CCS7 Overview

Stephanie SandersonJanuary 18, 2001

In the beginning, there was…PTS (Per-trunk signaling) – known as in-band signaling because the signaling and voice/data components are transmitted on the same trunkRequires the voice path to be established completely even if the call does not complete successfullyAlso known as Channel Associated Signaling (CAS)

End OfficeEnd Office

Single trunk:carries signalingtraffic and voice/data traffic

Now, there is…CCS7 (Common Channel Signaling Number 7) – a message-based signaling protocol that controls the information traveling between signaling points in a network.Two separate paths are used for transmitting information : one path for voice/data, one path for all signaling

End OfficeSignaling trunk

Voice trunks

End Office

Advantages of CCS7Voice/data traffic and signaling traffic travel on separate paths thus increasing bandwidth.Signaling traffic travels faster and can “look ahead” to make sure long distance and local channels are available.The improved call setup time results in increased network capacity.One signaling link can support several voice/data calls.The voice path is not set up until the signaling indicates a successful call set-up.

Specifications for CCS

CCS7 (Common Channel Signaling #7)North American standard for telecommunications applicationsDefined by American National Standards Institute (ANSI)

SS7 (Signaling System #7)Defined by International Telecommunications Union – Telecommunication Standardization Sector (ITU-T)ITU-T was formerly the International Consultative Committee for Telephone and Telegraph (CCITT)

CCS7 and SS7 are nearly identical and the terms are used interchangeably.

Entities of CCS7 Network

Service Switching Point A switching office with CCS7 voice trunk capability which can interact with service databases.

Service Control Point A network database which provides a centralized place for access to network information.

Signal Transfer Point A tandem point for messages destined to other nodes in the network

SCPSCP

SSP

SCP

STP

More CCS7 Entities

Signaling Link – Signaling connection between two nodes

Link Set – Set of links connection two nodes

Route – Signaling path from one node to another

BA

A B

BA

And More CCS7 EntitiesRoute Set – Set of all routes from one node to another which is made up of linksets

Associated Signaling – Direct Access

Quasi-Associated Signaling – Requires at least one STP tandem of the signaling messages

BA

BA

BA

CCS7 Network Configuration

SCPSSP

F Link

A Link

A Link

A LinksC Link C Link

B Link

B Links

B Link

A Link

A Links

A Link

C Link

D LinkD Links

D Link

E Link

SSP

STP

STP

STP

STP

STP

STP

SCPSCP

SCPSCPSCP

Example of GSM CCS7 Network

SCP

SCPSCP

SCPSCP

SCPSCPEIR

HLR

STP

STP

STP

DMS-MSC/VLR

BSSBSSBSS

BSS

BSS

BSSVLR

MSC

A LinkA LinkA Link

A Link

A Link

A LinkA Link

A LinkA Link

A Link

A Link B Link

A LinkB Link

C Link A Link

A Link

A Link

C Link

STP

In GSM, the BSS is an Access connection,therefore an A-link

B Link

CCS7 Link Notation TermsA link – Access link; SSP or SCP or STPB link – Bridge link; interconnects STPs in different regionsC link – Cross link; connects mated STPsD link – Diagonal link; connects primary and secondary STPs in different regionsE link – Extension link; connects an SSP to an STP in a different regionF link – Forward link; used for a fully associated link; directly connects an SSP to another SSP, or connects an SSP to an SCP without going through an STP

SS7 Software ArchitectureTransaction Capabilities Application Part – Layer used to provide services to support database transaction type applicationsSignaling Connection Control Part – Provides additional functions to the MTP to enhance routing capabilitiesMessage Transfer Part – Serves as a transport system providing reliable transfer of signaling messages between the network nodes

SCCP

MTP Level 3

MAP

DTAPBSSMAPTCAP ISUP

MTP Level 2

MTP Level 1

The Message Transfer PartServes as a transport system providing reliable transfer of signaling messages, in the correct sequence without loss or duplication between two signaling nodes of the network.Enables the significant information of the User Part to be transferred across the SS7 network to the required destination (Q.701)Encompasses the 3 lowest protocol layers:Level 1: Signaling Data Link Level (Physical Link)Level 2: Signaling Link LevelLevel 3: Signaling Network Level

MTP Level 1Concerned with the raw transmission of bits over a physical medium which can be satellite, microwave, fiber optics, etc.Transmission of bits occurs over a bi-directional path, comprised of two data channels operating together in opposite directions at the same data rate.The recommended bit-rate for ANSI is 56 kbps and for CCITT is 64 kbps, although this is dependant on the physical medium used.

