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© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
1
CNS 479R Voice Over IP
Signaling System 7Gary D. Brown, Guest Lecturer
Lecture 2
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
2
Signaling System 7 (SS7)
• Introduction
• SS7 Network Architecture
• SS7 Protocols
• SS7 Call Signaling
• VoIP Issues
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
3
SS7 Protocols
• Architecture– Definitions
• Functional Divisions– TDM-based Transport Protocols– IP-based Transport Protocols– Application-layer Protocols
• Call Control– Messages– Message Flows
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
4
SS7 Protocols—Architecture
OSI Model Signaling Point Functions SS7 Model
Layer 7
TCAPISUP Level 4
Layer 6
Layer 5
Layer 4 SCCP
Layer 3 MTP3 M3UA Level 3
Layer 2 MTP2 SCTP Level 2
Layer 1 MTP1 Ethernet Level 1
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• MTP1 Message Transfer Part Layer 1
• MTP2 Message Transfer Part Layer 2
• MTP3 Message Transfer Part Layer 3
• SCTP Stream Control Transmission Protocol (IP)
• M3UA MTP3 User Adaptation Layer (IP)
• SCCP Signaling Connection Control Part
• TCAP Transaction Capabilities Application Part
• ISUP ISDN User Part
• TUP Telephone User Part (not shown—obsolete)
SS7 Protocols—Definitions
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• TDM-based Transport (traditional)– MTP1– MTP2– MTP3
• IP-based Transport– Ethernet (IP network)– SCTP– M3UA
• Application Protocols– SCCP/TCAP– ISUP
SS7 Protocols—Functional Divisions
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• MTP1 Message Transfer Part Layer 1– Physical layer
• DS0 in T1 or E1 (A-link, F-link)• V.35 modem• Satellite link
– 64 Kbps bandwidth• 56 Kbps in “robbed-bit” T1 infrastructure
– ITU-T Q.702
SS7 Protocols—TDM-based Transport
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• MTP2 Message Transfer Part Layer 2– Data Link Control layer
• Error Detection and Correction (CRC-16)• Packet Sequencing• Link Status Indicators
– Signal Units• FISU (Fill-in Signal Unit)—permits immediate detection of link
error• LSSU (Link Status Signal Unit)—provides link status between
two SS7 nodes• MSU (Message Signal Unit)—carries higher level protocol
messages
– ITU-T Q.703
SS7 Protocols—TDM-based Transport
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• MTP3 Message Transfer Part Layer 3– Network layer
• Routes SS7 messages to their destination nodes• Reroutes SS7 messages during network failures
– Network Status• Signaling Link availability• Point Code (Node) availability• Congestion Control
– Levels 0 (None), 1 (Some) and 2 (Severe)– Initiates local congestion control on SS7 message reception
(“surge”)– Responds to remote congestion control indication by throttling
transmission
• Link Set Changeover and Changeback
– ITU-T Q.704
SS7 Protocols—TDM-based Transport
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• TDM-based Transport (traditional)– MTP1– MTP2– MTP3
• IP-based Transport– Ethernet (IP network)– SCTP– M3UA
• Application Protocols– SCCP/TCAP– ISUP
SS7 Protocols—Functional Divisions
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• SCTP Stream Control Transmission Protocol– Reliable Transport– Datagram and/or Stream– Redundancy
• Multiple Data Streams• Multi-homing (multiple IP addresses)• Active/Standby
– Efficiency• No “Head of Queue” Blocking• Short timeouts• Heartbeats
– IETF RFCs 2960, 3286, 3309
SS7 Protocols—IP-based Transport
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• M3UA MTP3 User Adaptation Protocol– Adaptation of SS7 Layer 4 protocols/MTP3 interface to
IP network transport• Appears as MTP3 interface to Layer 4 protocols• Appears as IP-based application to SCTP
– Maps between• SS7 Point Codes (OPC/DPC) and IP addresses• SS7 Point Codes with Circuit Identification Codes
(OPC/DPC/CIC) and IP addresses (optional)
– IETF RFC 4666
SS7 Protocols—IP-based Transport
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• TDM-based Transport (traditional)– MTP1– MTP2– MTP3
• IP-based Transport– Ethernet (IP network)– SCTP– M3UA
