ITU-BDT Regional Seminar on Fixed Mobile Convergence
and new network architectures for the Arab Region
Tunis, Tunisia, 21-24 November 2005
1.3.12/Part 1: Signalling Protocols and Evolving
Architectures for NGN
Riccardo Passerini, ITUITU-BDT
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Evolution of PSTN/ISDN to NGN
NGN (Next Generation Network) is believed to provide new opportunities for
and capabilities to the network and service providers. Considering that
existing networks have different life span and vast amount of capital has been
spent on them, complete replacement of their components is not considered to
be either advisable or possible. So, a phased approach should be considered
for evolution of existing networks to NGN.
PSTN/ISDN ( Public Switched Telephone Network/Integrated Services
Digital Network) being one of the first networks, is considered to be prime
candidate for evolution. For PSTN/ISDN evolution to NGN a phased
approach is considered
Different evolutionary Scenarios with PSTN/ISDN emulation (adaptation to
IP infrastructure) and with PSDN/ISDN simulation (session control over IP
interfaces and infrastructure) are presently under consideration in ITU in
order to providereference for the evolution to NGN
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1
Evolution Architecture
PSTN
IP
GW
LEX
IP
IP can be considered as a “ Data NGN ”
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Signalling Requirements to Support "IP telephony"
PSTN
STP
Transit
Exchange
PSTN
CA
MGC
MG
MGC
IP Network
STP
Transit
Exchange
MG
Local
Exchange
Local
Exchange
Phone (E.164)
Phone (E.164)
Network configuration A (phone to phone communication)
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Configuration A: Phone to Phone Communication (with IP
Transit Network)
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In the IWF (such as MG, MGC, SG function) between the PSTN and
IP network at the originating and terminating sides, control signalling
(ISUP - H.323 / SIP conversion) and information signalling (64-kbps
bearer - IP packet conversion) are converted. In the IP network, a call
is controlled by the H.323 / SIP protocol. A user dials a phone number
to identify the terminating phone terminal and also, in some cases,
additional information (e.g. through the use of prefix dialling) to select
an IP transit network.
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PSTN
STP
Transit
Exchange
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CA
MGC
MG
IP Network
Local
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IP Phone
Phone (E.164)
Network configuration B1 (IP phone to phone communication)
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Configuration B-1: IP Phone to Phone communication
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In the IWF (such as MGC, MG, SG functions) between a PSTN
and an IP network at the terminating side, the signalling protocol
(ISUP - H.323 / SIP conversion) and the user information (64-kbps
bearer - IP packet conversion) are converted. In the IP network, a
call is controlled by the H.323 / SIP protocol. The originating IP
phone user dials a phone number to identify the terminating
phone terminal.
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PSTN
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CA
STP
Transit
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MGC
MG
IP Network
Local
Exchange
IP Phone
Phone (E.164)
Network configuration B2 (phone to IP phone communication)
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Configuration B-2: Phone to IP Phone communication
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IP P h on e (ID )
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Configuration C: IP Phone to IP Phone communication
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Configuration C: IP Phone to IP Phone communication
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Network capabilities to support “IP telephony”
interworking between the PSTN and the IP network
Carrier A
Carrier B
PSTN
STP
Transit
Exchange
MGC
MG
CA
H323,SIP,etc
IP Network
CA
IP Network
RTP
Local
Exchange
Phone (E.164)
IP Phone
Network configuration connecting carriers (NNI between IP-IP network
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Carrier A
PSTN
Carrier B
ISUP(SS7)
STP
Transit
Exchange
SDH
CA
MGC
IP Network
MG
Local
Exchange
IP Phone
Phone (E.164)
Network configuration connecting carriers (NNI between PSTN-IP network
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Control Protocols for support of “IP telephony”
PSTN
STP
MGC
SG
Transit
Exchange
MG
H.323/SIP
H.248/
MEGACO
CA
IP Network
H.323/SIP
RTP/RTCP
Local
Exchange
Phone (E.164)
IP Phone
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TERMS AND DEFINITIONS
Media Gateway (MG): A media gateway converts the media provided by one type of network
to the format required by another type of network. Terminates voice calls on inter-switch
trunks from the PSTN, compresses and packetizes the voice data, and delivers compressed
voice packets to the IP network. For voice calls originating in an IP network, the media
gateway performs these functions in reverse order.
