modem signals or synchronous HDLC connections from a network (e.g.
SCN or xDSL network) and provides data access to the packet network.
Only those requirements specific to a NAS are described here.
Figure 1 provides a reference architecture for a Network Access
Server (NAS). Signaling comes into the MGC and the MGC controls the
NAS.
+-------+ +-------+
Signaling | | | |
-----------+ MGC + | AAA |
| | | |
+---+---+ +--+----+
| |
Megaco|_______________|
|
|
+---+---+ ~~|~~~
Bearer | | ( )
-----------+ NAS +-------( IP )
| | ( )
+-------+ ~~~~~~
Figure 1: NAS reference architecture
The Protocol must support:
a. Callback capabilities:
* Callback
b. Modem calls. The protocol must be able to specify the modem
type(s) to be used for the call.
c. Carriage of bearer information. The protocol must be able to
specify the data rate of the TDM connection (e.g., 64 kbit/s, 56
kbit/s, 384 kbit/s), if this is available from the SCN.
d. Rate Adaptation: The protocol must be able to specify the type
of rate adaptation to be used for the call including indicating
the subrate, if this is available from the SCN (e.g. 56K, or
V.110 signaled in Bearer capabilities with subrate connection of
19.2kbit/s).
e. Adaptable NASes: The protocol must be able to support multiple
options for an incoming call to allow the NAS to dynamically
select the proper type of call. For example, an incoming ISDN
call coded for "Speech" Bearer Capability could actually be a
voice, modem, fax, text telephone, or 56 kbit/s synchronous
call. The protocol should allow the NAS to report back to the
MGC the actual type of call once it is detected.
The 4 basic types of bearer for a NAS are:
1. Circuit Mode, 64-kbps, 8-khz structured, Speech
2. Circuit Mode, 64-kbps, 8-khz structured, 3.1-khz, Audio
3. Circuit Mode, 64-kbps, 8-khz structured, Unrestricted Digital
Transmission-Rate Adapted from 56-kbps
4. Circuit Mode, 64-kbps, 8-khz structure, Unrestricted Digital
Transmission
f. Passage of Called and Calling Party Number information to the
NAS from the MGC. Also, passage of Charge Number/Billing Number,
Redirecting Number, and Original Call Number, if known, to the
NAS from the MGC. If there are other Q.931 fields that need to
be passed from the MGC to the MG, then it should be possible to
pass them [9].
g. Ability for the MGC to direct the NAS to connect to a specific
tunnel, for example to an LNS, or to an AAA server.
h. When asked by the MGC, be able to report capability information,
for example, connection types (V.34/V90/Synch ISDN..), AAA
mechanism (RADIUS/DIAMETER/..), access type (PPP/SLIP/..) after
restart or upgrade.
11.2.6. Restricted Capability Gateway
The requirements here may also be applied to small analog gateways,
and to cable/xDSL modems. See also the section on access gateways.
The Protocol must support:
a. The ability to provide a scaled down version of the protocol.
When features of the protocol are not supported, an appropriate
error message must be sent. Appropriate default action must be
defined. Where this is defined may be outside the scope of the
protocol.
b. The ability to provide device capability information to the MGC
with respect to the use of the protocol.
11.2.7. Multimedia Gateway
The protocol must have sufficient capability to support a multimedia
gateway. H.320 and H.324 are characterized by a single data stream
with multiple media streams multiplexed on it.
If the mapping is from H.320 or H.324 on the circuit side, and H.323
on the packet side, it is assumed that the MG knows how to map
respective subchannels from H.320/H.324 side to streams on packet
side. If extra information is required when connecting two
terminations, then it must be supplied so that the connections are
not ambiguous.
