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Requirements for a Protocol between Media Gateway
Controller and Media Gateway (Reference Point 'N')
Status of this document
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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Abstract
This Internet-Draft contains an update of ETSI-TIPHON's work
on requirements for a protocol at the reference point N (in TIPHON
parlance) of the TIPHON reference configuration. Reference point N
designates the information flows required for control of a Media
Gateway in a decomposed Gateway [0].
This Internet-Draft supercedes the one submitted in November 1998
and represents consensus of working group 2 in TIPHON.
ETSI-TIPHON is working on standards to connect VoIP (Voice over IP)
networks and phone networks.
TABLE OF CONTENTS
1 Introduction
2 References
3 Definitions and abbreviations
3.1 Definitions
3.2 Abbreviations
4 Reference Configuration
5 Scope of Media Gateway Control
6 Media Gateway Examples
7 Protocol Requirements
7.1 Connection Control
7.2 Endpoint Attributes
7.3 Content Insertion
7.4 Event Detection and Notification
7.5 Other Requirements
7.5.1 Modularity and Extensibility
7.5.2 Resource Management
7.5.3 MGC-MG Association Management
7.5.4 Scripting
7.6 Security
8 Subjects for further study
9 Acknowledgements
10 Contact Address
1 INTRODUCTION
ETSI-TIPHON [12] is working on standards to connect VoIP (Voice over
IP) networks and phone networks. Of its now 7 working groups, working
group 2 (WG2) is on architecture and reference configurations which
are necessary for the delivery of telephone calls which originate in
- an IP network and are delivered to Switched Circuit Networks (SCN),
such as Public Switched Telephone Network (PSTN), Integrated Services
Digital Networks (ISDN) and Global System for Mobile communication
(GSM) networks (so-called TIPHON Scenario 1),
- an SCN and are delivered to an IP network (TIPHON Scenario 2),
- an SCN and are delivered to an SCN via an IP transit network
(TIPHON Scenario 3).
This Internet-Draft is the asciified version of, in ETSI parlance,
Technical Specification DTS/TIPHON-02005 Version V0.1.1, with status
15 January 1999 [0].
2 References
[0] DTS/TIPHON-02005 V0.1.1 (1999-01): "Telecommunications and
Internet Protocol Harmonization Over Networks (TIPHON); Requirements
for a Protocol at Reference Point N; Media Gateway Controller to
Media Gateway"; http://docbox.etsi.org/tech-org/tiphon/Document/
tiphon/07-drafts/wg2/DTS02005/V0.1.1/DTS02005%20V011.doc
[1] ITU-T Recommendation E.164 (1997): "The international public
telecommunications numbering plan".
[2] ITU-T Recommendation H.323 (1998): "Packet-based multimedia
communication".
[3] ITU-T Recommendation H.245 (1998): "Control protocol for
multimedia communication".
[4] ITU-T Recommendation H.225.0 (1998): "Call signalling protocols
and media stream packetization for packet based multimedia
communication systems".
[5] ITU-T Recommendation H.235 (1998): "Security and encryption for
H. series [H.323 and other H.245-based] multimedia terminals".
[6] TR 101 306: "Telecommunications and Internet Protocol
Harmonization Over Networks (TIPHON); Requirements for service
interoperability; Scenario 1".
[7] TS 101 324: " Naming and addressing; Scenario 1".
[8] ITU-T Recommendation G.711: "Pulse code modulation (PCM) of
voice frequencies".
[9] ITU-T Recommendation G.723.1: "Dual rate speech coder for
multimedia communications transmitting at 5.3 and 6.3 kbit/s".
[10] ITU-T Recommendation Q.931: "ISDN user-network interface layer
3 specification for basic call control".
[11] TS 101 313: "Telecommunications and Internet Protocol
Harmonization Over Networks (TIPHON); Network architecture and
reference configurations; Phase II: Scenario 1 + Scenario 2", Version
0.4.1, January 1999.
[12] http://www.etsi.org/tiphon. All ETSI-TIPHON documents are
publicly available for free through this website. TIPHON mailing
lists are also open to everyone.
3 Definitions and abbreviations
3.1 Definitions
- Gateway: A gateway is an endpoint on a network which provides for
real-time, two-way communication between terminals on an IP based
network and other terminals on switched circuit network.
- Media gateway: Provides the media mapping and/or transcoding
functions.
- Media gateway controller: Controls the Media Gateways.
- Signalling gateway: Provides the signalling mediation function
between the IP domain and the SCN domain.
