SS7 - Part 1 Rev A

Confidential and Proprietary to SingTel

SingTel – Innovative Mobile Network Provider of Choice

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Signaling No.7 in the GSM Network

Topics

• Introduction to Signaling• Message Transfer Part ( MTP )• ISDN User Part ( ISUP )• Signaling Connection Control Part ( SCCP )• Transaction Capabilities Application Part (TCAP)• Mobile Application Part (MAP)• IN Protocols• Other Protocols in GSM



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Reference Websites

• ITU-T (http://www.itu.int)

• 3GPP (http://www.3gpp.org/ftp)

• ETSI (http://www.etsi.org)

• GSM World (http://www.gsmworld.com)

• GSM Association (https://infocentre.gsm.org)

• Internet FAQ Consortium (http://www.faq.org)

• IETF (http://www.ietf.org)



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Ch 1: Introduction to Signaling– General

• Introduction• Channel Associated Signaling (CAS)• Common Channel Signaling (CCS)

– OSI Reference Model • Introduction• OSI Reference Model• Communication Process• Description of Layers



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Ch 1: Introduction to Signaling– Signaling System No. 7

• Introduction• Characteristics• User Parts• OSI Model and SS No.7

– Signaling Network • Terminology• Signaling modes• Signaling network planning objectives• Signaling network structure



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OBJECTIVES

– Describe the basic structure of Signaling System No. 7 and list at least 3 application layer protocols used in GSM

– Explain the basic functions of OSI reference model layers and the relationship between the model and Signaling System No. 7



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Introduction

– Signaling is … • Transfer of information and instruction relevant to

control and monitor telephony connections

– External and internal signaling• We will focus on external signaling between nodes.

– Purpose of signaling • Traffic control procedures, database communication,

network mgt procedures



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Signaling in Telecom Networks

Signaling

Access Signaling Trunk Signaling

Subscriber Line Signaling Channel Assoc. Signaling

Common Channel SignalingDigital Subscriber Signaling



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Access Signaling

• Subscriber Line Signaling• Signaling between telephony subscriber and Local Exchange• Use of on/off hook signals, dialed digits, information tones,

recorded announcements, and ringing signals.• Dialed digits ( decadic pulses, Dual Tone Multi Frequency)

• Digital Subscriber Signaling System No. 1 (DSS1)• Also known as D-channel signaling system.• Used for digital access lines only, i.e. ISDN.

• Signaling between the MS and Network in the GSM system



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Trunk Signaling• Channel Associated Signaling (CAS)

• speech channel (in-band), or a channel closely associated with a speech channel (out-band), is used for signaling

• Common Channel Signaling (CCS)• A dedicated channel, completely separated from

the speech channel is used for signaling. • 1 signaling channel in CCS can serve a large

number of speech channels• Rule of Thumb : 1 for 4096 voice circuits



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Channel Associated Signaling

• Line Signals• Whole duration of the call• Monitors connection status and traffic circuit• E.g. Seizure, Answer signals

• Register Signals• During Setup phase of call• Transfer address and category information.• E.g. Dialed B-number, A-category• Sent over a traffic circuit as a tone, which is a combination

of 2 out of 6 frequencies.



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Channel Associated Signaling

Line Signals

IdleSeizureSeizure ackB-answerClear backClear forwardRelease guardBlockingUnblocking

Register Signals

B-numberA-categoryA-numberEnd-of-pulsingControl signalsB-line statusEtc.

Signaling System R2 Multi-Freq Compelled R2



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Call Setup using CAS system

B-number

… conversation …

SEIZURE

SEIZURE ACK

B ANSWER

CLEAR BACKWARD

CLEAR FORWARD

RELEASE GUARD

NODE A NODE B



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System B

OSI Reference Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

System AUser A

Application

Presentation

Session

Transport

Network

Data Link

Physical

User B

Peer-to-peerprotocols

Physical Circuit



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Encapsulation / Decapsulation

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

PhysicalBits

DT Data DH

Data NH

Data TH

Data SH

Data PH

Data AH

Sender Receiver

Actual data transmission path



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DESCRIPTION OF LAYERS

• Application LayerProvides services for support of user’s application process and control of all communications between applications.

• Presentation LayerDefines how data is to be represented, that is, the syntax. Transforms syntax used in the application into a common syntax needed for communication between applications. Contains data compression as well.

• Session LayerControls the connection, synchronization and disconnection of the dialog. Allows presentation layer to set up checkpoints forretransmission.



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DESCRIPTION OF LAYERS

• Transport LayerPerforms end-to-end error detection/correction, and flow control and retransmission.

• Network LayerEstablishes, maintains, and release connections between network nodes and handles addressing and routing of circuits.

• Data Link LayerPerforms point-to-point error detection/correction, flow control and retransmission.

• Physical LayerProvides mechanical, electrical, functional, and procedural resources for activating, maintaining, blocking physical circuits.



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SS7 - INTRODUCTION

• SS7 protocol is drafted and specified by Comité Consultatif International Télégraphique et Téléphonique (CCITT), now known as International Telecommunication Union, Telecommunication Sector (ITU-T).

• Recommendations introduced in 4 revisions (1980 - Yellow, 1984 – Red, 1988 – Blue, 1992 – White)

• CCS was initially meant for telephony only, but has now evolved into non-telephony and non-connection related applications.