• Application Protocols– SCCP/TCAP– ISUP
SS7 Protocols—Functional Divisions
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• SCCP Signaling Connection Control Part– Transport interface between TCAP and MTP3– Supports connectionless, “non-circuit” based
messages– Traffic Management
• Maintains Signaling Point Status– Availability of Point Codes (SS7 Nodes)
• Maintains Subsystem Status– SCCP “user” availability (specific applications)
• Reroutes messages around failed subsystems (redundancy)
– ITU-T Q.711-Q.716
SS7 Protocols—Application Protocols
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• TCAP Transaction Capabilities Application Part– Non-circuit related transactions– Common interface for transaction-type applications
• Global Title Translation (800, 900 Numbers)
• Line Information Database (LIDB) Query
• Wire-line/wireless Local Number Portability Query
• Directory Assistance Query
– Protocol interface for application-oriented protocols• INAP (Intelligent Network Application Protocol)
• IS-41 (North American wireless call control protocol)
• MAP (Mobile Application Protocol—ITU)
– ITU-T Q.771-Q.775
SS7 Protocols—Application Protocols
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• ISUP ISDN User Part– Call Control Signaling (call setup and teardown)– National Variants
• ITU 88 (14-bit PC/12-bit CIC) ANSI 88 (24-bit PC/14-bit CIC)
• ITU 92 (14-bit PC/12-bit CIC) ANSI 92 (24-bit PC/14-bit CIC)
• ITU 96 (14-bit PC/12-bit CIC) ANSI 96 (24-bit PC/14-bit CIC)
• ETSI (14-bit PC/12-bit CIC) ANSI 2000 (24-bit PC/14-bit CIC)
• BT ISUP (14-bit PC/12-bit CIC) (United Kingdom and former colonies)
• China (24-bit PC/14-bit CIC)(ANSI MTP3/ITU ISUP)
• Telefónica (14-bit PC/12-bit CIC) (Spain, Portugal, South America)
– ITU-T Q.761-Q.769
SS7 Protocols—Application Protocols
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• ISUP Messages
• ISUP Call Control Message Flows– Call Setup– Call Teardown
SS7 Protocols—Call Control
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• Call Control Messages (most commonly used)– ACM Address Complete Message– ANM Answer Message– CCR Continuity Check Request– CFN Confusion– CON Connect– COT Continuity Message– CPG Call Progress Message– CRG Charge Information Message– IAM Initial Address Message– INF Information Message– INR Information Request Message– REL Release Message– RES Resume Message– RLC Release Complete Message– SAM Subsequent Address Message– SUS Suspend Message– USR User-to-User Information
SS7 Protocols—ISUP Messages
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• Circuit Management (most commonly used)– BLA Blocking Acknowledgement– BLO Blocking– CGB Circuit Group Blocking– CGBA Circuit Group Blocking Acknowledgement– CGU Circuit Group Unblocking– CGUA Circuit Group Unblocking Acknowledgement– CQM Circuit Group Query– CQR Circuit Group Query Response– GRS Circuit Group Reset– GRA Circuit Group Reset Acknowledgement– FAA Facility Accepted– FAC Facility– FAR Facility Request– FOT Forward Transfer– FRJ Facility Reject– IDR Identification Request– IRS Identification Response– RSC Reset Circuit– UBL Unblocking– UBA Unblocking Acknowledgement– UCIC Unequipped Circuit Identification Code
SS7 Protocols—ISUP Messages
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—ISUP Messages
• Message “Direction” Concept– Forward Direction
• Messages passed in the direction from the calling party to the called party
• Sometimes referred to as “downstream”
– Backward Direction• Messages passed in the direction from the called
party to the calling party• Sometimes referred to as “upstream”
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—ISUP Messages
• Common ISUP Header– All ISUP message have common message
header• Originating Point Code (OPC)
– Point code of switch transmitting original message
• Destination Point Code (DPC)– Point code of switch intended to receive message
• Circuit Identification Code (CIC)– Identifies circuit between transmitting switch and
receiving switch; not necessarily the original and intended switches!
