Chapter 7.1

Video Streaming QoS and QoE

(c) Tobias Hossfeld (Aug 2021)

This script and the figures are part of the following book. The book is to be cited whenever the script is used (copyright CC BY-SA 4.0):
Tran-Gia, P. & Hossfeld, T. (2021). Performance Modeling and Analysis of Communication Networks - A Lecture Note. Würzburg University Press.

The video buffer of a video client is modeled as M/M/1 queue with a trigger mechanism called $N$-policy, which allows study of the stalling behavior of HTTP video streaming.

The video contents are downloaded as HTTP segments which contain a certain video play time $B$. This video play time per HTTP segment is assumed to follow an exponential distribution with parameter $\mu=1/E[B]$.

Consider that the playback of a video consists of multiple HTTP segments. The HTTP segments are downloaded in-order and arrive at the client with rate $\lambda$. Assuming the interarrival times of HTTP segments at the video buffer follow an exponential distribution with rate $\lambda$.

In order to reduce the number of stalling events during playback, the video player uses a playback buffer. Video playback stops, if the buffer empties and is only, but immediately resumed, if the video buffer grows and reaches the level containing $N$ HTTP segments again. In the queueing literature, this strategy is referred to as $N$-policy.

The utilization of the system equals the offered load according to the utilization law $\rho = \frac{\lambda}{\mu}$ which corresponds to the ratio of the time when the video player (server) is busy. Hence, if $\rho<1$, the server may get idle and stalling occurs.

The system characteristics can be derived by means of continuous-time Markov chains.

Stall ratio

The stall ratio $\psi$ is the fraction of time the video player is idle.

$ \displaystyle \psi = 1 - \rho = 1 - \frac{\lambda}{\mu} $

Average stall duration

The average stall duration $L$ is the average idle duration - which is independent of the video play time per HTTP segment $E[B]$.

$ \displaystyle L = \frac{N}{\lambda} $

Stall frequency

The stall frequency $F$ is the number of interruptions per video playout time.

$ \displaystyle F = \frac{\mu-\lambda}{N} $

Quality of Experience

The QoE model takes into account the stall frequency $F$ and the average duration $L$ per stall based on subjective experiments. An exponential relationship according to the IQX hypothesis is observed. The parameter $\gamma$ reflects the minimum observed QoE value as measured in the subjective tests. Thereby, the video with the same stall characteristics is shown to several users who rate the QoE on a 5-point rating scale. For quantifying QoE, the mean opinion score (MOS) is used by averaging over the subjects. The QoE model found $\gamma=1.5$. In case no stalling occurs, the maximum QoE score of 5 is obtained, leading to the scale parameter $\alpha = 3.5$.

The crucial parameters of the IQX model are the sensitivity parameters $\beta_L$ and $\beta_F$ in the exponent which are weighting the influence of the average stall duration $L$ per interruption and the stall frequency $F$.

$ \displaystyle f(L,F) = \alpha e^{-\beta_F\cdot F - \beta_L \cdot L} + \gamma $

Subjective tests revealed: $\beta_L = 0.15 s^{-1}, \beta_F = 0.19 \cdot 30 s$ with units $[L]=s, [F]=s^{-1}$.

For many situations, $N=1$ leads to the best QoE.

QoE-optimal Threshold for N-Policy

The QoE-optimal threshold $N$ can be derived by considering the function

$ g(N) = \alpha e^{-\beta_F\cdot \frac{\mu-\lambda}{N} - \beta_L \cdot \frac{N}{\lambda}} + \gamma $

This leads to the QoE-optimal value through differentation of $g(N)$.

$ N = (\mu - \lambda)\cdot \lambda \cdot \frac{\beta_F}{\beta_L} $

It should be noted that in practice $N$ needs to be an integer value and $N\geq1$.

The optimal threshold $N$ of segments means that the video time $D = N \cdot E[B]$ is on average available in the buffer when the playout starts again after a video interruption. It can be seen that the curves are overlapping. Thus, the buffered video amount is the crucial component for the optimal playout. The $D$-policy is a different scheduling policy which works as follows. Again, the server (i.e. video player) is turned off at the end of a busy period and turned on when the cumulative amount of work (i.e. video play time) reaches the threshold $D$. In practice, even if the entire segment is not completely downloaded, the download starts if enough video contents are downloaded.