Introduction to Signaling System No. 7

Switching & Transmission Technologies,ICT, Islamabad

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Objective

SS7 system principleMessage transfer part

(MTP) principleISDN user part (ISUP)

message and application

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After this session, you will learn:

Signaling System No.7 (SS7)Signaling is used for connection control,

work coordination and “session” between communication devices

SS7 is formulated by CCITT and is widely applied in switched circuit networks (e.g. PSTN).

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Section 1 MTP

Section 2 ISUP

SS7 Functional Structure

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MACMTP1

MTP2

MTP3

SCCP

TCAP

TUP

ISUP

M3UA

IP

INAP

User part

Message Transfer Part

M2UASCTP

SS7 Functional StructureSS7 has two parts:

User Part (UP) Message Transfer Part (MTP).

MTP : enables reliable transfer of signaling messages between user functions. SS7 messages can be transmitted over

narrowband TDM transmission network (i.e. MTP),

over broadband IP network (the protocol used for transmitting SS7 over IP network is SIGTRAN.)

UP : the independent user part of different users it is the functional entity for various call

services6

MTPOverview, MTP1, MTP2MTP3

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MTP - OverviewThe main function

to provide reliable signaling message transfer in the signaling network

Feature : has measures to avoid or reduce message lossrepetition loss of sequence

MTP consists of three function levels: signaling data link (MTP1)signaling link function (MTP2)signaling network function (MTP3).

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MTP1 MTP1 definition:

Defining the physical, electric and functional characteristics of signaling data link, as well as access methods.

Equivalent to the physical layer of OSI seven-layer protocol structure, and used to generate and receive the signals over physical channels

Signaling Data Link :A bidirectional transmission channel for a

signaling.Composed of two-way data channels. The standard bit-rate is 64kbit/s

which also can be applied to the transmission link with lower rate (such as 4.8kbit/s) or that with higher rate (2048kbit/s)

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MTP2 MTP2 definition:

Having Signaling link function .

It is used to transmit signaling to data link, and cooperates with

MTP1 to ensure reliable signaling link between two directly

connected signaling points. Signaling link function can be sub-divided into

signaling unit delimitation

signaling unit location

error detection

error correction,

initial location

processor failure

level-two flow control

and signaling link error rate monitoring functions.10

MTPOverview, MTP1, MTP2MTP3

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MTP3Definition :

Network layer It implements the functions of the third layer of OSIused to transmit management messages to ensure

reliable transmission of signaling messages in case of faults in signaling link or signaling transfer point

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Signaling Message ProcessingMessage routing function:

To determine the outgoing signaling link for transmitting messages to destination point

Message identification function:To check whether the destination signaling

point in received message is a local office signaling point

Message allocation function:To distribute received messages (sent to

the point) to corresponding UPs

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Signaling Network ManagementSignaling service management:

To transfer signaling service from one link or route to one or multiple different links or routes, reset MTP in a signaling point or slow down the traffic in case of congestion

Signaling link management: To recover faulty signaling link, activate idle

(non-arranged) links, and deactivate arranged signaling links.

Signaling route management:To distribute the messages related to

signaling network status, block or unblock signaling routes

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Message FormatMTP has three basic signaling unit formats:

Message Signaling Unit (MSU) To transmit messages of respective UPs, signaling

network management messages as well as signaling network test and maintenance messages.)

Link Status Signaling Unit (LSSU) LSSU provides the link status information to

perform the connection and recovery of signaling links

Fill-In Signaling Unit (FISU) FISU is used to maintain normal operation of the

signaling link and implement fill-in function when no MSU or LSSU is transmitted over the signaling link.

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Section 1 MTP

Section 2 ISUP

ISUP SignalingOverviewCall process

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ISUP - OverviewISUP is one of UPs of Common Channel

Signaling System No.7. It provides necessary signal functions

for supporting basic bearer services and supplementary services of voice and non-voice purposes in the Integrated Service Digital Network (ISDN).

