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SYSTEM TRAINING

Introduction to SS7 Signalling

Training Document


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The information in this document is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This document is intended for theuse of Nokia Networks' customers only for the purposes of the agreement under which thedocument is submitted, and no part of it may be reproduced or transmitted in any form ormeans without the prior written permission of Nokia Networks. The document has beenprepared to be used by professional and properly trained personnel, and the customer

assumes full responsibility when using it. Nokia Networks welcomes customer comments aspart of the process of continuous development and improvement of the documentation.

The information or statements given in this document concerning the suitability, capacity, orperformance of the mentioned hardware or software products cannot be considered bindingbut shall be defined in the agreement made between Nokia Networks and the customer.However, Nokia Networks has made all reasonable efforts to ensure that the instructionscontained in the document are adequate and free of material errors and omissions. NokiaNetworks will, if necessary, explain issues which may not be covered by the document.

Nokia Networks' liability for any errors in the document is limited to the documentarycorrection of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FORERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL ORCONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of thisdocument or the information in it.

This document and the product it describes are considered protected by copyright accordingto the applicable laws.

NOKIA logo is a registered trademark of Nokia Corporation.

Other product names mentioned in this document may be trademarks of their respectivecompanies, and they are mentioned for identification purposes only.

Copyright Nokia Oyj 2003. All rights reserved.


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Contents

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Contents

1 Module objectives................................................................................ 4

2 Introduction.......................................................................................... 52.1 Standard messages............................................................................... 62.2 Implementation and evolution................................................................ 82.2.1 Drawbacks of the CAS system............................................................... 82.2.2 Common Channel Signalling (CCS)....................................................... 8

3 Common Channel Signalling System No. 7 ..................................... 103.1 Message Transfer Part (MTP).............................................................. 103.2 Telephone User Part (TUP) ................................................................. 113.3 Signalling Connection and Control Part (SCCP) .................................. 123.4 Summary ............................................................................................. 14

4 Additional SS7 protocols in GSM networks..................................... 154.1 Base Station Subsystem Application Part (BSSAP) ............................. 154.2 Mobile Application Part ........................................................................ 164.3 Transaction Capabilities Application Part (TCAP) ................................ 174.4 Summary ............................................................................................. 18

5 SS7 layers in GSM elements............................................................. 195.1 Protocol stack in the MSC.................................................................... 195.2 Protocol stack in the HLR, VLR, and EIR............................................. 205.3 Protocol stack in the BSC .................................................................... 20

6 Other signalling protocols in GSM ................................................... 21

7 Summary and Key Points.................................................................. 23

8 Review questions .............................................................................. 25


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1 Module objectives

After completing this module, the student will be able to:

Define the term signalling Describe the SS7 protocol stack and its functions

Identify the SS7 protocol stacks implemented in each GSM networkelement (BSC, MSC, and HLR)

without using any references.


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2 Introduction

Signalling in telecommunication networks has come a long way since the earlydays when a lady operator used to sit at the central exchange.

Telecommunication networks were relatively simple and the general procedureof setting up a call would go something like this:

You would pick up the handset of your telephone, electrical current would

flow to the exchange and a light would start blinking accompanied by a sound.

This would let the lady know that you are requiring service. She would plug inone connector to your terminal and the other to her headphone and inquire

about whom you wanted to talk to. After listening to your answer, she would try

to connect you to the person you wanted to talk with.

Then she would pull out the connector from your terminal and connect it toyour intended party. He would then hear his phone ringing. After he answers,

the lady will connect you to him. While you are talking, she will supervise thecall, and once the conversation is over (which will be indicated by another

light), she will pull out the plugs. That would be a typical scenario at atelephone exchange during the first half of this century.

%#!&?:^*(%/=

Figure 1. Signalling in the old days


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2.1 Standard messages

Soon after the invention of telephony, lady operators were replaced by

mechanical exchanges. And nowadays, so-called digital exchanges are in use.

