1. Signaling interworks in PSTN and Voice-over-IP networks
By Ni Zhang, Ziying Chen
March, 2002

2. 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.
The IP telephony may not be presented here as we always use voice as an example.

3. Terminology (Continued)
Signaling: exchange of information between involved components in the network required to provide and maintain services. For example, accessing a voice mailbox.

4. 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.
signaling
PSTN
IP network

5. 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.

6. Signaling Interwork Approaches
SS7 is transported to IP networks.
And more…
PSTN
Signaling Transport
IP network
Traditional SS7
Packetized SS7

7. 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
IP network
PSTN
voice IP packets
voice stream

8. Decomposed Gateway Architecture
VoIP gateway: Signaling Gateway(s), Media Gateway(s) and a Media Gateway Controller
Traditional SS7 signals
Signaling Gateway
SS7 IP packets
IP network
PSTN
Media Gateway Controller
control
Media Gateway
voice IP packets
voice stream

9. Signaling Gateway (SG)
Signaling Gateway relays, translates or terminates SS7 signaling. The process is named Signaling Transport.
Signaling
Gateway
SS7 signal packets (packetized SS7 messages)
Traditional SS7 signals

10. 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 encapsulated in IP packets
Media
Gateway
Voice stream

11. 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)

12. 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.

13. 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
Acknowledgement(ACK)
Command
Media Gateway

14. SS7 ISUP provides call control (1)
SS7 ISUP sets up a call: ISUP IAM.
STPX
STPY
1.IAM
SSPA
SSPB
caller
called party

15. SS7 ISUP provides call control(2)
SS7 ISUP sets up a call: ISUP ACM.
STPX
STPY
2.ACM
SSPA
SSPB
caller
called party

16. SS7 ISUP provides call control(3)
SS7 ISUP sets up a call: ISUP ANM.
STPX
STPY
3.ANM
SSPA
SSPB
caller
called party

17. SS7 ISUP provides call control (4)
SS7 ISUP releases a call: ISUP REL
STPX
STPY
1. REL
SSPA
SSPB
caller
called party
Assuming that the called party hangs up first.

18. SS7 ISUP provides call control(5)
SS7 ISUP releases a call: ISUP RLC
STPX
STPY
2.RLC
SSPA
SSPB
caller
called party
Assuming that the called party hangs up first.

19. SS7 ISUP & IP signaling interwork
It provides an interface for ISUP messaging between entities in the SS7 network and IP network.

20. SS7 ISUP & IP signaling interwork
Consider an example where a call originates from an IP network to the PSTN.
IP net
PSTN
Signaling Gateway
STP
Media Gateway Controller
Caller phone
SSP
Media Gateway 1 (MG1)
IP net
Media Gateway 2 (MG2)
Called phone

21. ISUP & IP signaling interwork example
Part 1 of a call set-up: ISUP IAM
IP net
PSTN k.
convert IAM packet to traditional SS7 IAM
Signaling Gateway
NotificationRequest: detect an off-hook event l a.
Off-hook k b.
Notify: off-hook detected l.
send SS7 IAM to PSTN j c.
NotificationRequest: collect a phone number
Media Gateway Controller d.
Dial digits
Caller phone c h f e.
Notify: collection of digits f.
CreateConnection a b
IP net g.
Acknowledge d
MG 2
MG 1 h.
CreateConnection g e i i.
Acknowledge j.
constructs SS7 IAM packet and sends to SG.

22. 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.
STP
Signaling Gateway a c.
Notify: ringing tone is detected f d.
NotificationRequest: detect the ringing tone.
Media Gateway Controller e.
Notify: ringing tone is detected. d b f.
Receive Notify
SSP
Caller phone
MG 1
MG2 IP e c
Called phone

23. 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
STP
Signaling Gateway b.
ModifyConnection: sets the connection to send/receive. a c.
Acknowledge d
Media Gateway Controller d.
Receive ACK b
SSP
Caller phone
MG 1
MG2 IP c
Called phone
conversation

24. ISUP & IP signaling interwork example
Call release: ISUP REL & RLC
SS7 ISUP REL
IP net
PSTN a.
convert SS7 REL to REL packet g b.
DeleteConnection to MG2
STP
Signaling Gateway c.
Acknowledge a d.
DeleteConnection to MG1 f e.
Acknowledge
Media Gateway Controller f.
construct a RLC packet, send it to SG. d b
SSP
IP net
Caller phone
MG 1
MG2 c e
Called phone g.
convert the RLC packet to traditional SS7 ISUP RLC, sends it to PSTN

