1 CNS 479R Voice Over IP Signaling System 7 Gary D. Brown, Guest Lecturer

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CNS 479R Voice Over IP

Signaling System 7Gary D. Brown, Guest Lecturer

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Signaling System 7 (SS7)

• Introduction

• SS7 Network Architecture

• SS7 Protocols

• SS7 Call Signaling

• VoIP Issues

• NG9-1-1 Architecture

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Introduction

• Public Switched Telephone Network (PSTN)– Time-Division Multiplexed (TDM) Technology– Wire-line Network– Wireless Network– SS7 Network

• Business Models

• Applications

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Wire-line Network

• Terminals– Telephones– Facsimile machines– Modems– Video Conferencing Systems– 143 Million (2004) in the United States– 1 Billion+ terminals worldwide

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Wire-line Network

• Switches– Central Office (End Office) Switches

• Connect terminals to PSTN (lines)• Interconnect with tandem switches (trunks)

– 142 Million (2004) in the United States

• 17,932 CO switches (2004) in the United States

– Tandem Switches• Interconnect CO and tandem switches• Form PSTN “back-bone” network

– National tandems– International tandems (same network between countries)– International Gateway tandems (different network types)

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Wire-line Network Architecture

Lines

Trunks

International Network

National Network

National Tandem

International Gateway Tandem

Central Office

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Wireless Network

• Wireless Terminals– Mobile Phones– Laptop Computers– Personal Digital Assistants (PDA)– 213 Million (2005) in the United States– 1 Billion+ worldwide

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Wireless Network

• Base Station Controllers (BSC)– Control multiple Antennas (cell towers)– TDM-to-radio conversion and frequency control– Limited switching capability

• Mobile Switching Centers (MSC)– Wireless equivalent of “Central Office” switch– Control multiple Base Station Controllers– Interconnect with tandem switches– 1,000+ in the United States

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Wireless Network Architecture

MSC


National Network

Central Office

National Tandem


Radio

Trunks

Lines

BSC

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SS7 Network

• Created in late 1970s—deployed 1980s– About same time as IP network

• Purpose-built for PSTN– Specifically designed for voice calls and

services

• Interconnects PSTN switches and networks– Wire-line CO and tandem switches– Wireless MSC– National/International networks

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SS7 Network Interconnection

MSC


National Network

Central Office

National Tandem


Radio

Trunks

Lines

BSC

SS7

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SS7 Network

• Interconnects all wire-line and wireless networks

• Interconnects with VoIP networks via SS7/VoIP signaling gateways

• Very important infrastructure network– AT&T’s “Black Monday” in 1980s

• Entire 1-800 network failed• Congressional hearings held to determine blame!

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PSTN Business Models

• Carriers or Service Providers– Local Exchange Carriers (LEC)

• Incumbent LEC (ILEC) (e.g. Qwest Communications)

• Competitive LEC (CLEC) (e.g. Veracity)

• Local Telephone Companies

– Inter-exchange Carriers (IXC)• Long Distance Telephone Companies (e.g. AT&T, Sprint)

• Switch-less Resellers

– Mobile Network Operators (MNO) (e.g. Sprint PCS, T-Mobile)

• Mobile Virtual Network Operators (MVNO) (e.g. TracPhone)

• Geographic Area/Coverage

• Interconnected via SS7 Network

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Applications

• POTS (Plain Old Telephone Service)

• 800 toll-free service

• 900 premium service

• Directory Assistance

• Operator Assistance

• Conferencing Service

• Local Number Portability (LNP)– Wireless Local Number Portability (WLNP)

• Line Information Database (LIDB)– Billed Number Screening (BNS)

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SS7 Network Architecture

• Network Elements (components)

• Redundancy

• Addressing

• Interconnections

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SS7 Network Elements

• Circuits– TDM-based– Voice Circuits– Signaling Circuits

Type T-Carrier (NA/Japan) Capacity

DS0 64 Kbps 1

T1 1.544 Mbps 24

T3 44.736 Mbps 672

Type Optical Carrier

OC-1 51.84 Mbps

OC-3 155.52 Mbps

OC-12 622.08 Mbps

OC-48 2.488 Gbps

OC-192 9.953 Gbps

Type E-Carrier (RoW) Capacity

DS0 64 Kbps 1

E1 2.048 Mbps 30/31

E3 34.368 Mbps 480/496

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• Nodes– Service Switching Point (SSP)

