Experiences Building Telepresence Systems
Federico Gil Professor of Computer Science
Keynote Speaker: Prof. Henry Fuchs
Adjunct Professor of Biomedical Engineering
University of North Carolina at Chapel-Hill
Scribe: Wei Cheng(National University of
Professor Henry Fuchs from University of North Carolina at Chapel
Hill gave the keynote speech titled "Experiences Building
Tele-presence System". The keynote first explained the term
tele-presence, of which the main aim of is to give users convincing
immersive experience. Henry then introduced tele-presence systems that
have been developed since 1997, underlining the main challenges in
developing such systems. The keynote concluded by highlighting
two key system challenges for immerse
environment: (i) a middleware, and (ii) a set of integrated OS components,
for common, lower level tele-presence functions, such as display
The keynote was interleaved with many questions from the audience.
Kevin Almeroth (UC Santa Barbara) asked why a new term ``tele-presence''
is coined (Answer from Henry: to emphasize human "sense").
Yih-Fan Chen (Robin, AT and T Research) argued that boundary exists
naturally between the virtual world and the real world and true
immersion cannot be achieved, to which Henry agreed.
Current solutions to virtual world are either not fully immersive or require wearing of
The role of using large displays in tele-presence systems were
discussed with questions from Dick Bulterman (CWI) and Damien Le Moal
(Hitachi). Henry pointed out that using large display helps users
in knowing the relative position and context within the data
displayed, and allows users to move around more freely.
Dick also asked about the relative importance of
audio and visual in tele-presence system. Henry pointed
out that the importance of audio/visual information is a psychological issue and depends on the
Damien asked why OS integration is important in large display system
when we can just develop dedicate applications. Henry argued that
system tasks (such as windows management) need to be adapted to the new display system, so only
an OS supporting the large display system can give users uniform
experience in both system tasks and dedicate applications.
Padmanabhan Pillai (Intel Research) and Damien asked about the
role of haptic and smell. Henry answered
that the sense of touch and smell are important in some cases (e.g.,
medical applications) but the current technology is limited. Haptic
devices confine movement to a small distance, and
the basis of smell (like RGB for color) are yet to be found.
Session 1: New Applications and Services
Discussion Lead: Wei Tsang Ooi (National University of Singapore)
Scribe: Saurabh Ratti (University of Ottawa)
Dynamic Overlay Multicast in Multi-Stream/Multi-Site 3D Collaborative Systems
Wanmin Wu from UIUC presented a 3D video collaborative system that
uses an P2P overlay for dissemination of content streams. Each peer
has the capability of producing and rendering multiple 3D
video streams (depth as the added dimension).
contribution of this work is an algorithm that
accounts for the contributing factor of streams
to the overall view when selecting peers. Experimental results
demonstrated that the algorithm performs more
efficiently when compared to using view proximity only.
The ensuing discussion on 3D video capturing systems revealed
current camera technology is very limiting in capturing larger areas,
as those currently being developed are being aimed at desktop usage,
with ranges of 1 - 3 meters.
Wei Tsang Ooi (National University of Singapore)
asked about the evaluation of visual quality of 3D video,
to which Wanmin replied that designing an objective visual quality metric
for 3D video remains an open challenge.
Web 2.0 Traffic Measurement: Analysis on Online Map Applications
Song Lin (Tsinghua University) presented analysis of
characteristics of captured traffic generated by four online map
applications. The findings of note are (i) the traffic follows
diurnal cycles, (ii) the majority of map application traffic
consists of image traffic (68.7% for Sogou), (iii) 40% of Google
Maps's traffic is from mash-up sources. Due to map hotspots and the
applicability of Zipf's law, the study suggests that CDN servers can
improve the performance of online map application at these hotspots,
especially when the hotspots would differ according to physical
location of users.
Wei Cheng (National University of Singapore) asked whether there was
any data supporting the conclusion that requests from different
physical locations have different map hotspots. Lin replied that
data supporting this is difficult to gather, but the conclusion
can be inferred from the
visual overlay of requests distribution on a geographical map. Wei
Tsang inquired what could online map service providers learned from
Lin mentioned two lessons. First, providing good API is critical in
encouraging other website to mash-up the map data and drive traffic
to the map server. Second, CDN and other caching methods are still
very relevant in such online applications.
