SS7 and SIGTRAN

Stephanie Williams

Agenda

Brief Current SS7 Network OverviewWhy SS7oIP? TechnologiesSigtran ProtocolDeployment Strategy/ Interim ArchitectureIP Core RequirementsSS7oIP Architecture for full scale DeploymentQ & A

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SS7 Network Today

• A-Links: Connect SSPs/ SCPs (end office switches/ databases) to STPs via 56K TDM/DS0

• B-Links: Connect STPs to STPs on the SS7 network via 56K TDM/DS0

• C-Links: Connect mated STP pairs together for management messaging and emergency re-routing over 56K TDM/DS0

• D-Links: Connect STPs to other carriers’ STPs for messaging off-net, 56K TDM/DS0

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SS7 Protocol Stack• MTP consist of 3 levels, its purpose is to reliably

transfer messaging across the SS7 network• MTP1-Physical interface (v.35 serial interface

running at 56K or 64K)• MTP2-ensures that messages are delivered in

sequence & error free (CRC-16)• MTP3-provides the message routing & failure

handling (adds DPC/OPC, performs Changeover/ Changeback, tries to restore failed links

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SS7 Protocol Stack Cont.

MTP1

SCCP

Network

TUP

Physical

Data Link

ASP

TCAP

BISUP

ISUP

MTP MTP2

MTP3

• SCCP-provides enhanced features to support circuit-related (GTT) & non-circuit-related signaling information (TCAP queries). SCCP is able to reach destination in the network by using SSN/DPC combos

• TCAP-used for query/retrieval of information from databases, uses SCCP transport, puts in request for data & waits for result

• ISUP-provides call set up & tear down, identifies circuits for voice path

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Why SS7oIP? • *Cost Savings-

Packet networks are less expensive than leased circuit networksReduction of infrastructure costs on leased lines & signaling ports

• Flexibility-Creates a many-to-many ratio for signalingMultiple signaling points via one network link

• Infrastructure Performance-Takes advantage of high speed & intelligent routing offered by IP transport networks

• Convergence-Offload or migrate legacy TDM-based signaling traffic to the IP backboneAccess IP or SS7 databases & nodes from same network

• New Business Opportunities-As SS7 & IP networks converge so do their businesses. SS7/IP is a key enabling technology

for new business opportunities in new markets

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SS7oIP Technologies• 2 distinctly different technologies:

*Sigtran ProtocolConversion of SS7 to Sigtran for transport over the IP network (Sigtran has built-in fail safes to aid the IP core)*TDMoIP Circuit EmulationEncapsulation of TDM circuit for signaling transport over the IP network without manipulating the SS7 protocol (this technology heavily relies on the stability of the IP core)

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IETF Sigtran

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IETF Sigtran Working Group

• Group of SS7 and SS7/IP infrastructure vendors designing SS7 over IP standards

• http://www.ietf.org/html.charters/sigtran-charter.html

• Tasked to create Transport and Stacks for reliable SS7oIP protocol suite

• SCTP (RFC2960): Protocol for reliable and sequenced delivery of SS7 MSUs

• Adaptation Layers: M2UA,M2PA,M3UA,SUAS. Williams 9

Sigtran Protocols• SCTP (Stream Control Transmission Protocol, RFC2960)-

transport layer that provides reliable data transfer• M2PA (MTP2-User Peer to Peer Adaptation, draft status)-

provides MTP3 with equivalent transport layer services as MTP2

• M2UA (MTP2-User Adaptation, RFC3331)- client/server protocol providing a gateway to legacy SS7 network for IP-based applications that interface at the MTP2 layer

• M3UA (MTP3-User Adaptation, RFC3332)- client/server protocol providing a gateway to legacy SS7 network for IP-based applications that interface at the MTP3 layer

• SUA (SCCP-User Adaptation, draft status)- client/server protocol providing a gateway to legacy SS7 network for IP-based applications that interface at the SCCP layer

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SCTP vs TCP• SCTP provides reliable transport, ensuring that data is

transported across a network without error and in sequence, like TCP

• Unlike TCP, the retransmission by SCTP of a lost message in one stream does not block the delivery of messages in other streams. The use of multiple streams within SCTP resolves the head of line blocking you see with the use of TCP