The Multimedia Gateway:
1) should support Bonding Bearer channel aggregation,
2) must support 2xB (and possibly higher rates) aggregation via
H.221,
3) must be able to dynamically change the size of audio, video and
data channels within the h.320 multiplex,
4) must react to changes in the H.320 multiplex on 20 msec
boundaries,
5) must support TCS4/IIS BAS commands,
6) must support detection and creation of DTMF tones,
7) should support SNMP MIBS as specified in H.341 [3]
a. If some of the above cannot be handled by the MGC to MG protocol
due to timing constraints, then it is likely that the H.245 to
H.242 processing must take place in the MG. Otherwise, support
for this functionality in the multimedia gateway are protocol
requirements.
b. It must be possible on a call by call basis for the protocol to
specify different applications. Thus, one call might be PSTN to
PSTN under SS7 control, while the next might be ISDN/H.320 under
SS7 control to H.323. This is only one example; the key
requirement is that the protocol not prevent such applications.
11.2.8. Audio Resource Function
An Audio Resource Function (ARF) consists of one or more functional
modules which can be deployed on an stand alone media gateway server
IVR, Intelligent Peripheral, speech/speaker recognition unit, etc. or
a traditional media gateway. Such a media gateway is known as an
Audio Enabled Gateway (AEG) if it performs tasks defined in one or
more of the following ARF functional modules:
Play Audio,
DTMF Collect,
Record Audio,
Speech Recognition,
Speaker Verification/Identification,
Auditory Feature Extraction/Recognition, or
Audio Conferencing.
Additional ARF function modules that support human to machine
communications through the use of telephony tones (e.g., DTMF) or
auditory means (e.g. speech) may be appended to the AEG definition
in future versions of these requirements.
Generic scripting packages for any module must support all the
requirements for that module. Any package extension for a given
module must include, by inheritance or explicit reference, the
requirements for that given module.
The protocol requirements for each of the ARF modules are provided in
the following subsections.
11.2.8.1. Play Audio Module
a. Be able to provide the following basic operation:
- request an ARF MG to play an announcement.
b. Be able to specify these play characteristics:
- Play volume
- Play speed
- Play iterations
- Interval between play iterations
- Play duration
c. Permit the specification of voice variables such as DN, number,
date, time, etc. The protocol must allow specification of both
the value (eg 234-3456), and well as the type (Directory
number).
d. Using the terminology that a segment is a unit of playable
speech, or is an abstraction that is resolvable to a unit of
playable speech, permit specification of the following segment
types:
- A provisioned recording.
- A block of text to be converted to speech.
- A block of text to be displayed on a device.
- A length of silence qualified by duration.
- An algorithmically generated tone.
- A voice variable, specified by type and value. Given a variable
type and value, the IVR/ARF unit would dynamically assemble the
phrases required for its playback.
- An abstraction that represents a sequence of audio segments.
Nesting of these abstractions must also be permitted.
An example of this abstraction is a sequence of audio segments, the
first of which is a recording of the words "The number you have
dialed", followed by a Directory Number variable, followed by a
recording of the words "is no longer in service".
- An abstraction that represents a set of audio segments and which
is resolved to a single segment by a qualifier. Nesting of
these abstractions must be permitted.
For example take a set of audio segments recorded in different
languages all of which express the semantic concept "The number you
have dialed is no longer in service". The set is resolved by a
language qualifier. If the qualifier is "French", the set resolves to
the French version of this announcement.
In the case of a nested abstraction consisting of a set qualified by
language at one level and and a set qualified by gender at another
level, it would be possible to specify that an announcement be
played in French and spoken by a female voice.
e. Provide two different methods of audio specification:
- Direct specification of the audio components to be played by
specifying the sequence of segments in the command itself.
- Indirect specification of the audio components to be played by
reference to a single identifier that resolves to a provisioned
sequence of audio segments.
11.2.8.2. DTMF Collect Module
The DTMF Collect Module must support all of the requirements in the
Play Module in addition to the following requirements:
a. Be able to provide the following basic operation:
- request an AEG to play an announcement, which may optionally
terminated by DTMF, and then collect DTMF
b. Be able to specify these event collection characteristics:
- The number of attempts to give the user to enter a valid DTMF
pattern.
c. With respect to digit timers, allow the specification of:
- Time allowed to enter the first digit.