Peer-Assisted Online Storage and Distribution: Modeling and Server Strategies
Yuen Feng (University of Toronto) presented, on behalf of the
authors, work on modeling a real world peer-assisted online storage
system called FS2You, which has become popular in China
with 500 GB to 1 TB of data transferred daily. The goal of the
work was to derive a lower bound for the average file download time
and evaluate the performance of different
bandwidth allocation strategies.
Using average file
download time and peer satisfaction level as metrics for evaluation,
it was found that FS2You's current strategy of allocating bandwidth
to file popularity has favorable performance, but not
when bandwidth is unconstrained.
Wei Cheng asked whether the file popularity in FS2You
follows the Zipf distribution, to which Feng replied that it does.
Robin asked about the assumption that peers have uniform
upload/download rate. Feng explained that this assumption is
a simplification to the model and is an area for
Session 2: P2P Streaming I
Discussion Lead: Ketan Mayer-Patel (University of North Carolina at Chapel
Scribe: Remo Meier (ETH Zurich)
Instead of the usual format where each presentation is followed by a
Q and A session, Session 2 is organized as a mini-panel, where each
presenter takes turn to present their work, followed by a 30 minutes
discussion on the work.
InstantLeap: Fast Neighbor Discovery in P2P VoD Streaming
Hassan Shojania (University of Toronto) presented the paper on
behalf of the authors. An on-demand P2P streaming systems have to
handle peers with different playback positions and, consequently, a
low block overlap among these peers. Hassan described InstantLeap, a
mechanism to quickly discover appropriate peers for a given playback
position, improving current systems with a typical O(log(#peers))
communication overhead to O(1) with high probability using a
lightweight indexing protocol. The protocol groups peers by their
playback locality into segments. Peers then maintain some connections
to other peers from the same segment and to some peers from other
segments using random exchanges of known peers.
Overlay Monitoring and Repair in Swarm-based Peer-to-Peer Streaming
Nazanin Magharei (University of Oregon) presented her work on
clustering of peers and their impact in peer-to-peer live streaming
systems using a random mesh as P2P overlay. Clusters occur, for
example, due to good connectivity among peers having the same
Internet Service Provider (ISP). Peers in a clusters may suffer from
poor content availability in their neighborhood as the cluster's
peers are mostly connected among each other and only to few outside
peers. The authors provide a mechanism to detect and break-up such
clusters. The proposed protocol groups peers by their distance to the
source and increases the number of connections to other peers and
swaps parents in case of problems. This allows reduction of the
buffer size, to improve playback quality, and to reduce the distance
to the source.
Adaptive Overlay Topology for Mesh-Based P2P-TV Systems
Michela Meo (Politecnica di Torino) presented their work addressing
the problems of heterogeneous bandwidth availability in P2P
live streaming systems. Like in the preceeding talks of this session,
a random mesh is used as overlay. The proposed protocol better
exploits high-bandwidth peers by increasing their connectivity and by
positioning them closer to the source. This way, the high-bandwidth
peers provide a backbone to quickly disseminate new data blocks,
benefiting all the peers.
The bandwidth of a peer is indirectly estimated based on the
fraction of used neighbor connections and without the peer having to
explicitly know or measure its bandwidth.
A short panel discussion with all the presenters was held at the end
of this session. The main topic of the discussion was the
locality-awareness of the proposed protocol, i.e., whether they are
aware of the underlying physical topology in order to avoid long
distance connections and to keep most traffic within the same ISP or
close-by ISPs. Ketan Mayer-Patel (UNC) compared the two work
presented by Michela and Nazanin -- one tries to bring structure into
the overlay while the other tries to remove structure. Mohamed
Hefeeda pointed out that increasing randomness in overlay
topology seems to be counter intuitive. Nazanin explained that,
without random links among peers, content might not flow to other
Padmanabhan asked how else can P2P applications become location aware
if ISP information is not available. The audience and presenters
mentioned several alternatives, including Network Weather Service,
CDN-based Relative Network Positioning, and the P4P project.
