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What
are H.323 ?
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Presented
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Copyright 2000© |
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What
is it?
H.323 is a standard that
specifies the components, protocols and procedures that provide
multimedia communication services - real-time audio, video, and data
communications - over packet-based networks (including the Internet).
H.323 is part of a family of ITU-T (International Telecommunications
Union) recommendations called H.32x that provides multimedia
communication services over a variety of networks. These standards
define how components that are built in compliance with H.323 set up
calls, exchange compressed audio and video, participate in multiunit
conferences, and operate with non-H.323 endpoints.
H.323 provides various services and, therefore, can be applied in a wide
variety of areas - consumer, business, and entertainment applications.
It can be applied in a variety of mechanisms - audio only (IP
telephony); audio and video (videotelephony); audio and data; and audio,
video and data. H.323 can also be applied to multipoint-multimedia
communications.
Microsoft and more than 120 other leading companies have announced their
intention to support and implement H.323 in their products and services.
This wide support establishes H.323 as the standard for audio and video
conferencing over the Internet.
The
evolution of H.323
The H.323 standard is
specified by the ITU-T Study Group 16. Version 1 of the H.323
recommendation - Visual Telephone Systems and Equipment for LANs that
provide a non guaranteed quality of service (QoS) - was accepted in
October 1996. It was, as the name suggests, heavily weighted towards
multimedia communications in a LAN environment.
At around the same time, innovators were beginning to experiment with
voice communications over the Internet. These initial attempts were
based on proprietary methods for setting up calls, compressing voice,
locating and alerting endpoints, and so forth. The absence of a standard
for voice over IP resulted in products that were incompatible. The H.323
standards seemed to be the fastest way to achieving interoperability
between the newly emerging voice clients.
With the development of voice over IP, new requirements emerged, such as
providing communication between a PC-based phone and a phone on a
traditional switched circuit network (SCN). Such requirements forced the
need for a standard for IP telephony. Version 2 of H.323 - Packet-based
Multimedia Communications Systems - was defined to accommodate these
additional requirements and was accepted in January 1998.
New features are being added to the H.323 standard, which will evolve to
Version 3 shortly. The features being added include fax-over-packet
networks, gatekeeper-gatekeeper communications, and fast-connection
mechanisms.
What's
in it? (or in other words: H.323 Components)
The H.323 standard specifies
4 kinds of components, which, when networked together, provide the
point-to-point and point-to-multipoint multimedia-communication
services:
Terminals
Terminals are the client
endpoints on the LAN that provide real-time bidirectional multimedia
communications. An H.323 terminal can either be a personal computer (PC)
or a stand-alone device, running an H.323 stack and the multimedia
applications.
It supports audio communications and can optionally support video or
data communications. H.323 specifies the modes of operation required for
different audio, video and/or data terminals to work together. Because
the basic service provided by an H.323 terminal is audio communications,
an H.323 terminal plays a key role in IP-telephony services, and is the
dominant standard of the next generation of Internet phones, audio
conferencing terminals, and video conferencing technologies.
The primary goal of H.323 is
to interwork with other multimedia terminals. In order to achieve this
interoperability:
1. All H.323 terminals must also support H.245, which is used to
negotiate channel usage and capabilities.
Three other components are required:
2. Q.931 for call signaling and call setup
3. A component called Registration/ Admission/ Status (RAS),
which is a protocol used to communicate with a gatekeeper
4. RTP/RTCP for sequencing audio and video packets.
Optional components in an H.323 terminal are video CODECs, T.210 data
conferencing protocols, and MCU capabilities
Gateways
A gateway connects two
dissimilar networks. An H.323 gateway provides connectivity between an
H.323 network and a non-H.323 network.
For example, a gateway can connect and provide communication between an
H.323 terminal and SCN networks (SCN networks include all switched
telephony networks, e.g., public switched telephone network [PSTN]).