The following simplified example demonstrates the basic steps of a call set-up:

1. Subscriber A wants to make a phone call. He lifts the receiver. An

electro-magnetical signal is sent to exchange 1, to which the wirelinephone is connected. The electro-magnetical signal indicates to exchange

1, that the subscriber requires a service.

2. The exchange generates a dial tone, with which it indicates its availabilityto subscriber A.

3. Subscriber A is dialling the telephone number of subscriber B. Thenumber is forwarded to exchange 1.

4. Exchange 1 is performing a number analysis. Based on the numberanalysis, the exchange can decide, how to serve subscriber A. If the

subscriber is requesting a service, which he is not allowed to use, theservice is not made available. For instance, if the subscriber has not paid

the telephone bill, the operator can restrict the offered services to

emergency services.Based on the number analysis, the exchange can decide, whether it can

serve the call locally, or whether the call has to be established via other

exchanges. This is the case in our example. Based on routing tables set bythe operator, exchange 1 is reserving transmission resources to exchange

2.

5. Then exchange 1 is transmitting a signalling message to exchange 2. The

message holds among other things the dial number and information aboutthe resources, which have be reserved on exchange 1s side.

6. Also exchange 2 is performing a number analysis. In our example, thecalled subscribers telephone is connected to exchange 2. Exchange 2seizes the trunk, and thus a bearer between exchange 1 and 2 is

established.

7. Exchange 2 is now transmitting a set-up message to telephone B.

8. Telephone B confirms the service request.

9. Then, it starts to ring.

10. It also sends an alert message to exchange B, which tells the exchange,

that the telephone is calling the called subscriber.

11. Exchange 2 is then informing exchange 1 about the successful link

establishment.

12. Exchange 2 is generating a ringing tone. Subscriber A hears the ringing

tone, which informs him, that a connection was established to subscriber

Bs telephone, and that telephone B is ringing.


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13. Subscriber B picks up his telephone. Telephone B sends a correspondingsignalling message to exchange 2

14. Exchange 2 terminates the tone generation. It then informs exchange 1

about the on-going call.

15. Now, a speech call can take place between telephone A and B.

subscriber A /telephone A

subscriber B /Telephone Bexchange 1 exchange 2

1. service request

2. dial tone

3. provide telephone number

4. number analysis &resource reservation

5. address information

6. number analysis &resource reservation

7. set-up message

8. set-up confirmation

9. ringing

10. alert message

12. ringing tone

11. address completemessage

13. connect message14. connect message

15. Conversation

Figure 2. Signalling operations

Telephones and switches are exchanging messages about the ongoing call. For

instance, between exchange 1 and 2, there exist transmission resources. Theyexchange messages to inform each other, which of the available transmission

resources to allocate to a specific call. They also have to inform each otherabout abnormal endings of calls. If not, one exchange would release the

transmission resources, while the other one is still blocking them. This alreadyindicates the importance to exchange messages between network elements of a

telecommunication network. These messages are called signalling and controlmessages.

A wide range of signalling systems exist. A signalling system represents a

specified set of rules on how network elements have to exchange signalling andcontrol information. Each signalling system must support messages for

Call set-up,

Call supervision,


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Call termination, and

Abnormal situation handling.

In Europe, the signalling system European Digital Subscriber Signalling SystemNo. 1 (E-DSS1) is often used between exchanges and telephones. Between

exchanges, the Common Channel Signalling System No. 7 (CCS#7, CS#7,SS#7, SS7)is nowadays the most common one. It was also adopted for GSM.

2.2 Implementation and evolution

As mentioned in the previous section, signalling in telecommunication systems

is basically a set of messages used for setting up, supervising and clearing thecall.

Many different factors have led to a variety of signalling systems being

developed in telecommunications networks.Different signalling standards were developed in different parts of the world.They were all doing the same task, but in a different way. This would obviously

mean that when a call originates in one network with one type of signallingimplementation and terminates in another network with another type of

signalling system, some compromise, or adaptation would have to be used. Due

to these kind of differences the then international governing body for

telecommunications, CCITT (now ITU), recommended the ChannelAssociated Signalling System (CAS) as the standard. In CAS, signallingmessages and user data/speech are transmitted on the same transmission

resource.