MTP Level 2Together with Level 1, this level provides a signaling link between two directly connected signaling points in the SS7 network.Functions are as follows:Transfer signaling information in variable length messages (20 – 279 bytes) called Signaling Units (SU) which come as MSUs, LSSUs, or FISUsSignaling Unit synchronization and alignmentError detection and correctionError monitoring by the FSN, BSN, FIB, BIBFlow Control

MTP Level 2 Message TypesMessage Signal Unit (MSU) – Used to transfer “user” signaling information from one SS7 node to another. This is the only signaling unit that is of interest in the upper layers (users) of MTP.Link Status Signal Unit (LSSU) – Used to report the status of a signaling unit. It is always placed at the beginning of the transmit queue.Fill In Signal Unit (FISU) – Used to “fill-in” gaps between “useful” messages. These are sent only when the transmit buffer is empty. FISU is mainly used for monitoring the status of a signaling link in the absence of any signaling traffic.

MTP Level 2 Message Structure

Flag Check Signaling Information Field SIO LI FSN BSN Flag

BIB

FIB

MSU

8 16 8 2 6 1 7 1 7 8 bits

transmission

direction

LSSU

LI FSN BSN Flag

BIB

FIB

2 6 1 7 1 7 8 bits

Flag Check

8 16

Status

8 or 16

FISUFlag Check

8 16

LI FSN BSN Flag

BIB

FIB

2 6 1 7 1 7 8 bits

Signaling Unit Fields

Flag – This is a specific predetermined byte which delimits two signaling units. Check – 16-bit Cyclic Redundancy Check (CRC) checksum is used for error detectionSignaling Information Field (SIF)“User” specific signaling message where the user has a specific format for SCCP, TCAP, or ISUPConsists of an integral number of octets (2 – 272)First seven octects are used for Routing Level (address of the message)

Signaling Unit Fields (cont)Signaling Information Octet (SIO) – Present only in the MSU and indicates:Different Message Types (Natl vs Intl)Priority of the MSUAdditional routing info to the SCCP for routing to different users of TCAP

Length Indicator (LI) – Indicates the number of octets between the end of the two unused octects to the start of the check field; Differentiates between the types of the SU:FISU – LI = 0LSSU – LI = 1 or 2MSU – LI = 3 to 63

Signaling Unit Fields (cont)Status – LSSU specific field; Indicates the status of the signaling link to the far end.Sequence Numbers (FSN, BSN) – From 0 to 127 and work as follows:Forward Sequence Number is the sequence number of a message which is sent.Backward Sequence Number is the sequence number of the message which is being acknowledged

Indicator Bits (FIB, BIB) – Used in conjunction with FSN and BSN for sequence and error control

MTP Level 3

Ensures reliable transfer of messages, even in the event of signaling link and STP failure. For elaboration, the two parts are:Signaling Message HandlingSignaling Network Management

Signaling Message Handling

Consists of three parts:Discrimination – Determines if the incoming message is destined for the point itself; If not, it will activate the routing functionRouting – Determines the outgoing signaling link on which an outgoing message is to be sent towards its final destinationDistribution – Delivers an incoming message to the appropriate MTP “user” once DISCRIMINATION has determined the message belongs on the node

Routing Level (SMH, cont)All functions of Signaling Message Handling are based on the routing address encoded in the first 7 bytes of the SIF of the MSU.Routing consists of 3 fields:Destination Point Code (DPC) – Indicates final destination of the message.Originating Point Code (OPC) – Indicates the originating node in the networkSignaling Link Selector (SLS) – When more than one link is available for transmission of a message between two nodes, the SLS id used to select a particular link; Messages which need to be sequenced have the same SLS

Routing Level (cont)