- Time allowed for user to enter each digit subsequent to the
first digit.
- Time allowed for user to enter a digit once the maximum expected
number of digits has been entered.
d. To be able to allow multiple prompt operations DTMF digit
collection, voice recording (if supported), and/or speech
recognition analysis (if supported) provide the following types
of prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
e. To allow digit pattern matching, allow the specification of:
- maximum number of digits to collect.
- minimum number of digits to collect.
- a digit pattern using a regular expression.
f. To allow digit buffer control, allow the specification of:
- Ability to clear digit buffer prior to playing initial prompt
(default is not to clear buffer).
- Default clearing of buffer following playing of un-interruptible
announcement segment.
- Default clearing of buffer before playing a re-prompt in
response to previous invalid input.
g. Provide a method to specify DTMF interruptibility on a per audio
segment basis.
h. Allow the specification of definable key sequences for DTMF
digit collection to:
- Discard collected digits in progress, replay the prompt, and
resume DTMF digit collection.
- Discard collected digits in progress and resume DTMF digit
collection.
- Terminate the current operation and return the terminating key
sequence to the MGC.
i. Provide a way to ask the ARF MG to support the following
definable keys for digit collection and recording. These keys
would then be able to be acted upon by the ARF MG:
- A key to terminate playing of an announcement in progress.
- A set of one or more keys that can be accepted as the first
digit to be collected.
- A key that signals the end of user input. The key may or may
not be returned to the MGC along with the input already
collected.
- Keys to stop playing the current announcement and resume playing
at the beginning of the first segment of the announcement, last
segment of the announcement, previous segment of the
announcement, next segment of the announcement, or the current
announcement segment.
11.2.8.3. Record Audio Module
The Record Module must support all of the requirements in the Play
Module as in addition to the following requirements:
a. Be able to provide the following basic operation:
- request an AEG to play an announcement and then record voice.
b. Be able to specify these event collection characteristics:
- The number of attempts to give the user to make a recording.
c. With respect to recording timers, allow the specification of:
- Time to wait for the user to initially speak.
- The amount of silence necessary following the last speech
segment for the recording to be considered complete.
- The maximum allowable length of the recording (not including
pre- and post- speech silence).
d. To be able to allow multiple prompt operations for DTMF digit
collection (if supported), voice recording (if supported),
speech recognition analysis (if supported) and/or speech
verification/identification (if supported) and then to provide
the following types of prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
e. Allow the specification of definable key sequences for digit
recording or speech recognition analysis (if supported) to:
- Discard recording in progress, replay the prompt, and resume
recording.
- Discard recording in progress and resume recording.
- Terminate the current operation and return the terminating key
sequence to the MGC.
f. Provide a way to ask the ARF MG to support the following
definable keys for recording. These keys would then be able to
be acted upon by the ARF MG:
- A key to terminate playing of an announcement in progress.
- A key that signals the end of user input. The key may or may
not be returned to the MGC along with the input already
collected.
- Keys to stop playing the current announcement and resume playing
at the beginning of the first segment of the announcement, last
segment of the announcement, previous segment of the
announcement, next segment of the announcement, or the current
announcement segment.
g. While audio prompts are usually provisioned in IVR/ARF MGs,
support changing the provisioned prompts in a voice session
rather than a data session. In particular, with respect to
audio management:
- A method to replace provisioned audio with audio recorded during
a call. The newly recorded audio must be accessible using the
identifier of the audio it replaces.
- A method to revert from replaced audio to the original
provisioned audio.
- A method to take audio recorded during a call and store it such
that it is accessible to the current call only through its own
newly created unique identifier.
- A method to take audio recorded during a call and store it such
that it is accessible to any subsequent call through its own
newly created identifier.
11.2.8.4. Speech Recognition Module
The speech recognition module can be used for a number of speech
recognition applications, such as:
- Limited Vocabulary Isolated Speech Recognition (e.g., "yes",
"no", the number "four"),
- Limited Vocabulary Continuous Speech Feature Recognition (e.g.,
the utterance "four hundred twenty-three dollars"),and/or
- Continuous Speech Recognition (e.g., unconstrained speech
recognition tasks).