2.2.1 Drawbacks of the CAS system

As a signalling system for setting up calls CAS was a very good system thatperformed quite well. A large number of telephone exchanges in the world are

still using this system, but its implementation is such, that it is only suitable forcases where traffic is low. Another problem with CAS is that it is not possible

to send signalling messages in the absence of a call. This causes bottlenecks and

wastes bandwidth.

2.2.2 Common Channel Signalling (CCS)

The CCITT (now the ITU) came up with a new recommendation for asignalling system, which was the Common Channel Signalling System Number

7. One of the main advantages of this system was that signalling did not have togo along the same path as the speech. It is abbreviated CCS7, CCS#7, SS7 or

simply C7, but they all refer to the same system.


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SS7 was developed in the beginning of the 1980s and is a Common ChannelSignalling system (CCS) with a signalling path bandwidth of 64 Kbits/s. The

term Common Channel Signalling indicates, that signalling information anduser data are transmitted via separate resources. Signalling messages are

transmitted via transmission resources, which are used for several hundreds up

to thousand calls. This is possible for three reasons: SS7 is packet orientated,i.e. signalling messages are sent as packets similar to IP packets in the Internet.The signalling load is low in comparison to the user data traffic. And there is

mainly need for signalling resources during the call set-up and call terminationphase. To use common resources results in a fairly high resource efficiency of

the signalling resources.

End points of SS7 signalling messages are exchanges, such as the MSC, butalso register elements, such as the HLR. Within SS7, the originator and the

receiver of SS7 signalling messages are called Signalling Points (SP). It

depends on the operator, whether Service Points can directly exchange SS7messages. Often, the routing of SS7 messages is done via Signalling Transfer

Points (STP). A Signalling Transfer Point receives SS7 messages, analyses the

destination address of the SS7 message, then forwards the message, followingrules set by the operator in the routing tables of the Signalling Transfer Point.A simple example can be seen in the figure below: There are three exchanges.

Each exchange holds a SS7 Signalling Point. The blue lines represent thetransmission resources, while the green lines represent the independent packet

oriented SS7 network. Exchange 1 can directly sent SS7 messages to exchange2. If the signalling link between the two exchanges fails, then there is still anoption to route the messages via the Signalling Transfer Point. Exchange 1 has

no direct signalling link to the Signalling Point of exchange 3. In this case, thesignalling messages must be always routed via the Signalling Transfer Point.

exchange 3

SP

exchange1

SP

exchange 1

SP

STP

SP Signalling PointSTP Signalling Transfer Point

Independent,packetorientedsignallingnetwork

SS7

Figure 3. Signalling Points and Signalling Transfer Points

It is modular in design, although the modules are not as clearly defined, as is the

case with the OSI 7-layer model, which it pre-dates. Let us take a closer look atthis system in the following sections.


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3 Common Channel Signalling SystemNo. 7

Originally, the Common Channel Signalling System No. 7 (hereafter referred toas SS7) consisted of two parts. The first part was responsible for transferring themessage within a signalling network. The second part was the user of thesemessages.

As an analogy we can compare it to two managers with their own message

runners. One manager writes a message, puts it in the envelope and gives it tothe messenger. The messenger in turn looks at the address on the envelope, and

gives it to the messenger of the other manager. The messenger of the receivingmanager looks at the address and gives it to his manager, who will then read and

act as necessary.

Figure 4. Message bearers taking the message to their managers

The initial phase of SS7 consisted of two parts:

Message Transfer Part-MTP(responsible for transferring messages)

Telephone User Part -TUP (user of messages).

3.1 Message Transfer Part (MTP)

We have so far established that signalling is used for setting up calls, and that

there are standard sets of messages, which are sent back and forth to helpfacilitate this. The part responsible for taking these messages from one network

element to another network element is known as the Message Transfer Part


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(MTP). The entire SS7 is built on the foundation of this MTP, which consists ofthree sub layers as shown in the figure below.

