Signaling interworks in PSTN and Voice-over-IP networks

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.

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
IP network
Signaling Transport
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
voice IP packets
Media Gateway
voice stream
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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
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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
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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)

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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.

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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
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SS7 ISUP provides call control (1)

• SS7 ISUP sets up a call ISUP IAM.

STPX
STPY
1.IAM
SSPA
SSPB
caller
called party
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SS7 ISUP provides call control(2)

• SS7 ISUP sets up a call ISUP ACM.

STPX
STPY
2.ACM
SSPA
SSPB
caller
called party
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SS7 ISUP provides call control(3)

• SS7 ISUP sets up a call ISUP ANM.

STPX
STPY
3.ANM
SSPA
SSPB
caller
called party
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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.
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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.
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SS7 ISUP IP signaling interwork

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

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SS7 ISUP IP signaling interwork

• Consider an example where a call originates from
  an IP network to the PSTN.

PSTN
IP net
Signaling Gateway
STP
Media Gateway Controller
Caller phone
SSP
Media Gateway 1 (MG1)
Media Gateway 2 (MG2)
IP net
Called phone
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ISUP IP signaling interwork example

• Part 1 of a call set-up ISUP IAM

PSTN
IP net
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
e.
Notify collection of digits
f
a
b
IP net
g.
Acknowledge
MG 2
d
MG 1
h.
CreateConnection
g
e
i.
Acknowledge
i
j.
constructs SS7 IAM packet and sends to SG.
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ISUP IP signaling interwork example

• Part 2 of a call set-up ISUP ACM

IP net
PSTN
SS7 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
MG 1
Caller phone
MG2
IP
c
e
Called phone
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ISUP IP signaling interwork example

• Part 3 of a call set-up ISUP ANM

IP net
PSTN
SS7 ISUP ANM
a.
convert traditional SS7 ANM to ANM packet
STP
Signaling Gateway
a
b.
ModifyConnection sets the connection to
send/receive.
c.
Acknowledge
d
Media Gateway Controller
d.
Receive ACK
b
SSP
MG 1
Caller phone
MG2
IP
c
Called phone
conversation
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ISUP IP signaling interwork example

• Call release ISUP REL RLC

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

• Call control is provided seamlessly between SS7
  and IP networks.

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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.

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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
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SS7 TCAP/IP Signaling interwork

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

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Architecture of TCAP/IP signaling (1)
IP network
PSTN
IP SCP
SS7 TCAP
Signaling Gateway
SS7 TCAP
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
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Architecture of TCAP/IP signaling (2)

• IP SCP, if it is present, provides the same
  functionality as SS7 SCP.
• TCAP doesnt go to Media Gateway.
• Signaling Gateway converts traditional SS7 TCAP
  to TCAP packets or does the reverse.

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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.

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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.

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

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The Network Model of Scenario 2
IP network
PSTN
Signaling Gateway
STP
SCP
SS7 TCAP
Packetized TCAP message
SS7 TCAP
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
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Scenario 2 IP entity communicates with SS7 SCP
PSTN
IP net
initiates a 1-800 call, sends to MG
Signaling Gateway
SS7 TCAP
SS7 TCAP
STP
a. 1-800 stream-gt 1-800 packets,
sends to MGC.
e
c
d
g
SCP
Packetized TCAP message
f
b. generates TCAP/IP packets, sends to SG
Media Gateway Controller
c. TCAP packets -gt 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 -gt TCAP packets, sends it
to MGC.
a
h. receives TCAP packets
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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.
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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.

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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 -gt IP Telephony -gt Internet Telephony

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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

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Reference

• SS7 ISUP tutorial and example from International
  Engineering Consortium (IEC), http//www.iec.org/o
  nline/tutorials/ss7/topic07.html
• SS7 TCAP tutorial and example from IEC,
  http//www.iec.org/online/tutorials/ss7/topic08.ht
  ml
• 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.t
  xt?number2719.
• Media Gateway Control Protocol (RFC2705), IETF,
  http//www.ietf.org/rfc/rfc2705.txt?number2705.
• Megaco Protocol Version 1 (RFC3015), IETF,
  http//www.ietf.org/rfc/rfc3015.txt..

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Thank you!
Any questions?

• Any Question?