Signaling interworks in PSTN
and Voice-over-IP networks
By Ni Zhang, Ziying Chen
March, 2002
Terminology

Voice over IP(VoIP) refers to the technology
of providing telephone calls (voice) over IP
networks.
 Telephony service: video, fax, voice mail…
 IP Telephony refers to the technology of
providing telephony service over IP.
networks.
Terminology (Continued)

Signaling: exchange of information between
involved components in the network
required to provide and maintain services.
For example, accessing a voice mailbox.
What is signaling interwork?

Different signaling protocols for telephony
(in different networks) work together. For
example, PSTN signaling works with IP
signaling, and a phone call can be set up
between the traditional PSTN and an IP
network.
PSTN
signaling
IP network
Why signaling interwork?

Two networks: PSTN and IP-based Internet.
PSTN uses SS7, the global standard. SS7
has its own architecture, while TCP/IP
architecture is different.
 We need telephony service everywhere.
 We want low price for telephony service.
An IP network can better utilize bandwidth,
so we pay less for the resource we use.
 We want more service.
Signaling Interwork Approaches

SS7 is transported to IP networks.
 And more…
PSTN
Traditional SS7
Signaling
Transport
IP network
Packetized SS7
VoIP Gateway
Out-of-band signaling in SS7 - signaling & voice channels.
 The VoIP gateway interfaces PSTN and IP networks for
signaling and voice delivering..

traditional SS7
signals
SS7 IP
packets
VoIP
gateway
PSTN
voice stream
IP network
voice IP
packets
Decomposed Gateway Architecture

VoIP gateway: Signaling Gateway(s), Media
Gateway(s) and a Media Gateway Controller
Traditional Signaling
SS7 signals Gateway
PSTN
SS7 IP
packets
Media Gateway
Controller
control
voice stream Media
Gateway
voice IP
packets
IP network
Signaling Gateway (SG)

Signaling Gateway relays, translates or
terminates SS7 signaling. The process is
named Signaling Transport.
Traditional SS7
signals
Signaling
Gateway
SS7 signal packets
(packetized SS7
messages)
Media Gateway (MG)

Media Gateway terminates circuit-switched
network media streams, packetizes the media data,
if it is not already packetized, and delivers the
packetized traffic to the packet-switched network.
It does the reverse function for the other traffic
direction.
Voice stream
Media
Gateway
Voice
encapsulated
in IP packets
Media Gateway Controller(MGC)

Media Gateway Controller controls Media
Gateway. For instance, resource management,
registration, etc. The controller is also named Call
Agent.
Media Gateway
Controller (MGC)
MGCP
Media Gateway(MG) …
Media Gateway (MG)
Media Gateway Control Protocol
(1)

Media Gateway Control Protocol (MGCP):
a mechanism for communication between
Media Gateway Controller and Media
Gateway.
 MGCP allows centralized control: one
controller controls multiple media
gateways.
Media Gateway Control Protocol
(2)

MGCP: a master/slave protocol. A media gateway executes
commands sent by MGC. There are 9 commands in
MGCP: NotificationRequest, Notify, CreateConnection,
ModifyConnection, DeleteConnection, etc.
Media Gateway Controller
Command
Acknowledgement
(ACK)
Media Gateway
SS7 ISUP provides call control (1)

SS7 ISUP sets up a call: ISUP IAM.
STP
X
1.IAM
SSP
A
caller
STP
Y
SSP
B
called party
SS7 ISUP provides call control(2)

SS7 ISUP sets up a call: ISUP ACM.
STP
X
SSP
A
caller
STP
Y
2.ACM
SSP
B
called party
SS7 ISUP provides call control(3)

SS7 ISUP sets up a call: ISUP ANM.
STP
X
SSP
A
caller
STP
Y
3.ANM
SSP
B
called party
SS7 ISUP provides call control (4)

SS7 ISUP releases a call: ISUP REL
STP
X
STP
Y
1. REL
SSP
A
caller
SSP
B
called party
Assuming that the called party hangs up first.
SS7 ISUP provides call control(5)

SS7 ISUP releases a call: ISUP RLC
2.RLC
STP
X
STP
Y
SSP
A
caller
SSP
B
called party
Assuming that the called party hangs up first.
SS7 ISUP & IP signaling interwork

It provides an interface for ISUP messaging
between entities in the SS7 network and IP
network.
SS7 ISUP & IP signaling interwork

Consider an example where a call originates from
an IP network to the PSTN.
Signaling Gateway
IP net
PSTN
STP
Media Gateway Controller
Caller
phone
Media
Gateway 1
(MG1)
IP
net
Media
Gateway 2
(MG2)
SSP
Called
phone
ISUP & IP signaling interwork example

