Video streaming on e-lab

Video Streaming
Rafael Bagagem Henriques
rhenriques@ipfn.ist.utl.pt

e-lab
Instituto Superior Técnico
Lisboa, Portugal
http://elab.ist.utl.pt

R. B. Henriques | Lisbon, 23 March 2012 | Video Streaming

Streaming multimedia
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Multimedia that is constantly received by, and
normally displayed to, the end-user
A media stream can be on demand or live
Usually applied to media that are distributed
over telecommunications networks
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Real world examples
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2

Radio and TV are inherently streaming
Youtube, on-line TV and Radios, etc.


OSI Model
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A set of seven layers that define the different stages
that data must go through to travel from one device to
another over a network


OSI Model – layers and
instances
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4

Layer – a collection of conceptually similar functions that provide
services to the layer above and receives service from the layer below.
Instance – on each layer an instance provides services to the
instances at the layer above and requests service from the layer
below; conceptually two instances at one layer are connected by a
horizontal protocol connection on that layer.


OSI Model – layers
description
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5

Physical Layer – electrical and physical
specifications (layout of pins, voltages, cable
specifications, hubs, network adapters …
Data Link Layer – functional and procedural means
to transfer data; detect/correct errors on Physical.
Network Layer – functional and procedural means
of transferring variable length data via one or more
networks; performs network routing functions;
perform fragmentation/reassembly, and report
delivery errors; maintain the QoS requested by the
Transport.
Transport Layer – provides transparent transfer of
data between end users, providing reliable data
transfer services to upper layers; controls reliability
of a given link through flow control, segmentation
and de-segmentation, and error control; keep track
of the segments and retransmit those that fail.

OSI Model – layers
description
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6

Session Layer – controls the dialogues between
computers; establishes, manages and terminates
the connections between the local and remote
application; provides for full-duplex, half-duplex, or
simplex operation, and establishes check-pointing,
adjournment, termination, and restart procedures.
Presentation Layer – independence from
differences in data representation (e.g., encryption)
by translating from application to network format,
and vice versa; works to transform data into the form
that the application layer can accept; formats and
encrypts data to be sent across a network, providing
freedom from compatibility problems.
Application layer – interacts with software
applications that implement a communicating
component; functions include identifying
communications partners and resource availability,
and communications synchronization.

Transport Layer (I)
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Most common protocols for media broadcast are UDP and TCP

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User Datagram Protocol (UDP)
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Unordered - the order in which they arrive cannot be
predicted.

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Lightweight - Small transport layer designed on top of IP.

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Unreliable: no concept of acknowledgement, retransmission
and time-out

Datagrams - Packets are sent individually and are
guaranteed to be whole if they arrive.


Transport Layer (II)
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Transmission Control Protocol (TCP)
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Reliable: manages message acknowledgement, retransmission
and time-out

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Ordered - the order is well known.

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Heavyweight - Large transport protocol designed on top of IP
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TCP requires three packets just to set up a socket, before any
actual data can be sent....

Streaming - nothing distinguishing where one packet ends and
another begins. Packets may be split or merged into bigger or
smaller data streams arbitrarily.


Streaming Media
Protocols
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MMS – Microsoft Media Services
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Deprecated in favour of RTSP
UDP or TCP

RTP/RTSP – Real Time Streaming Protocol
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PNM/PNA
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Real Audio/Networks proprietary

RTMP – Real Time Messaging Protocol
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Open, widely used

Proprietary protocol developed by Adobe Systems

Broadcasting raw data
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The number of bytes produced in one second is
given by:
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10FPS with 320 x 240 RGB image:

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Image width x Image height x Colours (typically
RGB=3 at 8 bits) x number of frames per second
(FPS)
2304000 bytes/s ~ 2.3MB/s ~ 18432000 bits/s ~
18.4Mbits/s


Problems with this
approach
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Typically a user has at home a bandwidth of
something between 2 and 24 Mbits/s
If 10 users are connected the bandwidth on the
server grows to 180Mbits/s!!!
Not so good solutions
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lower the image size
Use only one color (gray scale)


Solution
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Compress the image

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Two approaches
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Compress each image

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Compress over time
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This is clearly the most clever solution since most of the
time the images don't change that much!

CODECs are specialized algorithms to perform
this work


Frame compression
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Compress each frame:

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Compress over time:
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The difference between two the successive frames


Video compression
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Lossless
—
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Ideal but not feasible for video broadcast. The
bandwidth is still to high
Examples are: PNG, JPEG 2000, ZIP

Lossy
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Define the best relation between quality loss and
data reduction
Find a format which allows to maximize the
previous point


Lossy compression
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The big trick:
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Remove information from video that is not important
for human perception in order to achieve very high
compression rates while still keeping very good
visual quality.

Some video formats allow ratios of 200:1 !


Common Video Coding
Formats
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H.261

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H.263

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H.264
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x264

MPEG-4 AVC (Advanced Video Coding)
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Xvid, FFmpeg, DivX, x264...

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WMV
RealVideo

Why x264?
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Open source
H.264/MPEG-4 AVC video CODEC. Lots of
advanced encoding features

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200:1 ratios and more...