Padmanabhan also asked whether the overlays account for correlated
churn, e.g, when a large number of peers join and leave
simultaneously. Such churn, however, are not addressed by the
Session 3: OS and End Systems
Discussion Lead: Kevin Almeroth (UC Santa Barbara)
Scribe: Ishan Vaishnavi (Centrum voor Wiskunde en Informatica)
Random Network Coding on the iPhone: Fact or Fiction?
Hassan Shojania presented their experience of implementing random
network coding on the Apple iPhone and iPod Touch platform. The
presentation overviewed network coding technologies and the
challenges encountered in migrating the algorithm on a
mobile device, with special attention to the lack of
SIMD instruction set on the iPod. The presentation
compared the coding efficiency over iPod Touch and iPhone, and
presented the results of the feasibility study. The presentation also
compared CPU-based and a GPU-based
Robin inquired about the effects of network coding on
battery usage. Hassan replied that the interface to
power management on the current iPhone is not exposed.
He added that the CPU usage was, however, non
linear. Hassan said that they were looking forward to the new
processor in the forthcoming iPhone, which he believed had better SIMD
instruction set support.
SLIPstream: Scalable Low-latency Interactive Perception on Streaming Data
Padmanabhan started his presentation with the question
``What can one do in a room with 1000 cameras?''
The presentation then focused on
the scale of data and the amount of
processing required to extract some constructive information from
data from these cameras. The core of the presentation was a runtime system
called Sprout, used to identify and split parallel
tasks across hundreds of machines with a final goal of achieving low
latency for interactive applications with large data processing. The
presentation focused on processing event/gesture recognition in
videos in "interactive time" and identified three steps in the
process: (i) identification of low-level features, (ii) matching
low-level data against a training set to extract events of interest,
(iii) aggregation of these results into appropriate events/gestures.
Padmanabhan noted that the second step involves
a sweep over the stream in both time and space, which
can be parallelized (even within each frame), providing
significant reduction in latency.
Klara Nahrstedt (UIUC) asked how close they were to achieving
interactive time. Padmanabhan replied in short "very far" and said
that the acquiring time from the cameras was huge since they used
off-the-shelf systems. The presenter also added that the networks are not
optimized as yet to handle such sudden bursts of packets from
multiple synchronized cameras.
Dongyan Xu then inquired if they had looked into any
compiler techniques and generic code optimization techniques.
Padmanabhan said that they had not done any low-level parallelization
and had focused on parallelization at a higher level.
Server-Efficient High-Definition Media Dissemination
Philip W. Frey from IBM Research started with an
assumption that "the network is no longer the
bottleneck, but the memory to NIC bus is". The presentation focused
on reducing the data fetch request processing time on
server by removing the
current 4-way copy (two DMA copies and two CPU copies) and two
context switches required to send data to an incoming request. The
authors proposed new protocol based on RDMA (Remote Direct Memory
Access). The presentation showed the CPU usage in HTTP (with and
without kernel sendfile() support) and RTP-based servers. They then
presented their new client-driven protocol based on RDMA. Their
impressive results showed a zero CPU load using the protocol. They
also presented results showing lower context switches and lower
interrupt rates. For live-streaming applications,
direct memory transfer made their solution a zero-copy solution.
There are several clarifying questions from the audience. Philip
explained that RDMA works over TCP and the address space used in
the experiments is virtual. A
follow-up question was on the DMA transfer that maybe
required for virtual memory access. The speaker replied that
such memory access is a one-time cost.
Power Efficient Real-Time Disk Scheduling
Damien Le Moal (Hitachi Development Laboratory, Japan) spoke about a
new method of optimizing disk access in an effort to save power
consumption. The presentation distinguished between normal data
access and audio-visual (AV) data access targeted for set-top
boxes, but general enough for use in normal computing systems.