3.2 Abbreviations
ATM Asynchronous Transfer Mode
BRI Basic Rate Interface
DTMF Dual Tone Multi Frequency
GK GateKeeper
GW Gateway
IP Internet Protocol
MG Media Gateway
MGC Media Gateway Controller
NNI Network to Network Interface
PRI Primary Rate Interface
PSTN Public Switched Telephone Network
QoS Quality of Service
RTCP Real-time Transport Control Protocol
RTP Real-time Transport Protocol
SCN Switched Circuit Networks
SG Signalling Gateway
SS7 Signalling System N 7
UNI User to Network Interface
4. Introduction
Figure 1 shows the reference configuration as defined in [11].
Reference Point N designates the information flows required for
control of a Media Gateway in a decomposed Gateway.
+--------+ D +--------+ G +--------+
| GK |------| GK |------|Back End|
+--------+ +--------+ +--------+
| | /
A | C | ___F___/
| . . . . . . ./. . . . . . . .
+--------+ . +--------+ J +--------+ .
|H323 Ter| . | MGC |------| SG |------
+--------+ . +--------+ +--------+ .
\____B_. | .
.\ N | .
. \ | .
. +--------+ .
. | MG |-----------------.----
. +--------+ .
. . . . . . . . . . . . . . .
Figure 1: Basic call reference configuration
(MGC = Media Gateway Controller, MG = Media Gateway,
SG = Signalling Gateway, H323 Ter = H.323 Terminal)
5. Scope of Media Gateway Control
This subclause is a copy of subclause 6.7 of [11]
Information flows at reference point N shall include support for the
following functions:
1) creation, modification, and deletion of media stream connections
across the Media Gateway;
2) specification of the transformations to be applied to media
streams as they pass through the Media Gateway, when connections are
created and subsequently during the life of the connection;
3) requesting the insertion of tones and announcements into media
streams, either by explicit request of the Media Gateway Controller
or by indicating that insertion should begin and end with the
detection of specified events within the Media Gateway itself;
4) requesting the reporting of, and possibly specifying the actions
to take upon detection of specified events within the media streams.
6. Media Gateway Examples
Media Gateway examples include but are not limited to the following
applications:
- Trunk gateways that interface between SCN networks and Voice over
IP network. Such gateways typically interface to SS7 or other NNI
signalling on the SCN and manage a large number of digital circuits.
- Voice over ATM gateways which operate much the same way as voice
over IP trunk gateways, except that they interface to an ATM network.
- Access gateways that interface UNI interfaces like ISDN (PRI and
BRI) and traditional analogue voice terminal interfaces to a Voice
over IP network.
- Residential gateways are access gateways for a small number of
voice terminals that can be co-located with a cable modem or set top
box.
- Network Access Servers, which convert modem signals from an SCN
network and provide data access to the packet network.
- Multipoint Processing Units that provide multipoint connectivity
between terminals on SCN and packet networks.
- Interactive Voice Response systems that provide automatic voice
response and switching services in response to DTMF signals from the
SCN.
- IP Gateways that are used to interface either between
administrative domains which apply different policies (e.g. proxies),
or to transform media streams formats (e.g. transcoding).
- Fax Gateways
7. Protocol Requirements
7.1 Connection Control
In keeping with the capabilities of H.245, connection control shall
be able to provide at least the following types of connection:
- circuit to packet (IP);
- circuit to packet (ATM - H.323 Annex C operation);
- packet to packet
- circuit to circuit (e.g. circuit side fallback).
The connection mode may be unicast or multicast, or a connection may
be inactive. Typically connections are specified unidirectionally,
but may be specified in both directions at once. On internet
connections, IPv4 is mandatory and IPv6 may be supported.
On the circuit side, SS7 requires the ability to perform a loopback
for continuity testing. This requirement applies especially to
loopback through a circuit at the edge of the Media Gateway (path 1
in Figure 2), but as Figure 2 shows, four different types of loopback
are possible (e.g. fault isolation)
SCN Packet
+========================+
| |
---------------------------------------\
| |
path 2 <--------------------------------------/
| |
. . | . . . . . . . . . . . . . . . . . .
. | |
. . | . . . . . . . . . . . .|. . . . . > path 4
| |
path 1 -------------\ . . . . . . . . . .
| . |
<------------/ . . . . . . . . . > path 3
| |
+========================+
Figure 2 - Possible Types Of Loopback
Abstracting from this, the Media Gateway control interface shall
support the following capabilities for connection control:
1) Ability to identify the IP, ATM, and/or circuit local end-points,
and local connections within the Media Gateway between two or more of
its endpoints. Endpoints shall be designated using a hierarchical
naming construct.
2) It must be possible to wildcard the low-order portion of the
endpoint identifier (i.e. the port number for an IP transport address
or the low-order term of a circuit identifier), with the intent that
Media Gateway shall select a suitable endpoint instance and return
its full identity to the Media Gateway Controller.
3) Wildcarding shall be provided to allow a simultaneous reference to
multiple endpoints.
