signalling in dx200

NOC-SUR

Signalling in DX200

Training Document

CT6673en version 1 © Nokia Networks Oy 1 (62)

Signalling in DX200

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2 (62) © Nokia Networks Oy CT6673en version 1

Contents

Contents

1 Introduction...............................................................................51.1 Contents.....................................................................................51.2 Module objectives.......................................................................6

2 Signalling concept....................................................................72.1 Definition of signalling.................................................................72.2 Use of signalling.........................................................................82.3 Signalling types..........................................................................82.3.1 Circuit-related signalling.............................................................92.3.2 Non-circuit-related signalling....................................................10

3 Common Channel Signalling System No. 7.........................113.1 Common Channel Signalling....................................................113.2 CCS7........................................................................................113.3 CCS7 protocols vs. the OSI reference model...........................113.3.1 User part (common part)..........................................................133.3.2 Application part (GSM specific part).........................................133.4 CCS7 protocols in various network elements...........................14

4 Functional parts of the CCS7................................................154.1 User parts.................................................................................154.1.1 Telephone User Part (TUP) and National User Part (NUP).....154.1.2 ISDN User Part (ISUP).............................................................154.2 Application parts.......................................................................164.2.1 Base Station Subsystem Application Part (BSSAP).................174.2.2 Mobile Application Part (MAP)..................................................184.2.3 Intelligent Application Part (INAP)............................................204.3 Summary..................................................................................20

5 Signalling routing methods...................................................225.1 Message Transfer Part (MTP)..................................................235.2 Signalling Point (SP).................................................................235.3 Signalling Link (SL)...................................................................265.4 Signalling Link Set (SLS)..........................................................285.5 Signalling Transfer Point (STP)................................................315.6 Signalling Route (SR) and Signalling Route Set (SRS)............325.6.1 Load sharing.............................................................................365.6.2 Factors that effect the routing of a signalling message............375.6.3 Routing of a signalling message by the MTP...........................395.6.4 Example of MTP routing...........................................................425.6.5 Summary of MTP......................................................................445.7 Signalling Connection Control Part, SCCP...............................455.7.1 SCCP services.........................................................................455.7.2 SCCP routing function..............................................................465.7.3 Example of SCCP routing.........................................................50


Signalling in DX200

6 CCS7 management in the DX 200 MSC/VLR........................546.1 Creating an MTP......................................................................546.2 Creating an ISUP or TUP.........................................................556.3 Creating an SCCP....................................................................556.4 Creating SCCP subsystems.....................................................556.5 Creating new roaming contract.................................................556.6 Creating GT analysis................................................................556.7 MML interfaces.........................................................................56

7 Architecture..............................................................................57


Signalling routing methods

2 Introduction

This document explains how Common Channel Signalling No. 7 (CCS7) is used in the GSM network, especially in the NSS area. It also includes the necessary CCS7 protocols used in the BSC, MSC, HLR and PSTN and the function of each protocol.

Furthermore, it explains two different types of how the signalling message is routed from the origination to the destination over the GSM network. They are signalling routing by MTP-level 3 and by SCCP level. One example of SCPP routing is the roaming contract definition which is explained in the last part of this chapter.

2.1 Contents

To get an overview picture before you start to read this document, a list of the contents is introduced as a guideline:

The concept of signalling (types of signalling messages in the GSM)

Circuit-related and non-circuit related signalling

Summary of all CCS7 protocols in each GSM network element and also the PSTN exchange

Functions of user parts: ISDN User Part (ISUP), Telephone User Part (TUP) and application parts: Base Station Subsystem Application (BSSAP), Mobile Application Part (MAP) and Intelligent Network Application Part (INAP)

Possible ways to route signalling messages (ISUP message, MAP message, INAP message, etc.) in the GSM network: MTP routing or SCCP routing.

Signalling routing by MTP level

Signalling routing by Signalling Connection Control Part (SCCP) level

Global title (GT) analysis

Example of MTP routing

Example of SCCP routing on GT: roaming contract definitions

CCS7 management in the DX 200 MSC/VLR environment

Interfaces, versions and their functions

User interface of the DX 200 concerning the CCS7.


Signalling in DX200

2.2 Module objectives

After this module, the participant should be able to:

Give two examples of circuit-related signalling and four examples of non circuit-related signalling

List necessary CCS7 protocols needed in the MSC, HLR, BSC and the PSTN exchange

Tell the reason why some protocols have the name "User Parts" and some of them have the name "Application Parts"

Give an example case where SCCP routing is needed and MTP routing can not be used to route the signalling message to the destination

List at least one function of MTP-level 3, SCCP, MAP, BSSAP, ISUP and INAP

List the main purpose of IMSI analysis

Give two different IMSI analysis types and explain why and when these two types are used

Write a list of necessary definitions that have to be done in our MSC when a new MSC is added to the existing GSM network and this new MSC has a direct signalling connection to our MSC.



3 Signalling concept

In this chapter, we will start with the signalling concept. What is signalling? What are the functions of signalling? How many categories can signalling be divided into? And what is the recommended method for signalling transfer from/to or within a GSM network?

3.1 Definition of signalling

Signalling in general is defined as: any transfer of data that enables speech and data connections between users.

In the PSTN (Public Switched Telephone Network), signalling is needed only for call establishing, call release and call maintaining.

In GSM, on the other hand, the Subscriber Administration module shows that GSM provides a variety of basic services (short message service, fax, voice, data) and supplementary services (call barring, call forwarding, call hold, multiparty, etc.). From a cellular radio network administration point of view, GSM also supports mobility management of all users such as location update (when the user is in his home PLNM or in a visiting PLMN when roaming) and handover.

For example, when the subscriber tries to activate/deactivate/check the status of some supplementary service from the mobile station, the signalling is (in this case) used to update the supplementary services in the HLR, that is, signalling is not used for enabling speech or data connections as in the previous definition.

Furthermore, when the subscriber makes a location update, signalling is sent to update his/her location data in the VLR or HLR (if needed). Signalling does not aim to enable speech or data connection.

To make the definition of signalling more suitable for GSM, it should be as follows: "signalling is any transfer of data that enables speech and data connection between users and supports mobility management and GSM services handling".


Signalling in DX200

3.2 Use of signalling

In the PSTN, signalling is needed for call establishing, call release and call maintaining.

In the GSM system, signalling can also be independent from speech. The different functions of signalling are call control, control of services and charging control. Mobile networks have some special functions, such as location update, handover, subscriber administration and short message service.

We can divide the use of GSM signalling into three main categories:

handle speech and data connection (set up, supervise and release a call)

handle mobility management (location update, handover)

handle subscriber administration (including all basic and supplementary GSM services).

3.3 Signalling types

To be able to categorise all signalling messages into only two main types, we will use circuit identification as criteria (Figure 1).

circuit-related signalling

non-circuit-related signalling.

Origin Destination Circuit information

Origin Destination Other information

• Location data (LAI)• TMSI/IMSI• Short message• C-number (FORW)• Target cell (Handover)

CIC (Circuit Identification Code)1. Circuit-related

2. Non-circuit-related

Other information

Figure 1. Signalling message structure for circuit and non-circuit-related signalling



3.3.1 Circuit-related signalling

If the signalling message contains information about the circuit to be signalled and the origin and destination of the message, we will call it circuit-related signalling.

An example of circuit-related signalling is call set up signalling between two exchanges (Figure 2). When the originating exchange sends a call set-up signalling message to the terminating exchange, this message contains the MSRN and the Circuit Identification Code (CIC) to identify the circuit which is already reserved to be used for this call.

The CIC identifies the PCM and the time slot of the speech or data connection that the call set-up message is sent for.

2 0

0

X 0 0

22

D 0

X 0 D

D 2 0 2 X

GMSC/VLR

HLR

X 0

D 0

VMSC/VLR

A-sub

B-sub

1. SRI (B-MSISDN)

4. SRI Response (MSRN)

3. PRN Response (MSRN)

2. PRN (B-IMSI)5. Call setup (

CIRCUIT

CICMSRN + )

6. Paging

Non-circuit-related signalling

Circuit-related signalling

Figure 2. Example of circuit-related and non-circuit-related signalling


Signalling in DX200

3.3.2 Non-circuit-related signalling

Non-circuit-related signalling means that the signalling message does not contain the information about the circuit, instead it contains other information depending on the use of signalling.