The idea is to
have a separate file system for AV access, where
each AV data request would be associated with a deadline
it must meet. Thus, the system can hold all such requests in a queue
and schedule them together in a more efficiently re-ordered fashion
(according to seek distances). This increases the efficiency of the
system by reducing the seek distance between two consecutive
requests. The disk can also be spun down when the deadlines are far
Mohamed Hefeeda inquired if scheduling these real time events in such
a manner would adversely affect interactivity of multimedia
application. Damien replied that as long as the deadlines are
selected correctly, it will actually improve the interactivity since
the event is now certain to be scheduled before its deadline expires.
Session 4: Virtual Environments and Games
Discussion Lead: Kuan-Ta Chen (Academia Sinica)
Scribe: Pengpeng Ni (Simula Research Lab and University of Oslo)
A Delaunay Triangulation Architecture Supporting Churn and User Mobility in MMVE-s
Saurabh Ratti (University of Ottawa) presented, on behalf of the authors,
algorithm for dynamic construction of an overlay topology that
supports greedy routing in massively multi-user virtual environments.
The idea is to partition the topology into two non-overlapping
sets, where each set is updated in a separate phase using the other
set as a reference. After explaining the theoretical background,
Saurabh illustrated the algorithm steps and concluded with simulation
Dongyan Xu asked how to ensure the robustness of the algorithm
against when nodes leave.
Saurabh answered that the reliability would be
guaranteed if TCP is used. If UDP was used, node leave
would be detected in the next update circle.
Robin suggested installing a scanner in each geometrical region for
detecting the mobility of avatars. Saurabh argued that a
decentralized solution is more preferable than centralized
approach. Wei Tsang asked if there is any drawback of the algorithm.
The response from Saurabh was that the resumption time presented in
the simulation results is considerable. Although increasing
the number of nodes has little effect on the resumption time, the
baseline value of 1.1 seconds is not optimal, especially if the game has
multiple updates within a second.
Probabilistic Event Resolution with the Pairwise Random Protocol
John Miller (MSR and University of Cambridge) proposed the use of
Pairwise Random protocol (PRP), which uses secure coin flipping to
fairly determine the resolution of competition between two avatars in
distributed virtual environments. He started his talk with an
introduction to some related DVE security researches and explained
subsequently the three-way message exchange included in PRP protocol.
Performance analysis of three different variations of PRP was
presented. John concluded that the choice of the PRP
variations was a trade-off between overhead and security compromise.
In response to Dongyan Xu's question about the reliability of the
proposed approach, John said that the PRP protocol assumed reliable
transmission between the avatars.
Wei Tsang asked about the possibility of extending the PRP
protocol to more than two parties. The response was that it was
possible to decompose the message exchanges between multiple parties,
but the extension would be challenging due to the increased
complexity and performance overhead.
Ardalan Kangarlou (Purdue University) asked whether there was any
mechanism that could validate the hash value used in
PRP. John answered the exchanged message itself
can be used to prove the correctness of the delivered information.
Cross-Tree Adjustment for Spatialized Audio Streaming over Networked Virtual Environments
Ke Liang presented an algorithm that constructs and adjusts
multicast trees for spatialized audio streaming in a peer-to-peer
overlay network. The algorithm has two objectives: Maximize total
number of audio receivers, subject to nodes' bandwidth limits; while
minimizing the average latency of those receivers. Ke interpreted
their solution to the problem as achieving a
compromise between the two objectives via maximizing the number of
receiver that have the minimum latency. Extending from their previous
work, Ke proposed the cross-tree adjusting (CTA) algorithm that can
re-allocate the upload bandwidth incrementally for nodes with
bandwidth conflicts in all existing multicast trees. Simulation
results showed that CTA can achieve good performance in terms of
fairness and the total number of successful receivers that have low
Klara Nahrstedt asked if the cocktail effect had been considered in
the proposed algorithm. The cocktail party effect describes the
human's ability to focus one's listening attention on a single talker
among a mixture of conversations and background noises. Ke agreed on
that the cocktail party effect could be something for further
investigation, although it was not covered by their current work. A
suggestion to their future work from Klara was that the distance
between avatars in the virtual world could be taken into account when
constructing the multicast trees.
