A PREDICTIVE MULTI-CHANNEL MBAC TECHNIQUE FOR
ON-LINE VIDEO STREAMING
Pietro Camarda, Cataldo Guaragnella, Domenico Striccoli
D.E.E. Politecnico di Bari, via E. Orabona, 4, 70125 Bari, Italy
Keywords: Multimedia Streaming, Measurement Based Admission Control, Bandwidth Prediction.
Abstract: A measurement based admission control predictive technique is introduced for on line streaming systems
exploiting the GOP length demultiplexing of the aggregate bit stream in conjunction with a linear predictive
algorithm. Due to the long latency of the statistical aggregate, the predictive technique is able to predict the
bit rate over about two seconds of time. The prediction information is used in an admission control system
to estimate the bit rate and the margin with respect to the channel capacity in the proposed streaming
system. These measures have been used to estimate the overflow probability in a general aggregate
situation. Tests conducted over real video sequences confirm the feasibility of the proposed technique.
1 INTRODUCTION
Multimedia streaming applications (e.g., audio-video
streaming, Digital Video Broadcasting, UMTS
services, etc.) are rapidly growing in the actual
telecommunication world. They perform multimedia
reproduction while taking into account some user
feedback interactivity. In a multiplexed scenario,
several multimedia flows with a high bit rate
variability, data burstiness, self-similarity and Long
Range Dependence (LRD) properties (Garrett and
Willinger, 1994) are aggregated. Such a kind of
Variable Bit Rate (VBR) traffic makes very difficult
an optimal resource assignment and estimation, in
terms of bandwidth occupation and client buffers, to
properly manage video streaming.
An efficient aggregated VBR data transmission
assumes that multimedia flows are delivered with an
assigned Quality of Service (QoS), that is,
end-to-end delay, data losses, and jitter
specifications. To guarantee QoS requirements for
each of the aggregated flows, Call Admission
Control (CAC) schemes can be implemented, with
the purpose to verify that each flow, sharing link
bandwidth resources with other flows, can be
delivered to destination while respecting QoS
specifications. This task is performed whenever a
new flow requests admission in the network link. If
QoS requirements are respected, the new flow is
admitted, otherwise is rejected.
Admission Control algorithms need the
aggregate traffic to be accurately characterized,
through a priori determined traffic parameters, or
traffic measurements. In the first case,
“parameter-based”, in the second
“measurement-based” admission control algorithms
are implemented. As regards the first approach, the
main flow characteristics are a priori set to estimate
the amount of network resources needed by a flow
aggregation, including the new flow requesting
admission. The second approach is based on
aggregate traffic measurements, to estimate the
current link load.
Nevertheless, bursty compressed multimedia
traffic is very difficult to characterize, due to the
strong unpredictable nature of compressed aggregate
traffic, and relative traffic descriptors could provide
imprecise information for admission control
purposes (Crovella and Bestavros, 1996 and Floyd,
1997). This is particularly true for the
parameter-based approach, where traffic
characteristics are a priori set. This makes an
accurate traffic characterization hard to perform,
since admission control criterion does not properly
take into account dynamics of the aggregate running
into the network link. For bursty sources, this often
translates in a network link underutilization, or a
failed respect of QoS guarantees (Floyd, 1997).
Measurement Based Admission Control (MBAC)
algorithm class generally performs traffic evaluation
and bandwidth estimation through a dynamic
25
Camarda P., Guaragnella C. and Striccoli D. (2006).
A PREDICTIVE MULTI-CHANNEL MBAC TECHNIQUE FOR ON-LINE VIDEO STREAMING.
In Proceedings of the International Conference on Signal Processing and Multimedia Applications, pages 25-32
DOI: 10.5220/0001568300250032
Copyright
c
SciTePress
observation of traffic behaviour. This approach is
useful to take into account aggregate bandwidth
fluctuations by repeated “on-the-fly” measurements
in past periodic time intervals, and subsequent actual
bandwidth estimation, even if this generally comes
at a cost of a computation complexity overload. This
approach is particularly suitable in an on-line
context, where only part of the aggregate data set is
available for bandwidth estimation and actual load
prediction.
