Signaling interworks in PSTN Signaling interworks in PSTN and Voice-over-IP networksand Voice-over-IP networks

By Ni Zhang, Ziying ChenMarch, 2002

TerminologyTerminology

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)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?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 IP networksignaling

Why signaling interwork?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 ApproachesSignaling Interwork ApproachesSS7 is transported to IP networks.And more…

Traditional SS7

PSTN IP network

Packetized SS7

Signaling Transport

VoIP GatewayVoIP Gateway Out-of-band signaling in SS7 - signaling & voice channels. The VoIP gateway interfaces PSTN and IP networks for

signaling and voice delivering..

PSTN IP networkVoIP gateway

traditional SS7 signals

voice stream

SS7 IP packets

voice IP packets

Decomposed Gateway ArchitectureDecomposed Gateway Architecture VoIP gateway: Signaling Gateway(s), Media

Gateway(s) and a Media Gateway Controller

PSTN IP network

Signaling Gateway

Traditional SS7 signals

voice stream

SS7 IP packets

voice IP packets

Media Gateway Controller

Media Gateway

control

Signaling Gateway (SG)Signaling Gateway (SG)

Signaling Gateway relays, translates or terminates SS7 signaling. The process is named Signaling Transport.

Signaling

Gateway

Traditional SS7 signals

SS7 signal packets (packetized SS7 messages)

Media Gateway (MG)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.

Media

Gateway

Voice encapsulated in IP packets

Voice stream

Media Gateway Controller(MGC)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)

Media Gateway(MG) Media Gateway (MG)…

MGCP

Media Gateway Control Protocol Media Gateway Control Protocol (1)(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 Media Gateway Control Protocol (2)(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

Media Gateway

Command Acknowledgement(ACK)

SS7 ISUP provides call control (1)SS7 ISUP provides call control (1)SS7 ISUP sets up a call: ISUP IAM.

SSPA

SSPB

STPX

STPY

caller called party

1.IAM

SS7 ISUP provides call control(2)SS7 ISUP provides call control(2)

SS7 ISUP sets up a call: ISUP ACM.

SSPA

SSPB

STPX

STPY

caller called party

2.ACM

SS7 ISUP provides call control(3)SS7 ISUP provides call control(3)

SS7 ISUP sets up a call: ISUP ANM.

SSPA

SSPB

STPX

STPY

caller called party

3.ANM

SS7 ISUP provides call control (4)SS7 ISUP provides call control (4)

SS7 ISUP releases a call: ISUP REL

SSPA

SSPB

STPX

STPY

caller called party

1. REL

Assuming that the called party hangs up first.

SS7 ISUP provides call control(5)SS7 ISUP provides call control(5)SS7 ISUP releases a call: ISUP RLC

SSPA

SSPB

STPX

STPY

caller called party

2.RLC

Assuming that the called party hangs up first.

SS7 ISUP & IP signaling interworkSS7 ISUP & IP signaling interwork

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

SS7 ISUP & IP signaling interworkSS7 ISUP & IP signaling interwork Consider an example where a call originates from

an IP network to the PSTN.

Signaling Gateway

Media Gateway Controller

Media Gateway 1 (MG1)

Caller phone

Media Gateway 2 (MG2)

IP net

Called phone

PSTNIP net

SSP

STP

ISUP & IP signaling interwork exampleISUP & IP signaling interwork example Part 1 of a call set-up: ISUP IAM

Signaling Gateway

Media Gateway Controller

MG 1

Caller phone

MG 2IP net

PSTNIP netNotificationRequest: detect an off-hook event

a. Off-hook

a

b. Notify: off-hook detected

b

c. NotificationRequest: collect a phone number

cd. Dial digits

d

e. Notify: collection of digits

e

f. CreateConnection

f

g. Acknowledge

gh. CreateConnection

h

i. Acknowledgeij. constructs SS7 IAM

packet and sends to SG.

j

k. convert IAM packet to traditional SS7 IAM

kl. send SS7

IAM to PSTN

l

ISUP & IP signaling interwork exampleISUP & IP signaling interwork example Part 2 of a call set-up: ISUP ACM

Called phone

SSP

STPSignaling Gateway

Media Gateway Controller

MG 1 Caller phone

MG2IP

IP net PSTN

a

a. convert SS7 ACM to ACM packet

b

b. NotificationRequest: detect the ringing tone.

c

c. Notify: ringing tone is detected

d

d. NotificationRequest: detect the ringing tone.

e

e. Notify: ringing tone is detected.

f

f. Receive Notify

SS7 ACM

ISUP & IP signaling interwork exampleISUP & IP signaling interwork example

Part 3 of a call set-up: ISUP ANM

Called phone

SSP

STPSignaling Gateway

Media Gateway Controller

MG 1 Caller phone

MG2IP

IP net PSTNSS7 ISUP ANM

a

a. convert traditional SS7 ANM to ANM packet

b

b. ModifyConnection: sets the connection to send/receive.

c

c. Acknowledged

d. Receive ACK

conversation

ISUP & IP signaling interwork exampleISUP & IP signaling interwork example

Call release: ISUP REL & RLC

Called phone

SSP

STPSignaling Gateway

Media Gateway Controller

MG 1 Caller phone

MG2IP net

IP net PSTNSS7 ISUP REL

a

a. convert SS7 REL to REL packet

f

b

b. DeleteConnection to MG2

c. Acknowledge

c

d

d. DeleteConnection to MG1

e

e. Acknowledge

f. construct a RLC packet, send it to SG.

convert the RLC packet to traditional SS7 ISUP RLC, sends it to PSTN

g

g.