Session 5: Security
Discussion Lead: Klara Nahrstedt (UIUC)
Scribe: Liang Ke (National University of
End-to-End Secure Delivery of Scalable Video Streams
Mohamed Hefeeda presented their work on the problem of securing the
delivery of scalable video streams so that receivers can ensure the
authenticity (originality and integrity) of the video. Their focus in
the paper is on recent scalable video coding techniques, e.g.,
H.264/SVC, which can provide three scalability types at the same
time: temporal, spatial, and quality (or PSNR).
Mohamed presented an efficient authentication scheme
that accounts for the full scalability of video streams, enabling
verification of all possible substreams that can be extracted and
decoded from the original stream.
Dick Bulterman (CWI) wondered if there is any standard fee
needed to be charged to scalable video coding techniques. The
presenter responded that they are using the open source standard and
there are a lot of companies (e.g. Google) that have already use the
standard and the reference source code.
Secure Multimedia Content Delivery with Multiparty Multilevel DRM Architechture
Tony Thomas (Nanyang Technological University) presented a joint
watermarking and cryptographic mechanisms for securely delivering
multimedia content on multiparty multilevel digital rights
management (DRM) architecture, where content are
delivered by an owner to a consumer through several levels of
The presenter emphasised that license is more important
than content since the content will be encrypted before delivery
from the owner or distributors.
The authors proposed a mechanism that minimizes the
possible degradation of the quality of a content due to
embedding of watermark signals. Furthermore, in case the owner or a
distributor finds an unauthorized copy, they can identify the traitor
with the help of a judge.
Philip Frey (IBM Research GmbH) asked what is the advantage of
multi-layer design. Tony answered that the
multimedia content is distributed by multiple distributors which will
encrypt the contents before distribution. From the client's point of
view, it could track the distributor according to the content it
Alexander Eichhorn (Simula Research) asked why multiple layers of
license servers are used. The presenter
responded that it is due to business issues. Since
clients may be geographically distributed, multiple
distributors that may reencrypt the content are used.
Thus, client need to request different licenses from
different license servers.
Rapid Identification of Skype Traffic
In this talk, the presenter presented results of experimental work
using machine learning techniques (decision tree) to rapidly
identify Skype traffic. They use a number of
feature classes to classify IP flows as being Skype or non-Skype
flows. The feature classes they found most effective were the
interarrival times between IP packets frequently occurring IP packet
lengths less than 80 bytes. Their results show that using three
feature classes in a single classifier provided 98 percent precision
and 99 percent recall when using a window duration of 5 seconds or
more. The presenter emphasizes that their classifiers do not rely on
observing any particular part of a flow. They also report on the
performance of classifiers built using combinations of two of these
features and of each feature in isolation.
Amir Hassan Rasti Ekbatani (University of Oregon) asked the presenter
how to detect audio flows. The presenter answered that their
approach works on small sequence of any part of flow. Since they
make use of techniques of machine learning, they can identify Skype
traffic reliably with only a few seconds of traffic, whose
characteristics are extracted from short sliding windows and used to
train a classifier. Amir also asked if the authors tried using larger
packet size, to which Philip answered that they only used
SVOPC codec so far with packet size less than 80 bytes.
Michela asked what happened when Skype traffic has both audio
and video data (i.e. packet size is increased).
The presenter answered they still do not
have a good solution since their classifier is trained
from pure audio flows.
Session 6: Understanding and Improving User Experience
Discussion Lead: Mohamed Hafeeda (Simon Fraser University)
Scribe: Wei Cheng (National University of Singapore)
An Empirical Evaluation of VoIP Playout Buffer Dimensioning in Skype, Google Talk, and MSN Messenger
In VoIP applications, buffer size needs to be carefully chosen to
trade-off between audio quality and delay. Finding optimal buffer
size is challenging because many factors are involved. According to
their experimental results, real-life VoIP applications, such as
Skype, MSN, and gTalk, do not adjust the buffer size well. They
proposed a regression-based lightweight algorithm to compute the
optimal buffer size in real time.