An efficient MBAC scheme should thus provide
an intelligent admission decision, based on network
resource measurements, whose accuracy strongly
affect MBAC efficiency and robustness. MBAC
algorithms exploiting past traffic measurements and
QoS parameters a priori specified decide, based on
actual traffic estimation, the new flow admission or
rejection.
Several different strategies for traffic
measurements have been developed, each of them
strongly influencing the MBAC algorithm
performance. In Jamin et al (1997a) three different
MBAC techniques are compared. In Jamin et al
(1997b) a MBAC algorithm is proposed for a packet
delivery service with bounded delay requirements.
In this algorithm, admission decision is based on a
token bucket filter model, that is able to calculate
worst case delays. The algorithm has been tested in a
wide variety of scenarios, and including also LRD
traffic. The MBAC proposed in Casetti et al (1996)
presents a quite simple traffic estimation process in a
single-link scenario, able to achieve a high
bandwidth utilization, without violating, at the same
time, QoS guarantees, specified as delay bounds. In
Grossglauser and Tse (1999) the impact of some
parameters, like estimation errors, flow dynamics
(departures, arrivals, and bursty traffic time scales)
and system memory, on MBAC robustness are
analyzed. Their reciprocal interactions and impact
on QoS specifications are considered for a unified
framework. In Grossglauser and Tse (2003) a
MBAC algorithm is proposed, based on a time scale
decomposition of a flow aggregate, taking also into
account flow arrivals and/or departures. Bandwidth
fluctuations are decomposed into “fast time scale”
and “slow time scale” contributes, separated by the
“critical time scale” threshold. Aggregate
fluctuations slower than the critical time scale are
exploited for admission control policy while the
faster are used to estimate the necessary amount of
bandwidth needed to absorb high bandwidth
fluctuations in a brief time period.
In this paper a novel MBAC algorithm is
proposed. It is based on a set of aggregate traffic
measurements performed in past time intervals, to
derive statistically an estimation of the aggregate
bandwidth and of the available bandwidth margin,
necessary to absorb instantaneous peak rates, with
the same approach used in Grossglauser and Tse
(2003). Available bandwidth estimation is then
exploited to admit new flows without violating QoS
requirements of the whole aggregation.
The main novelty of the proposed algorithm lies
in the criterion adopted for aggregate bandwidth
estimation, that makes use of linear predictive
techniques. In particular, aggregate traffic
measurements are performed in time intervals
preceding the new flow request of admission. This
data set is exploited to statistically derive a
prediction of the aggregate behaviour in a future
time interval. This prediction is then combined with
the QoS parameter, that is, overflow probability, to
decide the new flow admission. In this context the
overflow probability is the probability that the whole
aggregate, included the new flow, exceeds the
available link bandwidth.
The linear prediction technique dynamically
performs an estimation of the statistical aggregate
characteristics; the algorithm utilized is based on a
“multi-channel” linear prediction, to determine the
predictable characteristics of a set N different
multiplexed video sources.
Predictive
bandwidth
estimation
subsystem
Measurement
Based
Admission
Control
subsystem
Figure 1: the processing scheme adopted in the MBAC
system.
As it will be more clear in the next sections, this
work aims at implementing a numeric filter that
properly describes the predictable portion of
aggregate traffic to quantify the overflow probability
for admission decision.
The paper is structured as follows. In Section 2,
the novel MBAC approach based on a bandwidth
prediction system, is presented and explained. In
Section 2.1 the predictive system is analyzed in
detail. Bandwidth prediction is then used in the
MBAC algorithm, whose principles are explained in
Section 2.2. In Section 2.3 the whole MBAC
algorithm is implemented. Section 3 provides some
significant numerical results. Finally, Section 4
gives some conclusions to the proposed work.
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2 THE MBAC PREDICTIVE
APPROACH
Statistical multiplexing of video sources allows an
efficient use of the channel bandwidth for the video
streaming application even if high frame variability
is present (Salehi et al, 1998).
It can be observed that the bit rate of MPEG
coded videos arranged in a composite bit stream
(statistically multiplexed transmission system)
presents a periodicity linked to GOP size: if the GOP
length is N, the multiplexed video stream bit rate
will still present a periodicity of N frames.
