TM5108-02A4EN01GLA01 Introduction to GSM Signaling

8/11/2019 TM5108-02A4EN01GLA01 Introduction to GSM Signaling

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Introduction to GSM Signaling

TM5108-02AEN01GLA01 © 2010 Nokia Siemens Networks

1

Contents

1 Module Objectives 3

2 Introduction 5

3 Common Channel Signaling System No.7 17

4

Additional SS#7 Protocols in GSM Networks 29

5 SS#7 Layers in GSM Elements 35

6 Other Signaling Protocols in GSM 45

7 Summary and Key Points 51





TM5108-02AEN01GLA01

© 2010 Nokia Siemens Networks2





3

1 Module Objectives





5

2 Introduction




TM5108-02AEN01GLA01


Signaling in telecommunication networks has come a long way since the early dayswhen a lady operator used to sit at the central exchange and switch the callsmanually on subscriber’s verbal requests. Telecommunication networks were

relatively simple and the general procedure of setting up a call would go somethinglike this:

You would pick up the “handset” of your telephone, electrical current would flow tothe exchange and a light would start blinking accompanied by a sound. This would letthe lady know that you are requiring service. She would plug in one connector to yourterminal and the other to her “headphone” and inquire about whom you wanted to talkto. After listening to your answer, she would try to connect you to the person youwanted to talk with.

Then she would pull out the connector from your terminal and connect it to yourintended 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 the call, and once theconversation is over (which will be indicated by another light), she will “pull out theplugs.” That would be a typical scenario at a telephone exchange during the first halfof this century.





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© Nokia Siemens Net works

Signalling in the Old Days

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http://www.iamprovincetown.com

Fig. 1




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2.1 Standard messages

Soon after the invention of telephony, lady operators were replaced by mechanicalexchanges. And nowadays, so-called digital exchanges are in use. The followingsimplified 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-magnetic signal is sent to exchange 1, to which the wireline phone is connected.The electro-magnetic signal indicates to exchange 1, that the subscriber requiresa service.

2. The exchange generates a dial tone, with which it indicates its availability tosubscriber A.

3. Subscriber A is dialing the telephone number of subscriber B. The number is

forwarded to exchange 1.

4. Exchange 1 is performing a number analysis. Based on the number analysis, theexchange can decide, how to serve subscriber A. If the subscriber is requesting aservice, which he is not allowed to use, the service is not made available. Forinstance, if the subscriber has not paid the telephone bill, the operator can restrictthe offered services to emergency services. Based on the number analysis, theexchange can decide whether it can serve the call locally, or whether the call hasto be established via other exchanges. Assuming that the call has to beestablished via other exchanges, based on routing tables set by the operator,exchange 1 is reserving transmission resources to exchange 2.

5. Then exchange 1 transmits a signaling message to exchange 2. The messageholds among other things the dialed number and information about theresources, which have been reserved on exchange 1’s side.

6. Also exchange 2 is performing a number analysis. In our example, the calledsubscriber’s telephone is connected to exchange 2. Exchange 2 seizes 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 2, which tells the exchange, that thetelephone is ringing.

11. Exchange 2 then informs exchange 1 about the successful link establishmentwith telephone B.

12. Exchange 2 generates a ringing tone which is sent back over the establishedbearer to Subscriber A. Subscriber A hears the ringing tone, which informs himthat a connection was established to subscriber B’s telephone, and thattelephone B is ringing.





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Signalling Operationssubscriber A /telephone A

subscriber B /Telephone B

exchange 1 exchange 2

1. service request

2. dial tone

3. provide telephonenumber

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

4. number analysis &resource reservation

Fig. 2




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13. Subscriber B picks up his telephone. Telephone B sends a correspondingsignaling 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.

Telephones and switches are exchanging messages about the ongoing call. Forinstance, between exchange 1 and 2, there exist transmission resources. Theyexchange messages to inform each other, which of the available transmissionresources to allocate to a specific call. They also have to inform each other aboutabnormal endings of calls. If not, one exchange would release the transmissionresources, while the other one is still blocking them. This already indicates theimportance to exchange messages between network elements of atelecommunication network. These messages are called signaling and controlmessages.

A wide range of signaling systems exist in present day telecommunication networks.A signaling system represents a specified set of rules on how network elements haveto exchange signaling and control information. Each signaling system must supportmessages for:

• Call set-up,

• Call supervision,

• Call termination, and

• Abnormal situation handling.

