| Online Conferencing Nuts & Bolts
BY TIM MAY
To attend a meeting or conference, most of us either walk down the hall to the nearest
conference room or make a long-distance call to an audio bridge provider. With the
development of the Internet, however, these practices are quickly becoming something of
the past. Over the next few years, increasing numbers of business meetings will occur
online through IP audio, video, and data technologies. However, while most people in the
computer industry have heard of online conferencing, few really understand what it is and
fewer still how it works.
The H.323 and T.120 series standards constitute the most significant technical issue in
conferencing because they are what will ensure the success of this type of application. As
in the case of fax machines, no matter how exciting the technology, until products from
different vendors could interoperate, faxing would never gain acceptance as a mainstream
business application. In conferencing, too, mainstream use of the Internet for
communication and collaboration is dependent on the ability of conferencing products to
seamlessly work with each other.
WHAT IS H.323?
H.323 is an umbrella recommendation from the International Telecommunications Union
(ITU) that sets standards for multimedia communications over local-area networks (LANs)
that do not provide a guaranteed Quality of Service (QoS). It is the newest member of a
family of ITU recommendations that cover video telephony and multimedia communications
over a variety of pipelines. Known as H.32X, this series includes H.320 and H.324, which
address ISDN and PSTN communications, respectively.H.323 is in many senses a derivative of H.320, a 1990 umbrella recommendation for video
telephony over switched digital telephone networks. H.323 borrows heavily from H.320's
structure, modularity, and audio/video codec recommendations.
The ITU's Study Group 16 approved Version 1 of the H.323 specification in 1996 and
Version 2 in February of 1998. The standard is broad in scope and includes both
stand-alone devices and embedded personal computer technology as well as point-to-point
and multipoint conferences. It addresses call control, multimedia management, and
bandwidth management for point-to- point and multipoint conferences. H.323 also addresses interfaces between LANs and other networks.
ARCHITECTURAL OVERVIEW
The H.323 standard defines four major components for a network-based communication
system: Terminals, Gateways, Gatekeepers, and Multipoint Control Units (MCUs).
Terminals
Terminals are the client endpoints on the LAN which provide for real-time, two-way
communications. All terminals must support voice communications; video and data are optional. H.323
specifies the modes of operation required for different audio, video, and/or data
terminals to interwork.
All H.323 terminals must also support H.245, which is used to negotiate channel usage
and capabilities. Three other components are required: Q.931 for call signaling and call
setup; a component called RAS (Registration/ Admission/Status) which is a protocol used to
communicate with a Gatekeeper; and support for RTP/RTCP for sequencing audio and video
packets.
Optional components in an H.323 terminal are video codecs, T.120 data conferencing
protocols, and MCU capabilities.
Gateways
The Gateway is an optional element in an H.323 conference. Gateways provide many
services, including a translation function between H.323 conferencing endpoints and other
ITU-compliant terminals. This function includes translation between transmission formats
(for example, H.225.0 to H.221) and between communications procedures (for example, H.245
to H.242). In addition, the Gateway translates between audio and video codecs and performs
call setup and clearing on both the LAN side and the switched circuit network side.
In general, the purpose of the Gateway is to reflect the characteristics of a LAN
endpoint to an SCN endpoint, and vice versa. The primary applications of Gateways are
likely to be:
- Establishing links with analog PSTN terminals.
- Establishing links with remote H.320-compliant terminals over ISDN-based switched
circuit networks.
- Establishing links with remote H.324-compliant terminals over PSTN networks.
Gateways are not required if connections to other networks are not needed, since
endpoints may communicate directly with other endpoints on the same LAN. Terminals
communicate with Gateways using the H.245 and Q.931 protocols. With the appropriate
transcoders, H.323 Gateways may support terminals that comply with H.310, H.321, H.322,
and V.70.
Many Gateway functions are left to the designer. For example, the actual number of
H.323 terminals that can communicate through the Gateway is not subject to
standardization.Similarly, the number of SCN connections, number of simultaneous independent
conferences supported, the audio/video/data conversion functions, and inclusion of
multipoint functions are left to the manufacturer. By incorporating Gateway technology
into the H.323 specification, the ITU has positioned H.323 as the glue that holds the
world of standards-based conferencing endpoints together.
Gatekeepers
Gatekeepers perform two important call-control functions that help preserve the
integrity of the corporate data network.The first is to address translation from LAN aliases for terminals and gateways to IP
or IPX addresses as defined in the RAS specification. The second function is bandwidth
management, which is also designated within RAS. For instance, if a network manager has
specified a threshold for the number of simultaneous conferences on the LAN, the
Gatekeeper can refuse to make any more connections once the threshold is reached. The
effect is to limit the total conferencing bandwidth to some fraction of the total
available; the remaining capacity is left for e-mail, file transfers, and other LAN
protocols.The collection of all Terminals, Gateways, and MCUs managed by a single Gatekeeper is
known as an H.323 Zone (Figure 4).
While a Gatekeeper is logically separate from H.323 endpoints, it is possible that
vendors may incorporate Gatekeeper functionality into the physical implementation of
Terminals, Gateways, or MCUs.