This connectivity of dissimilar networks is achieved by translating
protocols for call setup and release, converting media formats between
different networks, and transferring information between the networks
connected by the gateway.
A gateway is not required, however, for communication between two
terminals on an H.323 network. In general, the purpose of the gateway is
to reflect the characteristics of a LAN endpoint to an SCN endpoint and
vice versa. On the H.323 side, a gateway runs H.245 control signaling
for exchanging capabilities, H.225 call signaling for call setup and
release, and H.225 registration, admissions, and status (RAS) for
registration with the gatekeeper. On the SCN side, a gateway runs SCN-specific
protocols (e.g., ISDN and SS7 protocols).
Terminals communicate with gateways using the H.245 control-signaling
protocol and H.225 call-signaling protocol. The gateway translates these
protocols in a transparent fashion to the respective counterparts on the
non-H.323 network and vice versa. The gateway also performs call setup
and clearing on both the H.323 network side and the non-H.323 network
side. Translation between audio, video, and data formats may also be
performed by the gateway. Audio and video translation may not be
required if both terminal types find a common communications mode.
For example, in the case of a gateway to H.320 terminals on the ISDN,
both terminal types require G.711 audio and H.261 video, so a common
mode always exists. The gateway has the characteristics of both an H.323
terminal on the H.323 network and the other terminal on the non-H.323
network it connects. A gateway may be able to support several
simultaneous calls between the H.323 and non-H.323 networks. In
addition, a gateway may connect an H.323 network to a non-H.323 network.
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 connection,
the number of simultaneous independent conferences supported, the
audio/video/data conversion functions, and inclusion of multipoint
functions are left to the manufacturer. A gateway is a logical component
of H.323 and can be implemented as part of a gatekeeper or a MCU.
Gatekeepers
The gatekeeper is the most
important of the H.323 components. The gatekeeper's primary job is to
act as the central point for all calls within its zone and provide call
control services for registered H.323 endpoints.
Gatekeepers in H.323 networks are optional, but if they are present in
the network endpoints must use their services. Although they are not
required, gatekeepers provide important services such as addressing,
authorization and authentication of terminals and gateways; bandwidth
management; accounting; billing; and charging and may also provide
call-routing services.
The H.323 standards define mandatory services that the gatekeeper must
provide and specifies other optional functionality that it can provide.
Mandatory Gatekeeper
Functions
Address Translation:
Calls originating within an H.323 network may use an alias to
address the destination terminal. Calls originating outside the H.323
network and received by a gateway may use an E.164 telephone number to
address the destination terminal. The gatekeeper must be able to
translate the alias or the E.164 telephone number into the network
address for the destination terminal. The destination endpoint can be
reached using the network address on the H.323 network. The translation
is done using a translation table that is updated with Registration
messages.
Admission Control:
The gatekeeper can control the admission of the endpoints into the H.323
network. It uses RAS messages, admission request (ARQ), confirm (ACF),
and reject (ARJ) to achieve this. Admissions control may also be a null
function that admits all requests.
Bandwidth Control:
Gatekeepers must support the RAS bandwidth messages. How they
provide the bandwidth access or bandwidth management, however, is left
to the service provider or enterprise manager's individual policy. For
instance, if a network manager has specified a threshold for the number
of simultaneous connections on the H.323 network, the gatekeeper can
refuse to make any more connections once the threshold is reached. The
result is to limit the total allocated bandwidth to some fraction of the
total available, leaving the remaining bandwidth for data applications.
In many cases, any bandwidth requests will be honored, unless the
network or particular gateway is congested.
Zone Management:
A gatekeeper is required to provide the above functions-address
translation, admissions control, and bandwidth control-for terminals,
gateways, and MCUs located within its zone of control. The rules for
deciding which H.323 endpoints can register with a particular
gatekeeper, and the geographic or logical composition of a zone (the
collection of all components-terminals, gateways, and Multipoint Control
Units (MCUs)-managed by a single gatekeeper), are not specified by the
H.323 standard, but left to the discretion of the network designer.