The Speech Recognition Module must support all of the requirements in
the Play Module as in addition to the following requirements:
a. Be able to provide the following basic operation: request an AEG
to play an announcement and then perform speech recognition
analysis.
b. Be able to specify these event collection characteristics:
- The number of attempts to give to perform speech recognition
task.
c. With respect to speech recognition analysis timers, allow the
specification of:
- Time to wait for the user to initially speak.
- The amount of silence necessary following the last speech
segment for the speech recognition analysis segment to be
considered complete.
- The maximum allowable length of the speech recognition analysis
(not including pre- and post- speech silence).
d. To be able to allow multiple prompt operations for DTMF digit
collection (if supported), voice recording (if supported),
and/or speech recognition analysis and then to provide the
following types of prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
e. Allow the specification of definable key sequences for digit
recording (if supported) or speech recognition analysis to:
- Discard in process analysis, replay the prompt, and resume
analysis.
- Discard recording in progress and resume analysis.
- Terminate the current operation and return the terminating key
sequence to the MGC.
f. Provide a way to ask the ARF MG to support the following
definable keys for speech recognition analysis. These keys would
then be able to be acted upon by the ARF MG:
- A key to terminate playing of an announcement in progress.
- A key that signals the end of user input. The key may or may
not be returned to the MGC along with the input already
collected.
- Keys to stop playing the current announcement and resume playing
at the beginning of the first segment of the announcement, last
segment of the announcement, previous segment of the
announcement, next segment of the announcement, or the current
announcement segment.
11.2.8.5. Speaker Verification/Identification Module
The speech verification/identification module returns parameters that
indicate either the likelihood of the speaker to be the person that
they claim to be (verification task) or the likelihood of the speaker
being one of the persons contained in a set of previously
characterized speakers (identification task).
The Speaker Verification/Identification Module must support all of
the requirements in the Play Module in addition to the following
requirements:
a. Be able to download parameters, such as speaker templates
(verification task) or sets of potential speaker templates
(identification task), either prior to the session or in mid-
session.
b. Be able to download application specific software to the ARF
either prior to the session or in mid-session.
c. Be able to return parameters indicating either the likelihood of
the speaker to be the person that they claim to be (verification
task) or the likelihood of the speaker being one of the persons
contained in a set of previously characterized speakers
(identification task).
d. Be able to provide the following basic operation: request an AEG
to play an announcement and then perform speech
verification/identification analysis.
e. Be able to specify these event collection characteristics: The
number of attempts to give to perform speech
verification/identification task.
f. With respect to speech verification/identification analysis
timers, allow the specification of:
- Time to wait for the user to initially speak.
- The amount of silence necessary following the last speech
segment for the speech verification/identification analysis
segment to be considered complete.
- The maximum allowable length of the speech
verification/identification analysis (not including pre- and
post- speech silence).
g. To be able to allow multiple prompt operations for DTMF digit
collection (if supported), voice recording, (if supported),
speech recognition analysis (if supported) and/or speech
verification/identification and provide the following types of
prompts:
- Initial Prompt
- Reprompt
- Error prompt
- Failure announcement
- Success announcement.
h. Allow the specification of definable key sequences for digit
recording (if supported) or speech recognition (if supported) in
the speech verification/identification analysis to:
- Discard speech verification/identification in analysis, replay
the prompt, and resume analysis.
- Discard speech verification/identification analysis in progress
and resume analysis.
- Terminate the current operation and return the terminating key
sequence to the MGC.
i. Provide a way to ask the ARF MG to support the following
definable keys for speech verification/identification analysis.
These keys would then be able to be acted upon by the ARF MG:
- A key to terminate playing of an announcement in progress.
- A key that signals the end of user input. The key may or may
not be returned to the MGC along with the input already
collected.