Layer 1 Physical Connections

Message

Transfer

Part (MTP)

Signalling Message Handling

Layer 2 Data Link Control

Layer 3

Figure 5. Message Transfer Part layers

The lowest level, MTP layer 1 (physical connections), defines the physicaland electrical characteristics. MTP layer 2 (data link control) helps in error

free transmission of the signalling messages between adjacent elements. MTPlayer 3 (network layer) isresponsible for taking the message from any element

in a signalling network to any other element within the same network.

3.2 Telephone User Part (TUP)

The previous section explained the MTP. But who is the user who receives,

sends and acts on these messages? The answer is the Telephone User Part

(TUP).Those standard sets of messages that were mentioned previously are thestandard TUP messages that help to set up the call, to supervise and clear it.

For many the SS7 in the fixed telephone network consisted of only two parts,the MTP and the TUP. The CCITT (now the ITU) allowed for variations in

messages within one country alone. These variations were called theNational

User Part (NUP).


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Transport of signallingmessages within one network

Call Controlmessages

Layer 1

Layer 2

Layer 3

TUP

NUP

ISUP

MTP

MSC

SP

Layer 1

Layer 2

Layer 3

TUP

NUP

ISUP

MTP

PSTN exchange

SP

Physicalconnections Layer 1

Layer 2

Layer 1

Layer 2Data linkcontrol

Physicalconnections

Data linkcontrol

STP

Figure 6. Protocol stack of MTP and TUP/NUP/ISUP

With the introduction of the Integrated Services Digital Network (ISDN), which

has a broader capability than the PSTN, some extra sets of messages wererequired. These became known as theISDN User Part (ISUP). Whether it is

TUP, NUP or ISUP they are all doing the same job in helping to set up a call.

3.3 Signalling Connection and Control Part (SCCP)

The structure of SS7 with TUP/NUP/ISUP on top of the MTP was quite

satisfactory for speech call handling. However, with the passing of time and the

development of newer and more advanced technology, signalling requirementsalso started to become more stringent and demanding.

It was realised that the TUP/MTP combination alone was not sufficient when"virtual connections" became necessary. The MTP guarantees the transfer ofmessages from any "signalling point" in the signalling network to any other

"signalling point", safely and reliably. However, each message could reach the

destination signalling point by using different paths. This may cause situationswhere the order of messages that are received, are different from the original

sequence. When this order is important, there is need for establishing a "virtualconnection".


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Call Controlmessages

Layer 1

Layer 2

Layer 3

MTP

Exchange

SP

Layer 1

Layer 2

Layer 3

MTP

Exchange

SP

Physicalconnections Layer 1

Layer 2

Layer 1

Layer 2Data linkcontrol

Physicalconnections

Data linkcontrol

STP

SCCP SCCPSignalling Connection &

Control Part

e.g. ISUP e.g. ISUP

Offers following services to higher layers: non-call related signalling via networks of

different operators

connection-orientated network service

Figure 8. Location of the SCCP

As far as the fixed telephone network (the Public Switched Telephone Network,

PSTN) is concerned, this is all there is to SS7 and these protocol layers servetheir purpose very well. At the moment there is no other protocol in SS7 for

PSTN exchanges.

3.4 Summary

The MTP is the message transfer part. It is responsible for transferring

messages from one network element to another within the same network. Itconsists of three sublayers.

The TUPis the user part of the messages transferred by the MTP. These

messages deal with setting up, supervising and clearing the call connections. Ithas two variations: NUP and ISUP.

The SCCPis the signalling connection and control part. Its main function is toprovide virtual connections and connectionless signalling.


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4 Additional SS7 protocols in GSMnetworks

In GSM networks, signalling is not as simple as in the PSTN. There are extrasignalling requirements in GSM due to the different architecture of the networkthat requires a large amount of non-call-related signalling. In the first instancethe subscriber is mobile, unlike the PSTN telephone, which is always in one

place. Therefore, a continuous tracking of the mobile station is required, which

results in what is known as the location update procedure. This procedure is an

example of non-call-related signalling, where the mobile phone and the networkare communicating, but no call is taking place. This requires additional sets of

standard messages to fulfil the signalling requirements of GSM networks.