Optional Gatekeeper
Functions
Call-Control
Signaling:
The gatekeeper can route call-signaling messages between H.323
endpoints.
A gatekeeper can decide that it will process all call signaling
associated with the endpoints registered with it. This is known as the
Gatekeeper Routed Call Signaling Model. In this case, the call signaling
messages (Q.931) are routed through the gatekeeper between the
endpoints. Many service providers are expressing interest in this model
because it gives them more information that can be used for billing and
facilitates offering supplementary services.
A gatekeeper can decide to allow the call signaling messages to pass
directly between the endpoints. This model is known as Direct Endpoint
Call Signaling. Entities offering subscription-based services may prefer
this model. The gatekeeper decides which call signaling model will be
used for a particular call; an endpoint may express a preference, but
ultimately the gatekeeper decides which model will be used.
Call
Authorization:
When an endpoint sends call-signaling
messages to the gatekeeper, the gatekeeper can decide to accept or
reject the call, according to the H.225 specification. The reasons for
rejection may include the time of day, type of service subscription,
desire to access a restricted gateway, or lack of available bandwidth.
Bandwidth Management:
The Gatekeeper may reject calls from a terminal if it determines
that sufficient bandwidth is not available. This function also operates
during an active call if a terminal requests additional bandwidth. The
criteria for determining if bandwidth is available is outside the scope
of the H.323 standards and is, therefore, left to the discretion of the
H.323 service provider.
Call Management:
The gatekeeper may maintain information about all active H.323
calls so that it can control its zone by providing the maintained
information to the bandwidth-management function or by rerouting the
calls to different endpoints to achieve load balancing.
The gatekeeper may provide intelligent call management. For instance, it
may be known that a requested terminal is currently engaged in a call.
The gatekeeper could choose to redirect the call or, at a minimum, save
the call setup time by not attempting to establish a call to a busy
terminal.
MCU
(Multipoint Control Unit)
MCUs provide support for
conferences of three or more H.323 terminals (endpoints). All terminals
participating in the conference establish a connection with the MCU.
Under H.323, an MCU consists of a Multipoint Controller (MC) and zero or
more Multipoint Processors (MPs). The MC negotiates between terminals in
order to know what audio or video coder/decoder (CODEC) to use, and also
manages conference resources, by determining which, if any, of the audio
and video streams will be multicast. But, the MC doesn't 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. An MC may be
located within a Gatekeeper, Gateway, Terminal or an MCU.
The H.323 recommendation uses three concepts of multipoint conferences :
Centralized and Decentralized multipoint conferences, and Hybrid
multipoint conferences, which use a combination of centralized and
decentralized features.
Centralized multipoint conferences : This concept requires 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
controls the conference using H.245 control functions. The MP mixes the
audio, distributes data, switches and mixes video and sends the
resulting streams back to the participating terminals. The MP may also
converse between different codecs and bit rates and may use multicast to
distribute processed video.
Decentralized multipoint conferences : This concept makes use of
multicast technology. In this concept, terminals that participate in a
conference, multicast audio and video to other terminal that also
participate in the conference, without sending the data to an MCU, but,
still, the control is 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 incoming audio
and video streams, and they indicate the MC how many simultaneous video
and audio streams they can decode. The MP can provide video selection
and audio mixing in a decentralized multipoint conference.
Hybrid multipoint conferences : As described, this concept uses a
combination of centralized and decentralized features. H.245 signals and
an audio or video stream is processed through point-to-point messages to
the MCU. The remaining signal (audio or video) is transmitted to
participating H.323 terminals through multicast.
H.323 also supports mixed multipoint conferences in which some terminals
are in a centralized conference, others are in a decentralized
conference, and an MCU provides the bridge between the two types.
Communications
under H.323
Call
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 that 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.
Overall system control is provided by three separate signaling
functions: the H.245 Control Channel, the Q.931 Call Signalling Channel,
and the RAS Channel.