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• IAM Initial Address Message– Forward message indicating seizure of an outbound circuit and to
transmit number and other information
• SAM Subsequent Address Message– Forward message indicating additional called party number
information (ITU only)
• COT Continuity Message– Forward message indicating circuit continuity test is successful
• ACM Address Complete Message– Backward message indicating all address signals required for
routing call to called party have been received
SS7 Protocols—ISUP Messages
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• CPG Call Progress Message– Backward message indicating significant event has occurred
• ANM Answer Message– Backward message indicating the called party has answered
• REL Release Message– Forward or backward message indicating the circuit is being
released due to some reason
• RLC Release Complete Message– Forward or backward message, sent in response to a Release
message, indicating the circuit has been put into an “idle” condition
SS7 Protocols—ISUP Messages
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• SUS Suspend Message– Message indicating subscriber has disconnected and the call has
been suspended
• RES Resume Message– Message indicating subscriber has reconnected and the call has
been resumed
• RSC Reset Circuit Message– Message forces circuit into “idle” state; causes any call or circuit
seizure to be terminated
• CFN Confusion Message– Message indicates confusion about message just received;
usually happens during congestion or because of software bug
SS7 Protocols—ISUP Messages
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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• ISUP Messages
• ISUP Call Control Message Flows– Call Setup– Call Teardown
SS7 Protocols—Call Control
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—Call Setup Message Flow
Calling Party Called Party
Lifts handset
Plays dialtone
Dials number
IAM
backward talk path connected IAM
backward talk path connected
ACM
ACM Alerting (Ring)
Plays ringing
Called Party Answers
ANM talk path connected
ANM forward talk path connected
Stops ringing forward talk path connected
talk path connected
Tandem SwitchCalling Switch Called Switch
All circuits now connected and call is in conversation
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—IAM Message Flow
CO Switch
Tandem Switch
CO Switch
STP
Calling CO switch sends Initial Address Message (IAM)in forward direction to called CO switch via tandem switches, and
each switch seizes an outbound circuit and connects the backward talk path.
Tandem Switch
STP
IAM
IAM
IAM
IAM
IAM
IAM
IAM
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—ACM Message Flow
CO Switch
Tandem Switch
CO Switch
STP
Called CO switch alerts called party (ring),sends Address Complete Message (ACM) in backward direction to
calling CO switch via tandem switches, and calling CO switch plays ringing.
Tandem Switch
STP
ACM
ACM
ACM
ACM
ACM
ACM
ACM
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—ANM Message Flow
CO Switch
Tandem Switch
CO Switch
STP
Called party answers, called CO switch sends Answer Message (ACM)in backward direction to calling CO switch via tandem switches,
and all switches connect forward talk paths.
Tandem Switch
STP
ANM
ANM
ANM
ANM
ANM
ANM
ANM
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—Call Teardown Message Flow
Calling Party Called Party
Hangs up
REL
REL
RLC
RLC
Plays silence
Hangs up
All circuits now idled
Calling Switch Tandem Switch Called Switch
circuit idled
circuit idled
circuit idled
circuit idled
talk path disconnected
talk path disconnected
© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols
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SS7 Protocols—REL Message Flow
CO Switch
Tandem Switch
CO Switch
STP
Caller hangs up, calling CO switch sends Release Message (REL) in forward directionto called CO switch via tandem switches, and each switch sends Release Complete Message (RLC) in backward direction, disconnects the talk path and idles the circuit.
Tandem Switch
STP
REL
REL
REL
REL
REL
REL
REL
RLC
RLC
RLC
RLC
RLC
RLC
RLC