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The H.323 Revolution A Technology Review www…

Tags: accounting, caller identification, data entry, h 323, network capabilities, phone keypad, protocols, pstn networks, service capabilities, supplementary services, user experience,
Pages: 14
Language: english
Created: Mon May 6 10:06:51 2002

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The H.323 Revolution

A Technology Review

www.radvision.com
Contents
Background .................................................................................................................................... 3
Improved User Experience............................................................................................................ 4
Web-Based User Services ........................................................................................................... 4
Supplementary Services .............................................................................................................. 5
Data Entry via Phone Keypad ...................................................................................................... 6
Leveraging PSTN Networks .......................................................................................................... 6
Smooth Mapping of Supplementary Services .............................................................................. 6
Tunneling PSTN Signaling Protocols over H.323 ........................................................................ 6
Optimized Network Capabilities ................................................................................................... 7
Well-Managed Service Capabilities.............................................................................................. 7
Robust Call Session Accounting .................................................................................................. 7
Caller Identification Services ........................................................................................................ 7
Security and Privacy..................................................................................................................... 8
Generic Extensibility Framework.................................................................................................. 8
Enabling Scalability ....................................................................................................................... 8
Load Balancing and Redundancy ................................................................................................ 8
Massive Address Registration to a Gatekeeper........................................................................... 9
TCP Channel Multiplexing............................................................................................................ 9
More Efficient Setup Time ............................................................................................................ 9
Light Connectionless Signaling .................................................................................................. 10
RADVISION H.323 Enabling Technologies................................................................................ 11
Conclusion.................................................................................................................................... 12
Glossary of Terms ....................................................................................................................... 13

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The H.323 Revolution

Background
The H.323 protocol is best known as the original call signaling protocol that made real-
time voice and video over IP possible. Being the first solution to work, H.323 is the most
widely deployed protocol in the market, and through its veteran status and wide
acceptance provides telecommunication equipment with the benefits of a highly mature
and completely interoperable signaling solution.
Recently, other signaling protocols have become available on the market, although none
has reached the level of maturity and widespread deployment of H.323. Even though
other signaling protocols may have the advantage of being built on a more modern base,
the H.323 standard has kept abreast of the latest industry changes and has undergone a
metamorphosis from an early-day protocol that was mainly concerned with enabling voice
and video calls over limited bandwidth to a high-performance, feature-rich solution that is
deployable in large-scale carrier-class networks.
While the capability and performance boost of the H.323 version 4 protocol is impressive,
the standard leverages its early multi-media roots through its many annexes and open-
system plug-ins. These provide feature-rich and billable services that enhance the user
experience and provide financial incentives to Service Providers. The latest features of
the H.323 version 4 protocol can be grouped into the following general categories:
Improved user experience through web-based and advanced supplementary services
Improved PSTN signaling interworking
Advanced service capabilities for features such as privacy, security, and ease of
billing
Carrier-class network optimization features to enable required scalability and to
ensure service availability
Because of these new improvements, built on an extremely solid and proven foundation,
the H.323 version 4 protocol is an excellent solution for any real-time voice or video IP
implementation. This paper will describe the latest advances in the H.323 version 4
protocol and how they are being integrated in real-time voice and video over IP networks.

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The H.323 Revolution

Improved User Experience
Participants of IP voice or videoconferences expect to receive at least the same level of
reliable service and user experience that they are used to with other means of
communication such as telephones, e-mail, and instant messaging. When
communicating via the Internet, people enjoy web-enabled services such as click to dial,
address books, presence and instant messaging. When communicating via the
telephone, subscribers enjoy call forward, call waiting, hold and many other such
supplementary services.
H.323 addresses this market need. Of particular note is Annex K of H.323 version 4,
which enables H.323 applications to work in tandem with HTTP. Recommendation H.450,
which is part of the H.323 umbrella, provides an extensive range of useful supplementary
services.

