Koenraad Laevens

Increasing the user perceived quality for IPTV services

Currently, digital television is gradually replacing analogue TV. Although these digital TV services can be delivered via various broadcast networks (e.g., terrestrial, cable, satellite), Internet Protocol TV over broadband telecommunication networks offers much more than traditional broadcast TV. Not only can it improve the quality that users experience with this linear programming TV service, but it also paves the way for new TV services, such as video-on- demand, time-shifted TV, and network personal video recorder services, because of its integral return channel and the ability to address individual users. This article first provides an overview of a typical IPTV network architecture and some basic video coding concepts. Based on these, we then explain how IPTV can increase the linear programming TV quality experienced by end users by reducing channel-change latency and mitigating packet loss. For the latter, forward error correction and automatic repeat request techniques are discussed, whereas for the former a solution based on a circular buffer strategy is described. This article further argues that the availability of larger buffers in the network enables IPTV to better offer new services (in particular, time-shifted TV, network personal video recorder, and video-on-demand) than the competing platforms.

Traffic characteristics inside optical burst switched networks

We present an accurate model for burst traffic characteristics inside optical burst switched networks. At the edges of such networks, a number of IP packets are collected before being injected into the core as a single unit. The sizes of these so-called bursts critically impact the performance of the various network elements, so that a good understanding of their characteristics is an essential step in network performance engineering. A further abstraction of this assembly process results in a straightforward simulation model that can eliminate the need for packet level simulation, and thus reduce model complexity and simulation cost.

Modelling H.264/AVC VBR video traffic: comparison of a Markov and a self-similar source model

We compare the adequacy of two models for realistic video sources, namely the fractional Brownian motion (fBm) and Markov modulated fluid flow (Mmff) models. We use the effective bandwidth approach to get the probability that the buffer content exceeds a certain threshold. We use a formula in which the variance function, i.e., the variance of the traffic arriving in an interval of length τ, plays a central role and we model it as variance associated either with the Mmff or fBm model.We measure the variance function for an artificial source we construct, using Variable Bit Rate (VBR) H.264/AVC (Advanced Video Codec) video traces of real movies. There is a good correspondence between the buffer threshold exceeding probability obtained via trace-based simulations and the one predicted theoretically, based on the measured variance function. These we take as benchmark results against which we check both models—fBm and Mmff.First, we try to tune the model parameters such that their variance function matches the measured one over a large range of τ values, but this proves to be difficult. When matching the variance function over a short range of the τ of interest, we conclude that the Mmff model is better suited to model VBR video sources. We conduct an error sensitivity analysis with non-optimal model parameters and conclude that they do not influence considerably the buffer exceeding probability.Most reliable results are achieved with the measured variance function; if by some reason it needs to be modelled, the Mmff is preferred over the fBm model.

Quality-of-service differentiation and fairness in optical burst-switched networks

A mechanism that is often proposed for Quality-of-Service differentiation in Optical Burst-Switched networks is offset-time management. In this paper we identify and explain some undesired characteristics of this mechanism. The most important finding is that the burst drop probability differentiation that is attained for a given offset-time value strongly depends on the distribution of the burst durations. Hence control of the differentiation is difficult, since the distribution of burst durations is subject to changes all the time, depending on the traffic conditions at the edge of the OBS network. We also found that offset-time management slightly increases the unfairness within the lower priority classes, in the sense that longer bursts are dropped more frequently than shorter ones.

Analysis and design of a playout buffer for VBR streaming video

A critical component in the delivery of VBR streaming video over packet-based networks, is the playout buffer at the receiver side. As packets will incur a variable delay within the network, a mechanism is needed to dejitter the stream and assure that packets are played out at exactly the same rate at which they were generated by the sender.Packet loss at the playout buffer can be induced by either underflow or overflow, and we first identify the general conditions under which these events occur. These general rules require some knowledge on the network delay characteristics of an arbitrary packet, and are then further developed and refined, by taking a closer look at the case where a single delay bottleneck exists within the network.This analysis allows us to derive useful design rules, where we consider two cases, depending on whether or not the receiver has knowledge about the delay the first packet of the stream has incurred. We conclude by demonstrating that the traditional playout paradigm can be enhanced to guard against large initial delays.

Segmentation overhead in optical burst switching

In Optical Burst Switching (OBS), data packets at the edge of the network are aggregated into larger units identified as Data Bursts (DBs). When switching is performed synchronously in OBS core nodes (slotted switching), each DB has to be segmented at the switch input into fixed size units. Each unit is switched to a designated output in an optical time slot (a parameter of the optical switch). The purpose of our study is to give recommendations concerning the optimum size of these optical slots so as to minimize the overhead arising from the segmentation process. In order to estimate the total overhead due to this process we take into account the statistical distribution of the burst size, the possible padding of the last burst segment (to completely fill an optical slot) and the overhead due to the optical-slot preamble.

Video Quality Protection Strategies for HDTV in the Presence of Buffer Overflow

We investigate the impact of Forward Error Correction (FEC) and retransmissions on the video quality of packetized HDTV video flows with possible buffer overflow at the ingress node of the video network. The use of protection is a mixed blessing. On the one hand, it increases the packet rate associated with each video flow (due to overhead), which results in a higher buffer overflow probability. On the other hand, protection allows for the recovery of lost packets, which alleviates the problem of buffer overflow. It turns out that the overall impact of FEC and retransmission is very beneficial for the video quality. Moreover, retransmissions clearly outperform FEC, and can ensure HDTV video quality up to loads at the ingress node close to one.