Wei Tsang pointed out that some VoIP applications may apply time
compression to drop the silent period. He asked whether time
compression might affect buffer size estimation.
Kuan-Ta Chen said that estimating the buffer size
according to time shifting is the only thing that can be done currently.
Finding out whether time compression is used by reverse engineering
may work better but needs large number of experiments.
Mohamed asked which factor affects the user experience most as many
factors exist. Kuan-Ta answered that it is still an open question and
his personal opinion is that the codec may be the most important
Fine-Grained Scalable Streaming from Course-Grained Video
In scalable video streaming, the granularity of bit rate adaptation
with CGS is limited, while using MGS brings high signal overhead.
Pengpeng Ni proposed that by switching between layers quickly,
arbitrary bandwidth could be achieved without high
overhead. To understand and compare the user experience when
different switching patterns are used, the authors conducted a user study.
The result shows that frequent layer switching
is effective and could be better than quality downscaling, with
the switching period as a crucial design parameter.
Damien Le Moal asked whether the nominal frame rate is considered.
Pengpeng answered that only 25 and 12 fps is used in
user study as covering different frame rate variations in the
user study is time-consuming.
Saurabh asked whether the consistency of the user response is
considered. Pengpeng answered that during binary measurement, two
sequence may appear twice with different order (A, B and B, A), so it
could be used to test the consistency of user response.
Padmanabhan asked whether experiments are done on CRT since
LCD, which has lower refresh rate, may reduce the negative effect of
low frame rate. Pengpeng pointed out since the field study is done on
iPhone, only LCD is used.
Mohamed asked whether the lighting condition is considered and
whether the experiments are done following the ITU regulation.
According to Pengpeng, they find field study more interesting
than experiments in a controlled lab since the former is closer
to the real world usage.
Estimate and Serve: Scheduling Soft Real-Time Packets for Delay Sensitive Media Applications on the Internet
Ishan highlighted that scheduling at intermediate nodes is essential
in soft real-time applications.
he briefly introduced the traditional IntServ and DiffServ and their
disvantages, Ishan proposed a new method that compares the estimated
transmission time and deadline in each node and schedules the packet
with shortest per-hop time first. The main advantage of this method
is (i) bursts are better handled, (ii) packets cannot meet the
deadline are discarded to avoid wasting bandwidth, and (iii) the server
Alexander asked which router is used in the implementation
since this method requires clock synchronization and other computation.
Ishan answered that currently only Linux machines with IP forwarding
enabled are used, but they plan to use Cisco routers in the future.
Padmanabhan asked about misbehaving node that reports false
deadline requirement to gain advantage. Ishan argued that packets that
cannot meet deadline will be dropped, so a misbehavior flow may be
dropped to benefit other flows.
John Miller asked how to obtain the value of TTL and deadline. Ishan
answered that TTL value can be measured by nodes in the first round
trip transmission and the deadline is decided according to the
requirement of the application.
Session 7: P2P Streaming II
Discussion Lead: Dongyan Xu (Purdue University)
Scribe: Amir Hassan Rasti Ekbatani (University of Oregon)
Zebroid: IPTV Data Mining to Support Peer-Assisted VoD Content Delivery
Robin opened his talk by emphasizing on the difference
between Internet TV and IPTV. The former is applied to the best effort
delivery of video content over a broadband connection and the latter
is defined by ITU-T to provide security, reliability, and interactivity.
He showed AT&T's
architecture for IPTV, where optical
media connects the video source to the DSLAM switch, which are connected
to models using high bandwidth DSL links.
The uplink bandwidth of a DSLAM switch is 1 Gbps and thus only
supports a small number of IPTV users concurrently.
The speaker then explained their proposed peer-assisted system for IPTV, Zebroid, where
popular video-on-demand content is pre-striped on user set-top boxes (STB) during
idle hours. The STBs will then form a peer-assisted streaming overlay
to deliver the VoD content to each interested user during the peak hours.