Other information can be:

location data (LAI) and TMSI/IMSI in case of location update

short messages in case of sending or receiving a short message

forwarding numbers in case of activation of the call forward supplementary service

B-MSISDN in the Send Routing Information (HLR Inquiry) etc.

Exercise

Discuss with your neighbour whether the listed signalling functions involve call-related signalling or non-call related signalling and circuit-related or non-circuit-related signalling, respectively.

Call-related

non-call related

circuit-related

non-circuit related

Call establishmentCall maintenanceCall releaseCall controlControl of servicesCharging controlLocation updates

Subscriber admin.Short messages



4 Common Channel Signalling System No. 7

4.1 Common Channel Signalling

Common Channel Signalling (CCS) is a signalling method in which signalling information is conveyed over a single channel by addressed messages.

In the previous chapter we have learned that signalling information can be divided into two types: circuit-related and non-circuit related signalling. CCS supports both types of information.

4.2 CCS7

CSS7 is an internationally standardised Common Channel Signalling (CCS) system. It is optimised for operation in digital telecommunication networks such as GSM.

The Common Channel Signalling System No. 7 can be abbreviated CCS7, CCS#7, SS7 and SS No. 7, but they all refer to the same system. To avoid confusion we will only use the abbreviation CCS7 in this document.

One of the main advantages of this CCS7 is that the signalling does not have to go along the same path as the speech. Thus, CCS7 can support both circuit-related and non-circuit-related signalling as in Figure 2.

4.3 CCS7 protocols vs. the OSI reference model

Originally, the CCS7 consisted of two parts: The first part, the Message Transfer Part (MTP), was responsible for transferring the message within a signalling network. The second part, called the User Part, was the user of the MTP. This User Part was only used for circuit-related signalling.

However, the CCS7 was not created for only GSM so the definition of CCS7 in the previous paragraph did not suit the GSM concept. In GSM, CCS7 is also divided into two parts, but they are called the functional part and the message transfer part.


Signalling in DX200

Physical layer

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

MTP (levels 1-3)

User Part

(Common part)(PSTN, ISDN,

GSM)

User Part

OSI Reference Model Common Channel Signalling No. 7

Application Part

(GSM specific part)

SCCP

Layer

1

2

3

4

5

6

7

Level

1

2

3

4

Figure 3. CCS7 protocol stack vs. the OSI reference model

The user is the functional part who sends, receives and acts depending on received signalling information.

Example 1:

When a subscriber makes a call to the PSTN, the mobile sends the dialled number to the MSC via the BTS and BSC. Since only the BSC and MSC use CCS7, a functional part in the BSC will send the dialled number to the MSC, which will receive this dialled number by using another functional part in the MSC. In this example, the dialled number acts as signalling information and there are two functional parts in use (one is in the BSC and another in the MSC).

Example 2:

When a subscriber switches on a mobile, there is a location update request message sent from the mobile to the VLR that informs the VLR that the mobile now wants to become active in the GSM network. In this case, there are at least two functional parts in use (one is in the BSC and another in the VLR). It is also possible that the VLR will ask the HLR for subscriber data. In this case one more functional part has to be used in the HLR to be able to receive the signalling information from the VLR.

The functional part of CCS7 is divided into two parts (as shown in Figure 3):

user part (common part)

application part (GSM specific part).



4.3.1 User part (common part)

This part is commonly used in any type of digital telecommunication networks such as PSTN, ISDN and GSM. All signalling messages in this common part have the CIC defined. Thus, the common part is the only for circuit-related signalling part. Three different user parts are used in GSM:

Telephone User Part, TUP

National User Part, NUP

ISDN User Part, ISUP.

All user parts need only the services from the MTP level.

4.3.2 Application part (GSM specific part)

This part is only used in the GSM network. In GSM networks, signalling is not as simple as in the PSTN (only circuit-related signalling). GSM does not require only circuit-related signalling, but it also requires a large amount of non-call-related signalling such as location update, SMS, and handover.

Three application parts are used in GSM.

Base Station Subsystem Application Part, BSSAP

Mobile Application Part, MAP

Intelligent Network Application Part, INAP.

All application parts (BSSAP, MAP, INAP) need the services provided by the Signalling Connection and Control Part, SCCP. The SCCP will be explained later in this chapter.


Signalling in DX200

4.4 CCS7 protocols in various network elements

Figure 4. CCS7 protocols in various network elements

Referring to Example 1, when a subscriber makes a call, a functional part in the BSC sends the dialled number to a functional part in the MSC. The functional part in the BSC is the BSSAP and the functional part in the MSC is also the BSSAP. After the MSC analyses the dialled number, the MSC knows that the call has to be routed to the PSTN because it is a PSTN number. Thus, another functional part in the MSC will start sending call setup message (the dialled number together with the CIC are included) to the PSTN. The ISUP is the functional part in the MSC and in the PSTN in this case.

Referring to

Example 2, the location update message is sent from the mobile to the VLR via the BSC. The BSSAP is a functional part in both the BSC and VLR. If the VLR needs to ask subscriber data from the HLR, another functional part will be used in the VLR and HLR. This functional part is used only in the NSS area, as it in fact is a GSM specific functional part. Thus, a MAP is used in both the VLR and the HLR.

The details of each functional part will be discussed in the next chapter.



5 Functional parts of the CCS7

5.1 User parts

Figure 5. User parts (TUP, NUP, ISUP)

5.1.1 Telephone User Part (TUP) and National User Part (NUP)

The Telephone User Part (TUP) is the CSS7 protocol that provides signalling functions that are required in international telephone call control signalling. The TUP handles the normal call set-up and release of voice and data calls between the originating and destination exchanges, and handles abnormal situations by using its release procedures.

The ITU-T allowed that the TUP could be slightly modified to allow for national use within one country. These national TUP versions are known as National User Parts, NUPs. The variations are minor and very similar to the TUP. Therefore, we will only take a look at the TUP message structure.

5.1.2 ISDN User Part (ISUP)

The ISUP is the user part specifically defined for the ISDN. It provides the facilities for handling the ISDN services and supplementary services for voice and data applications.


Signalling in DX200

The ISUP is well suited for applications in dedicated telephone and circuit switched data networks, both national and international. Within the GSM network, the ISUP is used between the MSC and another MSC as well as between the MSC and the PSTN.

The ISUP is on level 4 in the CCS7 signalling system architecture. The MTP is used for carrying the ISUP information situated in the SIFs (Signalling Information Fields) of the MSUs (Message Signal Units) in the MTP.

5.2 Application parts

Figure 6. Application parts or SCCP subsystems (BSSAP, MAP, INAP)

The information in BSSAP, MAP and INAP messages is handled by the SCCP (Figure 6) so the application parts can be called SCCP subsystems. All application parts are specific to GSM networks. The TCAP is explained in the Appendix.



5.2.1 Base Station Subsystem Application Part (BSSAP)

The BSSAP is responsible for transferring the GSM specific messages between the MSC and BSC, and between the MSC and a mobile station. The BSSAP can be divided into two subparts, DTAP and BSSMAP.

DTAP (Direct Transfer Application Part) handles the transfer of the signalling information between the MSC and the MS such as call setup, alerting and location update request messages. They are all sent transparently via the BSC to the MS.

BSSMAP (Base Station Subsystem Application Part) handles the signalling functions between the MSC and the BSS and performs other procedures such as handover control and paging. The messages used in these procedures are sent directly from the MSC to the BSC.

From the examples of DTAP and BSSMAP it becomes clear that the BSSAP supports sending both circuit and non-circuit-related signalling by using SCCP-level services.

BSSA

P

BSSMAP

DTAP

BSC

L

MSC/VLR

MS

DTAP (Call setup, location update)

BSSMAP

(Paging)

Both circuit-related and non-circuit relatedsignalling messages.

Figure 7. BSSAP subparts (DTAP and BSSMAP)


Signalling in DX200

5.2.2 Mobile Application Part (MAP)

MAP is a GSM specific protocol for only non-circuit-related application inside the NSS area. MAP procedures can be location registration, handling of supplementary services, HLR inquiry, Inter-MSC handover, authentication, IMEI checking and support of short message services.

