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

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. signaling PSTN 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 Signaling Transport IP network Traditional SS7 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 IP network PSTN voice IP packets voice stream

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

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

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

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 Acknowledgement(ACK) Command Media Gateway

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

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

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

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.

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.

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. IP net PSTN Signaling Gateway STP Media Gateway Controller Caller phone SSP Media Gateway 1 (MG1) IP net Media Gateway 2 (MG2) Called phone

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.

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

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

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

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

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

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.

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 STP SSP 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 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

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 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 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 1-800 call is made by a PSTN user

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

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. 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 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 1-800 call in the IP network

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. 1-800 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

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.

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..

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