Web-Based User Services
One of the major innovations of H.323 version 4 is Annex K, which defines how HTTP
can be used for the transport of service control with call or non-call related H.323
signaling. This combination of best-of-breed protocols enables full-featured advanced
videoconference services to be accessed from a simple web browser.
With these new features an application can locate a pre-defined (personalized) URL and
enable the provision of a personalized service through the web. Examples of web-based
services that can be incorporated in applications are:
Address book navigation
Click to dial
Point and click for multi-party conference setup
Interactive Web/Voice Response
Web Multimedia Instant Messaging and Presence
Follow-me services

How it Works
During initial RAS signaling, Annex K of H.323 version 4 allows a central device, such as
a gatekeeper (or Softswitch with gatekeeper call control), to initiate a parallel HTTP
session to a predefined URL. In the RCF message, the gatekeeper may return a URL.
The parallel HTTP session, allows call control and non-call control services to be
incorporated into H.323 real-time voice and video over IP applications and devices. The
HTTP server maps HTTP events and call control actions in a manner transparent to the
endpoints. Further, H.323 version 4 allows various types of text formats, images and
1
sounds to be added and utilized dynamically.
The flow diagram in Figure 1 demonstrates address book navigation using combined
H.323 and HTTP protocol services.

1
As MIME types
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The H.323 Revolution

User Web Browser Terminal Gatekeeper Web Server
RRQ

RCF (URL)

Load (URL)

GET (URL)

200 OK
Display
Update
address Action
book GET (URL)
Write
200 OK address book
Display
Click to dial to
xxx using Action
address book (call to)
callto:xxx

ARQ (xxx)

Figure 1: Combining H.323 with HTTP

Supplementary Services
Supplementary services provide a variety of user facilities. Surveys indicate that many
vendors have already adopted the new supplementary services while key players regard
the addition of the latest advanced supplementary services as product differentiators.
Recommendation H.450 defines supplementary services. The first supplementary
services were "transfer", "forward" and "hold" (specified in H.450.2 to H.450.4). Many
vendors have implemented these already. Further, an enhanced suite of services
(H.450.5 to H.450.9) was published in H.323 version 3 followed by more advanced
services (H.450.10 to H.450.12) in H.323 version 4.
The following list details the supplementary services provided by H.450.5 to H.450.12:
Park and Pick-up (H.450.5) facilitates call center services and terminal mobility. A
user calling a call center is put into park mode until an agent is available for call pick-
up or a user can put a call into park mode at endpoint A and pick the call up at
endpoint B. For example, a user can start a call at a telephone, put it into park mode
and pick the call up at a videoconference terminal.
Call Waiting Indication (H.450.6) notifies a busy user that another caller is attempting
to connect.
Message Waiting Indication (H.450.7) provides a general-purpose mechanism for
notifying a user that a message such as voice mail, fax, or video mail is waiting.
Naming Indication (H.450.8) enables the exchange of readable descriptive text about
the endpoint user, such as the useful information that normally appears on a
business card.
Call Completion Services (H.450.9) signals the caller and the callee when both
parties are free to receive a call. Applies to "call completion for busy" or "no answer"
scenarios.

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Call Offering (H.450.10) enhances the Call Waiting service by notifying the called
user who can then accept the offered call or reject it. The called user can also accept
the offered call after putting the existing call on hold or terminating it.
Call Intrusion (H.450.11) enables the ranking of endpoints. A higher-ranking endpoint
can intrude in the call of a lower-ranking endpoint. Intrusion can be the breaking into
an existing call or silently monitoring the called user.
Common Information Additional Network (H.450.12) enables endpoints to exchange
information relating to the endpoint category and the supplementary services
supported by each. This information can be used by the endpoint or gatekeeper
applications to determine what supplementary services to use and how they can best
interoperate.

Data Entry via Phone Keypad
The Dual Tone Multi-Frequency (DTMF) method was defined in legacy telephony many
years ago by the ITU to enable the use of the phone keypad for data entry. For example,
DTMF is used when ordering a theater ticket by phone using a credit card. Each dialed
digit is decoded as two unique frequency tones mixed together, which the telephone
transmits as a DTMF signal to the remote phone. The remote phone encodes the dual-
tone back into an ASCII character, the central office or exchange recognizes these
signals and processes the requested service.
From the earliest versions H.323 provides DTMF support. In the latest ratified version 4,
H.323 enables an RTP payload for DTMF format and supports RFC2833 compliant
operation mode. The DTMF support is defined through H.245 version 7 message
exchange.