The operations defined within the MAP protocol are sent through the signalling network by using TCAP, SCCP and MTP services. For SCCP, MAP uses only a connectionless service.

5.2.2.1 MAP interfaces

The MAP interface enables the MSCs, HLRs, VLRs, and EIRs, to communicate smoothly and efficiently with each other. The GSM Technical Specification defines six different interfaces between network elements (Figure 8).

VLR

MSC

VLR

MSC

HLR

EIR

E G

F

B

B

D

C

Only non-circuit related signalling messages

Figure 8. MAP interfaces between the network elements

Interface B is situated between the MSC and the VLR. Messages concerning, for example, location updates, call control (check subscriber A info) go through this interface. In the DX 200 architecture the MSC and the VLR are integrated and thus the interface B is an internal interface.

Interface C is situated between the MSC and the HLR. Messages related to, for example, HLR-inquiries are sent via this interface.



Interface D is situated between the VLR and the HLR. Messages concerning, for example, location updates are sent via this interface.

Interface E is situated between two MSCs. It is used, for example, for inter-MSC handover related signalling.

Interface F is situated between the MSC and the EIR. Messages for IMEI check are sent via this interface.

Interface G is situated between two VLRs. It is used, for example, for location updates.

5.2.2.2 SCCP subsystems of MAP in the DX 200 architecture

In the DX 200 GSM implementation there are the following MAP SCCP subsystems:

MAP-M:MSC MAP. It handles messages going through the interfaces B, C, E,and F.

MAP-V:VLR MAP. It handles messages going through the interfaces B, D,and G.

MAP-H:HLR MAP. It handles messages going through the interfaces C and D.

MAP-E:EIR MAP. It handles messages going through interface F.

MTP

SCCP

TCAP

MAP-M MAP-V

MTP

SCCP

TCAP

MAP-H MAP-E

MSC/VLR HLR/AC/EIR

Figure 9. SCCP subsystems of MAP in the MSC/VLR and the HLR/AC/EIR


Signalling in DX200

5.2.3 Intelligent Application Part (INAP)

The INAP protocol is part of the Intelligent Network (IN) implementation and used for communication purposes between a Service Switching Point (SSP) and a Service Control Point (SCP). Both the SSP and SCP are part of the basic IN network architecture. Regarding the Nokia solution, these would be a DX 200 switch and a UNIX-server.

The operations defined within the INAP protocol are sent through the signalling network by using TCAP, SCCP connectionless service and MTP.

5.3 Summary

Part CCS7 user

Type of signalling

Required services from

Example Used between

User Parts

TUP Circuit-related signalling

MTPMessage Transfer Part

Setup (IAM)Supervise(ACM, ANM)Release (REL) a call

PSTN-MSCMSC-MSC

NUP

ISUP

Application Parts

BSSAP Both circuit-related and non-circuit-related

SCCPSignalling Connection Control Part

Paging, control handover and tracing procedure

MSC-BSC(BSSMAP)

Set up a call, authentication checking, activate/ deactivate supplementary services and send/receive short message services

MSC-MS(DTAP)

MAP Non-circuit-related

HLR inquiry (only MTC)IMEI checkingUpdate locationInter-MSC handoverInter-VLR inquiry

MSC-HLRMSC-EIRVLR-HLRMSC-MSCVLR-VLR

INAP Non-circuit-related

Initial Detection PointRelease call

SSP-SCPSCP-SSP



All user parts are commonly used in digital telecommunications networks (not only in the GSM network). The TUP and NUP are mainly used in the PSTN, but can also be used in GSM. The ISUP is introduced for handling services and supplementary services in the ISDN.

The only signalling information transferred by these three user parts is circuit-related signalling. To be able to transfer circuit-related signalling, MTP services are required. This means that:

the MTP should know how to route signalling messages from one user to another and vice versa. We will discuss about this in the next chapter.

All application parts are GSM specific parts. BSSAP is used only between the MSC and the BSC, and the MSC and the MS. MAP transfers only non-circuit-related signalling in the NSS area (MSC, VLR, HLR, EIR). INAP is needed for communication between the SSP and the SCP when there is IN implementation in GSM.

All GSM specific application parts need the SCCP services. This means that:

the SCCP should be able to perform routing functions for signalling messages which are sent from its users (SCCP subsystems)

only MTP services can not fulfil the application part requirement.

User PartsSCCP Subsystems

MTP

SCCP

TCAP

MAP-M MAP-V MAP-H MAP-E INAP

BSSAP

MTP

TUPNUPISUP

Figure 10. User parts and SCCP subsystems in GSM


Signalling in DX200

6 Signalling routing methods

In the summary of the previous chapter we learned that, to be able to transfer messages from both user parts and application parts, there are two levels that perform the routing function: the MTP (Message Transfer Part) level for user part messages and the SCCP level for application part messages.

Since the MTP was defined for general digital telecommunications networks such as the PSTN, the MTP is well suited for the TUP and NUP. But as the BSSAP, MAP and INAP are defined especially for GSM, a special protocol has to be established between the application parts and the MTP level. This special protocol is called the Signalling Connection Control Part, SCCP.

MTP

SCCP

Signalling messages from upper level

2.by SCCP

1.by MTP

to other network elements

Figure 11. Signalling routing methods (by MTP and by SCCP)

In this chapter the Message Transfer Part along with its different levels and the routing of the signalling message will be explained.



6.1 Message Transfer Part (MTP)

MTP is a functional part of the CCS7 system. It transfers signalling messages as required by all the users and performs necessary subsidiary functions (for example, error control and signalling security).

In this chapter we will take a closer look at the MTP. As described earlier, the MTP is responsible for the transmission of signalling messages. Hence the combination of signalling hardware, the part of the transmission network dedicated for CCS7 (signalling links) and the MTP definitions constitute the CCS7 signalling network.

The following introduction of the components of the signalling network is structured according to the order of making definitions in the MTP when creating the signalling network in a DX 200 MSC. As in other hierarchical systems, we have to define the basic elements first. The more complex objects can only be defined afterwards, as the basic elements are used as parameters for the complex objects in the DX 200 user interface.

6.2 Signalling Point (SP)

When talking about the network elements in terms of signalling, we call them Signalling Points. A signalling point is a node of the signalling network, which is able to send and receive signalling messages.

A SP is not necessarily a ”whole” network element, that is, User or Application Parts are not necessarily defined so that the SP is able to function as a potential creator or addressee of a signalling message. It is also possible to define network nodes that have only a routing task, like the transmission node where only the MTP and possibly also the SCCP are defined.

Signalling points that can be used as a link to other signalling points are called Signalling Transfer Points irrespective of whether or not they perform more than just MTP functions. SPs that do not forward so called "transfer messages" are Signalling End Points (SEP).

SEP STP SEP

Figure 12. Signalling Point types


Signalling in DX200

Typically the network elements are configured as in the following table:

Network Signalling Point

MSC STP

HLR SEP

BSC SEP

PSTN STP

If we look at a certain signalling message, we have one signalling point that creates the message and one signalling point for which the message is meant. The signalling point that sends the signalling message is called Originating Point. The signalling point receiving the signalling message is called Destination Point (DP).

Note

These terms are functions of a signalling point related to a single message, not signalling point types as the above mentioned SEP and STP, which are terms used irrespective of a single signalling message.

All signalling points within a network are identified by Signalling Point Codes, SPCs, which are unique within a signalling network. As we discovered already in chapter 3, the MTP uses the SPC to route a message. The SPC of the originating point and the destination point are called Originating Point Code (OPC) and Destination Point Code (DPC) respectively. Both are included in a signalling message.

OPC

DPC

SIO

CIC

.....

Figure 13. Circuit-related signalling message (simplified)1

All signalling points that stand in a signalling relationship with each other and the signalling links interconnecting them build a Signalling Network.

In terms of MTP definitions we have to give the following information about a signalling point: the signalling network, the SPC and its format, the name of the

1 The Circuit Identification Code (CIC) is the identification of the certain speech circuit that the circuit-related signalling is for. Its value represents the PCM and time slot number in an unambiguous way.



SP, and the SP type. As soon as a signalling point is created in a signalling network, the network exists from the SP point of view. The opposite view also holds; if a SP is not created in a network, the network does not exist for the SP.