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Applications in SoftX3000

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PSTN

SoftX3000

TDM

TMGTMG

PSTN TDM IP Core

ISUP

SoftSwitch

SIP/BICC

ISUP

Protocol Stack

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MACMTP1

MTP2

MTP3

ISUP

M2UA

M3UA

IP

SCTP

User part

Message transfer part

ISUP SignalingOverviewCall process

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Successful Call Setup

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Calling PartyOriginationg

ExchangeTransit

DestinationExchange

Called Party

Setup IAMIAM

Setup

Address Complete(no indication)

Address Complete(no indication)

Alerting

Alerting

Call Progress(Alerting)Call Progress

(Alerting)

ANMANM

Connect

Connect Answered(off hook)

Dial

Ring

Ring

Connect Ack

Connect AckSpeech or Data

Successful Call Release (1)

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Calling Party OriginationgExchange

Transit DestinationExchange

Called Party

DisconnectREL REL

RLC

Release

RLC

RLC

Disconnect

Calling PartyClears

OnHook

Successful Call Release (2)

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Calling Party OriginationgExchange

Transit DestinationExchange

Called Party

DisconnectREL REL

RELEASEDisconnect

Called Party Clears

RLCRLC

OnHook

SummarySS7 signaling includes two parts: Message

Transfer Part (MTP) and User Part (UP).The MTP consists of MTP1, MTP2 and

MTP3ISUP is one of UPs of SS7. It provides

necessary signal functions for supporting basic bearer services and supplementary services of voice and non-voice purposes in the integrated service digital network.

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Scaleable Network

Fewer capacity upgrades (especially complex upgrades) are required in the signalling network as traffic levels increase. This is because the bandwidth of the SIGTRAN connection is far greater.

Improved SLS loadsharing within link-set

Having fewer high capacity signalling links in a link-set leads to more even SLS loadsharing. For instance you no longer need to ensure all SLS values are present at the STP in order to evenly loadshare messages to an end point (not the case with a 16x64k link-set), thus making network design far simpler.

SCCP Class 1 load-sharing

Class 1 messages rely on the SLS value to ensure in-sequence delivery at the far end. This usually means that no more than 16 links can be used by a signalling element to transmit Class 1 messages (even if more than 16 links are deployed). Also random SLS type features can not be deployed on the network to ease link load imbalances. Again having fewer high capacity signalling links helps to overcome this issue.

Reasons to deploy SIGTRAN

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Fewer network elements

The SS7 bottleneck can now be removed by deploying SIGTRAN connectivity to signalling elements (especially relevant for databases (HLR) and IN elements). Instead the capacity of an element will now be governed by its CPU capacity.

Greater capacity per element can lead to a reduction in the number of elements required in the network, thus leading to reduced network complexity and cost.

Cost

SIGTRAN can reduce cost by removing the need to purchase expensive SS7 stacks, associated interfaces and licences on signalling end elements.

For the equivalent bandwidth at the same level of service, IP transport can be much cheaper than the cost of traditional TDM links.

TDM links are traditionally distance sensitive while IP transport is typically priced on bandwidth rather than distance

Reasons to deploy SIGTRAN

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Single Link-set

SIGTRAN can remove the need to deploy ‘Multiple Point Code’ and ‘Duplicate Point Code’ features which are typically used to increase the number of signalling links / link-sets between two signalling elements.

Removing the need to deploy such features can lead to a reduction in cost and network complexity.

Failover

M3UA can be used to provide failover of signalling traffic between an active and standby call server which are deployed using a single point code. This is particularly relevant when deploying a Release 4 network.

Future Proof

Many of the advantages gained from the use of SIGTRAN could also be gained through the use of High Speed TDM links (capacity, loadsharing etc), however SIGTRAN is far more future proof as it is mandated for use in Release 4 and Release 5 networks (All IP core).

Reasons to deploy SIGTRAN

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Case Study: 3G ImpactToday the vast majority of SS7 traffic volume in a GSM mobile network is authentication and location update traffic (routed via STPs).

UMTS authentication requires more signalling messages (of a larger size) due to the use of 128 byte encryption keys.

UMTS is mainly deployed in areas of high population and 3G coverage (though improving) can be limited resulting in large numbers of 3G-to-2G handovers.

If dedicated 3G MSC’s are deployed then a full location update (inc authentication) is required when handing over between 2G and 3G networks.

If integrated 2G–3G MSC’s are deployed with similar coverage area’s then 2G-3G inter-MSC handovers (resulting in a location update) can be reduced, however this typically leads to smaller MSC coverage areas which will lead to more 2G-2G 3G-3G Inter-MSC handovers and therefore more location updates.