In Europe, the signaling system European Digital Subscriber Signaling System No. 1(E-DSS1) is often used between exchanges and telephones. Between exchanges,the Common Channel Signaling System No. 7 (CCS#7, CS#7, SS#7, SS#7) is themost common protocol. It has also been adopted for GSM as it supports a widerange of mobile related procedures.





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2.2 Implementation and Evolution

As mentioned in the previous section, signaling in telecommunication systems isbasically a set of messages used for setting up, supervising and clearing the call.

Many different factors have led to a variety of signaling systems being developed intelecommunications networks.

Different signaling standards were developed in different parts of the world. Theywere all doing the same task, but in different ways. This would obviously mean thatwhen a call originates in one network with one type of signaling implementation andterminates in another network with another type of signaling system, somecompromise, or adaptation would have to be used. Due to these kinds of differencesthe then international governing body for telecommunications, CCITT (now ITU),

recommended the Channel Associated Signaling System (CAS) as the standard. InCAS, signaling messages and user data/speech are transmitted on the sametransmission resource.

2.2.1 Drawbacks of the CAS system

As a signaling system for setting up calls CAS was a good system that performedquite well. A large number of telephone exchanges in the world are still using thissystem, but its implementation is such that it is only suitable for cases where traffic islow. Another problem with CAS is that it is not possible to send signaling messagesin the absence of a call. This causes bottlenecks and wastes bandwidth.

2.2.2 Common Channel Signaling (CCS)

The CCITT (now the ITU) came up with a new recommendation for a signalingsystem, which was the Common Channel Signaling System Number 7. One of themain advantages of this system was that signaling did not have to go along the samepath as the speech. It is abbreviated CCS7, CCS#7, SS#7 or simply C7, but they allrefer to the same system.

SS#7 was developed in the beginning of the 1980s and is a Common ChannelSignaling system (CCS) with a minimum signaling path bandwidth of 64 Kbits/s. The

term “Common Channel Signaling” indicates, that signaling information and user dataare transmitted via separate resources. Signaling messages are transmitted viatransmission resources, which are used for several hundreds up to thousand calls.This is possible for three reasons: SS#7 is packet oriented, i.e. signaling messagesare sent as packets similar to IP packets in the Internet. The signaling load is low incomparison to the user data traffic. And there is mainly need for signaling resourcesduring the call set-up and call termination phase only.

End points of SS#7 signaling messages are exchanges, such as the MSC, but alsoregister elements, such as the HLR. Within SS#7, the originator and the receiver ofSS#7 signaling messages are called Signaling Points (SP). It depends on the

operator, whether Service Points can directly exchange SS#7 messages. Often, therouting of SS#7 messages is done via Signaling Transfer Points (STP). A SignalingTransfer Point receives SS#7 messages, analyses the destination address of the




TM5108-02AEN01GLA01


SS#7 message, then forwards the message, following rules set by the operator in therouting tables of the Signaling Transfer Point.

A simple example can be seen. There are three exchanges. Each exchange holds a

SS#7 Signaling Point. The blue lines represent the transmission resources, while thegreen lines represent the independent packet oriented SS#7 network. Exchange 1can directly sent SS#7 messages to exchange 2. If the signaling link between the twoexchanges fails, then there is still an option to route the messages via the SignalingTransfer Point. Exchange 1 has no direct signaling link to the Signaling Point ofexchange 3. In this case, the signaling messages must be always routed via theSignaling Transfer Point.





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© Nokia Siemens Networks

Signalling Points and Signalling Transfer Points

exchange 3

SP

exchange 1

SP

exchange 2

SP

STP

SP Signalling PointSTP Signalling Transfer Point

Independent,packet-oriented

signallingnetwork

SS7

Fig. 3




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The Signaling System #7 (SS#7) is used in the GSM PLMN for NSS signaling,furthermore for signaling between the NSS and the BSS, OMS (optional) andexternal ISDN.

For studying SS#7, we can compare it with the layers of OSI model. OSI Layers 1,2and 3 correspond to SS#7 Level 1, Level 2 and Level 3 respectively. For OSI Layer 4to 7, in ss#7 we define Level 4.