This particular case makes the bit rate time series
“cyclostationary” over limited time intervals, and
somehow “predictable”; the N frame-step
stationarity can be exploited to predict the future
behavior of the bit rate over a predictable time
window.
The predictive nature of the video aggregate can
be fruitfully exploited for designing MBAC system
for on-line application in video streaming systems. It
can be subdivided into two main parts:
the predictive system, used to predict the
bandwidth total allocation upon a future time, based
on the bandwidth measures accumulated on a
previous limited observation time;
the admission control system.
Figure 1 pictorially represents the MBAC
system.
2.1 The Predictive Multi-channel
System
Predictive bandwidth estimation is carried out by
standard predictive algorithm, with a multi-channel
approach. The proposed system performs several
predictions of the bit rates for several future time
lags in order to estimate the whole bandwidth
allocation required over a time-finite length
observation window.
As it can be seen by figure 2, a quasi-periodicity is
still present in the aggregate bit stream, and such
characteristic comes from the intrinsic nature of the
MPEG video encoded stream. The GOP quasi
periodicity allows to think the process as a cyclo-
stationary process, so that statistical parameters can
be computed separately on each stationary random
process extracted by subsampling the aggregate bit
rate time series with GOP length time step.
Figure 2: Time evolution and spectrum of a 6 streams CIF
aggregate.
aggregate
ch. 1
ch. 2
ch. 12
T
T
Figure 3: the channel splitting system; 12 sub-channels
depend on the 12 GOP length.
Figure 3 represents a 12 (GOP size) channel
demultiplexer of the aggregate bit rate information.
Each channel will undergo a time prediction to
estimate the bandwidth allocation of each channel
video sub-stream. All the measures will contribute to
the prediction of the whole aggregate video
bitstream on a window extended over the predictable
time length of the process at hand. Figure 4 reports a
time average evolution of the process correlation
function (normalized auto-covariance function of
each sub-samples process).
Of course, such analysis can be still valid over a
limited time duration, depending on the correlation
of the process at hand.
The first step to this goal is thus oriented to the
estimation of the time average correlation length of
each sub-channel bandwidth time series. At this aim,
each channel normalized covariance function
(correlation coefficient) has been carried out. As
long as the process we are dealing with is not
properly stationary, several estimates of each
channel correlation functions have been averaged to
estimate the time correlation length.
In the proposed approach, the time correlation length
has been chosen as the time lag where the
correlation function is greater than 0.5. Prediction
A PREDICTIVE MULTI-CHANNEL MBAC TECHNIQUE FOR ON-LINE VIDEO STREAMING
27
can thus be carried out over time extending not
further than 2 seconds. This means, at our full time
resolution (25 frames/s), 25×2 = 50 frames, so that,
for each channel, only 4 frames can be considered as
the goal of our predictive technique.
a) Sub-channel-1 correlation function
b) detail of the correlation function: samples only 5 samples result
above the 0.5 value.
Figure 4: a) Time averaged correlation function of the
sub-channel 1 process and b) detail of the correlation
length.
The predictive scheme for time prediction of
bandwidth allocation is reported in figure 5. After
demultiplexing the data stream, filtering of each
channel output is carried out. The filtering procedure
is performed to extend process stationarity over
longer time intervals, as subsampling tend to reduce
the correlation between successive measures of
bandwidth allocation (Shanmugan and Breipohl,
1988). This operation, in any case is straightforward
and requires no computation: if a rectangular
impulse response is used in the filter, the low-passed
channel time series is at each time instant the sum of
all the bandwidth measures along the widow length.
This measure is simply obtainable by tracking the
required transmission buffer filling, as the buffering
at the transmission end cannot be avoided.
The data to be transmitted are simply rearranged
in 12 sub-fluxes and each channel buffer filling is
tracked to be used as the data input of the
multi-channel prediction of the whole aggregate bit
rate.
For each channel time series, a multistep
predictive technique is implemented.