Part 1 of a call set-up: ISUP IAM
IP net
k. convert IAM packet
to traditional SS7
IAM
Signaling Gateway
NotificationRequest:
detect an off-hook event
a. Off-hook
l
k
l. send SS7
IAM to PSTN
b. Notify: off-hook detected
j
c. NotificationRequest:
Media Gateway Controller
c
Caller
phone
a
d
f
b
MG 1
e g
h
IP
net
PSTN
collect a phone number
d. Dial digits
e. Notify: collection of
digits
f. CreateConnection
g. Acknowledge
MG 2
i
h. CreateConnection
i. Acknowledge
j. constructs SS7 IAM
packet and sends to SG.
ISUP & IP signaling interwork example

Part 2 of a call set-up: ISUP ACM
SS7 ACM
IP net
PSTN
a. convert SS7 ACM to ACM packet
b. NotificationRequest: detect
the ringing tone.
c. Notify: ringing tone is
detected
d. NotificationRequest: detect
the ringing tone.
e. Notify: ringing tone
Signaling Gateway a
f
Media Gateway Controller
d
is detected.
f. Receive Notify
Caller
phone
STP
b
SSP
MG 1
e
IP
MG2
c
Called
phone
ISUP & IP signaling interwork example

Part 3 of a call set-up: ISUP ANM
SS7 ISUP ANM
IP net
PSTN
a. convert traditional SS7
ANM to ANM packet
Signaling Gateway a
b. ModifyConnection: sets the
connection to send/receive.
c. Acknowledge
STP
d
Media Gateway Controller
d. Receive ACK
b
Caller
phone
MG 1
c
conversation
IP
MG2
SSP
Called
phone
ISUP & IP signaling interwork example

Call release: ISUP REL & RLC
SS7 ISUP REL
IP net
a. convert SS7 REL to REL packet
b. DeleteConnection to MG2
c. Acknowledge
d. DeleteConnection to MG1
e. Acknowledge
g
STP
Signaling Gateway
a
f
Media Gateway Controller
f. construct a RLC
d
b
packet, send it to SG.
Caller
phone
PSTN
MG 1
SSP
e
IP
net
c
MG2
g. convert the RLC packet to traditional SS7 ISUP RLC, sends it to PSTN
Called
phone
SIP complements MGCP in signaling interwork


When media gateways controlled by different controllers support a call, the originating
controller invites the terminating one to join a SIP session. The controllers talk via SIP.
MGCP is only used between the media gateway controller and the media gateway.
PSTN
STP
SG 1
SG 2
SIP
MGC 1
MGCP
SSP
Caller
phone
PSTN
IP network
MG 1
STP
MGC 2
MGCP
IP network
MG 2
SSP
Called
phone
SS7 ISUP & IP signaling
interwork

Call control is provided seamlessly between
SS7 and IP networks.
Introduction to SS7 TCAP
 TCAP
Transaction Capabilities Application Part
- a user part of SS7
- supports non-circuit-related telephony
transactions in SS7 networks
e.g. prepaid calls, 800-number service, etc.
A example of TCAP messaging

A 1-800 call is originated from a PSTN user
within the SS7 network.
SCP
Caller
Telephone
1.A 1-800 call
4. TCAP Response
3.TCAP Query
5. TCAP Response
SSP
STP
2.TCAP Query
SS7 TCAP/IP Signaling interwork

It provides an interface for TCAP messaging
between entities in the SS7 network and IP
network.
Architecture of TCAP/IP signaling (1)
PSTN
IP network
IP SCP
SS7 TCAP
SS7
Entity
SS7 TCAP
STP
Signaling
Gateway
Packetized
TCAP message
Media Gateway
Controller
media stream
Media
Gateway
If needed, a SCP (IP SCP) can be built
in the IP network, which provides the
same functionality as SS7 SCP.
IP packets
IP network
Architecture of TCAP/IP signaling (2)
• IP SCP, if it is present, provides the same
functionality as SS7 SCP.
• TCAP doesn’t go to Media Gateway.
• Signaling Gateway converts traditional SS7 TCAP
to TCAP packets or does the reverse.
Inter-domain Scenarios for TCAP/IP
Signaling Interwork
Three major inter-domain scenarios for TCAP/IP
messaging:
•
•
•
SS7 entity communicates with IP SCP. For example, a
TCAP request comes from a SS7 STP to the IP SCP for
service.
IP entity communicates with SCP in SS7 network. For
example, a packetized TCAP message from the IP network
goes to the SS7 SCP for service.
IP signaling transports between two SS7 networks. For
example, the IP network is used to transport TCAP
messages between SS7 networks.
Scenario 1: SS7 entity communicates
with IP SCP
SCP is located in IP network – IP SCP.
 The call starts in SS7 network, which requires noncircuit-related transaction service, e.g. 1-800 call.
 SSP in SS7 originates the TCAP request. Then the
SSP, which is assisted by STP, communicates with
the IP SCP through Signaling Gateway.
 Note that Media Gateway and Media Gateway
Controller are not involved in this scenario.

