© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IP Signaling System 7 Protocols 1 CNS 479R Voice Over IP Signaling System 7

© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IPSignaling System 7 Protocols

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CNS 479R Voice Over IP

Signaling System 7Gary D. Brown, Guest Lecturer

Lecture 2


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Signaling System 7 (SS7)

• Introduction

• SS7 Network Architecture

• SS7 Protocols

• SS7 Call Signaling

• VoIP Issues


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SS7 Protocols

• Architecture– Definitions

• Functional Divisions– TDM-based Transport Protocols– IP-based Transport Protocols– Application-layer Protocols

• Call Control– Messages– Message Flows


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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


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• MTP1 Message Transfer Part Layer 1



• 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


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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


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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


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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



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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



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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


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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


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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


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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



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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)



• 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



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• ISUP Messages

• ISUP Call Control Message Flows– Call Setup– Call Teardown

SS7 Protocols—Call Control


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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


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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



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• 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”


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• 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!


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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



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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



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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



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• ISUP Messages

• ISUP Call Control Message Flows– Call Setup– Call Teardown

SS7 Protocols—Call Control


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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


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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


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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


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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


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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


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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

Documents

© 2007-2008 Gary D. Brown UVSC CS 479R – Telecommunications and Voice Over IP Signaling System 7 Protocols 1 CNS 479R Voice Over IP Signaling System 7