SS#7 originally was developed for call related applications. For this purpose the 4level model of SS#7 was created:

Level 1 (Physical & electrical attributes)

• defines physical and electrical attributes of the transmission

Level 2 (Signaling Data Link)

• guarantees the error free transmission on the SS#7 transmission line (errordetection, error correction, error rate monitoring)

Level 3 (Signaling Network functions)

• routing of messages

• network management (e.g. congestion handling)

These 3 levels form the Message Transfer Part MTP. The MTP is the base of

SS#7. The MTP transports the data generated by upper layers.

Level 4 (User Parts)

• responsible for call related messages (e.g. ISDN User Part ISUP, Telephone UserPart TUP, Data User Part DUP)

In contrast to the 4 level model for non-call related signaling the 7 layer model isused. Here the tasks of the level 4 are further divided into Layer 4 to Layer 7.

In GSM the Layer 4 to 7 are usually implemented by the Signaling ConnectionControl Part SCCP, Transaction Capability Application Part TCAP, Mobile ApplicationPart MAP and BSS Application Part (BSSAP).





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Physical & electrical attributes

Signaling Data Link

Signaling Network functions

MTPMessage

TransferPart

ISUPISDN

User Part

Layer

TUPTelephoneUser Part

DUPData

User Part

MAPMobile

ApplicationPart

TCAP

SCCP

TCAP: Transaction Capability Application PartSCCP: Signaling Connection Control Part

7

4 - 6

3

2

1

3

2

1

Level

4

Signaling System #7

Fig. 4




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17

3 Common Channel Signaling System No.7




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Originally, the Common Channel Signaling System No. 7 (hereafter referred to asSS#7) consisted of two parts. The first part was responsible for transferring themessage within a signaling network. The second part was the user of these

messages.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 to themessenger. The messenger in turn looks at the address on the envelope, and gives itto the messenger of the other manager. The messenger of the receiving managerlooks at the address and gives it to his manager, who will then read and act asnecessary.

The initial phase of SS#7 consisted of two parts:

• Message Transfer Part -MTP (responsible for transferring messages)

• Telephone User Part -TUP (user of messages).-





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Message Bearers and Managers

Fig. 5




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3.1 Message Transfer Part (MTP)

We have so far established that signaling is used for setting up calls, and that thereare standard sets of messages, which are sent back and forth to help facilitate this.The part responsible for taking these messages from one network element to anothernetwork element is known as the Message Transfer Part (MTP). The entire SS#7 isbuilt on the foundation of this MTP, which consists of three sub layers.

The lowest level, MTP layer 1 (physical connections), defines the physical andelectrical characteristics. MTP layer 2 (data link control) helps in error freetransmission of the signaling messages between adjacent elements. MTP layer 3(network layer) is responsible for taking the message from any element in a signalingnetwork to any other element within the same network.

For many, the SS#7 in the fixed telephone network consisted of only two parts, theMTP and the TUP. The CCITT (now the ITU) allowed for variations in messageswithin one country alone. These variations were called the National User Part (NUP).





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SS7 - MTP

Layer 1 PhysicalConnections

Message

TransferPart (MTP)

Signalling MessageHandling

Layer 2 Data Link Control

Layer 3

Fig. 6




TM5108-02AEN01GLA01


With the introduction of the Integrated Services Digital Network (ISDN), which has abroader capability than the PSTN, some extra sets of messages were required.These became known as the ISDN User Part (ISUP). Whether it is TUP, NUP or

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





23

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SS7 - MTP / TUP / ISUP

Transport of signallingmessages within one network

Call Control messages

TUP

NUP

ISUP

SP

Physicalconnections

Data linkcontrol

STP

L1 L1’

L2 L2’

Layer 1

Layer 2

Layer 3

TUP/ISUP

TUP

NUP

ISUP

SP

Layer 1

Layer 2

Layer 3

TUP/ISUP

Physicalconnections

Data linkcontrol

Fig. 7




TM5108-02AEN01GLA01


3.2 Signaling Connection and Control Part (SCCP)

The structure of SS#7 with TUP/NUP/ISUP on top of the MTP was quite satisfactoryfor speech call handling. However, with the passing of time and development ofnewer and more advanced technology, signaling requirements also started tobecome more stringent and demanding.

It was realized that the TUP/MTP combination alone was not sufficient when "virtualconnections" became necessary. The MTP guarantees transfer of messages fromany "signaling point" in the signaling network to any other "signaling point", safely andreliably. However, each message could reach the destination signaling point by usingdifferent paths. This may cause situations where the order of messages that arereceived, are different from the original sequence. When this order is important, thereis need for establishing a "virtual connection".