These additional protocol layers are:

Base Station Subsystem Application Part (BSSAP)

Mobile Application Part (MAP) Transaction Capabilities Application Part (TCAP).

4.1 Base Station Subsystem Application Part(BSSAP)

The first of these additional protocol layers is the Base Station SubsystemApplication Part (BSSAP). This layer is used when an MSC communicates with

the BSC and the mobile station. Since the mobile station and the MSC have to

communicate via the BSC, there must be a virtual connection; therefore theservice of the SCCP is also needed.

The authentication verification procedure and assigning a new TMSI all takeplace with the standard sets of messages of the BSSAP. Communication

between the MSC and the BSC also uses the BSSAP protocol layer. Therefore,the BSSAP serves two purposes:

MSC-BSC signalling

MSC-MS signalling.


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Controlmessages

MTP

BSC

MTP

MSC

Physicalconnections

Data linkcontrol

Signalling Connection &Control Part

Used to exchange requests and responses between

BSC and MSC

encapsulate messages to be exchangedtransparently between MS and MSC

Layer 1

Layer 2

Layer 3

SCCP

BSSAP

Layer 1

Layer 2

Layer 3

SCCP

BSSAP

BSSAPBase Station Subsystem Application part

Figure 9. Location of the BSSAP in SS7

4.2 Mobile Application Part

The example of a location update procedure mentioned previously is not

confined only to the MSC-BSC section, it spans multiple PLMNs. In case of a

first time location update by an international roaming subscriber (where he isnot in his home network), the VLR has to get the data from the subscribers

HLR via the gateway MSC of the subscribers home network.

While a mobile terminated call is being handled, the MSRN has to be requestedfrom the HLR without routing the call to the HLR. Therefore, for these cases

another protocol layer was added to the SS7 called the Mobile ApplicationPart (MAP). MAP is used for signalling communication between NSS

elements.

Note

The MSC-MSC communication using MAP is used only in case of non-call-

related signalling. For routing a call from one MSC to another MSC, TUP orISUP is still used.


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4.3 Transaction Capabilities Application Part (TCAP)

In MAP signalling, one MSC sends a message to an HLR, and that message

requests (or invokes) a certain result. The HLR sends the result back, which

may be the final result, or some other messages might also follow (or it mightnot be the last result). These invocations and results that are sent back and forth

between multiple elements using MAP need some sort of secretary to managethe transactions. This secretary is called the Transaction Capabilities

Application Part (TCAP). This completes the SS7 protocol stack in the GSMnetwork and its functions.

The SS7 picture is now complete.


Transaction CapabilitiesApplication Part

M

TP

GMSC

MTP

HLR

Physicalconnections

Data link

control

Signalling Connection &Control Part

Secretary function for higher layer, e.g.when the higher layer transactionconsists of a set of messages, whichhave to be exchanged between thepeer entities in an orderly way.

Layer 1

Layer 2

Layer 3

SCCP

TCAP

Layer 1

Layer 2

Layer 3

SCCP

TCAP

MAP MAPMobile

Application Part

Non-call- related requests and

responses, e.g. GMSC interrogationfor the MSRN.

Figure 10. MAP and TCAP


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4.4 Summary

Protocol Name Function

MTP Message Transfer Part Responsible for transferring an SS7 messagefrom one network element to another withinthe same signalling network.

TUP

NUP

ISUP

Telephone User Part

National User Part

ISDN User Part

User parts of MTP. They send, receive,analyse and act on the messages deliveredby the MTP. All of these are Call ControlMessages that help in setting up, supervisingand clearing a call.

SCCP Signalling Connection andControl Part

Protocol layer responsible for making virtualconnections and making connectionlesssignalling across multiple signalling networks.

BSSAP Base Station SubsystemApplication Part

Protocol layer responsible for communicatingGSM specific messages between the MSCand the BSC, and the MSC and the MS.