Leveraging PSTN Networks
PSTN networks have evolved over several decades. Due to the capital invested in it, its
large installed base, and its solid reliability, PSTN will continue to be used for many
years. In order to leverage past investments in PSTN, Service Providers need to be able
to provide seamless services between PSTN and IP. H.323, and version 4 in particular,
facilitates greater interoperability with legacy domains.

Smooth Mapping of Supplementary Services
In many cases, the architects of H.323 adopted the same approach as ISDN. This makes
interoperability easier. For example, H.323 uses the same call control signaling protocol
(Q.931) as ISDN for call establishment and teardown. H.450 supplementary services use
the same logic as QSIG.
QSIG is the most common protocol used to communicate between PBXs in a PSTN
network. The H.323 supplementary services suite H.450 has been based on QSIG logical
procedures. That is, H.323 supplementary services use exactly the same logic as QSIG.
So when a supplementary service needs to be provided between a PSTN network and an
IP network (via a gateway) the inter-network mapping is straightforward and simple.

Tunneling PSTN Signaling Protocols over H.323
As part of the effort to interoperate with PSTN, H.323 version 4 specifies a generic
means of tunneling signaling messages in any H.225.0 call control message. This
method allows non-H.323 signaling information to be transported over H.323 networks.

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The H.323 Revolution

Consequently, protocols such as QSIG (Annex M1), ISUP (Annex M2) and DSS1 (Annex
M3) can be transmitted by tunneling over H.323 networks.

Optimized Network Capabilities
Gateways compatible with H.323 version 4 can interface with one or more PSTN
switches and associate trunk groups or carrier information into the call signaling path.
This means that the right gateway can be chosen for the right task. For example, if an
endpoint chooses to send fax data, the call can be routed to a gateway that only knows
how to send a fax instead of routing the call to a sophisticated gateway that can handle
complex video calls.
The Desired Protocols feature defined in H.323 version 4 allows an endpoint through
various H.225.0 messages to indicate what protocols (such as fax, H.320, or T.120) it
desires to utilize during the course of a call. If an endpoint has indicated a list of desired
protocols to its gatekeeper, the gatekeeper attempts to locate a gateway that can provide
support for the desired protocols.

Well-
Well-Managed Service Capabilities
As IP voice and video conferencing becomes mainstream, Service Providers require
more secure and well-managed service capabilities including robust call session
accounting, call services such as caller identification and restriction, caller authorization,
and protection against eavesdropping.

Robust Call Session Accounting
By combining the features available in H.323 Version 4, Service Providers can now
manage call session accounting. The following features can be used to improve
accounting and billing information:
User Authentication (see Security and Privacy below).
During call setup information from the newly specified H.225.0 fields and optionally,
from H.248, provides credit-related capabilities. An endpoint can receive user credit
or debit information from a gatekeeper before and after call establishment.
A gatekeeper may specify a time limitation and an endpoint can limit the user's call
duration accordingly. For instance, an endpoint may disengage the call when the time
or credit on the user's account is exhausted.
A gatekeeper can send balance-related announcements to the endpoint and indicate
a call duration limit to the endpoint.
A gatekeeper can indicate the call billing mode, which can be credit mode or debit
mode.
Annex G of H.225.0 (added in H.323 version 2) allows administrative domains to
make a request to other domains to provide information about the resource usage of
a specific call.

Caller Identification Services
H.323 version 4 provides caller identification services, including:
Calling party number presentation and restriction
Connected party number presentation and restriction
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The H.323 Revolution

Called (alerting) party number presentation and restriction
Busy party number presentation and restriction

Security and Privacy
Recommendation H.235 defines authentication as "the provision of assurance of the
claimed identity of an entity". User authentication is accomplished by adding a signature
to RAS and Q.931 messages with an agreed shared secret key known only to the
endpoint (user) and the central device.
H.235 Version 2 defines a multimedia encryption security profile. This allows privacy
through the prevention of eavesdropping. Eavesdropping on media is prevented through
the use of H.245, which presents RTP media streams with well-known encryption
algorithms.

