Voice-over-IP banking on H.323
October 20, 1998 ELECTRONIC ENGINEERING TIMES via
NewsEdge Corporation : Conventional wisdom
is that the days of circuit-switched voice are
drawing to an end. Packet voice-specifically,
voice-over-IP (VoIP)-is the wave of the
future. Consider these developments:
- Next-generation telcos such as Qwest and
Level 3 are defying traditional carriers by
offering voice services over high-speed IP
(Internet Protocol) backbone networks.
- Internet Service Providers (ISPs) are
transforming themselves into Internet
Telephony Service Providers (ITSPs).
- Enterprises are beginning to see the
savings from integrating with their voice and
data networks.
There is no doubt that VoIP is on an upward
growth curve.
Yet for a long time VoIP had no standard,
resulting in products that were incompatible
and confining IP voice services to a niche.
To enable widespread deployment and
associated cost savings, there must be a
standard that allows multivendor product
interoperability-and the H.323 standard from
the International Telecommunications Union
(ITU) is emerging as the de facto solution to
this problem. The VoIP Forum recently
ratified an implementation agreement that
defined an interoperability profile based on
H.323. Other standards and vendor
organizations such as ETSI TIPHON and the
Internet Engineering Task Force (IETF) are
also working to ensure interoperability with
H.323.
Essentially, H.323 was designed to be a
technology for multipoint-multimedia
communications over packet-based
networks, which include IP-based networks,
such as the Internet. It can be applied in a
variety of ways-audio only (IP telephony, or
VoIP); audio and video (videotelephony);
audio and data; and audio, video and data.
Of these, the first-VoIP-is clearly the driver
for its current popularity and stratospheric
growth. Longer term, the multimedia aspects
will be important in delivering value-added
communication services over Internet
Protocol networks.
H.323 has been around for a few years now.
Version 2-packet-based multimedia
communications systems-was defined in
January 1998 to accommodate additional
requirements for VoIP. New features
currently being added to H.323, such as
intergatekeeper communications and
fast-connection mechanisms, will result in a
new version's being stan- dardized in the
next year or so.
H.323 specifies four kinds of components
that, when networked, enable point-to-point
and point-to-multipoint multimedia
communications: terminals, gateways,
gatekeepers and multipoint control units.
A user can employ the H.323 terminal for
real-time bidirectional multimedia
communications. A terminal must support
audio communications and can optionally
support video or data communications. It can
be either a PC or a standalone device,
running an H.323 stack and multimedia
applications. The primary goal of an H.323
terminal is to be compatible with terminals on
other multimedia networks-and H.323
terminals may be used in multipoint
conferences. Because the basic service
provided by an H.323 terminal is audio
communications, it plays a key role in
IP-telephony services.
Gateways to interoperability
One goal for development of the H.323
standard was interoperability among
multimedia services networks via gateways.
Although gateways are not required for
communications between two terminals in an
H.323 network, they facilitate
interoperability. They connect two dissimilar
networks and perform any network or
signaling translation required for
interoperability. They also translate protocols
for call setup and release, convert media
formats among networks and transfer
information between H.323 and non-H.323
nets. Translation among audio, video and
data formats may also be performed by the
gateway.
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. For example, a gateway can
connect and provide communication among
an H.323 terminal and SCN networks
(including all switched telephony networks,
e.g., PSTN).
Gatekeepers provide such services as
addressing, authorizing and authenticating
terminals and gateways; managing
bandwidth; accounting; billing; and charging.
They may also route calls. Although
gatekeepers in H.323 networks are optional,
if they are present terminals and gateways
must use their services, including translating
addresses, controlling admissions, controlling
bandwidth and managing zones.
The gatekeeper supports bandwidth control
and can route call-signaling messages
between H.323 end points.
When an end point sends call-signaling
messages to the gatekeeper, the gatekeeper
may accept or reject the call.
Multipoint control units support conferences
of three or more H.323 terminals. All
terminals participating in a conference
establish a connection with the control unit,
which manages conference resources,
negotiates among terminals to determine the
appropriate audio or video codec, and,
possibly, handles the media stream.
An H.323 zone is a collection of all terminals,
gateways and multipoint control units
managed by a single gatekeeper. A zone
includes at least one terminal, has one
gatekeeper and may include gateways or
multipoint control units. A zone may be
independent of network topology and may be
composed of multiple network segments that
are connected via routers or other devices.
Although H.323 is independent of the packet
network and the transport protocols it
specifies protocols for audio codecs, video
codecs, H.225 RAS, H.225 call signaling and
H.245 control signaling. It also specifies the
use of two protocols-the real-time transport
protocol (RTP) and real-time transport
control protocol (RTCP)-defined by the IETF
for real-time media transport.
RTP provides end-to-end delivery services of
real-time audio and video. It provides
payload-type identification, sequence
numbering, time stamping and delivery
monitoring.
RTCP provides control services. Its function
is to provide feedback on data distribution
quality.
G.711 supported
An audio codec encodes the audio signal
from the microphone for transmission on the
transmitting H.323 terminal and decodes the
received audio code, which is sent to the
speaker on the receiving H.323 terminal. All
H.323 terminals must support at least one
audio codec (G.711). A video codec encodes
video from the camera for transmission on
the transmitting H.323 terminal and decodes
the video code that is sent to the display on
the receiving terminal. Support of such video
codecs as H.261 is optional.
H.225 registration, admission and status
(RAS) is the protocol between end points
(terminals and gateways) and gatekeepers.
The RAS is used to register, control
admission, change bandwidth and status,
and disengage procedures between end
points and gatekeepers. H.225 call signaling
establishes a connection between H.323 end
points. H.225 call signaling sets up
connections between H.323 end points over
which real-time data can be transported.
For example, H.225 protocol messages are
carried over TCP in an IP-based H.323
network. If there is no gatekeeper in the
H.323 network, H.225 messages are
exchanged between the end points. When a
gatekeeper exists in the network, the H.225
messages are exchanged either directly
between the end points or between the end
points after routing through the gatekeeper.
The method chosen is decided by the
gatekeeper during the RAS admission
message exchange. The admission messages
are exchanged between end points and the
gatekeeper on RAS channels.
The most popular H.323 interworking is IP
telephony, when the underlying network of
H.323 is an IP network and the interoperating
network is SCN. SCN includes PSTN and ISDN
networks.
Copyright c 1998 CMP Media Inc.
By Rajeev Gupta, Engineering Director,
Emerging Technologies, Trillium Digital
Systems Inc., Los Angeles
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10-19-98, p. PG72>> |