Flag Check Signaling Information Field SIO LI FSN BSN Flag

FIB

BIB

User Specific Information Routing Level

MSU

7 bytes

SLS Origination Point Code Destination Point Code

3 bits 5 bits 3 bytes 3 bytes

Network Identifier Network Cluster Cluster Member

Zone Area Network Signaling Point

ANSI

ETSI

Signaling Network ManagementWhen a failure in a signaling link or STP occurs, reconfigurations are carried out so that the messages are not lost, duplicated or put out of sequenceSignaling Network Management does this in three functions:Signaling Traffic Management – Diverts signaling traffic from the unavailable link or route to one or more alternative links or routesSignaling Route Management – Distributes information about the signaling network status in order to block or unblock signaling trafficSignaling Link Management – Controls the locally connected signaling links by link Activation, Deactivation and Restoration

Signaling Connection Control Part

Enhances services of the MTP by providing addressing capability to User services through routing by DPC and SSN or GTSubsystem Number (SSN) – Local addressing information used by SCCP to identify each of the SCCP users at a nodeGlobal Title (GT) – Addresses, such as dialed digits, that do not contain information that would allow routing by MTP; Requires translation capability to translate the GT into a DPC and SSN

Services of SCCPProvides four classes of service beyond addressing

enhancements:Class 0 – Basic Connectionless Class: User-to-User information blocks (called Network Services Data Units, NSDUs) are delivered independently without sequencingClass 1 – Sequenced Connectionless Class: NSDUs are delivered in sequenceClass 2 – Basic Connection Oriented Class: Transfer of NSDUs is performed by setting up a temporary or permanent signaling connection.Class 3 – Flow Control Connection Oriented Class: Capabilities of Class 2 with flow control, the detection of message loss and misqueuing

Transaction Capabilities Application Part

Provides a connectionless environment which can be used by an application at one SS7 node to invoke execution of a procedure in another nodeConsists of two sublayers: Transaction portionComponent Portion

Transaction Portion (TCAP)

Provides means of associating messages with a specific Application Process transactionConsists of the package type and the transaction identifier which uniquely defines the transaction on both sides.

Package Type

Transaction ID

Component 1

Component 2

Component n

TransactionPortion

ComponentPortion

Transaction Portion (TCAP,cont)

TCAP defines six package types:UnidirectionalQuery with permissionQuery without permissionConversation with permissionConversation without permissionResponse

Component Portion (TCAP)

Ensures components are formatted and exchanged properlyContain either requests for action or a response to a requested actionIdentifies an operation with instructions for the receiving endFour component types: Invoke, Return Result, Error, Reject

Package Type

Transaction ID

Component 1

Component 2

Component n

TransactionPortion

ComponentPortion

Integrated Services Digital Network User Part (ISDN-UP or ISUP)

Provides the signaling functions that are needed to support the basic bearer service and supplementary bearer service for switched voice and non-voice applications in an ISDN environmentMTP layer transfers the signaling information between ISUP parts located at different nodes

SCCP

MTP Level 3

MAP

DTAPBSSMAPTCAP ISUP

MTP Level 2

MTP Level 1

ISUP (cont)

Allows voice and data to be integrated into a single signalAll ISUP messages include:Message TypeMandatory Fixed PartMandatory Variable PartOptional Part

ISUP Message Format

Flag Check SIF SIO LI

FIB FSN

BIB BSN Flag

ISUP Message Routing Level

Optional PartMandatory Variable Part

Mandatory Fixed Part

Message Type CIC

Max. 272 bytes

Telephone User Part (TUP)

Pre-cursor to ISUP (not as “smart”); Main differences in messaging:Either IAM or IAI can be used for call setupISUP REL message contains a certain release cause whereas any number of Unsuccessful Backward Messages are used in TUP

The SIF of the MSU contains TUP specific information and a TUP label

TUP Message Format

Flag Check SIF SIO LI FSN BSN Flag

FIB

BIB

TUP MessageInformation Elements

H1Message Type

H0Message Group

RoutingLabel

8x(n-6) 4 4 40 or 64

SS7 Layers in DMS Software

MTP Level 1: LIU7MTP Level 2: LIU7MTP Level 3: LIU7/LIMSCCP: LIU7/CMTCAP: CMISUP/TUP: CM and DTC7

Tables related to SS7 DatafillCLLI : Common Line Language IdentifierTRKGRP : Trunk GroupTRKSGRP : Trunk SubgroupTRKMEM : The members of the trunk group and the physical resource that will be used (ie: which PDTC, which PCM30 of the PDTC, which timeslot of the PCM30)C7NETWRK : The point code of the node and the network to which it belongsC7RTESET : The route sets, the routes within each routeset and the destination point code (DPC).C7LKSET : Defines each link set and associates them with the Far End Point Code (FEPC).C7LINK : Each link in the linkset and it associated HW (LIU7)