4) Ability to request the selection of ports for both RTP and RTCP
reception on the packet side. The numerical values of RTP and RTCP
ports may be non-subsequent numbers. [2]
5) Ability to convey the requested QoS parameters applicable on the
packet side of a media stream connection.
6) Ability to convey the requested bearer capabilities applicable on
the SCN side of a media stream connection.
7) Ability to convey QoS statistics for an established connection at
any time during the call and at teardown.
8) Ability for the Media Gateway to autonomously report if the QoS on
a given connection has fallen below a specified value.
7.2 Endpoint Attributes
The protocol shall be able to convey the following endpoint
attributes:
1) Media protocol used (RTP, fax-protocol, ...)
2) Payload type (e.g. codec),
3) Codec-related attributes like packetisation interval, jitter
buffer size and silence suppression where appropriate
4) Generation of comfort noise during silent periods.
5) Application of encryption/decryption and identification of the
encryption schemes.
6) Echo cancellation
7) Lawful interception of the content of a specified media stream.
8) It should be possible to support additional endpoint attributes as
they are defined without modifying the existing semantics.
The protocol shall allow
- the specification of the endpoint attributes when the connection is
created;
- modification of endpoint attribute values for an already-existing
connection (e.g. in response to a H.245 FlowControlCommand or
ReplacementFor OLC (OpenLogicalChannel) ).
7.3 Content Insertion
The protocol shall support
- the ability to request the sending of a specified tone or
announcement, at any time, in a specified direction
- the specification of the conditions under which the sending should
stop
- the ability to request muting of a media stream.
- the ability for the controller to request the Media Gateway to
insert and detect tones as required for SS7 continuity testing and
other forms of testing.
7.4 Event Detection and Notification
The protocol shall support the ability to instruct the Media Gateway
to detect, notify and possibly act upon specified events.
Examples of events are:
- DTMF tones,
- off-hook, on-hook
7.5 Other Requirements
As a general requirement, the protocol shall allow the Media Gateway
to indicate to the controller whenever some or all of a request
cannot be executed. It shall be possible for the Media Gateway to
indicate the general nature of the problem and to provide further
details, possibly including vendor-specific content.
7.5.1 Modularity and Extensibility
It is essential that the protocol be both modular and extensible. Not
all implementations may wish to support all of the possible
extensions. This will permit light-weight implementations for
specialized tasks where processing resources are constrained.
The protocol shall provide the means whereby a controller can
determine the capabilities supported by a particular Media Gateway.
The protocol shall support backwards compatibility as new versions
are released.
The protocol shall allow the possibility of vendor-specific
extensions.
7.5.2 Resource Management
The protocol shall provide the means for the Media Gateway Controller
to determine resource availability within the associated Media
Gateway. The protocol shall allow for unsolicited messages between
the Media Gateway and Media Gateway Controller. Optionally, this
capability may allow for queries during regular operation. The means
by which remaining capacity is quantified is for further study.
It shall be possible for the Media Gateway to indicate to the
controller that it lacks sufficient resources to carry out a given
command.
It shall be possible for the Media Gateway Controller to audit the
commitment of resources to connections, to ensure that all
commitments are valid. It shall further be possible for the
controller to order that specific resource assignments be cleared if
it finds that they are invalid.
It shall be possible for the Media Gateway Controller to audit the
connection state of connections in Media Gateways with which it is
associated.
It shall be possible for the Media Gateway to report changes in
operational status of significant resources from in-service to out-
of-service and vice versa. This is especially required for
transmission facilities terminating on the Media Gateway.
7.5.3 MGC-MG Association Management
- The protocol shall provide the means to establish and remove an
MGC-MG association between a specific controller and a specific Media
Gateway.
- It shall be possible for a Media Gateway to establish an MGC-MG
association with an alternate Media Gateway Controller if its
currently associated controller becomes unavailable.
- It shall be possible for either the Media Gateway or the Media
Gateway Controller to detect loss of the control association.
7.5.4 Scripting
The protocol shall provide the means to download scripts to be
executed autonomously by the Media Gateway.
7.6 Security
The protocol shall allow for mutual authentication at the start of an
MGC-MG association , and for preservation of the integrity and
confidentiality of control messages once the association has been
established.
8. Subjects for further study
- Support for H.323 fast start
9. Acknowledgments
This draft is the work of Working Group 2 of ETSI-TIPHON. Many thanks
to all contributors.
10. Contact Address
Jozef Vandenameele
Alcatel Bell NV
Francis Wellesplein 1
B-2018 Antwerpen, Belgium
Tel: + 32-3-240 4361
Fax: + 32-3-240 9999 or 9961
email : Jozef.Vandenameele@btmaa.bel.alcatel.be
or send mail to the mailing list of ETSI-TIPHON WG 2:
TIPHON_WG2@list.etsi.fr
(to subscribe, send "subscribe TIPHON_WG2" to listserv@list.etsi.fr)
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