SIP complements MGCP in signaling interworkSIP 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.

Called phone

SSP

STP

Caller phone

SSP

STP

MGC 1 MGC 2

SG 1 SG 2

MG 1 MG 2

MGCP

IP network

IP network PSTNPSTN

SIP

MGCP

SS7 ISUP & IP signaling SS7 ISUP & IP signaling interworkinterwork

Call control is provided seamlessly between SS7 and IP networks.

Introduction to SS7 TCAPIntroduction to SS7 TCAP

TCAPTransaction 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 messagingA example of TCAP messaging A 1-800 call is originated from a PSTN userwithin the SS7 network.

Caller Telephone

SSPSTP

SCP

5. TCAP Response

4. TCAP Response

2.TCAP Query

3.TCAP Query

1.A 1-800 call

SS7 TCAP/IP Signaling interworkSS7 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)Architecture of TCAP/IP signaling (1)

Signaling Gateway

SS7 TCAP

Media GatewayController

Media Gatewaymedia stream IP packets

PacketizedTCAP message

IP SCP

IP network

STPSS7 Entity

SS7 TCAP

If needed, a SCP (IP SCP) can be built in the IP network, which provides the same functionality as SS7 SCP.

IP networkPSTN

Architecture of TCAP/IP signaling (2)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 Inter-domain Scenarios for TCAP/IP Signaling InterworkSignaling 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: Scenario 1: SS7 entity communicates SS7 entity communicates with IP SCPwith 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. 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 1The Network Model of Scenario 1

Signaling Gateway

SS7 TCAP

Media GatewayController

Media Gateway

media stream IP packets

Packetized TCAP message

IP SCP

IP network

STPSSP

SS7 TCAP

PSTN user

An example: A 1-800 call is made by a PSTN user

IP netPSTN

1-800 call

Scenario 1: Scenario 1: SS7 entity communicates with IP SCPSS7 entity communicates with IP SCP

Signaling Gateway

STPSSP

PSTN user

IP SCP

PSTN IP net generates SS7 TCAP message, sends to STP

SS7 TCAP SS7 TCAPa

a sends to SG

Packetized TCAP message

b

b TCAP ->TCAP packets sends to IP SCP

1-800 call

c

c responds TCAP, sends back to SG

d

d TCAP packets->TCAP Sends back to STP

e sends to SSP

ef

f receives TCAP

Scenario 2: Scenario 2: IP entity IP entity communicates with communicates with SS7 SCPSS7 SCP

The SCP is located in SS7 as traditional. The call starts in IP network, which requires non-

circuit-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 2The Network Model of Scenario 2

Signaling Gateway

SS7 TCAP

Media GatewayController

MediaGateway

1-800 call

1-800 packets

PacketizedTCAP message

STPSCP

SS7 TCAP

Caller phone

An example: A phone makes a 1-800 call in the IP network

IP networkPSTN

Scenario 2: IP entity Scenario 2: IP entity communicates with SS7 SCPcommunicates with SS7 SCP

Signaling Gateway

Media GatewayController

Media Gateway

STP

Caller phone

PSTN IP net

SS7 TCAP

1-800 call

initiates a 1-800 call, sends to MG

PacketizedTCAP message

a

a. 1-800 stream-> 1-800 packets, sends to MGC.

1-800 packets

b

b. generates TCAP/IP packets, sends to SG

d

SS7 TCAP

d. sends to SCP

e

e. responds TCAP,sends back to STP

h. receives TCAP packets

h

f. sends to SG

f

g. SS7 TCAP -> TCAP packets, sends it to MGC.

g c

c. TCAP packets ->TCAP, sends to STP

SCP

Scenario 3 in TCAP/IP signaling interworkScenario 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)

PSTNIP network

SCPSSP

TCAP request

TCAP response

Due to limited time, this complicated scenario is not illustrated here.

SS7 TCAP & IP signaling SS7 TCAP & IP signaling interworkinterwork

Non-circuit-related transactions for service like 800 number, calling card, etc, are provided seamlessly between SS7 and IP networks.

VoIP developmentVoIP 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 ResearchIssues 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…

ReferenceReference 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!Thank you!

Any Question?Any questions?Any questions?