The increase in location updates creates major signalling link capacity issues at both the MSC and HLR leading to the need to deploy multiple signalling link-sets or high speed signalling links such as SIGTRAN

Measurements taken inside a big European PLMN show that a 3G subscriber requires between 6 to 12 times more of SS7 bandwidth compared to a 2G subscriber.

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Case Study: Release 4 Networks

SG

MSSMSS

MGWMGW

HLR

STP

INServices

HLR

SMC

M3ua

M3ua

M3ua

M3ua

M3ua

TDM

TDM

TDM

VoIP

In a Release 4 network both voice and signalling is designed to be delivered over an IP core network.

SIGTRAN M3UA has been adopted by switch vendors to carry signalling traffic between elements (MSS-MGW, MSS-MSS, and MSS-STP).

The larger VLR sizes deployed by Release 4 MSS vendors (1-2 million subscribers) also creates issues of signalling link capacity, which again necessitates the deployment of SIGTRAN.

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What is SIGTRAN?

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SIGTRAN Protocol Architecture

Application Protocol

An adaptation module, that supports the lower layer primitive interface required by a signalling application protocol.

(M2PA, M3UA, SUA etc)

A common signalling transport protocol that supports a common set of reliable transport functions (SCTP)

Standard IP.

IP Transport

Common SignallingTransport

Adaptation module

Application Protocol

SIGTRAN

(RFC 2719)

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SCTP

SUAM3UA

M2PA

IP

MTP2

MTP1

MTP3

TDM SIGTRAN

Protocol Stacks

SCCP

TCAP

ApplicationMAP/ CAP/ INAP

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SCTP

M3UAM2PA

IP

MTP2

MTP1

MTP3

TDM SIGTRAN

ISUPBICC

Protocol Stacks (Circuit Related)

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SCTP : Stream Control Transport Protocol Provides reliable delivery of messages over a packet switched network, much like TCP but

has several advantages in the telecommunications environment. RFC 2960, October 2000 RFC 3309 Checksum Change, September 2002

M2PA : MTP2 Peer-to-Peer Adaptation Layer Supports the transport of SS7 MTP3 signalling messages over IP, using the services of the

SCTP. Primarily used for point to point Trunk applications such as STP C-Links where high

capacity or link aggregation is required. No change to signalling network architecture / topology; just bigger pipes (Similar in

principle to 2meg HS TDM links). Retains strong SS7 resilience due to simple implementation and because the MTP3 layer

remains unchanged. M2PA has sequence numbers to support link change-over/ change-back, thus preventing

message loss. RFC 4165, September 2005

Protocol Definitions

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STPSGW

STPSGW

SignallingElement

SignallingElement

SCTPIP

M2PA

MTP3

MTP1

MTP2 MTP2

MTP1

SCCP

TCAP

MAP/ CAP/ INAP

IP TDMTDM

C-Link

M2PA: End to End message flow

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Protocol Definitions

M3UA : MTP3 User Adaptation Layer Allows for the Transport of MTP3 user parts (ISUP/ SCCP and above) over IP using the

services of the SCTP. Designed to provide IP signalling capabilities to signalling end points in a distributed

architecture. Adopted by switch vendors for Release 4 connectivity between MSS-MGW, MSS-MSS

and MSS-STP. Although M3UA replaces MTP3 it retains some MTP3 features and services helping it to

maintain some of the traditional Telco resilience. RFC 3332, September 2002

SUA : SCCP User Adaptation Layer Allows for the Transport of SCCP user parts (TCAP + Applications) over IP using the

services of SCTP. Designed For Use Between a Signaling Gateway and an IP Resident Database. Looses some of the traditional SS7 resilience (that is provided by MTP3 layer) and is

therefore perhaps more suited to a database / server environment. RFC 3868, October 2004

Other Protocols:

M2UA: MTP2 User Adaptation layer

TUA: TCAP user adaptation layer

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STPSGW

SignallingElement

SignallingElement

SCTP

IP

M3UAMTP3

MTP1

MTP2

SCCP

TCAP

MAP/ CAP/ INAP

IPTDM

M3UA: End to End message flow

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STPSGW

SignallingElement

SignallingElement

SCTP

IP

SUAMTP3

MTP1

MTP2

SCCP

TCAP

MAP/ CAP/ INAP

IPTDM

SUA: End to End message flow

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