MAP Mobile Application Part A GSM specific protocol for non-call-relatedapplications between NSS elements.

TCAP Transaction CapabilitiesApplication Part

Protocol layer responsible for providingservice to MAP by handling the MAPtransaction messages between multipleelements.


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5.2 Protocol stack in the HLR, VLR, and EIR

The registers are not responsible for call control. Therefore, the TUP/ISUP is

not necessary. In addition, the registers do not communicate directly with the

BSC and so the BSSAP is not needed either, which leaves the MTP, SCCP,TCAP and MAP as the signalling protocols in the HLR, VLR, and EIR.

5.3 Protocol stack in the BSC

The BSC only needs the BSSAP, but since the BSSAP needs the services of the

SCCP, which in turn needs the MTP, the BSC contains the MTP, the SCCP and

the BSSAP.


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6 Other signalling protocols in GSM

As we have already seen, the GSM core network elements use SS7 (SignallingSystem No. 7) to pass signalling messages between them.

TDMA

FDMA

LAPDm

RR

MM

CM

TDMA

FDMA

LAPDm

RR

L1

LAPD

BTSM

LAPD

BTSM

RR

MTP

SCCP

BSSAPP

MM

CM

BSSAP

TCAP

SCCP

MTP

MS MSC

BSCBTS

CM Connection Management BTSM Base Transceiver Station Management

MM Mobility Management BSSAP BSS Application Part

RR Radio Resource Management SCCP Signalling Connection Control Part

LAPDm Link Protocol MTP Message Transfer Part

L1

Um Abis A

TUPNUPISUS

MAP

HLR

external

ex-

change

Figure 12. Signalling in GSM

Between the BSC and the BTS, a signalling protocol known as LAPD (Link

Access Protocol for the ISDN "D" channel) is used. This is the same protocolthat is used in ISDN networks between the customer and the network. This

protocol is also used the exchange requests and responses between the BSC andthe TRAU.

Between the mobile station and the BTS, the LAPD is used with smallmodifications to cope with the characteristics of the radio transmission medium.

This protocol is known as LAPDmwhere the "m" denotes modified.

The LAP-D message structure is similar to SS7, but it does not supportnetworking capabilities, therefore, it is used for point-to-point connections.

As can be seen with the figure above, a signalling protocol is required tonegotiate to radio resources to be used for dedicated signalling and user datatransport. The protocol is called Radio Resource (RR)management protocol,

and its messages are exchanged between the MS and the BSC via LAPDm andLAPD. RR in the above figure indicates, that some radio resource management


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tasks can be directly performed between the MS and the BTS.The BTS determines, which radio interface resources are allocated to the MS

for dedicated traffic. But also the BTS must be informed about it. This is onecause for the existence of the BTSM (BTS Management) protocol.

What about mobility management? Mobility Management (MM)comprisestasks such as location update and authentication. These are messages that areexchanged directly between the MS and the MSC/VLR. As can be seen above,

there are other network elements in the transmission path of the mobility

management messages, such as the BTS and the BSC. But these networkelements transparently transmit the higher layer mobility management

messages. Similar to the STP in SS7, they take the signalling message, and

forward it to the next entity. The MS and the MSC/VLR are the so-called peerentities of mobility management messages.

The same is true for Connection Management (CM)messages. Connection

management includes call set-up messages, alerting message, etc., i.e. messagesnecessary for call control. Also SMS and supplementary services are managed

with the help of the connection management.


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7 Summary and Key Points

The following table highlights the function of the SS7 protocol in every GSMnetwork element capable of processing SS7.

MSC BSC HLR

MTP Transfer of SS7 messagesbetween different networkelements.

Transfer of SS7messages betweendifferent networkelements.

Transfer of SS7messages betweendifferent networkelements.

TUP/ISUP Setting up, supervising, andclearing call connections.

Unavailable. Unavailable.

SCCP Connectionless signallingand virtual connections.

Virtual connectionbetween MSC and MS.