Media Gateway Controller (MGC): Controller that controls the parts of the call state that
pertain to connection control for the media channels within a media gateway. A media
gateway controller handles the registration and management of resources at the media
gateway(s). A media gateway controller exchanges ISUP messages with central-office
switches via a signaling gateway (described below). Because vendors of media gateway
controllers often use off-the-shelf computer platforms, a media gateway controller is
sometimes called a softswitch.
Signalling Gateway (SG): A signalling gateway provides transparent interworking of
signalling between switched-circuit and IP networks. The signalling gateway may terminate
PSTN/SS7 signalling or relay messages over an IP network to a media gateway controller or
another signalling gateway. Because of its critical role in integrated voice networks,
signalling gateways are often deployed in groups of two or more to ensure high availability
Call Agent (CA): Function that controls the provision of services to users.
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TERMS AND DEFINITIONS
Telephone Number Mapping (ENUM): Protocols for mapping telephone numbers to IP phone
identifiers (i.e. E.164 numbers to URIs).
IP Network:An IP network is a network that uses IP technologies to transport information. It
may be a Private IP network, or a Carrier’s network.
Phone: Phone refers to a PSTN terminal.
IP Phone: IP phone refers to a terminal (e.g. dedicated voice terminal or multipurpose personal
computer) that is connected directly (e.g. Through an Ethernet interface or an xDSL line) to an
IP network.
IP telephony:“IP telephony” is a service that enables the exchange of voice information,
primarily in the form of packets, using IP protocols.
Internet Telephony: The combination of the term ‘Internet’ with the term ‘telephony’ is
regarded as a specific use of the Internet, rather than a service. The Internet offers many
capabilities to users, including the ability to carry bi-directional speech in real time or near
real time. This is considered to be an Intrinsic capability of the Internet and not
a telecommunications service.
(Note) Internet telephony is a particular application of the Internet and therefore falls outside of
the scope of this document.
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8
CONTROL PROTOCOLS FOR SUPPORT OF “IP TELEPHONY”
This section describes the protocol stacks used for the call control and
media transport in “IP telephony”.
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Protocol Stacks (1)
IP phone
/MGC
1)
CA
2)
MGC
H.323/SIP
H.248/MEGACO/
SCTP/TCP/UDP
TCP/UDP
IP
IP
Layer 2
Layer 2
Layer 1
Layer 1
MG
Protocol stack (example for configurations B-1 and B-2)
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Protocol Stacks (2)
MG
3)
IP phone
4)
SG
MGC
SS7 Upper Layer
RTP/RTCP
SCTP
UDP
Adaptation Layer
IP
IP
Layer 2
Layer 2
Layer 1
Layer 1
Protocol stack (example for configurations B-1 and B-2)
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General Framework for migrating Telephony networks
towards Next Generation Networks (NGN)
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General Framework for migration to NGN
(The Essential Report on IP telephony – ITU-D
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Step 1: use of today’s TDM based network for voice telephony and Internet
access
Step 2: consolidation of switching and access equipment;
Step 3: introduction of Voice-over-Packet technology for trunking;
Step 4: introduction of Voice-over-Packet technology in access and CPE
Step 5: multimedia services and new applications;
Step 6: end-of-life replacement of legacy infrastructure and migration to all-IP
signalling.
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Step 1: PSTN for Voice and Internet
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Step 3: Voice over Packet for Trunking
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Step 5 : Multimedia
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ApGW
Class 4
AS
SCP
Class 5
SS7/IP
(SIGTRAN)
MM
PortalSoftswitch
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WWW
STP
SS7
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IP, ATM
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IP
Voice
Data
IP
VoP
Signalling
Step 6 : The full NGN
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Protocols: Status of studies being undertaken by
international standards organizations
Call control
Protocol
Media control
H.323
ITU-T SG16
H.323
SIP
IETF SIP-WG
RFC3261
H.248/MEGACO
RTP/RTCP
IETF MEGACO-WG
RFC3015
ITU-T SG16
H.248
IETF MMUSIC-WG
RFC1889
RFC3398
Note : For the latest draft document, see
URL of SIPPING-WG
IETF SIPPING-WG
Interwork
SIP-ISUP inter-working
ITU SG11
TRQ.2815 supplement 45
ITU SG11
Q.1912.5
Note: http://www.ietf.org/html.charters/sipping-charter.html
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ANNEX:
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15