Tables related to SS7 Datafill (cont)

C7TIMER : Various Timing Values for SS7 for Error Monitoring (MTP Level 2)ISUPDEST : Associates the CCS7 trunk with the routesetC7TRKMEM : Defines the CIC of each ISUP trunk memberADJNODE : Allows the operating company to identify restrictions present at connected exchanges. The index specified is used in table TRKSGRP.C7UPTMR : Defines the specific user part protocol timer values to be used by the trunkgroup as defined within table TRKSGRP.C7CNGSTN : Congestion Levels data for Signaling Network Management (MTP Level 3)FAILMSG : Associates the cause value received on one protocol type to map to a DMS treatment or release to be applied to another protocol type during an interworking

Tables related to SS7 Datafill (cont)

TMTMAP : Associates what action is to be performed when a DMS treatment is to be applied to a particular protocol typePETATTR : Allows the customer to selectively remove optional ISUP parms from outgoing signalsPETSIG : Specifies ISU-T ISUP signaling optionsPETSERVS : Contains the services for that trunk groupLTCINV : PDTCs are defined here as well as the C-side links, the physical location of the PM and hardwareLIUINV : Defines the location and properties of the LIU7sLTCPSINV : Defines the P-side of the PM (ie, carriers and spans)CARRMTC : Defines the properties of the carrierC7NETSSN : Defines the far end subsystems or the Remote SSC7LOCSSN : Defines the local subsystem for a particular node

ISUP Message Type CodesMessage Type

Message Description Code (byte)

IAM Initial Address Message 01

SAM Subsequent Address Message 02

INR Information Request 03

INF Information 04

COT Continuity 05

ACM Address Complete Message 06

CON Connect 07

FOT Forward Transfer 08

ANM Answer Message 09

REL Release 0C

SUS Suspend 0D

ISUP Message Type Codes (cont)Message Type

Message Description Code (byte)

RES Resume 0E

RLC Release Complete 10

CCR Continuity Check Request 11

RSC Reset Circuit 12

BLO Blocking 13

UBL Unblocking 14

BLA Blocking Acknowledgement 15

UBA Unblocking Acknowledgement 16

GRS Circuit Group Reset 17

CGB Circuit Group Blocking 18

CGU Circuit Group Unblocking 19

ISUP Message Type Codes (cont)Message Type

Message Description Code (byte)

CGBA Circuit Group Blocking Acknowledgement 1A

CGUA Circuit Group Unblocking Acknowledgement 1B

CMR Call Modification Request 1C

CMC Call Modification Complete 1D

RCM Call Modification Reject 1E

FAR Facility Request 1F

FAA Facility Accepted 20

FRJ Facility Reject 21

FAD Facility Deactivated 22

FAI Facility Information 23

LPA Loop Back Acknowledgement 24

ISUP Message Type Codes (cont)Message Type

Message Description Code (byte)

CSVR Closed User Group Selection and Validation Request

25

CSVS Closed User Group Selection and Validation Response

26

DRS Delayed Response 27

PAM Pass Along Message 28

GRA Circuit Group Reset Acknowledgement 29

CQU Circuit Query 2A

CQR Circuit Query Response 2B

CPG Call Progress 2C

USR User-to-User Information 2D

UCIC Unequipped Circuit Identification Code 2E

CFN Confusion 2F

ISUP Message Type Codes (cont)

Message Type

Message Description Code (byte)

FAC Facility 33

CRA Circuit Reservation Acknowledgement E9

CRM Circuit Reservation Message EA

CVR Circuit Validation Response EB

CVT Circuit Validation Test EC

EXM Exit Message ED

TUP Message GroupsMessage Group

Description

FAM Forward Access Messages

FSM Forward Setup Messages

BSM Backward Setup Messages

SBM Successful Backward Setup Info Messages

UBM Unsuccessful Backward Setup Info Messages

CSM Call Supervision Messages

CCM Circuit Supervision Messages

GRM Circuit Group Supervision Messages

CNM Circuit Network Management Messages