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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…

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terminology
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
Terminology (Continued)
  • Signaling: exchange of information betweeninvolved 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.

signaling

PSTN

IP network

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 approaches
Signaling Interwork Approaches
  • SS7 is transported to IP networks.
  • And more…

PSTN

Signaling Transport

IP network

Traditional SS7

Packetized SS7

voip gateway
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
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 (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 (MG)
  • Media Gateway terminates circuit-switchednetwork 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(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 (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
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 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 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 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 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 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
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 interwork20
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

f.

CreateConnection

ISUP & IP signaling interwork example

IP net

PSTN

  • Part 1 of a call set-up: ISUP IAM

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

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 example22
ISUP & IP signaling interwork example

SS7 ACM

IP net

PSTN

  • Part 2 of a call set-up: ISUP ACM

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 example23
ISUP & IP signaling interwork example

SS7 ISUP ANM

IP net

PSTN

  • Part 3 of a call set-up: ISUP ANM

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 example24
ISUP & IP signaling interwork example

SS7 ISUP REL

IP net

PSTN

  • Call release: ISUP REL & RLC

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
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 interwork26
SS7 ISUP & IP signaling interwork
  • Call control is provided seamlessly between SS7 and IP networks.
introduction to ss7 tcap
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 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
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
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
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
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
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
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 scp35
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
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
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 scp38
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

SSP

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

SCP

TCAP response

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

ss7 tcap ip signaling interwork40
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
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
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
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
Thank you!

Any Question?

Any questions?