H.225 Call Signaling
The H.255 standard defines a
layer that formats the transmitted video, audio, data, and control
streams for output to the network, and retrieves the corresponding
streams from the network. As part of audio and video transmissions,
H.225 uses the packet format specified RTP and RTCP specifications for
the following tasks:
Logical framing - defines how the protocol frames the audio and video
data into packets for transport over a selected communications channel.
Sequence numbering - determines the order
of data packets transported over a communications channel.
Error detection
After initiating a call, one
or more RTP or RTCP connections are established:
RTP (Real Time Transport Protocol) provides end-to-end delivery
services of real-time audio and video. RTP is typically used to
transport data via UDP. RTP, together with UDP, provides
transport-protocol functionality: payload-type identification, sequence
numbering, time stamping, and delivery monitoring.
RTCP (Real Time Control Protocol) provides control services, and
mainly feedback on the quality of the data distribution.
The Call Signalling Channel
uses Q.931 to establish the connection between two terminals:
Q.931
This protocol defines how
each H.323 layer interacts with peer layers, so that participants can
interoperate with agreed upon formats. The Q.931 protocol resides within
H.225. As part of H.323 call control, Q.931 is a link layer protocol for
establishing connections and framing data. Q.931 provides a method for
defining logical channels inside of a larger channel. Q.931 messages
contain a protocol discriminator that identifies each unique message
with a call reference value and a message type. The H.225.0 layer then
specifies how these Q.931 messages are received and processed.
H.255 Registration,
Admission, and Status
The H.225.0 standard also
includes registration, admission, and status (RAS) control. RAS is the
protocol between endpoints (terminals and gateways) and gatekeepers
which makes the connections between them available. The RAS is used to
perform registration, admission control, bandwidth changes, status, and
disengage procedures between endpoints and gatekeepers.
RAS is not used if a Gatekeeper is not present.
H.245 Control
Signaling
This standard provides the
call control mechanism that allows H.323-compatible terminals to connect
to each other.
The H.245 Control Channel is a reliable channel that carries control
messages governing operation of the H.323 endpoint, These control
messages carry information related to the following:
Capabilities exchange
Opening and closing of logical channels
used to carry media streams
Preference requests
Flow-control messages
General commands and indications
There is only one H.245 Control Channel per call.
Audio
CODECs
An audio CODEC encodes the
audio signal from the microphone for transmission on the transmitting
H.323 terminal and decodes the received audio code that is sent to the
speaker on the receiving H.323 terminal. Because audio is the minimum
service provided by the H.323 standard, all H.323 terminals must have at
least one audio CODEC support, as specified in the ITU-T G.711
recommendation (audio coding at 64 kbps).
Additional audio and video CODECs provide a variety of standard bit
rates, delay, and quality options that are suitable for a range of
network selections. For example: G.722 (64, 56, and 48 kbps), G.723.1
(5.3 and 6.3 kbps), G.728 (16 kbps), and G.729 (8 kbps).
Video
CODECs
A video CODEC encodes video
from the camera for transmission on the transmitting H.323 terminal and
decodes the received video code that is sent to the video display on the
receiving H.323 terminal. Because H.323 specifies support of video as
optional, the support of video CODECs is optional as well. However, any
H.323 terminal providing video communications must support video
encoding and decoding as specified in the ITU-T H.261 recommendation.
T.120
Data Communications
H.323 specifies T.120
services for data communications and conferencing within and next to an
H.323 session. This T.120 support means that data handling can occur
either in conjunction with H.323 audio and video, or separately.
T.120 can use the H.225 layer to send and receive data packets or simply
create an association with the H.323 session and use its own transport
capabilities to transmit data directly to the network. Data from
conferencing programs, such as file transfer and program sharing, use
T.120 support to operate in conjunction with H.323 connections. Also,
H.323-compatible products interoperate with data conferencing products
developed under the T.120 specification.