• Unlike TCP, SCTP ensures the sequenced delivery of user messages within a single stream

• Unlike TCP, SCTP supports Multi-Homing for added redundancy and faster retransmission of non-acknowledged packets

• Unlike TCP, SCTP supports built-in heartbeat (destination check)• Unlike TCP, SCTP supports a security cookie against SYN flood

attack• SCTP supports Selective Acknowledgements (SACK)

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SCTP Peer-to-Peer SS7 (M2PA) Protocol Architecture

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SS7SS7ApplAppl

MTP3MTP3

MTP2MTP2

MTP1MTP1

MTP2MTP2

MTP1MTP1

Link Protocol

Link Protocol

SS7SS7ApplAppl

MTP3MTP3

MTP2MTP2

MTP1MTP1

MTP2MTP2

MTP1MTP1

MTP2MTP2

MTP1MTP1

MTP3MTP3

MTP2MTP2

MTP3MTP3

MTP1MTP1

SCCPSCCPGTTGTT

SCCPSCCPGTTGTT

SS7SS7 SS7SS7

M2PAM2PA

SCTPSCTP

M2PAM2PA

SCTPSCTPTransport

Peer

IPIP IPIP

IPIP

SCCPSCCPGTTGTT

SCCPSCCPGTTGTT

SSP STP

SS7oIP Device SS7oIP Device

M3UA/SUA Signaling Gateway IP SS7 End Nodes

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MTP3MTP3 M3UAM3UA

SCTPSCTP

IPIP

MTP2MTP2

MTP1MTP1

MTP3MTP3

MTP2MTP2

MTP1MTP1

SCCPSCCPTCAPTCAP

TUP

ISUP

IS-41

IPNetwork

NNIIFF

SCCPSCCPGTT

M3UAM3UA

SCTPSCTP

IPIP

SCCPSCCPTCAPTCAP

TUP

ISUP

MAP IS-41

SCTP/IPSSP/STPSSP/STP ASP/DBASP/DBSS7

MAP

IPNetwork

SUASUA

SCTPSCTP

IPIP

TCAPTCAPMAP

MTP3MTP3

MTP2MTP2

MTP1MTP1

SCCPSCCPTCAPTCAP

MAP

MTP3MTP3SCTPSCTP

IPIP

MTP2MTP2

MTP1MTP1

SCCPSCCP SUASUANNIIFF

GTT

SS7 Stack Comparison to Sigtran

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•SS7 stack compared with Sigtran adaptation layers

•Note that the MTP3 layer remains unchanged with transport

•M2PA allows TWTC to keep the existing SS7 topology and use IP to transport SS7 messages i.e. signaling links become virtual

•SUA is optimized to carry transactional content signaling (TCAP). M3UA can also transport SCCP/TCAP, SUA eliminates more of the SS7 stack,makes better use of IP routing,requires less SS7 network overhead

M3UA/SUA Standard Features

• Flexible Routing configurationM3UA-DPC,OPC,SIO,ISUP,CIC range,GTTSUA-DPC,OPC,SI=SCCP,SSN,GTT

• Fully compliant traffic mode operation• Fully compliant signaling network management

operation• Fully compliant ASP state & traffic maintenance

operation• Fully compliant point-code sharing• Acknowledgement of heartbeat• Signal-Gateway Mate Protocol-used to exchange

necessary state information to act as mated pairS. Williams 15

SS7 over IP Network Design Resilience

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Level 1:

• IP routing protocols can detect path outages and re-route

Level 2:

• When SCTP Multi-homing detects unresponsiveness in the current IP path, it changes the IP path used for the session by changing the source and/or destination IP address of the session. During this process, the SCTP session remains active and MTP3 is unaware of the path change.