As mentioned earlier, the distribution function of the MTP requires a second address, which identifies the MTP users. These protocols are called ”services” and the address Service Information Octet (SIO).

Caution

The SIOs may be defined before or after creating the SPC but have to be defined before making reference to the signalling network, that is, when creating signalling link sets (see below).

For a functioning signalling network on MTP level only the SIOs identifying the receiving programs of Signalling Network Testing and Maintenance (SNT) and Signalling Network Management (SNM) messages are needed. If telephone calls have to be possible, however, also the other services (SCCP, TUP/ISUP) have to be identified by their SIOs.


Signalling in DX200

6.3 Signalling Link (SL)

A PCM time slot reserved for signalling between two neighbouring signalling points is called a Signalling Link. Signalling links constitute the basic elements of signalling between two signalling points.

NE1 NE2

Signalling in TSL 64-4

65

66

64

67

All speech time slots handledby TSL 64-4

96

97

95

98

Figure 14. Common channel signalling in time slot 64-4

Time slot 0 of an external PCM is reserved for 0synchronisation and alarms. Thus it can not be used as a signalling link. Otherwise the selection of time slots used for signalling is not restricted. The only exception is the connection between the MSC and BSC, where time slot 16 is used for historical reasons.

This is because the Transcoder between the MSC and BSC (in NOKIA implementation it is placed at this location, other configurations are possible) usually compresses the information in all other time slots except for TSL 16 unless otherwise configured. However, this configuration might be impossible in older or non-NOKIA equipment. The signalling data would not be interpretable anymore after compressing it with the speech-oriented algorithms of the Transcoder. It is common practice still, even without regarding the Transcoder, to place the signalling link into time slot 16.

There can be several signalling links between two signalling points. It is recommended to have at least two signalling links in two different PCM lines connected to two different signalling units, which share the outgoing traffic. Then, in case of a failure of some equipment (PCM, signalling terminal or



signalling unit), the other signalling link will still be working. Without taking special measures (which will not be discussed here) the maximum number of signalling links between two neighbours within one signalling network is restricted to 16. This should be more than enough in the most cases.

<ZNCI;

DX 200 MSC03 1998-09-13 09:30:45

INTERROGATING SIGNALLING LINK DATA

TERM LOG LOG PARAM LINK LINK SET PCM-TSL UNIT TERM FUNCT UNIT TERM SET

---- --------- -------- ------------------ ---------- -----

0 16 HLR01 88-01 CCSU-2 1 0 4041H 1 0

1 16 HLR01 90-01 CCSU-0 3 0 4042H 0 0

2 17 BSC01 80-16 BSU-2 1 0 4131H 1 0

3 18 PSTN1 65-01 CCSU-2 0 0 4041H 0 0

4 19 BSC02 81-16 BSU-1 1 0 4132H 0 0

5 20 PSTN2 66-01 CCSU-2 2 0 4041H 2 0

COMMAND EXECUTED

Figure 15. Signalling link definitions in an MSC

When creating a signalling link we do not yet have to define the remote SP to which it interconnects. This will be done in the next step. What we have to define instead is hardware related data, that is, which signalling unit to connect (determines the internal PCM) and which time slot in which external PCM to use.

Note

The number of the external PCM implicitly already identifies the connected remote SP at the other end of the PCM.

The definition of a signalling link results in the creation of a semipermanent connection through the Group Switch (GSW) between an external and internal PCM and includes the external PCM-TSL as a circuit in a special circuit group that the system reserves for signalling.


Signalling in DX200

Legend Figure 15

LINK signalling link number

LINK SET number and name of the signalling link set which the link is assigned to (see next chapter)

PCM-TSL external PCM number and time slot of the link

UNIT type and index of the signalling unit handling the link

TERM index of the signalling terminal (automatically assigned)

TERM FUNCT

ordering number of the link within a multichannel terminal (automatically assigned, e.g. AS7-U: 0 through 3)

LOG UNIT internal unit identification (automatically assigned)

LOG TERM ordering number of the link within a signalling unit (automatically assigned)

PARAM SET number of the selected signalling link parameter set

Caution

Before creating a signalling link, we have to pay attention that the selected time slot is not reserved for other purposes (included in another circuit group, for example for speech circuits) and that the selected signalling unit has enough free capacity (signalling terminal) to handle the signalling link.

In order to identify the signalling link, the system internally uses a number, the so-called Signalling Link Number (SLN). This internal number is most likely different at the other end of the line, since usually the next free number is assigned. When creating the signalling link this next free number depends on the order of creating signalling links and the amount of already created links at either end.

6.4 Signalling Link Set (SLS)

All signalling links directly connecting two signalling points within one signalling network make up a Signalling Link Set. For each signalling network that a signalling point exists in, the SP can have as many SLSs as it has connections to other network elements.



<ZNSI:NA0;

EXECUTION STARTED

DX 200 MSC03 1998-09-13 09:30:55

INTERROGATING SIGNALLING LINK SET DATA

NET SP CODE H/D LINK SET LS STATE LINK SLC PRIO

--- ---------- --------- -------- -------------

NA0 0020/00032 18 PSTN1 AV 3 0 0

NA0 0064/00100 20 PSTN2 UA 5 0 0

LINK TEST NOT ALLOWED

NA0 0320/00800 17 BSC01 AV 2 0 0

NA0 0384/00900 16 HLR01 AV 0 0 0

1 1 0

NA0 044C/01100 19 BSC02 UA 4 0 0

COMMAND EXECUTED

Figure 16. Signalling Link Set definitions in an MSC (five link sets, only the one towards the HLR contains more than one link)

The system uses the SLS as a list of resources to choose from. If the routing function of the MTP in NE1 (see Figure 18) has decided to send a message to NE2, a signalling link is selected from the link set defined in NE1 for NE2. The signalling load is generally shared over all links and is evenly distributed by means of a certain distribution mechanism.

Legend Figure 16

NET Signalling network the SLS is defined for

POINT H/D SPC of the neighbour node in hexadecimal and decimal format

LINK SET Name and number of the SLS defined

LS STATE State of the SLS (AV: available, UA: unavailable); as soon as at least one link is active, the SLS automatically becomes available

LINK Signalling link number (internal identification)

SLC Signalling link code (external identification)

PRIO Priority (max. 0, min. 15) of signalling link within link set


Signalling in DX200

NE1 NE265

66

64

67

= Signalling Link (TSL dedicated to signalling)= PCM

SLS from NE2 to NE1(defined in NE2)

SLS from NE1 to NE2(defined in NE1)

Signalling Link is included inneither SLS it is not used inthe same signalling network ornot at all

96

97

95

98

Figure 17. Signalling Link Sets have to be defined from both sides and include all usable links between two neighbours (in one signalling network)

The Signalling Link Code (SLC) is used to identify a specific signalling link within a link set for both neighbours. This number is necessary as the signalling link number (SLN) is only used internally for administrational purposes and is possibly different on both sides of the link.



SLN SLC

same at both ends? not necessarily yes

value range HLR, MSC: 0..299

BSC: 0..99

0..15

Usage Internally In messages

The SLC has to be the same at both ends otherwise the link is not usable. It is needed for Signalling Network Testing and Maintenance (SNT) and Signalling Network Management (SNM) messages, which govern and handle the availability of a link.

6.5 Signalling Transfer Point (STP)

As stated before, signalling can be transmitted using a different route than the actual speech connection. For availability reasons it is very common not only to define redundant signalling links directly between two signalling points, but also to utilise an indirect connection via a Signalling Transfer Point, STP. If all direct links between the two elements become faulty, the indirect path through the STP might still be available.

OP

STP

DP

quasi-associatedsignalling link

associatedsignalling link phone call

identified by CIC

Figure 18. Associated signalling uses the same path as the speech, quasi-associated signalling is routed independently


Signalling in DX200

Depending on whether or not the signalling message uses the same route as the speech connection we call the signalling either associated signalling or quasi-associated signalling.

Note

These terms can only be applied in connection with circuit-related signalling, for example when sending a call set-up message.

The Circuit Identification Code (CIC) identifies the PCM and the time slot of the speech connection the call set-up message is for. The destination point is notified to reserve resources for this call and to continue with the call set-up towards the next network element, if necessary.