The Network Model of Scenario 1
PSTN
IP net
IP SCP
SS7 TCAP
SSP
SS7 TCAP
STP
1-800 call
Signaling
Gateway
Packetized
TCAP message
Media Gateway
Controller
PSTN
user
media stream
Media
Gateway
IP packets
An example: A 1-800 call is made by a PSTN user
IP network
Scenario 1: SS7 entity communicates with IP SCP
PSTN
f
SSP
generates SS7
TCAP message,
sends to STP
IP net
STP e
a sends to SG
d
b TCAP ->TCAP packets
sends to IP SCP
SS7 TCAP
a
SS7 TCAP
Signaling
Gateway b
c responds TCAP, sends
back to SG
1-800 call
PSTN
user
d TCAP packets->TCAP
Packetized
TCAP message
Sends back to STP
c
IP SCP
e sends to SSP
f
receives TCAP
Scenario 2: IP entity communicates with
SS7 SCP

The SCP is located in SS7 as traditional.
 The call starts in IP network, which requires noncircuit-related transaction, e.g. 1-800 call.
 Media Gateway Controller (IP entity) originates
the TCAP request. Then the controller
communicates with the SS7 SCP through
Signaling Gateway and SS7 STP.
The Network Model of Scenario 2
PSTN
SCP
IP network
Signaling
Gateway
STP
SS7 TCAP
SS7 TCAP
Packetized
TCAP message
Media Gateway
Controller
1-800 packets
Media
Gateway
1-800 call
An example: A phone makes a 1-800 call in the IP network
Caller
phone
Scenario 2: IP entity communicates with SS7 SCP
PSTN
SS7 TCAP
e
SCP
IP net
SS7 TCAP Signaling
STP
Gateway
d
g
initiates a 1-800
call, sends to MG
a. 1-800 stream-> 1-800
packets, sends to
c
Packetized
TCAP message
f
Media Gateway
Controller
h
c. TCAP packets ->
b
TCAP, sends to STP
1-800 call
Caller
phone
a
b. generates TCAP/IP
packets, sends to SG
1-800 packets
Media
Gateway
MGC.
d. sends to SCP
e. responds TCAP,
sends back to STP
f. sends to SG
g. SS7 TCAP -> TCAP
packets, sends it to MGC.
h. receives TCAP packets
Scenario 3 in TCAP/IP signaling interwork

IP signaling transports between two SS7 networks. For
example, the IP network is used to transport TCAP
messages between SS7 networks.
PSTN (e.g. a
local exchange
carrier)
PSTN
IP network
TCAP
request
SSP
SCP
TCAP
response
Due to limited time, this complicated scenario is not illustrated here.
SS7 TCAP & IP signaling
interwork

Non-circuit-related transactions for service
like 800 number, calling card, etc, are
provided seamlessly between SS7 and IP
networks.
VoIP development

The convergence of voice (i.e. circuit-switched)
and data (i.e. packet-switched) networks for
offering seamless voice and multi-media services
is rapidly progressing.
 VoIP -> IP Telephony -> Internet Telephony
Issues for Future Research




Addressing and routing for IP telephony, e.g. how do we
address the IP SCP (if there is one) so that SS7 entities can
send it messages?
QoS for voice delivery in IP networks
Performance issues for signaling interwork, e.g. because
there is a lot of messaging, is the delay acceptable?
More…
Reference








SS7 ISUP tutorial and example from International Engineering Consortium (IEC),
http://www.iec.org/online/tutorials/ss7/topic07.html
SS7 TCAP tutorial and example from IEC, http://www.iec.org/online/tutorials/ss7/topic08.html
SS7 Tutorial, Performance Technologies, http://www.pt.com/tutorials/ss7/.
Russell, Travis. Signaling System #7. New York : McGraw-Hill, 1995. TK5103.8.R88 1995.
U. Black, Advanced Internet Technologies, Prentice Hall, 2001, TK 5105.8865 .B52 2001.
Framework Architecture for Signaling Transport (RFC2719), IETF,
http://www.ietf.org/rfc/rfc2719.txt?number=2719.
Media Gateway Control Protocol (RFC2705), IETF,
http://www.ietf.org/rfc/rfc2705.txt?number=2705.
Megaco Protocol Version 1 (RFC3015), IETF, http://www.ietf.org/rfc/rfc3015.txt..
Thank you!
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questions?
Question?