Level 3:

• If SCTP/IP can not correct the issue, MTP3 is notified to initiate standard MTP3 rerouting procedures (standard SS7)

Resilience Hierarchy

IP Routing

SCTP Multi-Homing

MTP

IP Routing

SCTP Multi-Homing Examples

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208.4.2 208.4.4

208.4.2 208.4.4ACTIVE SCTP SRC-DEST Combo

M2PA/SCTP link 0

STANDBY SCTP SRC-DEST Combo

local-peer 5000

10.120.122.6

10.120.123.6

local-peer 5000

10.120.122.22

10.120.123.22

10.120.122.6

10.120.123.6

10.120.122.22

10.120.123.22

MTP2Link 0

MTP2Link 0

Redundant Path IP Network

MTP2Link 0

MTP2Link 0

SS7/IP Device SS7/IP Device

SS7/IP Device SS7/IP Device

Deployment Strategy/ Interim Architecture

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Deployment Strategy-City

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5E

Sonus

SS7/IP

SS7/IP

TDMMigrate links ontoSS7/IP platform after X months

IP CoreNew IP links soak period

•Each A-link city will have 1 or 2 SS7/IP devices during deployment with ½ the city’s A-links riding over IP

•IP link soak period & TDM migration of remaining links-TBD

•Savings are realized only after TDM link(s) are disconnected

Deployment Strategy-STP City

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Portland SS7/IP

SS7/IP

ColumbusSS7/IP

SS7/IP

TDM TDM

IP Core

½ (B-links)(A-links)(C-links)

½ (B and C-links)

STPSTP

Interim Architecture• A-Link Strategy

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5ESS

SS7oIP ConversionDevice

SS7oIP ConversionDevice

IP Core

Sonus

TDM

TDMLocalLoop

LocalLoop

56K Circuit

DS1 Circuit

QOS implemented withinIP Core

latigid

Mux

latigid

latigid

Mux

Mux

Sample City with Link Costs

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DRM

SS7oIP ConversionDevice

SS7oIP ConversionDevice

LocalLoop

LocalLoop

latigid

latigid

Mux

Mux

IP Core

QOS implementedwithin IP Core

SS7oIP ConversionDevice

SS7oIP ConversionDevice

STP

STPlatigid

latigid

latigid

latigid

Mux

Mux

Mux

Mux

PTLD

PHNX

$117MRC

$1540.95 MRC

T1 ENT. FAC.$117 MRC

$490.78 MRC

$200MRC

T1 ENT. FAC.$117 MRC

Interim Architecture• STP City Interim

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SSP

SSP

SSP

SS7oIP Devices

SS7oIP Devices

SS7oIP Devices

A/B/C-LinksSS7 DS0s muxeddown from DS1s

SS7oIP Device

SS7oIP Device

SS7oIP Device

SS7oIP Device

A-LinksDedicated DS0s

DS0s muxedup to DS1sAll links depicted: half of each SS7 location links on IP

IP BackboneNetwork

IP BackboneNetwork

TDM Device

TDM Device

TDM Device

TDM Device

TDM Device TDM Device

TDM Device

IP Core Requirements• COS implemented throughout IP core to guarantee bandwidth for

SS7 messaging• Physically diverse/ redundant access paths into the core for each

link• Latency from furthest SP on SS7 network to STP-IP core cannot

inject > 50 msecs (WC) of latency on roundtrip• Security• QOS measurements required for signaling traffic• Router outages must meet MTTR on service affecting SS7 outages• Network congestion must give SS7 signaling priority• Dropping MSU packets is unacceptable• Sub-Second reroute in the core must be in place for IP network

failuresS. Williams 24

SS7oIP Full Scale Deployment

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Full Scale Deployment• Once all trials and “soak periods” have completed-the goal

is to migrate all A,B, & C-links to IP network• Savings realization on TDM link costs can happen only

after the TDM links have been disconnected (30-45 day window for discos)

• NOC personnel (SS7 group) must be thoroughly trained on the new equipment

• IP Maintenance/ Transport group must be kept in loop and available to aid the SS7 support team in the NOC (24X7-if issue has been isolated to underlying IP transport )

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Key Areas for Testing• SCTP (Sigtran) association management & config• IP vs. SS7 traffic priority & contention• Link & link set failure • Reroute capabilities• Coexistence of SS7 & IP-based links within a common linkset• QoS/ COS capabilities• Troubleshooting & debug capabilities• Vendor interoperability• Latency measurements *Requires either module in INET/ NeTracker (lab) for monitoring

capability (if TWTC chooses to look at the Sigtran protocol/ SS7 monitoring still valid on TDM end of SS7/IP circuit

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Q & A

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