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Available for free in every operating system

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Supported by most popular video players:
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Quicktime, VLC, Windows Media Player 12,
mplayer


Video sizes
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QQVGA 160 × 120

quarter of a quarter of VGA

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HQVGA 240 × 160

half of a quarter VGA

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QVGA

320 × 240

e-lab videos (quarter of VGA)

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VGA

640 × 480

VGA

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VGA (Video Graphics Array)

XGA

1024 × 768

Extended Graphics Array


Best bit rates
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As discussed before, the output of the CODEC
must maximize the quality of the image at the
lowest bandwidth
8 kbit/s — telephone quality (using speech
codecs)

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5 Mbit/s — DVD quality

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120kbit/s — elab videos
15 Mbit/s — HDTV quality


Multicast vs Unicast
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Multicast
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It is the perfect solution... but NO Internet Provider
will allow multicast on its networks

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Streams the same media to all the receivers

It only works in private networks

Unicast
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Multiple connections for the same stream
The only solution


Streaming servers
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Microsoft Media Services
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Not free
In the past only support the MMS protocol (now uses RTSP)

Adobe Flash Media Streaming Server 3
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Not free

Darwin Streaming Server (DSS)
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Shares the same code base as QuickTime Streaming Server

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Open source
Streams MPEG-4


e-lab structure diagram
Video Streaming Server

22


DSS – Reflector
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Allows to deliver live broadcast to clients
Client connects to DSS which then reflects the
connection to the desired broadcast
Separate program sends RTP packets to DSS
with the video
UDP packets are sent from the source
DSS makes the bridge between the
broadcaster and the client


Session Description
Protocol (SDP)
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DSS stores files, describing each broadcaster, saved with the
SDP format
Most important information:
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Media type (payload)

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Bit rate

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Port of the source

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IP of the source

Title

There are programs which automatically generate these files


SDP Example
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v=0

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o=- 1204476969191332 1204476969191334 IN IP4 192.168.0.123

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s=GAMMA

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b=RR:0

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t=0 0

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a=tool:vlc 1.1.13

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a=type:broadcast

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m=video 2026 RTP/AVP 96

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c=IN IP4 192.168.0.2

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b=AS:80
(red lines can be optional)

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Connection flow
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Client connects to DSS with an URL like:
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rtsp://elabmc.ist.utl.pt/gamma.sdp

DSS parses the SDP file and connects it to the desired
source (typically an UDP port)
On the previous example the video is being sent to the
port 2026 of the 192.168.0.2 from 192.168.0.123
Client also uses the SDP file to check the media type
(in the previous case 96=MPEG4)


Video broadcast
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In order to send the video to the streaming
server one needs
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Drivers

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Generic API to access the images from the
cameras

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Encoder

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Cameras

Broadcaster


Cameras
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In the case of remote laboratories, commercially
available webcams are the perfect solution
Problem is that a lot of webcams only work under
Windows
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Costs money

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Imply encoding under Windows
Highly unreliable

Some webcams are supported quite well under Linux


Video 4 Linux (V4L)
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Video capture API for Linux;

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Unifies the way programs access the cameras

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Closely integrated with the Linux kernel

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V4L is in its second version (V4L2)
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Include a compatibility mode for V4L1
Better use V4L2 native devices/drivers


Linux webcams
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Look for cameras which already have a driver
in the kernel tree
Camera drivers must support native V4L2
All the cameras supported by the PWC (Phillips
USB Webcam Driver for Linux) project are a
good choice
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Philips
Logitech Quickcams


Video encoding
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The ideal program grabs the video using V4L2

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Encodes using MPEG-4

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Sends the data using RTP to the streaming
server
Automatically writes an SDP file which can be
placed on the DSS
All these features are present on the VLC
project


VLC
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Open-source

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VLC live streaming
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audio/video capture utility that can capture and
encode audio and video in real-time
results can be written to either an .mp4 file,
transmitted onto the network via either unicast or
multicast, or both simultaneously


VLC internals
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Download source from
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http://www.videolan.org/vlc/
Configure, make and install

Gentoo Linux OS:
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emerge vlc

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USE=”X ffmpeg libv4l2 live rtsp stream v4l2 x264”

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Each broadcaster uses a specific configuration command

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The program is launched with the following syntax:
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vlc + “configurations” (with GUI)
cvlc + “configurations” (without GUI)


VLC configurations
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Entire command

su elab --command "cvlc -vvv v4l2:///dev/video1 :input-slave=alsa:// :v4l2-standard=0 :file-caching=300
--sout '#transcode{vcodec=h264,vb=80,fps=10,scale=1,width=320,height=240,acodec=none}'
':rtp{proto=udp,dst=192.168.0.2,port-video=20002,ttl=127,name=SCUBA,sdp=file:///home/elab/webcam_streaming/sdps/scuba.sdp}'
--no-sout-audio" &

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Parts of the command
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Open the webcam:
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Stream output (transcoding):
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':rtp{proto=udp,dst=192.168.0.2,port-video=20002,ttl=127,name=SCUBA,
sdp=file:///home/elab/webcam_streaming/sdps/scuba.sdp}'

No audio:
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'#transcode{vcodec=h264,vb=80,fps=10,scale=1,width=320,height=240,acodec=none}'

RTP and SDP configurations:
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cvlc -vvv v4l2:///dev/video1 :input-slave=alsa:// :v4l2-standard=0 :file-caching=300

--no-sout-audio

Launching VLC at
e-lab computer cluster
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Each broadcaster have a script file with VLC
commands corresponding to the respective webcams
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Information about the streaming port number is
located in a web-page:
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/etc/local.d/vlc.start

elab1.ist.utl.pt

All broadcaster execute the script file located in their
/etc/local.d directory by executing local daemon:
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/etc/local.d/vlc.start

which is executed by:

/etc/init.d/local start

Remarks
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This set-up allows to have a complete, opensource and free video broadcasting solution
It can be easily adapted to broadcast other kind
of sources (stored files, movies, music, etc.)
Required software
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Darwin Streaming Server

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Linux installation
VLC


The final picture

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Video streaming on e-lab

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