H.320 Videoconferencing
The International Telecommunication Union's (ITU's) H.320 standard is applied mostly to dedicated circuit-based
switched network (point-to-point) connections of moderate
or high bandwidth, such as through the medium-bandwidth ISDN
digital
phone protocol
or a fractionated high bandwidth T1 lines.
- Bandwidth. The most common media
is ISDN. Each 128Kbps ISDN line requires use of 2
phone lines
(each 64Kbps) in
parallel.
The
industry norm is to use 3 ISDN lines (i.e. connect at 3x128Kbps
= 384
Kbps), which is also a quarter of a 1.44 Gbps T1 line.
This normally gives near-TV level quality (30 fps, CIF or
4CIF image).
We have 4 ISDN lines in one two rooms with Polycom systems,
and for the demo you saw how clear both sound and video are
at this bandwidth. There is thus no reason to go above 512Kbps
for videoconferencing, or for H.320, below 128Kbps.
- H.320 video. Support for the
ITU H.261
standard is required, and H.263 is optional. In addition
to these rather conservative but effective (for higher bandwidth)
codecs, the
default for the standard in image quality is CIF (354x288
pixels with color, though "high definition" 4CIF of 708x576
is also supported), and for frame rate is
30 fps (but can be
15 fps or values between these extremes such as 20 fps).
The lower frame rates are more relevant if the connection
is only 128 Kbps or 256 Kbps. The standard also support multiple
cameras, and control of zoom/pan/tilt for both local and
remote cameras.
- H.320 audio. Targets targets
voice (e.g., 50 Hz to 4 KHz). Codecs include the more basic
G.711 (pulse code modulation requiring
48-64 Kbps of the connection) and the newer higher quality
G.722 (7 KHz voice with 64/32/16 Kbps). Clever lower-bandwidth
codecs with more aggressive compression (such as G.723.1
at 5.3 or 6.3 Kbps and G.728 (coding at 16 Kbps using low-delay
linear prediction)) are more relevant for H.324
systems.
- H.320 shared data. The
optional T.120 standard includes a shared whiteboard, chat,
and file transfer. See the H.323 discussion for more details.
- H.320 control features. In addition
to zoom/pan/tilt, there are connection protocols, prioritization
for bandwidth distribution between audio, video and data
(audio has highest priority), and specifications for the
structure of multipoint connections.
Multipoint connections distribute equal bandwidth. For instance,
with our 512Kpbs capacity, we'd coordinate a
four site conference with each connection at 128Kbps, and a
three-site conference at at 256Kbps (as long as each of the
two other sites can handle 256Kbps).
Throughout
the
1990s,
federal
grants
programs
helped
support the implementation of T1 hub-spoke networks to rural
communities, primarily for civic, educational and health
needs. These were initially very expensive, and because of
lack of interoperability between major vendors each state would
have to pick one company. As systems H.320 emerged,
this all changed. The
H.320
protocol
now sees
widespread
use for conferencing
of
all types,
ranging
from
telemedicine
consultations
between a tertiary and rural hospital to our routine use
of H.320 for our RERC meetings. It is the pillar for most
hub-spoke networks.
For telehealth applications, an especially
useful feature is both
local
and
remote control of cameras -- remote control by specialists
is part of the reimbursement guidelines for the United States’ Medicare
program.
Virtually all of the newer H.320 products are also
H.323 compliant, and can facilitate transmission of one
or more channels of data (e.g., signals, images, records, presentations).
This is true, for example, with our Viewstation MP systems
(Polycom).
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