Since the PCM number is also only valid locally, that is, like the SLN it can be different on both sides, the CIC is based on another numbering of the PCMs which is agreed between the two neighbours: the CCSPCM number.

No matter which way the message takes, the Destination Point will clearly understand from where (OPC), and in which time slot of which PCM (CIC), the call will come in.

6.6 Signalling Route (SR) and Signalling Route Set (SRS)

Signalling messages are not always exchanged between neighbours, as usually in circuit-related signalling. For non-circuit related signalling especially, the destination point (for example an HLR) can under some circumstances be reached only through a chain of network elements. Therefore, the CCS7 signalling system provides the means to route a message through the signalling network towards distant signalling points.

A Signalling Route (SR) is a pre-determined path, consisting of a succession of signalling points (or signalling transfer points respectively) and the interconnecting signalling links, that a message takes through the signalling network between the OP and the DP.

Since it would have been quite inflexible and wasteful to store the data of the whole path (towards every other necessary signalling point in the network) in every signalling point, another approach was taken: The system works according to the receive and forward principle.



OP

SRS containing all possible routes (neighbours) to reach DP from OP (as defined in OP)

SLS from OP to DP(as defined in OP)

SLS from OP to STP(as defined in OP)

DP

STP

Figure 19. One of the two possibly defined signalling route sets (grey area) in the OP (the other one can be defined towards the STP and happens to contain the same routes). The route set contains neighbouring signalling points, not signalling links or link sets

The only information that an OP and STP needs is which neighbouring signalling point a message has to be forwarded to in order to reach the identified destination of the message. The neighbour represents the whole route.

The responsibility of delivering the message is delegated to this neighbour, as the path beyond that point can not be influenced. By using this method the complexity of the routing task decreases considerably to a level of neighbour-to-neighbour relationships.

The system in every signalling point only has to keep a list of all possible destination points and the corresponding neighbour(s) responsible for forwarding the message. Since we can have multiple possible routes through the network we can also store more than one neighbour in that list.


Signalling in DX200

SRS #1

SRS #2

SRS #3

SRS #4

SP Data

SRS #5

<ZNRI:NA0;

DX 200 MSC03 1998-09-13 09:31:05

INTERROGATING SIGNALLING POINT DATA

NET SP CODE H/D NAME RS STATE PAR SET--- ------------ ------- -------- ---------NA0 0020/00032 PSTN1 AV 0LOAD SHARING BETWEEN SIGNALLING ROUTES DENIED

ROUTES: SP CODE H/D NAME STATE PRIO --------------- ---- ------ ---- NA0 0020/00032 PSTN1 AV-EX 7 NA0 0064/00100 PSTN2 AV-SP 6

NET SP CODE H/D NAME RS STATE PAR SET--- ------------ ------- -------- ---------NA0 0064/00100 PSTN2 AV 0 LOAD SHARING BETWEEN SIGNALLING ROUTES DENIED

ROUTES: SP CODE H/D NAME STATE PRIO --------------- ---- ------ ---- NA0 0064/00100 PSTN2 AV-EX 7 NA0 0020/00032 PSTN1 AV-SP 6

NET SP CODE H/D NAME RS STATE PAR SET--- ------------ ------- -------- ---------NA0 0320/00800 BSC01 AV 1 LOAD SHARING BETWEEN SIGNALLING ROUTES DENIED

ROUTES: SP CODE H/D NAME STATE PRIO --------------- ---- ------ ---- NA0 0320/00800 BSC01 AV-EX 0

NET SP CODE H/D NAME RS STATE PAR SET--- ------------ ------- -------- ---------NA0 0384/00900 HLR01 AV 0 LOAD SHARING BETWEEN SIGNALLING ROUTES DENIED

ROUTES: SP CODE H/D NAME STATE PRIO --------------- ---- ------ ---- NA0 0384/00900 HLR01 AV-EX 0

SS7 SUBFIELD INFONET SP CODE H/D SP NAME SP TYPE STAND COUNT BIT LENGTS--- ------------------ ------- ------- ----- ----- ----------NA0 0BB8/03000 MSC3 STP CCITT 1 14 OWN SP

NET SP CODE H/D NAME RS STATE PAR SET--- ------------ ------- -------- ---------NA0 044C/01100 BSC02 UA 1 LOAD SHARING BETWEEN SIGNALLING ROUTES DENIED

ROUTES: SP CODE H/D NAME STATE PRIO --------------- ---- ------ ---- NA0 044C/01100 BSC02 UA-INS 0

COMMAND EXECUTED

Figure 20. Signalling Route Set definitions in an MSC



As you see in Figure 20, no link set or signalling link is mentioned in the route set definitions. This might seem surprising as, in everyday life, the term route normally suggests the way rather than the destination.

A signalling route is an entity of the signalling network which possesses its own working state and which must not be mistaken with the signalling link set connecting us with the neighbour.

If the signalling link set towards the signalling point indicated by the signalling route is available, the signalling route can also be activated and become available. If the signalling link towards a neighbour is available, the route can still be unavailable due to problems behind that neighbour. Due to security considerations, the activation of a route is a manual process.

Legend Figure 20

ROUTE SET DATA Identification of the destination

NET Signalling network the SRS is defined for.

SP CODE H/D DPC (destination point code) in hex and decimal format.

NAME Name given for the destination in the local system.

RS STATE State of the SRS (explained below); as soon as at least one route is available, the SRS automatically becomes available.

PAR SET Number of the selected SRS parameter set.(1 = A interface)

info line When creating the route set, the sharing of the signalling load among all routes of that route set can be allowed or denied; several notes can be output here.

ROUTE DATA Identification of the neighbour

SP CODE H/D STPC (transfer point code) in hex and decimal format.

NAME Name given for the neighbour in the local system.

STATE State of the SR (explained below) as manually set with MML.

PRIO Priority of the route (max. 7, min. 0) within the route set; unless load sharing is agreed and there is more than one route with the highest priority, the route with the highest priority carries the traffic.


Signalling in DX200

Legend Figure 20 (cont.)

POINT DATA Identification of the own signalling point

NET Signalling network the own SP is defined in.

SP CODE H/D Own SPC in hex and decimal format.

SP NAME Own name as given locally.

SP TYPE Indicates if the own SP is created as an end point (SEP) or a transfer point (STP).

SS7 STAND Indicates the used signalling standard: mainly if the SPC is composed according to CCITT rules (14bit SPC), ANSI or CHINA standards (both 24 bit SPC).

SUBFIELD INFO Tells about the grouping of the SPC bits into max 3 groups (subfields) and how many bits per subgroup are allocated.

6.6.1 Load sharing

If we have more than one route defined for a destination, the system has to be advised how to handle the redundant routes. There are two ways as to how this can be done. The first one is to always use one route as the primary route and the others as spare routes. Only if the primary route becomes unavailable a spare route is activated. Using this method, all routes within one route set should be given different route priorities. The system then chooses the route with the highest priority from all the available routes.

In Figure 19 there is a triangle situation between the MSC, PSTN1 and PSTN2 as previously depicted in Figure 18, where the routes leading directly to a network element are the primary routes and the indirect routes are spare routes.

The second possibility is called load sharing. In this case those routes which are selected to share the signalling load have to have the same priority and load sharing has to be allowed for the route set. The system then distributes the load evenly over all routes participating in the load sharing if no other route has a higher priority. Usually, only two routes are used for loadsharing.

Even though this seems to be a good idea, one has to be careful with this method. Since the definitions in the other network elements are sometimes not so well known, there is the danger in complex networks of loops being created. A loop results in a part of the messages circulating the network and either not reaching its destination on time, or perhaps never arriving at all (messages have maximum lifetime).



Caution

The danger of creating loops is the reason why the first method is more favourable. Each route is designed to handle the complete load, load sharing is denied and a spare route is used only when the primary route fails.

6.6.2 Factors that effect the routing of a signalling message

There are three main factors that have to be taken into account every time a message is transferred.

Factor 1:A message should be transferred to the correct destination, for example, a location registration message is sent between MAP-V in the VLR and MAP-H in the HLR.

