping, error control,
adaptive synchronization and smoothing.

Conti and Gregori study bandwidth allocation schemes for the transmission
of VBR video traffic with deterministic guarantees, i.e. no packets loss
and no missed deadlines. They propose and investigate a policy which
attempts to keep the quality of video constant by varying the
transmission rate (constant quality), while the video quality transmission
is reduced to cope with network congestion.

Zhang, Nelakuditi, Aggarwal and Tsang discuss techniques of selectively
discarding frames of prestored video  at the server during network
congestion to obtain an optimal Quality of Service at the client.
Packets may be lost in a network during periods of congestion. Nikolaidis,
Hasan and Marvasti discuss error resilient video codecs as an adaptation
technique to recover from  packet losses in an ATM network.

The paper by Mehaoua, Boutaba, Pu, Rasheed, and Leon-Garcia discusses
techniques to transport video over an enhanced ATM UBR best effort
service.  They propose a new best effort ATM service for optimal
transmission of video.
The last paper by Hadar and Cohen proposes an algorithm to smooth out the
burstiness of video traffic by dividing it into a number of
intervals.

Before closing the editorial, we'd like to explicitlky acknowldege the time
and expertise of the following reviewers who helped us maintaining a high quality 
for the special issue:
Guojun Lu, Stuart Dunstan, Bin Qiu, Phillip Branch, Samar Sing, Jim Breen,
Sid Ray, David Taubman, Mark Hedley, Jaga Indulska, Aruna Seneviratne,
Harsha Sirisena, Shahadat Khan, Ahmet Sekercioglu, Gamze Seckin, Pedro Cuenca,
Luis Orozco Barbosa, and Naoto Matoba