Connectionlesssignalling.

BSSAP GSM signalling with BSCand MS.

GSM signalling withMSC.

Unavailable.

MAP GSM specific signalling withHLR and other MSC.

Unavailable. GSM specific signallingwith MSCs and otherHLRs.

TCAP Service provider to MAP. Unavailable. Service provider toMAP.

A virtual connection uses packet type switching principles and the connection

only exists when packets or messages are being transferred. In the simplest formof packet switching each packet is regarded as a complete transaction in itself.

This is known as the connectionless mode as there is no sense of a connectionbeing set up before communication begins, and the network treats each packetindependently. Some applications, however, involve the transfer of a sequence

of packets, for which the connection-oriented approach is more appropriate.In this case, a virtual connection is established by an initial exchange of "set-up"

packets between the communicating terminals. During the data transfer, eachpacket associated with a connection is passed over the same route through the

network.

Key points

Signalling is the transfer of information between subscriber interface

points and the network, and between different network elements to help

establish a call.

Signalling information is interchanged as standard sets of messages that

was developed and standardised into the present SS7 system.


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GSM networks need non-call related signalling, which is possible withSS7.

The SS7 used in PSTN networks is not sufficient to fulfil the signalling

requirements of GSM networks, thus new protocols specific to GSM

were developed. The MTP is the basis of SS7, and it is responsible for transferring

signalling messages from one element to another within the samesignalling network.

The TUP/ISUP are the user parts of the MTP that handle call control.

The SCCP is needed for virtual connections and connectionless

signalling.

The BSSAP is used for signalling between MSC-BSC and MSC-MS.

The MAP is needed for signalling between MSC-HLR, MSC-VLR, HLR-

VLR (and MSC-MSC in the case of non-call related signalling).

The Link Access Protocol in D channel (LAP-D) provides a point-to-point signalling capability. It is used between the BTS and BSC, and in amodified version between the MS and the BTS (LAP-Dm).


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8 Review questions

In the following questions, please select one alternative that you think is thebest answer for the particular question.

1. Which of the following is nota signalling function?

a. To analyse the dialled digits.

b. To digitise the user's speech before transmission.

c. To make speech path connections.

d. To inform the user of the progress of the call.

2. Which of the following was a drawback of CAS signalling?

a. It supported only call related signalling.b. It required one signalling channel for every PCM line.

c. It was not possible to have many different signalling messages.

d. All of the above.

3. Which of the following are advantages of SS7 over CAS signalling?

a. It can send call set-up messages.

b. One signalling channel can support 10 000 traffic channels.

c. It can support non-call-related signalling.

d. All of the above.

4. Which of the following is not a need for SS7 signalling?

a. The need to supervise a call.

b. The need to make circuit reservations.

c. The need to clear connections when the call is over.

d. The need to transfer charging information.


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5. Which of the following signalling requirements is specific to GSMnetworks only?

a. The ability to reserve circuits in the outgoing direction.

b. The ability of one signalling channel to handle calls in other

physically different cables.

c. The ability to transport service dependent messages acrossswitching exchanges.

d. The ability to perform non-call-related signalling procedures.

6. Which of the following combinations of SS7 protocols is not present in

PSTN exchanges?

a. MTP, SCCP.

b. MTP, ISUP.

c. MTP, TUP.d. MTP, SCCP, TCAP, MAP.

7. Which pair of network elements in the GSM network does not both have

SS7?

a. MSC, HLR.

b. BSC, HLR.

c. MSC, BSC.

d. BTS, HLR.


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Review questions

8. Which of the following pictures is correct?

a.

MTP

SCCP

ISUP

MAP

TCAP BSSAP

b.

MTP

TCAP

BSSAP

MAP

SCCP TUPNUP

ISUP

c.

MTP

SCCP

BSSAP

MAP

TCAP TUPNUP

ISUP

d.

MTP

SCCP

MAPBSSAP

TCAPTUPNUP

ISUP

04-2gsystra_introduction to ss7 signalling_6-66869

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