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General Structure of SS No.7

Other UserParts SCCP ISDN

User PartTelephonyUser Part

Signaling data link

Signaling networkmanagement

Signaling messagehandling

Signaling link function

Message Transfer Part

User Parts

Level 1

Level 2

Level 4

Signaling network functions



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CHARACTERISTICS

• High Flexibility

• High Capacity

• High Speed

• High Reliability

• Economical



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USER PARTS

• User Parts contains functions dealing with processing of signaling information before and after it is transmitted throughthe signaling network.

• User does not refer to the subscriber, but the user of the MTP.

• Only User Part of the same type can communicate with one another.



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Some of the MTP User Parts

• ISUP (ISDN User Part)Provides control functions / signaling to deal with ISDN subscriber calls and related functions

• TUP (Telephony User Part)Provides all necessary functions / signaling for dealing with a telephony user.

• DUP (Data User Part)Used for File Transfer and Related Signaling



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SS No.7 Protocols in GSM

Users of ITU-T SS No.7

MTP

SCCP

TCAP

MAP CAP BSSAP ISUP TUP



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Introduction of SCCP

• While MTP provides a low overhead facility for connectionless transport of telephony, GSM requires additional services e.g. expanded addressing and reliable message transfer.

• SCCP was added to CCITT Red Book in 1984.

• SCCP provides connectionless (CL) and connection-oriented (CO) network transport services to other functions that are not related to users, e.g. network control.



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Users of SCCP services• Transaction Capabilities (TC)

- Provides mechanisms for transaction-oriented applications and functions.

• Operation and Maintenance Application Part (OMAP)- Specifies network mgt functions and messages related to O&M.

• Application Service Element (ASE)- Blocks to support higher layer applications e.g. MAP, CAP.- MAP used in switching system part of GSM network.- CAP used for control of standardized IN services.

• Base Station System Application Part (BSSAP)- Signaling functions between MSC and BSC.



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OSI Model & SS No. 7

Application

Presentation

Session

Transport

Network

Data Link

Physical Signaling Data Link

Signaling Link

Signaling network

SCCP

User PartsASE

TCAPM

TP

NS

P

NSP : network service partASE : Application Svc Element



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SIGNALING NETWORK TERMINOLOGY

Signaling Point (SP)- Identified by the NI and Signaling Point Code (SPC)- Originating Point Code (source identified by OPC)- Destination Point Code (sink identified by DPC)

Signaling Transfer Point (STP)- Signaling Point that is able to route signaling messages- Only MTP is used; upper levels are not used.

Signaling Link (SL)- Packet data link that connects two SPs- Theoretically, 1 SL supports 5000 connections.- In reality, a maximum of 500 connections are controlled

by one SL due to security reasons.



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SIGNALING NETWORK TERMINOLOGY

Link Set (LS)- A number of parallel Signaling Links (SP) connecting the

same Signaling Points (SP).- Maximum of 16 signaling links inside a link set.

Signaling Route (SR)- Predetermined path between an Originating Point and

Destination Point.- Link set assigned to carry traffic to a particular

destination.

Signaling Route Set (SRS)- All signaling routes used for traversing between an

Originating Point and Destination Point.



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Signaling Network Elements

SP 3

SP 1 SP 2

Link Set

Link Set

Signaling Link 0

Signaling Link 1

Route Set to SP 2• Direct or• via SP 3



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SIGNALING MODES

SP

SPSP

STP

Node A

Node C

Node B

Node D

Associated signaling mode

Quasi-associated signaling mode

SignalingSpeech

CIC

MSU



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SIGNALING NETWORK PLANNING OBJECTIVES

Main Parameters

1. Simple Network Structure

2. Reliability

3. Short Delay Times

4. Reasonable Costs

A simple network structure is normally achieved by arranging the signaling network in a few hierarchical levels.

flexibility for future growth/scalability



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SIGNALING NETWORK STRUCTURE

STP

STP

SP

Complex network generally has a hierarchical structure of STPs



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Parameters affecting Network Design

• Capacity• Number of signaling links• Signaling message-transfer-time• Message throughput-capacity

• Network Performance• Number of Signaling Points and delay times

• Availability / Reliability• Ability of network to provide service due to

STP failure



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Network Design Guidelines

• Each SP that is not a STP is connected to at least two STPs of the higher level.

• Each STP of the lower level is connected to at least 2 STPs of the higher level.

• STPs in the higher level are fully meshed.



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Signaling Network Link Types

Link Types

A-link (“access”) : Connects SP (e.g. MSC) to STP

C-link (“cross”) : Interconnect mated STP nodes

B-link (“bridge”) & D-link (“diagonal”): Interconnect peer STP pairs

E-link (“extended) : Connects SP to another STP pair forenhanced reliability

F-link (“fully associated”) : Connects 2 SP



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Signaling Network Link Types

SP

SPSTP

STP

STP

STP

SP

SP

SPSTP

STP Another network

SP

A-link

B-link

C-link

D-link

F-link

E-link

SS7 - Part 1 Rev A

Documents

isdn user part

common channel signaling

originating point

destination point

signaling network

speech channel

signaling system

signaling points