H.323
Protocol Stack
H.323 uses both reliable and
unreliable communications.
Control signals and data require reliable transport because the signals
must be received in the order in which they were sent and cannot be
lost. To achieve reliable tranmission, H.323 uses TCP, which is a
reliable, connection-oriented protocol. This guarantees sequenced,
error-free, flow-controlled transmission of packets. However, this
reliability can cause delays in transmission and reduce throughput. TCP
is used by H.323 for the H.245 Control Channel, the T.120 Data Channels
and the Call Signaling Channel.
Control signals and data require reliable transport because the signals
must be received in the order in which they were sent and cannot be
lost. To achieve reliable tranmission, H.323 uses TCP, which is a
reliable, connection-oriented protocol. This guarantees sequenced,
error-free, flow-controlled transmission of packets. However, this
reliability can cause delays in transmission and reduce throughput. TCP
is used by H.323 for the H.245 Control Channel, the T.120 Data Channels
and the Call Signaling Channel.
In conferences with multiple audio and video streams, unreliable
transport via UDP uses IP Multicast and the Real-Time Protocol (RTP)
developed by the Internet Engineering Task Force (IETF) to handle
streaming audio and video. IP Multicast is a protocol for unreliable
multicast transmission in UDP. RTP works on top of IP Multicast, and was
designed to handle the requirements of streaming audio and video over
the Internet. A header containing a time-stamp and a sequence number is
attached to each UDP packet. With appropriate buffering at the receiving
station, timing and sequence information allows the application to
eliminate duplicate packets; reorder out-of-sequence packets;
synchronize sound, video and data; and achieve continuous playback in
spite of varying latencies. RTP needs to be supported by Terminals,
Gateways, and MCUs with Multipoint Processors.
The Real-Time Control Protocol (RTCP) is used for the control of RTP.
RTCP monitors the quality of service, conveys information about the
session participants, and periodically distributes control packets
containing quality information to all session participants through the
same distribution mechanisms as the data packets.
The
Importance of H.323
Interoperability - H.323 establishes methods for receiving clients to
communicate capabilities to the sender. It also establishes common call
setup and control protocol. All above help the users to conference
without worrying about compatibility at the receiving point.
This network independence of H.323 assures that even when network
technology evolves, and bandwidth-management techniques improve, H.323
will be able to take advantage of those enhanced capabilities.
H.323 is not tied to any hardware or operating system. H.323-compliant
platforms will be available in many sizes and shapes, including
video-enabled personal computers, dedicated platforms, IP-enabled
telephone handsets, cable TV set-top boxes and turnkey boxes.
H.323 sets multimedia standards for the existing infrastructure (i.e.
IP-based networks), and therefore allows customers to use multimedia
applications without changing their network infrastructure.
An H.323 conference can include endpoints with different capabilities.
For example, a terminal with audio-only capabilities can participate in
a conference with terminals that have video and/or data capabilities.
Furthermore, an H.323 multimedia terminal can share the data portion of
a videoconference with a T.120 data-only terminal, while sharing voice,
video, and data with other H.323 terminals.
H.323 provides multiple audio and video CODECs that format data
according to the requirements of various networks, using different bit
rates, delays, and quality options. Users can choose the codecs that
best support their computer and network selections.
Although
H.323 can support conferences of three or more endpoints without
requiring a specialized multipoint control unit, MCU's provide a more
powerful and flexible architecture for hosting multipoint conferences.
Multipoint capabilities can be included in other components of an H.323
system.
H.323 supports multicast transport in multipoint conferences: In
multicast a single packet is sent to a subset of destinations on the
network without replication (In contrast to unicast or broadcast, in
which the network is used inefficiently as packets are replicated
throughout the network).
H.323 has the support of many computing and communications companies and
organizations, including Intel, Microsoft, Cisco, and IBM. The efforts
of these companies will generate a higher level of awareness in the
market.
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