Factor 2:A destination should receive an error-free message or, if there are some errors, the destination should be able to detect and correct these errors as it is called error handling technique. For example, MAP-H in the HLR should receive the correct IMSI, which is included in the location registration message.

Factor 3:Since there can be more than one signalling link between two adjacent signalling points and more than one route towards the same destination, the MTP should know which signalling route and signalling link to use. Sometimes, a signalling link may fail because of transmission or traffic overload problems, or because of how the MTP informs the adjacent nodes that this signalling link is unavailable and please use the other available link instead. Therefore, the MTP must have some kind of management for signalling traffic, signalling links and signalling routes.

These three factors are handled by three levels of the MTP as shown inthe following figure.


Signalling in DX200

Figure 21. MTP levels

The MTP contains the functions of the three lowest levels of the CCS7 architecture: level 1: signalling data link functions; level 2: signalling link functions; and level 3: signalling network functions.

Level 1:Signalling data link functions define physical and electrical characteristics of the signalling link The standard signalling link is a 64 kbit/s time slot on a 2.048 Mbit/s digital connection. Bit is a unit of transfer data in this level.

Factor 1 and factor 3 need a function of MTP level 1.

Level 2:The signalling link functions provide a reliable method of transferring data between two adjacent signalling points (error detection, correction and synchronisation). The information transfer unit on this level is a Signal Unit (SU). Factor 2 and factor 3 need a function of MTP level 2.

Level 3:Contains two function blocks: Message Handling and Network Management. The data transfer unit on this level is Message.

The Message Handling function block is responsible for the routing and distribution of the received signal units as well as of the signal units to be sent. Factor 1 requires the message handling function.

The Network Management function block contains all procedures and functions for maintaining the signalling network in case of loss of a signalling link, link set or route set as well as mechanism for traffic control. Factor 3 requires the network management function.



6.6.3 Routing of a signalling message by the MTP

We will start with level 2 of the MTP and continue with level 3. As level 1 is just a physical level it will not be discussed here.

6.6.3.1 MTP level 2

To be able to understand how MTP level 2 performs error and synchronisation handling between two adjacent points, the basic structure of a signal unit (SU) should be explained.

Signal units contain additional information to guarantee a secure transmission. This additional information enables the following level 2 functions:

Delimitation of signal units by means of flags

Flag imitation prevention by bit stuffing

Error detection by checking bits included in each signal unit

Error control by re-transmission and signal unit sequence control by means of explicit sequence numbers in each signal unit and explicit continuous acknowledgements

Signalling link failure detection by means of signal unit error rate monitoring and signalling link recovery by means of special procedures.

6.6.3.2 Level 3 of the MTP

After level 2 receives a signal unit (for example an MSU or an LSSU), it will interpret that SU. If no error is detected and a SU is the MSU, the MSU will be routed to the level 3 function block Signalling Message Handling. If there is no error, the LSSU will be sent to the level 3 function block Signalling Network Management as shown in Figure 22.


Signalling in DX200

Are thereany errors?

MSU

No

Message handling Network Management

Are thereany errors?

LSSU

YesYesError

handling

No

Level 2

Level 3

TUP ISUP SCCP

Network management messages

To othernetworkelement

Figure 22. Functions of MTP level 2 and level 3

Signalling message handling

Questions:

1. Why does only the MSU (and not the FISU or the LSSU) need a message handling function?

As we already know, the FISU and LSSU are sent between two adjacent nodes only. The LSSU contains the signalling link status so it should be routed directly to the network management. The FISU is used only when there is no MSU or LSSU sent in the link.

1. What is the purpose of message handling?

To find out two things:

The right destination (that is, whether the destination of the message is this network element or another).

If the destination of the message is this network element, the next thing to find out is the right user. The recipient of the message depends on the message type. If the message is used for network management, the message handling will send it to the network management function block. If the message is of ISUP type, it will be sent to the ISUP level and if it is a SCCP message, message handling will send it to the SCCP level.



2. What are the necessary processes in message handling?

There are three processes needed in message handling function:

Message discrimination: decides whether the incoming message is for its own signalling point or not by checking the Destination Point Code (DPC) contained in the message.

Message distribution: determines upon receipt of a signalling message to which user part the signalling is to be delivered by using the Service Information Octet (SIO).

Message routing: decides which signalling link to use for each outgoing signalling message by using the Signalling Link Selection (SLS) or the Signalling Link Code (SLC).

Discrimination

DPC = own SPC

Routing

Distribution

DPC own SPC¹

Network ManagementSCCPTUPISUP

MTP users

From MTP level 2To MTP level 2

Signalling MessageHandling

Figure 23. Signalling message handling

The discrimination function evaluates the Destination Point Code (DPC) of the MSU:

DPC = own SPC -> distribution function

DPC <> own SPC -> routing function.

The distribution function checks the Service Information Octet, SIO to find out the suitable user part.

The routing function finds the suitable signalling link for sending the signal unit to another network element.

Message handling can only route out a message which contains three bits of necessary information: DPC, SIO and SLS or SLC. DPC is not the only bit


Signalling in DX200

needed for routing purposes; the Originating Point Code (OPC) is also needed when the destination wants to reply to the original source of the message, so it can use the OPC which is contained in the received message.

6.6.4 Example of MTP routing

Discrimination

DPC = own SPC

Routing

Distribution

ISUP in MSCIAM (DPC,CIC)ISUP

MTP

ISUP in PSTNIAM (DPC,CIC)

X0 D0 D20

MSC/VLR

PSTNexchange-A

IAM (CIC+dialled digits)

DPC SIO = ISUP

Figure 24. MTP routing example

Figure 24 shows an example of a call to the PSTN originating from a mobile subscriber. The MSC/VLR sends the Initial Address Message (IAM) to a PSTN exchange by the ISUP. The ISUP uses the message handling function of the MTP to route out the IAM to a PSTN exchange and in PSTN exchange. The message handling function is needed in the PSTN exchange to receive and deliver a message to the correct user part (ISUP) by using the Service Information Octet (SIO) contained in the message.



Signalling network management

The signalling network management message contains a label and management functions of the signalling link as shown in Figure 25.

D O C U M E N T T Y P E

T y p e U n i t O r D e p a r t m e n t H e r eT y p e Y o u r N a m e H e r e T y p e D a t e H e r e

H 1 H 0 S L C O P C D P C

4 b i t s 4 b i t s 4 b i t s 1 4 b i t s 1 4 b i t s 4 b i t s

O P C = O r i g i n a t i n g p o i n t c o d eD P C = D e s t i n a t i o n p o i n t c o d eS L C = S i g n a l l i n g l i n k c o d eH 0 = H e a d i n g c o d e i n d i c a t i n g w h i c h m e s s a g e g r o u p t h e m e s s a g e b e l o n g s t oH 1 = H e a d i n g c o d e i n d i c a t i n g t h e m e s s a g e w i t h i n t h e g r o u p i n q u e s t i o n

S u b s e r v i c ef i e l d 0 0 0 0

F i r s t b i t t r a n s m i t t e d

F C K S I F S I O L IFI

BF S N

BI

BB S N FM T P

m e s s a g e

U s e r i n f o r m a t i o ns u b f i e l d

Figure 25. Structure of the network management messages

There are three subsidiary functions to carry out signalling network management:

Signalling Traffic Management

Signalling Link Management

Signalling Route Management.

Signalling Traffic Management is responsible for the availability of a signalling link or a signalling route by using the following procedures:

Changeover

Changeback

Forced rerouting

Controlled rerouting

Signalling traffic flow control

Signalling traffic usage limitations (inhibiting, uninhibiting, inhibit test procedure)

Restarting a signalling point.


Signalling in DX200

Signalling Link Management controls the signalling links and is responsible for state changes by using the following procedures:

Signalling link activation

Signalling link restoration

Signalling link deactivation

Signalling link set activation.

Signalling Route Management is responsible for the availability of a destination by using the following procedures:

Transfer prohibited: indicating the unavailability of a destination

Transfer allowed: indicating the availability of a destination

Transfer controlled: indicating the overload situation of a destination

Route set test: testing the state of a signalling route set

Transfer restricted: indicating the restricted availability of a route.

The management functions use the LSSU2s to perform operations such as changeover between signalling links or routes and restore failed signalling links.