• Central Office Switches (CO)• Mobile Switching Centers (MSC)• [Access] Tandem Switches (AT)• Controls calls• Asks for services

– Signal Transfer Point (STP)• Forwards SS7 Messages

– Service Control Point (SCP)• Provides services


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SS7 Network

SSP

STP

SCP

IP Network

LAN

Router

Server

≡

Service Switching Point

Signaling Transfer Point

Service Control Point

Switches between computers

Forwards packets

Provides services

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CO Switch

Tandem Switch

MSC Switch

STPHome Location Register (HLR)

Voice Circuits Voice Circuits

SS7 Messages SS7 MessagesSS7 Messages

SS7 Messages

Global Title Translation

(GTT)

SS7 Messages

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SS7 Signaling Links

• Physical Connection– DS0 channel in T1/E1 circuit (very common)

– V.35 modem (older technology)

– IP network (newer technology)

• Bandwidth– 56Kbps (robbed-bit T1)– 64Kbps (clear channel T1 and E1)

• Link Sets– Dual data links for Redundancy

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STPPair

STPPair

SS7 Signaling Links/Redundancy

• A-links – links between STP A-links – links between STP and SSP or SCP endpointsand SSP or SCP endpoints

• B-links – bridge links between B-links – bridge links between mated pairs of STPsmated pairs of STPs

• C-links – cross link between C-links – cross link between STPs within STP pairSTPs within STP pair

• D-links – diagonal links D-links – diagonal links between mated STP pairs in between mated STP pairs in different networksdifferent networks

• E-links – extended links E-links – extended links between SSP and alternate between SSP and alternate STPSTP

• F-links – links between SSPsF-links – links between SSPs

SCP SCP

SSPSSPVoice Circuits

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SS7 Addressing

• Point Codes and Circuit Identification Codes– ANSI

• 24-bit point code (16,777,216 nodes)• 14-bit circuit identification code (16,384 circuits)• North America, Japan, China

– ITU• 14-bit point code (16,384 nodes)• 12-bit circuit identification code (4,096 circuits)• Rest of World (RoW)

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SS7 Addressing

• Origination Point Code (OPC)– Sender of SS7 message

• Destination Point Code (DPC)– Receiver of SS7 message

• Circuit Identification Code (CIC)– Identifies physical circuit between SSPs– Circuits have common numbering between SSPs

• SS7 Call Signaling Messages contain OPC, DPC and CIC

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SS7 Interconnections

• SSP-to-SSP (switch-to-switch)– F-link used to directly interconnect switches– Uses one DS0 channel of T1 circuits between

switches– Sometimes used in North America


SS7 F-link

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• SSP-to-STP (switch-to-SS7 network)– Uses A-link to interconnect SSP and STP– Uses one DS0 channel of T1 or E1 circuit—no

channels used for voice– Commonly used throughout the world—required in

Europe


SS7 A-links

SSP

SSPSTP STP

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• SS7 Signaling Transport Companies– AT&T Wholesale

• Was SNET (Southern New England Telephone)– Connecticutt ILEC

• ISUP Gateway Service• LNP Gateway Service• 8XX Gateway Service• SOA (Service Order Administration) Service

Bureau– Ports subscribers to other carriers (LNP)

• CLASS (Custom Local Area Signaling Services) gateway

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• SS7 Signaling Transport Companies– VeriSign

• Illuminet (purchased in 2001)• 15 STP Pairs• Largest independent U.S. SS7 transport network• Interconnects with nearly all carriers in North,

Central and South America

– IXCs (e.g. Sprint LD, MCI)– LECs (Qwest, Verizon)

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

Mobile phone dials 1-800 number. MSC verifies phone is a customer.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

MSC asks for Global Title Translation of 1-800 number, which gives actual destination.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

MSC switch passes call information to and requests circuit from Tandem switch.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

Tandem switch passes call information to and requests circuit of CO switch.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

Circuit path completed and called party alerted.Answer supervision returned when called party answers and billing starts.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

Call in conversation.

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Wireless 1-800 Call

CO Switch

Tandem Switch

MSC Switch



(GTT)

Call torn down when one party disconnects.Circuits are released and billing terminates.

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Signaling System 7 (SS7)

Next lecture on SS7 Protocols used between SS7 network elements

Documents

1 CNS 479R Voice Over IP Signaling System 7 Gary D. Brown, Guest Lecturer