Generic Extensibility Framework
The extensible framework provides a common method for feature negotiation that
operates over multiple domains and may be managed and configured by different
operational entities. The framework allows new features to be readily added to the
protocol without affecting the underlying (H.225.0) core specification.
An extended service can be defined on one side without knowing whether the other side
has the capability for this feature to operate successfully. If the other side has the
capabilities, it can provision the extended service. If the other side does not have the
capabilities, the extended service request will simply be ignored without affecting the call.
The generic extensibility framework is an optional feature of H.323 Version 4.

Enabling Scalability
Looking toward the future, as VoIP networks grow, scalability issues and the support of
large numbers of users need to be addressed. New features in H.323 Version 4, such as
the ability to register many aliases to a gatekeeper, have overcome problems previously
encountered in very large-scale systems. Q.931 multiplexing and load balancing facilities
reduce resource consumption and boost call handling performance.

Load Balancing and Redundancy
For the purposes of ensuring system availability, redundancy, and scalability, H.323
defines an entity called an Alternate Gatekeeper. A gatekeeper may utilize multiple
physical or logical devices as Alternate Gatekeepers. When a gatekeeper becomes
overloaded it can balance the load by redirecting the registered endpoint to an Alternate
Gatekeeper. Figure 2 shows how an endpoint registers with a gatekeeper. The
gatekeeper rejects the registration but adds the address of an Alternate Gatekeeper to
the rejection message.

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The H.323 Revolution

Endpoint GK 1 GK 2

RRQ

RRJ
(with IP address of alternate gk)

RRQ

RCF

Figure 2: Alternate Gatekeeper

Massive Address Registration to a Gatekeeper
Previous versions of H.323 were not efficient when a gateway or MCU had to register
very large numbers of alias addresses to a gatekeeper. This inefficiency was caused by
the size limitation of UDP packets. H.323 version 4 has addressed and resolved this
problem through an "Additive Registration" mechanism, an incremental registration
process by which an endpoint registers to a gatekeeper using several messages. When
Additive Registration is used, the gateway or MCU registers to a gatekeeper by sending
an initial list of aliases in an RRQ. Subsequently the gateway or MCU sends additional
RRQs until the full list of alias addresses is complete.

TCP Channel Multiplexing
By default in H.323 TCP is used for call signaling (H.225.0) and call control (H.245)
connections in the call. TCP assures reliable signaling over error-prone Internet
connections. It also assures connection state auditing for accurate service charges
required by Service Providers. However, each TCP connection requires a large amount
of operating system resources and CPU time. By using Q.931 multiplexing the same TCP
connection can be used for multiple call setups saving TCP resources and improving
performance.
The Call Signaling Channel carries H.225.0 call control messages. The Call Reference
Value (CRV) associates the message with the call and allows the Call Signaling Channel
to carry signaling for many concurrent calls.

More Efficient Setup Time
Since H.323 version 2, the 1.5 Round Trip Time (RTT) for call establishment connect
time has been recognizably faster than many other VoIP protocols. Since H.323
messages use ASN.1 PER (Packet Encoding Rules), call setup messages require less
bytes than any other VoIP protocol. In H.323 version 4 call setup can perform fast
connect and establish an H.245 session, via tunneling, in parallel. Hence call setup can
be performed even faster than before.

This means that you can set up an audio call in both directions (as before) while also fully
2
exchanging terminal capabilities with the endpoints (in three messages ). You can thus
set up two-way audio and video without any further signaling, thereby improving
performance and cutting down on signaling time.

Light Connectionless Signaling
Annex E (defined in H.323 version 3) is a generic protocol for performing reliable
3
message exchange over UDP. UDP can be used instead of TCP to carry Q.931
messages. This gives better control of parameters such as time-outs, retransmision, and
peer failure detection and also enables automatic multiplexing.