The heading codes H0 and H1 identify the message type such as changeover and out of service.

The user information subfield contains additional information depending on the message type.

6.6.5 Summary of MTP

The MTP level can route the signalling message from the source and destination only if the label is known. A label consists of DPC, OPC and SLS or SLC. It means that if the source knows the DPC, it can use the MTP for routing purposes. If, however, the source does not know the DPC of the destination, but knows a number which can be used to identify the destination, the source has to use routing by the SCCP level instead of routing by the MTP level.

2 Link State Signal Unit, LSSU

The LSSU is used to control the signalling link. It contains information about the state of the signalling link. The possible states are link out of service or busy. The LSSU can be sent only between two adjacent nodes.



6.7 Signalling Connection Control Part, SCCP

As the application parts are GSM specific, the MTP does not fit them. Non-circuit-related signalling messages, for example location registration, HLR inquiry and short message service, require services provided by the SCCP.

6.7.1 SCCP services

The SCCP provides two different services to the application parts.

BasicConnectionless

Service

SequencedConnectionless

Service

BasicConnection-

orientedService

Flow ControlConnection-

orientedService

ConnectionlessServices

Connection-orientedServices

0 1 2 3

Services provided by the SCCP

Protocol Class

Figure 26. SCCP services

The overall set of services is grouped into:

connection-oriented services

connectionless services

Connection-oriented services provide the means to form a virtual connection between two network elements. The messages which are sent from one SPC to another can be connected as a single communication link, and the routing operations are made only during the connection establishment.

Connectionless services provide the applications a way to transfer signalling messages through the signalling network without exactly knowing the network structure. These services are used for very short transfers of data between network elements.

When using the connectionless service, all messages must be routed separately.

In GSM networks both service groups are used. The Mobile Application Part, MAP, uses only connectionless service (mostly service class 0; only inter-MSC handover uses service class 1). The Base Station Subsystem Application Part, BSSAP, uses the connection-oriented service class 2 and the connectionless


Signalling in DX200

service uses service class 0. The Intelligent Application Part, INAP, uses only connectionless service as shown in Figure 27.

Figure 27. Use of SCCP services

6.7.2 SCCP routing function

The messages coming from the SCCP subsystems (BSSAP, MAP and INAP) have to be processed by the SCCP. The information contained in the message determines which way the message is routed.

If there is a label (DPC, OPC, SLS) inside, the SCCP will not perform any special routing function. It just sends a message through the MTP and uses the normal message handling function of MTP level 3. We call this type of routing SCCP routing on label.

If, on the other hand, a label is not contained in the message but there is an address of the destination contained in a message instead, the SCCP will perform a routing function by using this address instead of the label. The address is called a Global Title (GT). Thus, this routing type is called SCCP routing on Global Title (GT).



BSSAP MAP INAP

Is DPC contained in the message?

Yes No (there is a GT)

Routing on label(do nothing)

Routing on GT(GT analysis)

Message handling

Application parts

SCCP

MTP

Figure 28. SCCP routing by label and by GT

BSSAP MAP INAP

RI = GT

Application parts

SCCP

MTP

SIO = SCCP

GT Analysis

Other DPC

Discrimination

DPC = own SPC

Routing

Distribution

DPC is not own SPC

RIRI = SSN

DPCOwn SPC

SSN

Figure 29. Signalling routing (MTP and SCCP)


Signalling in DX200

6.7.2.1 Global Title (GT)

A Global Title (GT) is an address used to identify the destination of the signalling message. The GT is sent by application parts (MAP-V, MAP-M, MAP-H and MAP-E) and analysed by the SCCP level.

Most global titles are in the format of MSISDN (E.164), such as a short message service centre address (SMSC), HLR, VLR, MSC and EIR numbers.

Global Title Format: Numbering Plan Digits

E.164 358 60 2210

E.164 86 138 00100500

HLR address

SMSC address

Figure 30. GT examples

Only location update for a roaming subscriber, E.214 or a hybrid number will be used instead of E.164.

As an example of using E.164, in case of location update for a new visitor (HPLMN subscriber), the VLR will send "Update Location" message to the HLR so that the destination of this message is identified by the HLR number. When the HLR replies, the message contains the VLR number as the destination.

If the new visitor is a subscriber roaming from another operator, the VLR will not know the HLR number of this subscriber. The VLR will use a hybrid number, which is a result of an IMSI analysis, to route the "Update Location" message to the HLR of the other operator network.

Furthermore, a GT is used to route a signalling message to the right destination. Thus, a GT itself should have enough information to route a message towards a destination: Country Code (CC) and National Destination Code (NDC). The CC and NDC are in both E.164 and E.214.

E.214 (Hybrid number): operator number + MSIN

It consists of operator number and the MSIN. Operator number is a unique number that identifies the GSM operator. Normally we can use the CC + the NDC to identify the GSM operator. However, some countries can not use only the CC + NDC to identify their network. They need to add a few more digits.



Table 1. Hybrid numbers for different operators

Operator CC+NDC Operator number

MSIN (part of E.212)

Hybrid number (E.214): operator number + MSIN

Finnish 35850 35850 1234567890 35850+1234567890

China A 86139 86139 0000000002 86139+0000000002

China B 86130 86130 0000001234 86130+0000001234

All GSM operators in Finland can only use CC + NDC to tell that a number can be used to route a signalling message to their network. In their case the E.214 is CC + NDC + MSIN.

6.7.2.2 GT Analysis

GT analysis is used to analyse a GT and identify which way a signalling message should be routed out of the exchange.

Because the SCCP uses the MTP service, one of the outputs of GT analysis is the Destination Point Code (DPC), which is a necessary parameter for message handling function on MTP level 3. The other output of GT analysis is the Routing Indicator (RI) which is used to inform the next exchange whether the GT analysis has to be performed or not.

GTAnalysis

Numbering Plan

Digits

DPC

RI

When the DPC receives the message• RI = SSN, there is no need for GT analysis.• RI = GT, the GT analysis has to be performed.

Figure 31. Global title analysis


Signalling in DX200

6.7.3 Example of SCCP routing

6.7.3.1 Location update

IMSI analysis

Let us take a look at what happens when a subscriber makes a location registration.

If the VLR does not have the subscriber data before, the subscriber's IMSI will be analysed by the means of an IMSI analysis. The purpose of the IMSI analysis is to find out where the subscriber's HLR is. When the VLR knows the subscriber's HLR, the VLR will send a request message to the subscriber's HLR for subscriber data including authentication and GSM services.

The application part responsible for sending "update location" message to the HLR is MAP-V (VLR). MAP uses the connectionless service of SCCP. On the SCCP level, there are two types of signalling routing. Thus, the IMSI analysis can use both SCCP routing on label and SCCP routing on GT depending on who the subscriber is.

IMSI analysis with routing on GT

If the subscriber is roaming subscriber, the VLR does not know the Destination Point Code (DPC) of his HLR. Therefore, the VLR needs IMSI analysis to translate the IMSI (E.212) to a hybrid number (E.214). The hybrid number will be analysed later in the GT-analysis.

IMSIAnalysisIMSI or E.212 Hybrid no. or E.214

MCC MNC MSIN Operator No. MSIN

Figure 32. IMSI analysis with output as Hybrid number (E.214)

IMSI analysis with routing on GT is always used with roaming subscribers as in Figure 33.



IMSIAnalysis

IMSI or E.212

Hybrid no. or E.214

GTAnalysis

Message handling

SCCP routing on GT

MTP

Figure 33. IMSI analysis with routing on GT

IMSI analysis with routing on label

If the subscriber is a home subscriber, the use of IMSI analysis with routing on label depends on whether the VLR knows the DPC of the HLR or not. If the VLR knows the DPC of the HLR, it can use routing on label. Otherwise it has to use routing on GT. In this case the result of the IMSI analysis is the DPC of the HLR as shown in Figure 34.

IMSIAnalysisIMSI or E.212 DPC of the HLR

MCC MNCMSIN

Figure 34. IMSI analysis with output as DPC of the HLR

IMSI analysis with routing on label can be used with home subscribers if the VLR knows the DPC of the HLR.


