The problem of controlling access to multimedia multicasts requires the distribution and maintenance of keying information. Typically, the problem of key management is considered separately from the problem of distributing the rekeying messages. Multimedia sources provide two approaches to distributing the rekeying messages associated with securing group communication. The first, and more conventional approach employs the use of a media-independent channel to convey rekeying messages. We propose, however, a second approach that involves the use of a media-dependent channel, and is achieved for multimedia by using data embedding techniques. Compared to a media-independent channel, the use of data embedding to convey rekeying messages provides enhanced security by masking the presence of rekeying operations. This covert communication makes it difficult for an adversary to gather information regarding the group membership and its dynamics. In addition to proposing a new mode of conveyance for the rekeying messages, we introduce a new message format that is suitable for multicast key management schemes. This new message format uses one-way functions to securely distribute new key material to subgroups of users. An advantage of this approach over the traditional message format is that no additional messages must be sent to flag the users which portion of the message is intended for them, thereby reducing communication overhead. We then show how to map the message to a tree structure in order to achieve desirable scalability in communication and computational overhead. Next, as an example of the interplay between the key management scheme and the mode of conveyance, we study the feasibility of embedding rekeying messages using a data embedding method that has been recently proposed for fractional-pel video coding standards such as H.263 and MPEG-2. Finally, since multimedia services will involve multiple layers or objects, we extend the tree-based key management schemes to include new operations needed to handle multilayer multimedia applications where group members may subscribe or cancel membership to some layers while maintaining membership to other layers.

A joint source-channel coding (JSCC) scheme for robust progressive image transmission over broadband wireless channels using orthogonal frequency division multiplexing (OFDM) systems with spatial diversity is proposed for the application environments where no feedback channel is available such as broadcasting services. Most of current research about JSCC focuses on either binary symmetric channels (BSC) or additive white Gaussian noise (AWGN) channels. To deal with fading channels in most previous methods, the fading channel is modeled as two state Gilbert-Elliott channel model and the JSCC is normally aimed at the BER of bad channel status, which is not optimal when the channel is at good status. By using diversity techniques and OFDM, the frequency selective fading effects in broadband wireless channels can be significantly decreased and we show that subchannels in OFDM systems approach Gaussian noisy channels when the diversity gain gets large; as a result, the system performance can be improved in terms of throughput and channel coding efficiency. After analyzing the channel property of OFDM systems with spatial diversity, a practical JSCC scheme for OFDM systems is proposed. Simulation results are presented for transmit diversity with different numbers of antennas and different multipath delay and Doppler spread. It is observed from simulations that the performance can be improved more than 4 dB in terms of peak signal-to-noise ratio (PSNR) of the received image Lena and the performance is not very sensitive to different multipath spread and Doppler frequency.

An integrated framework of optimal rate allocation for video coding is presented in the case of transmission over wireless channels without feedback channels. For a fixed channel bit rate and finite number of channel coding rate, the proposed scheme can find the optimal source and channel coding pair and corresponding robust video coding scheme such that the expected end-to-end distortion of video signals can be minimized. With the, assumption that encoder has the stochastic channel information, the proposed scheme takes into account robust video coding, channel coding and packetization, error concealment techniques altogether. Simulation results show the accuracy and optimality of the proposed method.

The problem of controlling access to multimedia multicasts requires the distribution and maintenance of keying information. The conventional approach to distributing keys is to use a channel independent of the multimedia content. We propose a second approach that involves the use of a data-dependent channel, and can be achieved for multimedia by using data embedding techniques. Using data embedding to convey rekeying messages can provide an additional layer of security when compared with the traditional approach. We then introduce multicast key distribution, and employ a recent tree-based key distribution scheme to exhibit the factors involved in transmitting keys using data embedding.

Joint source and channel coding (JSCC), multicarrier modulation (MCM) and diversity are some key techniques for multimedia transmission over wireless channels. We investigate a joint source channel matching (JSCM) scheme for progressive image transmission in the MCM system using multiple antennas. The focus of this work is on the effect of the practical channel estimation schemes on the JSCM algorithm. This is different from other research on JSCC assuming perfectly known channel information. We propose a model-based channel estimation scheme and use it.

Joint source and channel coding (JSCC), multicarrier modulation (MCM) and diversity are some key techniques for multimedia transmission over wireless channels. We first investigate the channel estimation problem for the MCM system with multiple transmitter antennas. A model based channel estimation approach is proposed to identify the multiple channels simultaneously. We then apply the model based channel estimation in a joint source and channel matching (JSCM) scheme for the robust progressive image transmission. It is shown that the channel estimation affects not only the image decoding at the receiver but also the rate allocation at the transmitter. It is important to jointly consider the channel estimation and JSCM scheme to improve the quality of image transmission.

We propose a novel data embedding scheme for fractional-pixel based video coding algorithms such as H.263 and MPEG-2. By modifying the motion estimation procedure at fractional-pixel precision, two bits data can be embedded in a motion vector (MV) for an inter-mode coded macroblock (MB). For half-pixel precision motion estimation, the resulted bitstream is compatible with the current video coding standards. Performance of the proposed data embedding scheme in terms of compression efficiency is also studied. As an application example of the proposed data embedding scheme, an error-resilient video coding scheme is also presented where some redundant information, which is used to protect MVs and coding modes of MBs in one frame, is embedded into the MVs in the next frame. When errors occur in one group of blocks (GOB), the decoder can exactly recover the MVs of the corrupted GOB if the next frame can be received correctly. Simulations show that the proposed scheme has better performance than standard H.263 coding scheme for transmission over error-prone channels.

The Real Time Protocol (RTP) is an application-layer protocol with growing popularity in use with multimedia traffic on the Internet. This paper presents the idea of error-correcting gateways that can operate in real time and with software only. These gateways would be very useful in multimedia streaming over heterogeneous networks or for reliable multicasting when suitable forward error correction codes are implemented. Two different schemes were tested and the results compared. Various other application scenarios are also discussed in this paper.