We will take a closer look at these issues.
Video Networking - quest for speed
The VoD network consists of one or more video server systems connected
to home set-top-boxes via high speed backbone networks.
There would be local servers that contain a database holding current
popular video(to ease net traffic on the backbone). The set-top-box
serves two functions:
Sending the customer's demands to the local server
Processing video directed to the home by the local server
The customer is able to request operations on the video such as video
selection, pause, rewinding etc. These are processed by the set-top-box
and sent to the local server. The local server processes the request if
possible, otherwise it relays the request to a video archive server,
much as a hierachy system.
Digital video/audio can be compressed and stored on hard disk and
advertised for users on the network.
Multiple archive servers can simultaneously be running over the network
depending on the bandwidth available.
There are several viable ways of networking Video-on-Demand.
The major schemes are: ISDN, ADSL and ATM.
ISDN (Integrated Services Digital Network)
ISDN is intended to replace POTS and differs in
the telephone company central switches, software and other equipment.
It allows a single wire or optical fiber to carry voice, digital network
services and video in digital form rather than analog (POTS).
An ISDN-2 BRI (Basic Rate Interface) line consists of two B-channels (bearer)
that provide 64 kbps (kilobits per second) transmission speed, or
a total of 128 kbps data per BRI line. It also has a D-channel (delta)
for control and syncronisation.
The popular V.32bis modem operates at 14.4 kbps or approximately one-tenth
the speed of ISDN. With the same modem, ISDN, being digital (POTS is analog)
provides a clearer signal than analog. "Noisy" POTS analog lines reduce
14.4 maximum modem bandwidth.
ADSL (Asymmetric Digital Subscriber Loop)
ASDL runs over twisted pair copper(POTS) and transmits over 6.00
Mbit/s(Simplex) or 576 Kbit/s(Duplex), that is, it would support a voice call
(or fax), Video-on-Demand, Internet access and video conferencing over
a single line.
ADSL is important as regional fibre lines won't be available for another
40 years, as laying new cables take time. Companies researching on VoD are
taking ADSL very seriously.
ATM (Asynchronous Transfer Mode)
ATM is a current research hot topic.
It is a method for dynamic allocation of bandwidth using a fixed 53 byte
(of which 5 form the header) packet (cell), known also as "fast packet".
The cells use charactoristics of both time-division-multiplexing
of transmission media, and packet
switching of data networks. A "virtual path" is set up
through the involved switches when two endpoints wish to communicate.
This provides a bit-rate independant protocol that can be implemented
on several network types.
The encryptor operates at speeds ranging from 1.544 Mbps, with a
capability to evolve to 622 Mbps.
ATM speeds could operate up to 2.2 Gbps over a cell-switched network,
being limited to the medium, since its technology os not tied to
any specific data rate.
However, ATM requires wideband fiber/coax cables and is currently
being implemented for national back-bone and long-distance carriers.
Types of Communications Networks
| Type |
Copper Bitrate
(Mbit/s) |
Fiber Bitrate
(Mbit/s) |
Cable requirements |
| ISDN |
0.128 (BRI) |
2.00 (PRI) |
Copper/Coax/Fibre |
| HDSL |
0.800 |
2.00 |
Copper/Coax/Fibre |
| ADSL |
1.536 |
- |
Copper |
| ADSL 2 |
6.00 |
- |
Copper |
| ATM |
?51.0 |
155 (?future 2500) |
Copper/Coax/Fibre |
Media Formats - setting a standard
There are several digital video formats used currently. These incorporate
high compression factors, bearing in mind that video will be transmitted
over relatively narrow bandwidths. The major formats are:
Apple Quicktime
Apple is working with a number of partners on the development of an interactive
television set-top box, based on the PowerPC processor. In addition, Apple is
developing its own interactive television set-top box, based on QuickTime
architecture, that is currently being tested by:
MPEG (Moving Pictures Experts Group)
MPEG is a group of people that meet under
ISO (the International Standards Organization). They generate standards for
digital video (sequences of images in time) and audio compression.
MPEG works jointly with the ITU-TS for ATM Video Coding. MPEG also
collaborates with representatives from EBU, ITU-RS, SMPTE, and the North
American HDTV community.
MPEG-1 was originally designed for delivery of video to consumer devices at
single speed CD-ROM data rates, and is therefore lower
resolution and lower quality than MPEG-2, which was designed for delivery
of broadcast and HDTV quality video. MPEG-4 is an ongoing Very-Low-Bitrate coding
that explores new algorithms including fractal compression and morphing.
MPEG standards comparison
| |
Bitrate |
Application |
| MPEG-1 |
1.15 Mbit/s |
CD-ROM video (150kbytes/sec) |
| MPEG-2 |
4.00 Mbit/s |
broadcast and HDTV video |
| MPEG-3 |
- |
dropped (HDTV) |
| MPEG-4 |
<64 Kbit/s |
Very-Low-Bitrate applications |
MPEG-1 is actually inadequate because it does not contain timing information to
regulate frame rate. It assumes synchronous delivery, hence applicable to
home CD-ROM video only.
MPEG video has a compression ratio of about 26:1 while that for
audio goes up to 7:1.
Real time MPEG encoding(and decoding, of course) hardware exist, costing
from US$10000.
MPEG-2 becomes the Standard
The Grand Alliance was formed in May 1993 by seven organizations (AT&T Corp., General
Instrument Corp. (GI), Massachusetts Institute of Technology (MIT), Philips Consumer
Electronics, David Sarnoff research center, Thomson, Zenith Electronics Corp.) to evaluate
technologies and to decide on key elements that will be at the heart of the best of the best
HDTV system. They agreed on the MPEG-2 Video and Systems syntax with interlaced
and progressive modes.
A consortium of 85 European companies also signed an agreement
in 1993 to fund a video broadcasting project using MPEG-2.
It is likely that MPEG-2, and possibly MPEG-4 (when released) will become
international standards for full-motion video.
Summary
Technology
The types of network suitable for Video-on-Demand are ADSL and ATM.
ISDN (the highest currently is PRI) does not meet VoD bandwidth, but it is suitable for video conferencing.
To date, trials are conducted fairly extensively on ADSL and ATM, though
eventually large scale VoD would require ATM and the laying of fibre cables.
Quicktime video is less commonly used (2 known trials) compared to
MPEG (over 18).MPEG-1 and MPEG-2 are standards for full-motion compression,
but MPEG-2 is the common standard for current trials and can be seen as the
international digital video format.
MPEG-4, a proposed Very-Low-Bitrate standard, has also started work, and could
lay the way for new coding standards.
Conclusion
The concept of Video-on-Demand is certain, though there are several paths
it may follow. Looking at current research and trials, VoD is likely to be
initially implemented over ADSL technology until ATM becomes available. MPEG
has conquered huge grounds as far as standards go and make great efforts
to ensure that they remain as international standards for moving picture coding.
Video-on-Demand is set to become the future home entertainment system, though it
isn't about to happen 'by storm' before ATM is realized. Much work is ongoing,
and we should expect to see results within the next two years!
Sources
Comments and suggestions to DP Quek (d.quek@ic.ac.uk)
Article 2
A New kind of Entertainment
A New kind of Entertainment
How are the Clients(set-top-boxes) and Server(Central video-base) networked?