Zebroid also tries to predict the probability of a node failure which
happens when an STB is turned off and provision a level of redundancy
The simulation-based evaluation shows that their proposed peer-assisted
delivery is only possible for the 8Mbps and 12 Mbps neighborhoods and only
when the number of requesting peers is at most 8.
Robust Live Media Streaming in Swarms
Remo Meier started by talking about the challenges introduced by the
P2P application model, including peer heterogeneity, robustness
against randomness and selfishness, as well as system's fairness
The presenter mentioned the features of a hypercubic overlay including
simplicity of construction and maintenance as well as neighbor
Remo then presented the proposed P2P streaming mechanism in which
the peers form
multiple hypercubic trees and push the content blocks down the tree
to reach a quarter of the peers. In the next phase, with a
BitTorrent-like pull-based mechanism, the content is distributed
among all peers. Using simulation-based evaluation, Meier showed
that their proposed system successfully limits free riders.
Peers who share too few packets often suffer from underflows. He
suggested that by using layered audio/video codecs, it is possible to
accommodate heterogenenous peer groups.
Providing Statistically Guaranteed Streaming Quality for Peer-to-Peer Live Streaming
Lisong Xu opened his talk by positing that
a common problem for P2P streaming was the ``best-effort'' quality.
Due to user dynamics and system complexity, achieving a
guaranteed quality is hard.
The speaker defined the paper's goal as
providing statistical guarantee on the overall upload bandwidth of a
P2P streaming system using admission control.
The paper uses a queueing model used based on heterogeneous
upload bandwidth and includes user dynamics as well as admission
control, and defines the statistical
guarantee problem as deciding whether to admit a new peer
to maintain the probability of having enough upload
Toward this end, they show that a dynamic admission control algorithm
that exploit state information leads to
lower rejection rate with the same level of guarantee.
The speaker also explore admission control's sensitivity to user lifetime
distribution and user arrival process.
and concluded that there is a
fundamental tradeoff between user rejection rate and user behavior
The session continued with a question/answer panel including all three speakers.
Mohamed asked the last speaker
where the admission control algorithms were to be executed. Lisong pointed out
that their paper had assumed a central entity responsible for admission control.
Mohamed then asked the first speaker whether in Zebroid provides
multiple encoding rates or scalable oding for the same TV channel
to accommodate users who want to receive HD or SD
versions of each channel. Robin Chen was not sure whether
multiple encoding rates were provided, yet he pointed out that in future
such schemes would be necessary to provide IPTV service to mobile devices.
Ardalan asked Remo about the
possibility of unbalanced delivery tree.
Remo responded that due to randomness of IDs, unbalanced tree is rare,
however, their algorithm periodically ensure that the tree is balance.
raised the concern of energy consumption at the set-top boxes
in Zebroid during the idle hours when the boxes would be otherwise off
or stand-by. Robin Chen explained that the set-top boxes have the potential
to be the next battlefield for a variety of applications similar to cell
phones and could stay on most of the time.
Hassan asked Robin about the mention of erasure code in his presentation.
Robin clarified that erasure
code is necessary to compensate for the peers that are not up
during the content push time or at the peak hour. However, such mechanism
was not implemented into Zebroid yet.
Saurabh asked Robin about the level of peer outage considered.
Robin explained that they assume
80% of the peers are up at all times, and this rate is based on
Mohamed asked Robin if data pushed to each client has any
correlation with the usage history of that user. Chen responded that
they plan to include a mechanism to predict the usage of each user
and consider that prediction in pushing the data and then refining
the predictions gradually with the user's choices. Discussion ensued
about the privacy issues on this approach.
Philip asked Remo about ID conflict between clients since IDs are assigned
randomly. Remo Meier explains that although the probability is low,
they had proposed a mechanism to ensure that does not happen in another
Dongyan then asked the panelists how they assessed their proposed
systems' robustness. Robin mentioned that they could configure the system
to ensure that the user had no access to the set-top box, while Remo pointed out that
their approach designed each peer's behavior to be as greedy as
possible, therefore they would not expect any manipulation to change the
code to make it more selfish.