2
One in each direction plus an ACK.
3
TCP is a closed protocol provided with the operating system so that you cannot change parameters.
4
Ratified after H.323 version 4 was ratified (May 2001)
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The H.323 Revolution

RADVISION H.323 Enabling Technologies
The RADVISION technology business unit provides a complete real-time voice and video
over IP solution for VoIP developers to create any type of VoIP products compliant with
the latest ITU H.323 V.4 protocol discussed in this paper. These solutions include:
H.323 Protocol Toolkit
H.323 Gatekeeper Toolkit
ProLab H.323 Test Manager
Reference Applications
Embedded Gateway Solutions
Optional add-on technologies
The RADVISION H.323 Protocol Toolkit implements all mandatory H.323 features and
offers the most extensive implementation of ITU standard for optional capabilities. The
RADVISION H.323 Protocol Toolkit offers exceptional flexibility for building innovative IP-
based communication products and services with unique value-added functionality. The
current RADVISION release, version 4.1, is H.323 version 4 compliant and supports the
most recently ratified annexes while maintaining full backwards compatible.
The H.323 Gatekeeper Version 4.1 is a full-featured server solution compliant with the
latest H.323 standard and completely interoperable with H.323 gateway and endpoint
solutions. This toolkit supports optional and supplemental services including the full set of
services defined in H.450 version 4, security as defined in H.235 version 2, as well as
optional add-on technologies such as the Open Supplementary Protocol (OSP) and a
Firewall/NAT traversal solution.
The H.323 Protocol and H.323 Gatekeeper Toolkits include test applications in source
code so that developers can integrate the RADVISION solution into their applications
quickly and confidently. The test applications are fully functioning software programs that
can be copied and modified, or used as a baseline reference for correct stack behavior.
To assure protocol compliance and behavior in high traffic loads of developed products
RADVISON provides the ProLab H.323 Test Manager. ProLab is a network based testing
tool specifically designed to aid the developer during all phases of system development.
ProLab H.323 simulates calls, setups and teardowns to test standards compliance, call
loading and interoperability.
For embedded applications such as IP phones and residential gateways, RADVISION
provides an Embedded Gateway Solution that facilitates integration with system-on-chip
devices. The embedded solution includes media control, an API interface glue in addition
to the embedded H.323 stack.
A full-featured IP Telephony network will be multi-protocol and will support equipment
handling all of the modern IP protocols such as SIP, Megaco and MGCP in addition to an
H.323 solution. RADVISION, therefore, provides protocol toolkits to support these
solutions so that developers can implement the protocols of choice--all from a single
reliable common base of API layers available on multiple operating systems. In addition,
the RADVISION Embedded Gateway Solution provides a protocol abstraction layer
whereby the same embedded application can be developed in many variants, each
supporting a different signaling protocol, but without significant changes to the embedded
software application.

Conclusion
Today H.323 is much more than a legacy IP telephony protocol. The new H.323 version 4
enables full-featured and well-managed real-time voice and video over IP with a rich set
of add-on user services. The combination of H.323 and HTTP introduces a new level of
user experience and enables the creation of a new breed of services.
H.323 version 4 makes enhanced integration with PSTN networks possible, using similar
PSTN logic schemes for both basic and advanced call services.
For Service Providers, H.323 version 4 provides a rich set of services for user
authentication, user privacy, billing, service creation as well as scalability.
H.323 dominates the VoIP industry, with billions of minutes of voice and video traffic
every month. H.323 version 4 is the professional answer of choice for addressing market
needs--full-featured, well-managed conferencing and advanced service creation, all
within an extremely stable, robust, and interoperable environment.

Learn about RADVISION enabling technologies for H.323 at:
http://www.radvision.com/f_products/f2_h323.php3?prod=H.323+Toolkit

Glossary of Terms

Abbreviation Description

Annex K HTTP Based Service Control Transport Channel

Annex L Stimulus-signaling procedures between H.323 terminals and a Feature
Server functional entity.

Annex Q H.281/H.224 based FECC

DTMF Dual-Tone Multi Frequency

H.235 v2 Security and Encryption for H-Series (H.323 and other H.245 Based)
multimedia terminals

H.245 Call control signaling protocol for determining capabilities and for issuing
open and close channels commands

H.320 The ITU standard for videoconferencing over digital networks such as
ISDN

H.450 Protocol for the support of Supplementary Services in H.323

HTTP Hypertext Transfer Protocol

ISDN Integrated Services Digital Network

PER Packet Encoding Rules

PSTN Public Switched Telephone Networks

Q.931 Call control signaling protocol for the establishment and tear down of
calls

QSIG A global signaling and control standard for Private Integrated Network
Exchange

T.120 An ITU standard for real-time data conferencing (sharing data among
multiple users).

TCP Transmission Control Protocol

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