Linear prediction assumes that, stated b
n
the n-th
sample of a predictable time series in additive white
gaussian noise (AWGN, w
n
), it can be estimated
from its M previous samples x
n-1
,..., x
n-M
by the
relation (Proakis and Manolakis, 1996):
1
M
nknkn
k
bcxw
=
=
⋅+
(1)
The equation in (1) can be solved by means of a
minimum mean squared error criterion. To obtain a
compact description of the problem and a direct
solution, matrix notation can be introduced:
X
cb
= (2)
where X is the [N×M] data matrix of the signal
time series,
c is the [M×1] linear prediction
coefficient vector and
b
is the [N×1] vector of the
data to be predicted. Let us remember that N
represents the length, in frames, of the aggregate
periodicity. The number of equations of (2) is much
higher than the number of coefficients of the linear
prediction filter.
The solution in the mean squared error sense can
be obtained by the use of the pseudo-inverse of X,
giving:
(
)
1
TT
cXXXb
=
⋅⋅
(3)
This allows to compute the coefficients of the
linear FIR predictor filter. This filter thus predicts x
n
(the elements in b vector) from its M preceding
values [x
n-1
, ..., x
n-M
]. The same procedure can be
established to predict the x
n+1
, x
n+2
and so on in the
future. Going a step further, we can perform a joint
estimate of the next four samples [x
n
, ..., x
n+3
],
basing on the observation of a certain number M of
preceding channel bandwidth measures. At this aim,
equation (4) calculates a joint four-step prediction
measure for each channel:
(
)
1
TT
CXXXB
⋅⋅
(4)
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aggregate
ch. 1
ch. 2
ch. 12
T
T
Tx buf 1
Predictive
System
Tx buf 2
Tx buf 12
BW allocation
Buffer filling
Data
Figure 5: the predictive system scheme. The orange lines
indicate the measures used in the predictive system.
In (4) C is a 4 columns vector of M coefficients.
Each column in the vector contains the M
coefficients of a linear prediction filters for the four
successive channel bandwidth measures to be
estimated.
The four bit rate measures predicted by (4) for
the 12 channels are then summed together, thus
obtaining the total number
0
B
of bits predicted over
a period T of two seconds. This gives us a predictive
estimation of the total channel bandwidth, obtained
as
0
/
B
T
.
This measure is then used to construct a
predictive MBAC system for on line applications as
described in the next section.
Figure 7 represents the cumulative bandwidth
estimated in the predictable observation window
both for the true aggregate and for the predicted. The
difference is the prediction error.
2.2 The MBAC System
In this work, a look-ahead system, able to take care
of the predictable information about the bit rate time
evolution is proposed, to try to build up a
Measurement Based Admission Control system.
The assumed transmission medium is
characterized by a constant maximum channel
capacity, C.
Both statistics of the prediction error and the
predicted (low passed) composite video stream for
all the cases of K-streams composite video have
been computed.
The MBAC system, basing on the information
about the acquired statistics of the composite video,
should be able to accept or deny the request of a end
user for the addition of a new stream in the video
aggregate. The acceptance of the user request should
not compromise the performances of the aggregate
video stream of any user, so that a measurement
based approach should be able to guarantee, within a
given probability, that the inclusion of a new stream
in the aggregate doesn’t lower too severely the
whole system performances.
Figure 6: the scheme used in each channel prediction
system. The red time samples are the goal of the predictive
system.
Figure 7: Predicted Vs true time series of the whole video
aggregate in the prediction window (about 2 seconds).
An efficient MBAC system should be able to use
as much as possible the available channel capacity,
so that the higher the number of videos in the
aggregate bit streams, the higher the obtained
performances of the streaming system.
Usually MBAC techniques use few statistic
parameters such as the peak bit rate, the average bit
rate and the standard deviation bit rate to define a
procedure for the admission, so that a low bandwidth
efficiency can be often experienced to be able to
guarantee QoS.
In this work we deal with on line streaming. Not
all the video stream is available, so that statistics for
the generic N-video aggregate case are used to
define the MBAC admission, in conjunction with the
acquired statistics on the past observation of the
process at hand.
The definition of the MBAC system needs a
conservative margin with respect to C, to manage all
the unpredictable bit rate requirements in the K-
streams composite video. Such a margin should be
neither too strict, to avoid too frequent overflow of
A PREDICTIVE MULTI-CHANNEL MBAC TECHNIQUE FOR ON-LINE VIDEO STREAMING
29
the available channel capacity C, nor too mild, to
avoid low bandwidth efficiency of the on line
streaming system.
