University of Konstanz
Graduiertenkolleg / PhD Program
Computer and Information Science

Graduation Talks

title

Efficient multimedia transmission over packet erasure channels

speaker

Shakeel Ahmad, University Konstanz
Konstanz, Germany

date & place

Wednesday, 04.07.2007, 15:15 h
Room C252

abstract

Packet erasure channels like the Internet provide transportation service in a best-effort manner without guaranteed Quality of Service (QoS). To cope with packet losses, a widely used mechanism is the TCP/IP protocol. While TCP/IP works well for non real-time data, e.g., file transfer, its relevance for real-time applications is questionable. This is the case, for example, when a user wants to download a 3D mesh in a certain time period or in live video streaming where each video frame has to be delivered and displayed in a certain time period for continuous playback. For such applications, forward error correction (FEC) may be a better candidate. The basic principle of FEC is to add redundant n - k symbols to the original k symbols. These symbols are then encapsulated into channel packets and sent to the receiver. At the receiver side, all k source symbols can be recovered if at least k symbols are received.

The first part of the talk describes an FEC based system to transmit 3D meshes over packet erasure channels. Given a number of available layers of source data and a transmission bit budget, we propose an algorithm that determines how many layers should be sent and how many FEC protection bits should be allocated to each transmitted layer such that the expected distortion at the receiver is minimum. In contrast to the previous approach, which uses exhaustive search, the time complexity of our algorithm is linear in the transmission bit budget.

In the second part of the talk, we present a live video streaming system for packet erasure channels. In situations where the packet loss rate is time-varying and hard to predict, fixed-rate FEC becomes inefficient due to the mismatch between the estimated loss rate and the actual loss rate. To overcome this problem, we propose a scheme based on rateless FEC codes combined with acknowledgment. We show that the proposed approach outperforms fixed-rate FEC.