SIP / ISUP interconnection
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SIP / ISUP interconnection. Gonzalo Camarillo. [email protected] ASRL Advanced Signalling Research Lab. Telephone Services. GSTN (General Switched Telephone Network). IP Telephony. IP Telephony. GSTN. Contents. Brief SS7 introduction (architecture).

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SIP / ISUP interconnection

Gonzalo Camarillo

[email protected]

ASRL

Advanced Signalling Research Lab.


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Telephone Services

  • GSTN (General Switched Telephone Network)

  • IP Telephony

IP Telephony

GSTN


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Contents

  • Brief SS7 introduction (architecture)

  • Gateway architecture ( SG, MGC, MG )

  • ISUP - SIP interworking issues and proposed

solutions


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Different architectures

GSTN architecture:

Intelligence in the network

Dumb terminals

SIP architecture:

Move the intelligence to the edge


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Signalling in GSTN

Signalling in the network (between switches)

User to network signalling

DSS 1

ISUP

DSS 1


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DSS-1

Q. 931

Protocol between the user and the network

Much simpler than ISUP.

Q. 921

Handles the transport of the messages


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Signalling in GSTN

SS7 (System Signalling No. 7)


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General ISUP<->SIP Conversion

Voice Stream

Signalling

SG

SIP Server

ISUP/MTP

STP

ISUP/IP

SIP

SIP

MGC

IP network

SIP

MGCP

SS7

SIP Client

MG

DSS 1

E1/T1

RTP


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Architecture of the distributed gateway

ISUP

Signalling Gateway

IP side

ISUP/IP

GSTN side

SIP

Media Gateway Controller

MGCP

Voice stream

Voice stream

Media Gateway


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Signalling Gateway

Function:

  • To find the appropriate media gateway controller

Signalling GW

ISUP

ISUP/IP

ISUP/IP

ISUP/IP

ISUP/IP

MGC

MGC

MGC

MGC

MGCP

SIP

MGCP

SIP

MGCP

SIP

MGCP

SIP


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Media Gateway

Mapping between different voice formats

Control

Protocol

GSTN

RTP


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Media Gateway Controller for distributedMedia Gateways

Functions

  • Actual mapping between protocols (ISUP-SIP)

  • Control the Media Gateway/s

  • Authentication, Authorization and Accounting (AAA)


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Experiences implementing ISUP to SIP mapping

  • Different flavours of ISUP (message flow)

  • Overlap signalling (SAMs)

  • In band announcements and ring-back tone

  • ISUP transparency

  • Billing in forwarding situations

  • Tracing calls (malicious, SOS)


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Different message flows

ITU ISUP message exchange during a call

ANSI ISUP: CPG before ACM


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Overlap signalling (SAMs)

European phone numbers does not have a fixed length.

Example:

+358 9 299 1

Finland Helsinki Ericsson Front desk

+358 9 299 3371

Finland Helsinki Ericsson Camarillo’s

The ingress gateway cannot know when the number is complete.


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Proposed solution

  • Ingress gateway waits for SAMs with more digits

  • a certain amount of time.

  • Timer = 0

  • Heavy signalling traffic may be generated

  • Timer = 5 secs ( inter-digit timeout)

  • User is not that patient


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In-band announcements and ring-back tones

  • ‘Your call is being forwarded to an automatic voice mail’

  • Ring-back tone

  • Busy tone

Proposed solution: 183 SIP response

SIP Client

MG

DSS 1

E1/T1

RTP


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IP network

ISUP transparency(SIP bridging)

Voice Stream

SG

SG

Signalling

ISUP/MTP

ISUP/MTP

ISUP/IP

ISUP/IP

SIP

MGC

MGC

STP

STP

MGCP

MGCP

E1/T1

RTP

E1/T1

MG

MG


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ISUP transparecy( SIP bridging)

ISUP messages in SIP bodies (MIME),

SIP INFO method, reliable 1xx responses ...

Trusted parties, confidentiality, ISUP flavours...


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Billingscenarious

Isabel calls from Spain to my GSM mobile:

+358 40 702 35 35 (Finland’s international prefix)

Isabel pays the call leg Spain-Finland

I pay the call leg Finland-Atlanta

SIP is end to end … where are the call legs??


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Tracing calls

  • SOS services ( physically closest center )

  • Malicious calls

An IP address or a SIP URL do not give any useful information for catching the “bad guys”.


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Further information

IETF internet drafts ( www.ietf.org )

  • draft-ietf-sigtran-mime-isup-00.txt

  • draft-ietf-mmusic-sip-multipart-00.txt

  • draft-zimmerer-mmusic-sip-isup-mime-00.txt

  • draft-ietf-mmusic-sip-info-method-01.txt

  • draft-ietf-mmusic-sip-100rel-00.txt

  • draft-roach-mmusic-sip-provisional-media-00.txt

  • draft-donovan-mmusic-183-00.txt

  • draft-roach-mmusic-sip-pstn-require-header-00.txt

  • draf-zimmerer-mmusic-sip-bcp-t-00.txt

  • draft-roach-sip-isup-parameters-00.txt

  • draft-camarillo-mmusic-sip-isup-bcp.txt


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