The definition of a suitable margin with respect
to the channel capacity C of the system is carried out
basing on the prediction statistics for several N-
aggregate streams, with different values of N.
The channel capacity overflow probability event
can depend on two possible causes:
overflow coming from a peak in the prediction
error;
overflow coming from the peak of the required
bandwidth to deliver the composite video
stream.
The probability of the union of these events can
be approximated by the sum of the two probabilities.
As we are searching for a conservative MBAC
algorithm, we can assume valid the following
relation:
qsi
PPP≈+
(5)
where P
q
is the required overflow probability, P
s
is the overflow contribute coming from the signal
component and P
i
is the overflow probability
coming from the prediction error.
As a further requirement for the MBAC system,
we assume the P
i
probability be small with respect to
P
s
:
is
PP
α
(6)
with α a coefficient to be defined, so that from
(3.1):
(1 )
qs
PP
α
=+ (7)
In our experiments, we assume that
i
P be small if
compared with the probability of the overflow event
due to the signal specificity, so that we can choose
P
i
an order of magnitude smaller than
s
P ; thus, we
suppose
0.1
α
= .
With such an assumption, we can draw the
graphics for the P
s
as a function of the bit rate for
several orders K of the composite video stream.
By drawing the polynomial fitting curves of the
bit rate as function of the P
s
, we can compute the bit
rate of the composite video signal as a function of
the overflow probability P
s
.
2.3 The MBAC Implementation
The overflow probability can be used to define a
MBAC system; the global bit rate of the K-streams
composite video has to be considered; to take care of
the two components of the bit rate, the predicted bit
rate is computed as the sum of two components: the
estimated bit rate in the prediction window and a
margin with respect to the channel capacity, to avoid
the frequent occurrence of the overflow event.
The predictable bit rate is estimated as
previously described, while the margin depends on
the number of aggregated streams and on the value
of the predicted bit rate.
Two different measures have been carried out,
one over the predicted signal and the other over the
prediction error.
Let
f
n
(b) be the probability density function (pdf)
of the prediction error with respect to the bit rate,
and
F
n
(b) its corresponding cumulative distribution,
the quantity
(
)
(
)
b-FbF
ncn
1
=
represents the
probability that the event “the difference between
estimated bit rate and the true one is greater than b”.
This probability will be used in the proposed MBAC
algorithm.
To accept a new streaming client, the available
composite video bit rate must satisfy the relation:
(
)
(
)
1,
ss
BR N P Br P C
α
+
(8)
where
BR and Br
Δ
are evaluated by (9). BR is a
function of the overflow probability Ps, and
represents the composite video stream bit rate in the
hypothesis the new stream is accepted and
multiplexed together with the given K-streams
composite video, to originate a (K+1)-streams
composite video, and
Br
Δ
is the corresponding
margin with respect to the channel capacity C to
absorb the peaks of the BR due to the
unpredictability of the phenomenon.
Whenever
BR Br C
+
Δ< the new client will be
considered as admissible to the service: the
probability of overflow will be estimated and sent to
the client that, considering such information, might
decide whether to accept the service or deny it.
The equality can be assumed in (8) as a limit
condition to determine the searched
P
s
value.
For each K, and defined α in (6), the
complementary curves of the probability as a
function of the bit rate can be approximated by
polynomial functions in the variable P
s
; we compute
a single polynomial function for the unpredictability
of the bit rate, ΔBr, and a family of curves for the N-
streams cases of composite video streams.