A Gatekeeper is not required in an H.323 system. However, if a Gatekeeper is present,
it is mandatory that terminals make use of the services offered by Gatekeepers. These
services, which are defined in RAS, include address translation, admissions control,
bandwidth control, and zone management.
Gatekeepers can also play a role in multipoint connections. To support multipoint
conferences, users would employ a Gatekeeper to receive H.245 control channels from two
terminals in a point-to-point conference; when the conference switches to multipoint, the
Gatekeeper can redirect the H.245 control channel to a multipoint controller, the MC. The
Gatekeeper need not process the H.245 signaling; it only needs to pass it between the
terminals or between the terminals and the MC. A valuable alternative would be for the
Gatekeeper to route the H.245 streams between endpoints, thus providing greater control
over the call.
LANs that contain Gateways should also contain a Gatekeeper to translate incoming E.164
addresses into Transport Addresses. Because a Zone is defined by its Gatekeeper, H.323
entities that contain an internal Gatekeeper require a mechanism to disable the internal
function so that when there are multiple H.323 entities that contain a Gatekeeper on a
LAN, the entities can be configured into the same Zone.
Multipoint Control Units
The Multipoint Control Unit (MCU) supports conferences between three or more endpoints.
Under H.323, an MCU consists of a Multipoint Controller (MC), which is required, and zero
or more Multipoint Processors (MP). The MC handles H.245 negotiations between all
terminals to determine common capabilities for audio and video processing. The MC also
controls conference resources by determining which, if any, of the audio and video streams
will be multicast.
The MC does not deal directly with any of the media streams. This is left to the MP,
which mixes, switches, and processes audio, video, and/or data bits. MC and MP capabilities can exist in a dedicated component or as part of other H.323
components.
MULTIPOINT CONFERENCES
Multipoint conference capabilities are handled in a variety of methods and
configurations under H.323. The recommendation uses the concepts of centralized and
decentralized conferences, as described in Figure 5.
Centralized multipoint conferences require the existence of an MCU to facilitate a
multipoint conference. All terminals send audio, video, data, and control streams to the
MCU in a point-to- point fashion. The MC centrally manages the conference using H.245 control
functions that also define the capabilities for each terminal.
The MP does the audio mixing, data distribution, and video switching/mixing functions
typically performed in multipoint conferences and sends the resulting streams back to the
participating terminals. The MP may also provide conversion between different codecs and bit rates, and may use
multicast to distribute processed video. A typical MCU that supports centralized
multipoint conferences consists of an MC and an audio, video, and data MP.
Decentralized multipoint conferences make use of multicast technology.
Participating H.323 terminals multi-cast audio and video to other participating
terminals without sending the data to an MCU. Note that control of multi-point data is
still centrally processed by the MCU, and H.245 control channel information is still
transmitted in a point-to-point mode to an MC.
Receiving terminals are responsible for processing the multiple incoming audio and
video streams. Terminals use H.245 Control Channels to indicate to an MC how many
simultaneous video and audio streams they can decode. The number of simultaneous
capabilities of one terminal does not limit the number of video or audio streams that are
multicast in a conference. The MP may also provide video selection and audio mixing in a
decentralized multipoint conference.
COMMUNICATIONS UNDER H.323
Communications under H.323 can be considered a mix of audio, video, and control
signals. Audio capabilities, Q.931 call setup, RAS control, and H.245 signaling are
required. All other capabilities, including video and data conferencing are optional. When
multiple algorithms are possible, the algorithms used by the encoder are derived from
information passed by the decoder during the H.245 capability exchange.
H.323 terminals are also capable of asymmetric operation (different encode and decode
algorithms) and can send/receive more than one video and audio channel.
Control
The call-control functions are the heart of the H.323 terminal. These functions include
signaling for call setup, capability exchange, signaling of commands and indications, and
messages to open and describe the content of logical channels. All audio, video, and control signals pass through a control layer which formats the
data streams into messages for output to the network interface. The reverse process takes
place for incoming streams. This layer also performs logical framing, sequence numbering,
error detection, and error correction as appropriate to each media type. The Q.931, RAS,
and RTP/RTCP protocols perform these functions.
Overall system control is provided by three separate signaling functions: the H.245
control channel, the Q.931 call signaling channel, and the RAS channel.
The H.245 control channel is a reliable channel that carries control messages governing
operation of the H.323 entity, including capabilities exchange, opening and closing of
logical channels, preference requests, flow control messages, and general commands and
indications. Capabilities exchange is one of the fundamental capabilities in the ITU recommendation;
H.245 provides for separate receive and transmit capabilities as well as for methods to
describe these details to other H.323 terminals. There is only one H.245 control channel per call.
The call signaling channel uses Q.931 to establish a connection between two terminals.
The RAS signaling function performs registration, admission, bandwidth changes, status,
and disengage procedures between end-points and Gatekeepers. RAS is not used if a
Gatekeeper is not present.