25. 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
PSTN
IP network
SG 1
SG 2
STP
STP
SIP
MGC 1
MGC 2
MGCP
MGCP
SSP
SSP
MG 1
MG 2
IP network
Caller phone
Called phone

26. SS7 ISUP & IP signaling interwork
Call control is provided seamlessly between SS7 and IP networks.

27. Introduction to SS7 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.

28. A example of TCAP messaging
A call is originated from a PSTN user
within the SS7 network.
SCP
Caller
Telephone
1.A call
4. TCAP Response
3.TCAP Query
5. TCAP Response
STP
SSP
2.TCAP Query

29. SS7 TCAP/IP Signaling interwork
It provides an interface for TCAP messaging between entities in the SS7 network and IP network.

30. Architecture of TCAP/IP signaling (1)
PSTN
IP network
IP SCP
SS7 TCAP
SS7 TCAP
Signaling Gateway
SS7 Entity
STP
Packetized
TCAP message
Media Gateway
Controller
If needed, a SCP (IP SCP) can be built in the IP network, which provides the same functionality as SS7 SCP.
Media Gateway
media stream
IP packets
IP network

31. 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.

32. 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.

33. Scenario 1: SS7 entity communicates with IP SCP
SCP is located in IP network – IP SCP.
The call starts in SS7 network, which requires non-circuit-related transaction service, e.g 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.

34. The Network Model of Scenario 1
PSTN
IP net
IP SCP
SS7 TCAP
SS7 TCAP
Signaling Gateway
SSP
STP
Packetized
TCAP message
1-800 call
Media Gateway
Controller
PSTN user
Media Gateway
media stream
IP packets
IP network
An example: A call is made by a PSTN user

35. Scenario 1: SS7 entity communicates with IP SCP
generates SS7
TCAP message,
sends to STP
PSTN
IP net f
a sends to SG d
STP e
SSP
Signaling Gateway
b TCAP ->TCAP packets
sends to IP SCP
SS7 TCAP a
SS7 TCAP
c responds TCAP, sends
back to SG b
1-800 call
d TCAP packets->TCAP
Sends back to STP
Packetized
TCAP message
PSTN user
IP SCP c
e sends to SSP
f receives TCAP

36. Scenario 2: IP entity communicates with SS7 SCP
The SCP is located in SS7 as traditional.
The call starts in IP network, which requires non-circuit-related transaction, e.g call.
Media Gateway Controller (IP entity) originates the TCAP request. Then the controller communicates with the SS7 SCP through Signaling Gateway and SS7 STP.

37. The Network Model of Scenario 2
PSTN
IP network
Signaling Gateway
SCP
STP
SS7 TCAP
SS7 TCAP
Packetized
TCAP message
Media Gateway
Controller
1-800 packets
Media
Gateway
1-800 call
Caller phone
An example: A phone makes a call in the IP network

38. Scenario 2: IP entity communicates with SS7 SCP
PSTN
IP net
initiates a 1-800
call, sends to MG
SS7 TCAP
Signaling Gateway
SS7 TCAP
STP e
a stream-> 1-800
packets, sends to
MGC. c d g
SCP
Packetized
TCAP message f
b. generates TCAP/IP packets, sends to SG
Media Gateway
Controller
c. TCAP packets ->
TCAP, sends to STP b h
1-800 packets
d. sends to SCP
e. responds TCAP,
sends back to STP
1-800 call
Media Gateway
Caller phone
f. sends to SG
g. SS7 TCAP -> TCAP
packets, sends it to MGC. a
h. receives TCAP packets

39. 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
PSTN (e.g. a local exchange carrier)
IP network
TCAP request
SSP
SCP
TCAP response
Due to limited time, this complicated scenario is not illustrated here.

40. 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.

41. 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

42. 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…

43. Reference
SS7 ISUP tutorial and example from International Engineering Consortium (IEC),
SS7 TCAP tutorial and example from IEC,
SS7 Tutorial, Performance Technologies,
Russell, Travis. Signaling System #7. New York : McGraw-Hill, TK R
U. Black, Advanced Internet Technologies, Prentice Hall, 2001, TK B
Framework Architecture for Signaling Transport (RFC2719), IETF,
Media Gateway Control Protocol (RFC2705), IETF,
Megaco Protocol Version 1 (RFC3015), IETF,

44. Thank you!
Any questions?
Any Question?