- Keys to stop playing the current announcement and resume speech
verification/identification at the beginning of the first
segment of the announcement, last segment of the announcement,
previous segment of the announcement, next segment of the
announcement, or the current announcement segment.
11.2.8.6. Auditory Feature Extraction/Recognition Module
The auditory feature extraction/recognition module is engineered to
continuously monitor the auditory stream for the appearance of
particular auditory signals or speech utterances of interest and to
report these events (and optionally a signal feature representation
of these events) to network servers or MGCs.
The Auditory Feature Extraction/Recognition Module must support the
following requirements:
a. Be able to download application specific software to the ARF
either prior to the session or in mid-session.
b. Be able to download parameters, such as a representation of the
auditory feature to extract/recognize, for prior to the session
or in mid-session.
c. Be able to return parameters indicating the auditory event found
or a representation of the feature found (i.e., auditory
feature).
11.2.8.7. Audio Conferencing Module
The protocol must support:
a. a mechanism to create multi-point conferences of audio only and
multimedia conferences in the MG.
b. audio mixing; mixing multiple audio streams into a new composite
audio stream
c. audio switching; selection of incoming audio stream to be sent
out to all conference participants.
11.2.9. Multipoint Control Units
The protocol must support:
a. a mechanism to create multi-point conferences of audio only and
multimedia conferences in the MG.
b. audio mixing; mixing multiple audio streams into a new composite
audio stream
c. audio switching; selection of incoming audio stream to be sent
out to all conference participants.
d. video switching; selection of video stream to be sent out to all
conference participants
e. lecture video mode; a video selection option where on video
source is sent out to all conference users
f. multi-point of T.120 data conferencing.
g. The ability for the MG to function as an H.323 MP, and for the
MGC to function as an H.323 MC, connected by this protocol
(MEGACOP/H.248). It should be possible for audio, data, and
video MG/MPs to be physically separate while being under the
control of a single MGC/H.323 MC.
12. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[2] ITU-T Recommendation Q.2630.1, AAL type 2 Signalling Protocol
(Capability Set 1), December 1999.
[3] ITU-T Recommendation H.341, Line Transmission of Non-Telephone
Signals, May 1999.
[4] ATM Forum Technical Committee, af-vtoa-0083.001, Voice and
Telephony Over ATM to the Desktop Specification, March 1999.
[5] ITU-T Recommendation H.323v3, Packet-based Multimedia
Communications Systems (includes Annex C - H.323 on ATM),
September 1999.
[6] ATM Forum Technical Committee, af-saa-0124.000, Gateway for
H.323 Media Transport Over ATM, May 1999.
[7] ITU-T Recommendation T.140, Protocol for Multimedia Application
Text Conversation, February 1998.
[8] ITU-T Recommendation V.18, Operational and Interworking
Requirements for DCEs Operating in Text Telephone Mode, February
1998.
[9] ITU-T Recommendation Q.931, Digital Subscriber Signalling System
No. 1 (DSS 1) - ISDN User - Network Interface Layer 3
Specification for Basic Call Control, May 1998.
14. Acknowledgements
The authors would like to acknowledge the many contributors who
debated the Media Gateway Control Architecture and Requirements on
the IETF Megaco and Sigtran mailing lists. Contributions to this
document have also been made through internet-drafts and discussion
with members of ETSI Tiphon, ITU-T SG16, TIA TR41.3.4, the ATM Forum,
and the Multiservice Switching Forum.
15. Authors' Addresses
Nancy Greene
Nortel Networks
P.O. Box 3511 Stn C
Ottawa, ON, Canada K1Y 4H7
Phone: (514) 271-7221
EMail: ngreene@nortelnetworks.com
Michael A. Ramalho
Cisco Systems
1802 Rue de la Port
Wall Township, NJ
Phone: +1.732.449.5762
EMail: mramalho@cisco.com
Brian Rosen
Marconi
1000 FORE Drive, Warrendale, PA 15086
Phone: (724) 742-6826
EMail: brosen@eng.fore.com
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