Audio
Audio signals contain digitized and compressed speech. The compression algorithms
supported by H.323 are all proven ITU standards. Although H.323 terminals must support the
G.711 voice standard for speech compression, the G.723 and G.729 codecs are gaining
industry support.
Video
While video capabilities are optional, any video-enabled H.323 terminal must support
the H.261 codec; support for H.263 is optional. Video information is transmitted at a rate
no greater than that selected during the capability exchange. H.261, which provides
a measure of compatibility across many of the different ITU
recommendations (Table 1), is used with communication channels that are multiples of 64
Kbps (P=1,2,3...30.). H.261 calls for fully encoding some frames and for coding only the
difference between a frame and the previous frame in other cases. Motion compensation, which improves image quality, is an H.261 option.
Data
Data conferencing is an optional capability. When supported, data conferencing enables
collaboration through applications such as shared whiteboards, application sharing, and
file transfer. H.323 supports data conferencing through the T.120 specification (see the
sidebar entitled The T.120 Standard: What Is It?). An ITU standard, T.120 addresses point-to-point and multipoint data
conferences. It provides interoperability at the application, network, and transport level.
H.323 VERSION 2
Approved in February of 1998, version 2 of the H.323 standard addresses deficiencies in
version 1 and introduces new functionality within existing protocols, such as Q.931,
H.245, and H.225, as well as new protocols. The most significant advances of version 2 are
in security, fast call setup, supplementary services, and T.120/H.323 integration.
Security
In development for months, the H.235 standard addresses four general issues when
dealing with security: authentication; integrity; privacy; and non-repudiation.
- Authentication
is a mechanism to make sure that the
endpoints participating in the conference are really who they say they are.
- Integrity
provides a means to validate that the data
within a packet is indeed an unchanged representation of the original data.
- Privacy/Confidentiality
is provided by encryption
and decryption mechanisms that hide the data from eavesdroppers so that if it is
intercepted, it cannot be viewed.
- Non-repudiation
is a means of protection in such a
case as: someone denying that they participated in a conference when you know they were
there.
Fast Call Setup
Using version 1 of H.323, when a call was placed from one endpoint to another, streams
were not always immediately available. This resulted in a second or two delay in
connection time. With H.323 version 2 and the introduction of Fast Call Setup, this
problem has been eliminated. In addition, Fast Call Setup reduces the number of H.323
messages needed to establish a call. This improvement provides call setup times that
approach SCN call setup times.
Supplementary Services
Supplementary Services for H.323, namely Call Transfer and Call Diversion, have been
defined by the H.450 series. H.450.1 defines the signaling protocol between H.323
end-points for the control of supplementary services; however, it does not define any PDUs
(Protocol Data Units). H.450.2 defines Call Transfer and H.450.3 Call Diversion. Call Transfer allows a call
established between end-point A and endpoint B to be transformed into a new call between
end-point B and a third endpoint, endpoint C. Call Diversion provides the supplementary
services call forwarding unconditional, call forwarding busy, call forwarding no reply,
and call deflection.
T.120/H.323 Integration
Although the first version of H.323 addressed the integration of T.120 with H.323, the
call setup scenarios were somewhat complex and unclear. Version 2 of H.323 addresses this problem by requiring endpoints that support both
T.120 and H.323 to lead the call with H.323. Further, version 2 states that T.120 is an
optional part of an H.323 conference and that enabling T.120 is at the discretion of each
H.323 endpoint.
WHY IS H.323 IMPORTANT?
The H.323 recommendation, which is comprehensive, yet flexible, includes configurations
for voice-only headsets, voice, and video games, full multimedia videoconferencing
stations, and a number of other applications. H.323 applications like these are set to
grow into the mainstream market for several reasons.
As mentioned briefly above, by pro viding device-to-device, application-to-application,
and vendor-to-vendor interoperability, H.323 allows a customer's products to interoperate
with other H.323-compliant products. In addition, H.323 provides standards for
interoperability between LANs and other networks. These types of interoperability are
critical to the long-term success of conferencing and collaborative applications.
H.323 also sets multimedia standards for the existing infrastructure (i.e., IP-based
networks). Designed to compensate for the effect of highly variable LAN latency, H.323
allows customers to use multimedia applications without changing their network
infrastructure. With H.323, network managers can easily handle network loading by restricting the
amount of network band-width available for conferencing. Multicast support also reduces band-width requirements.
Finally, H.323 has the support of many computing and conferencing companies and
organizations, including Intel, Microsoft, and Netscape. Their efforts will continue to
generate a higher level of awareness in the market. As more and more people look to the
Internet as a way to find information quickly, reduce travel, and speed their daily
business activities, H.323-based conferencing applications are poised to take off.
Tim May is the toolkit product manager at DataBeam Corporation.
Long known as a pioneer in communications industry standards, DataBeam is a leading
developer of remote collaboration, training, and IP telephony technologies. The company's
OEM products, the H.323 and T.120 toolkits and new Multimedia Telephony Platform Series,
enable third-party communications, networking, telephony, and conferencing products with
real-time collaboration capabilities.
For more information, visit the company's Web site at www.databeam.com.
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