Thus we have:
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() ( )
() ()
0
0
,
L
n
isns
n
L
n
sns
n
B
rP Br P c P
BR N P a K P
α
=
=
Δ=Δ=
=⋅
(9)
Assuming to be in proximity of the channel
capacity, with a K-streams composite video, the
algorithm steps consist in:
1. Compute the probability Pso due to the (K+1)
case finding the zero of the polynomial
equation BR(Pso) = C:
()
0
1, ( 1)
L
n
s
on so
n
CBRK P aK P
=
=+= +
2. Compute the margin with respect to the
channel capacity for the (K+1)-streams
composite video, Br(Pso):
()
0
1, ( 1)
L
n
s
on so
n
Br K P c K P
α
=
Δ+= +
3. Compute the new maximum bit rate to be
assumed for the smooth signal, BR1:
()
1 S
BR C Br P
α
=−Δ
4. Compute the minimum value assumed at the
new computed bit rate for the (K+1) case,
P
min
, as the solution of the polynomial
equation BR
1
=BR(K+1,P
min
):
1min min
0
(1,) (1)
L
n
n
n
B
RBRK P aK P
=
=+= +
5. Compute the estimated overflow probability
as:
(1 )
qs
P
P
α
=+
3 EXPERIMENTAL RESULTS
The experimental results we present all refer to real
data of composite videos obtained by statistically
multiplexing MPEG encoded YUV-CIF videos, with
GOP of 12 frames and 30 fps. All the created
composite video streams have been obtained
randomly mixing sources with different statistics,
and composite videos are limited to K=20 sources
composite video. Figure 8 refers to the obtained
complementary cumulative distribution function for
the cases of K multiplexed sources, with K in the set
[1,20].
Curves in figure 8 and 9, representing the
complementary cumulative distribution functions for
the aggregate predicted bit rate and errors
respectively, have been polynomial fitted to give
analytical expression in (9).
Two sets of K-streams composite videos time
series bit rates have been produced: a training one,
used to obtain curves in figures 8 and 9, and a test
set, used to obtain experimental results.
The test of the proposed algorithm has been
carried out on the test set of K-streams composite
videos to estimate the expected overflow event.
A capacity C communication channel has been
assumed, and a number of video, K, in the
composite video stream has been selected in order
be close to the channel capacity and give a
negligible overflow probability. A request of service
by a (K+1)-th client has been assumed, and the
overflow probability for that client has been
estimated as previously described.
Figure 8: 1–F
s
(BR): complementary cumulative
probability functions of the bit rates for several orders of
composite videos.
Repeated experiments in the same conditions
have been carried out. The experimental measure of
the overflow probability for the test cases have been
computed as the fraction between the overflow
experienced time duration and the total duration of
the (K+1)-streams composite videos.
The obtained overflow probability is mostly well
predicted by the proposed algorithm.
Figure 10 presents the histogram of the obtained
probability values for 100 cases of (K+1)-streams
composite videos in the test set. The fraction of test
cases showing an experienced overflow probability
greater than the estimated one results lower than
15% in our simulations; also, the difference in
probability value is never much higher than the
estimated one.
A PREDICTIVE MULTI-CHANNEL MBAC TECHNIQUE FOR ON-LINE VIDEO STREAMING
31
Figure 9: 1–F
n
(BR): cumulative complementary
probability functions of the prediction error
(unpredictability) of the bit rates for several orders of
composite videos.
4 CONCLUSIONS
In this paper, a new MBAC algorithm for VBR
multimedia streaming has been proposed. It makes
use of an innovative bandwidth prediction technique
to more efficiently admit new flows in a network
link with aggregate traffic already running, without
violating the QoS requirements of the whole
aggregation.
Figure 10: Histogram of tested sequences: the estimated
probability (signaled by the red arrow) is lower than the
experienced one only for the 15% of the total test cases.
The proposed bandwidth prediction technique
exploits the particular, quasi-stationary nature of
aggregate traffic to predict bandwidth utilization in a
relatively short time period. This approach is
particularly useful to take into account aggregate
dynamics quickly varying in time. Furthermore, this
straightforward predictive technique can be fruitfully
implemented in advanced video smoothing systems,
where aggregate video transmission consists of
Constant Bit Rate (CBR) pieces. The proposed
predictive technique can thus be exploited to
establish the maximum CBR size allowed for video
smoothing in the next two seconds.
The predictive bandwidth estimation technique is
the core of the MBAC algorithm. The predicted
bandwidth information is utilized to perform a new
flow admission, that respects the given aggregate
overflow probability (QoS parameter).
Performed simulation show the effectiveness of
the proposed MBAC for a wide number of
simulation scenarios. Experimental results show that
the observed overflow probability almost never
exceeds the correspondent QoS parameter, testifying
a good performance of the proposed algorithm and
its ease of use in several practical scenarios.
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