Signalling in DX200

IMSIAnalysis

IMSI or E.212

DPC of the HLR

Message handling

SCCP routing on label

MTP

Figure 35. IMSI analysis with routing on label

6.7.3.2 International roaming, the first location update

The MS is roaming in country B and performs a location update for the first time.

1. The MS sends its Temporary Mobile Station Identity, TMSI and Location Area Identity, LAI, which is identified by the visited MSC/VLR as numbers which are not related to the own PLMN. Therefore the IMSI of the subscriber is requested from the MS.

2. The IMSI can not be used for signalling routing through the PSTN and international network to the home HLR of the subscriber. Thus the MSC/VLR makes an IMSI analysis shown in Figure 33, which results in a new number, defined in the ITU, E.214. This number is a hybrid of MSISDN and IMSI numbers, and it can be used for routing purposes. The part of the number identifying the destination network element is called the global title.

3. The hybrid number is sent to the SCCP level as an address. The SCCP performs a global title analysis, which results in a Destination Point Code, DPC, understood by the MTP level 3. The result of a global title analysis is normally a SPC of a gateway network element, which also belongs to another signalling network. Additional information is added; for example, the Routing Indicator, RI, which contains information about whether the receiving network element has to perform another global title analysis (RI=GT) or not (RI=SSN).

4. The signalling message can be routed using MTP level 3 routing (DPC) until the gateway network element is reached. After that the message is distributed to the SCCP level.



5. The SCCP level performs according to the Routing Indicator value another global title analysis, which results in a new DPC, adds again a Routing Indicator into the SCCP address and sends the message through the intermediate network such as a PSTN. Only the PSTN exchange, which has a SCCP routing function, can perform GT analysis shown in Figure 36.

6. The message is sent through all the networks between the visiting and the home PLMN of the subscriber.

7. Finally, the message reaches the HLR and the global title analysis points to the signalling point itself. The message is sent to the proper subsystem, which in the case of location update is the MAP-H.

8. The acknowledgement messages are routed by using the MSC-ISDN number.

Figure 36. SCCP routing at PSTN (both on label and on GT)


Signalling in DX200

7 CCS7 management in the DX 200 MSC/VLR

In this chapter, we will take a look at how to create an MTP, user parts, an SCCP, and SCCP subsystems in the DX 200 MSC/VLR. We will learn the necessary procedures and the MML commands needed for implementing them.

Figure 37 shows the procedure when a new MSC is added to a network and has direct connection to our MSC.

Figure 37. Necessary procedure when new MSC is added to network

7.1 Creating an MTP

1. Create signalling link(s) (SL) – ZNC.

2. Create signalling link set (SLS) – ZNS.

3. Create signalling route set (SRS) – ZNR.

4. Activate states of signalling link(s), signalling route(s) – ZNL, NV.



7.2 Creating an ISUP or TUP

1. Create Service Information Octet (SIO) for ISUP or TUP if it does not exist – ZNP.

2. When the ET is connected, define the User Part for that ET so ET knows that it is an ISUP-ET or a TUP-ET – ZWUC.

7.3 Creating an SCCP

1. Check Service Information Octet (SIO) for SCCP. If it does not exist, create it – ZNP.

2. Define SCCP – ZNFD.

3. Activate SCCP state – ZNG.

7.4 Creating SCCP subsystems

1. Add SCCP subsystem(s) (BSSAP, MAP-M, MAP-V, MAP-H, MAP-E or INAP) – ZNFD or ZNFB.

2. Activate SCCP subsystem state(s) – ZNH.

7.5 Creating new roaming contract

1. Create new PLMN for new roaming contract operator – ZMXA.

2. Create IMSI analysis (IMSI => Hybrid number) – ZCFC.

3. Create GT analysis – ZNA, ZNB.

7.6 Creating GT analysis

1. Create GT results – ZNA.

2. Create GT analysis – ZNB.


Signalling in DX200

7.7 MML interfaces

DX 200 MSC03 2000-12-15 09:36:44

SS7 NETWORK ADMINISTRATION

? ..... DISPLAY MENU

A ..... GLOBAL TITLE RESULT HANDLING

B ..... GLOBAL TITLE ANALYSIS HANDLING

C ..... SIGNALLING LINK DATA HANDLING

E ..... SIGNALLING NETWORK STATE INTERROGATION

F ..... SCCP DATA HANDLING

G ..... SCCP STATE HANDLING

H ..... SCCP SUBSYSTEM STATE HANDLING

L ..... SIGNALLING LINK STATE HANDLING

M ..... CCS7 LEVEL 3 PARAMETERS

N ..... SIGNALLING ROUTE SET PARAMETER HANDLING

O ..... SIGNALLING LINK PARAMETER HANDLING

P ..... SERVICE INFORMATION DATA HANDLING

R ..... SIGNALLING ROUTE SET DATA HANDLING

S ..... SIGNALLING LINK SET DATA HANDLING

V ..... SIGNALLING ROUTE STATE HANDLING

Z; .... RETURN TO MAIN LEVEL

Figure 38. SS7 network administration command class



8 Architecture

CCSUBSU

ET

ET

ETHLR

PSTN

BSC

ECUGSW

CCMU

Figure 39. CCS7 architecture


Signalling in DX200

Abbreviations

ACM Address Complete Message

ANM Answer Message

ANU Answer Signal

BIB Backward Indication Bit

BSN Backward Sequence Number

BSSAP Base Station Subsystem Application Part

CBK Clear Back Signal

CC Call Control

CCS7 Common Channel Signalling Number 7

CFU Call Forwarding Unconditional

CIC Circuit Identification Code

CK Check Bits

CLF Clear Forward Signal

CONN Connect Message

DPC Destination Point Code

DUP Data User Part

F Flag

FIB Forward Indication Bit

FISU Fill In Signal Unit

FRL Forced Release

FSN Forward Sequence Number

GSW Group Switch

H0 Heading Code 0

H1 Heading Code 1


Appendix

IAI Initial Address Message with additional information

IAM Initial Address Message

IMEI International Mobile Equipment Identity

IMSI International Mobile Subscriber Identity

ISUP ISDN-User Part

LAI Location Area Identity

LI Length Indicator

LSSU Link State Signal Unit

LU Location Update

MAP Mobile Application Part

MSISDN Mobile Subscriber International ISDN Number

MSRN Mobile Station Roaming Number

MSU Message Signal Unit

MTP Message Transfer Part

NE Network element

OPC Originating Point Code

OSI Open System Interconnection

PLMN Public Land Mobile Network

PSTN Public Switched Telephone Network

REL Release Message

RI Routing Indicator

RLC Release Complete Message

RLG Release Guard Signal

RST Route Set Test

SA Subscriber Administration


Signalling in DX200

SAO Subsequent Address message with One signal

SCCP Signalling Connection Control Part

SF Status Field

SIF Signalling Information Field

SIO Service Information Octet

SLC Signalling Link Code

SLS Signalling Link Selection

SMS Short Message Service

SPC Signalling Point Code

SS Supplementary Services

SSN Subsystem Number

TCAP Transaction Capabilities Application Part

TFA Transfer Allowed

TFP Transfer Prohibited

TMSI Temporary Mobile Station Identity

TUP Telephone User Part


Appendix

Appendix

Transaction Capabilities Application Part (TCAP)

In GSM networks the Transaction Capabilities Application Part, the TCAP, is the interface between the Mobile Application Part, MAP and the SCCP.

Because the connectionless services of the SCCP only send datagrams a function enabling a virtual dialogue between two network elements is needed.

The TCAP provides all functions that enable this virtual dialogue.

The TCAP contains the following functions:

Component Handling

Transaction Handling.

Component Handling provides the assignment between an operation sent by MAP and the received answer.

Example: HLR-enquiry

MSC sends towards the HLR: INVOKE (Send Routing Information)

HLR answers: RESULT (MSRN)

The following components are used:

INVOKE operation

RESULT related to invoke

ERROR

Transactions Handling binds these components into a logical dialogue.

The real TCAP message looks like:

MSC -> HLR: BEGIN [ INVOKE (Send Routing Information) ]

HLR -> MSC: END [RESULT (MSRN) ]

The following TCAP-messages are defined:

BEGIN dialogue

CONTINUE dialogue

END dialogue

ABORT dialogue


Signalling in DX200

Figure 40. TCAP-dialogue


signalling in dx200

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