Virtual connections use a "connection oriented" protocol that provides sequencenumbers to enable the messages to be placed in the correct order at the distant end.

Another instance of when the TUP/MTP structure is inefficient is when a signalingmessage has to be sent across multiple networks in the absence of a call. The MTPis capable of routing a message within one network only. The case of setting up a callacross multiple networks is not the same as signaling across the same network. Thesignaling goes leg by leg according to the call. But in the absence of a call, the MTPcannot route a signaling message across multiple networks.

The solution to these two problems was the creation of another protocol layer on top

of the MTP, which was called the Signaling Connection and Control Part (SCCP).The SCCP takes care of virtual connections and connectionless signaling. Note thatthe tasks of TUP and SCCP are different, and thus they are parallel to each other,but both use the services of MTP.

As far as the fixed telephone network (the Public Switched Telephone Network,PSTN) is concerned, this is all there is to SS#7 and these protocol layers serve theirpurpose very well. At the moment there is no other protocol in SS#7 for PSTNexchanges.





25

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Virtual Connections

B

A

VirtualConnection using

“ConnectionOriented”

SCCP

OriginatingSignallingPoint

SignallingTransfer Point

Signalling

TransferPoint

DestinationSignallingPoint

MTP

MTP

MTP

SignallingTransferPoint

Fig. 8




TM5108-02AEN01GLA01


3.3 Summary

The MTP is the message transfer part. It is responsible for transferring messagesfrom one network element to another within the same network. It consists of threesublayers.

The TUP is the user part of the messages transferred by the MTP. These messagesdeal with setting up, supervising and clearing the call connections. It has twovariations: NUP and ISUP.

The SCCP is the signaling connection and control part. Its main function is to providevirtual connections and connectionless signaling.





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SS7 - SCCP



TUP

NUP

ISUP

SP

Physicalconnections

Data linkcontrol

STP

L1 L1’

L2 L2’

Layer 1

Layer 2Layer 3

TUP

NUP

ISUP

SP

Layer 1

Layer 2

Layer 3

Physicalconnections

Data linkcontrol

SCCP

e.g. ISUP

SCCP

e.g. ISUP

Signalling Connection &Control Part

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

of different operators• connection-orientated network service

Fig. 9




TM5108-02AEN01GLA01






29

4 Additional SS#7 Protocols in GSM

Networks




TM5108-02AEN01GLA01


In GSM networks, signaling is not as simple as in the PSTN. There are extrasignaling requirements in GSM due to different architecture of the network thatrequires a large amount of non-call-related signaling. In the first instance the

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 inwhat is known as the location update procedure. This procedure is an example ofnon-call-related signaling, where the mobile phone and the network arecommunicating, but no call is taking place. This requires additional sets of standardmessages to fulfill the signaling 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 Subsystem ApplicationPart (BSSAP). This layer is used when an MSC communicates with the BSC and themobile station. Since the mobile station and the MSC have to communicate via theBSC, there must be a virtual connection; therefore the service of the SCCP is also

needed.The authentication verification procedure and assigning a new TMSI all take placewith the standard sets of messages of the BSSAP. Communication between the MSCand the BSC also uses the BSSAP protocol layer. Therefore, the BSSAP serves twopurposes:

• MSC-BSC signalling – Known as BSSMAP

• MSC-MS signalling – Known as DTAP

BSS Management Application Part (BSSMAP) is used for transmission of RadioResource RR related information. This includes transactions like Paging, Assignment

/ Blocking of traffic channels, Handover Resource Request / Indication, Cipher Start,Queuing etc.

Direct Transfer Application Part (DTAP) handles messages to be exchangedbetween the MSC and the MS. These Mobility Management MM and Call Control CCmessages are transparent for the BSS, such as Location Update, Call Setup etc.





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SS7 - BSSAP



TUP

NUP

ISUP

SP

Physical connections

Data link control

Layer 1

Layer 2

Layer 3

TUP

NUP

ISUP

SP

Layer 1

Layer 2

Layer 3

SCCP

BSSAP

SCCP

BSSAP

Signalling Connection &Control Part

Used for

• exchange requests and responsesbetween BSC and MSC• encapsulate messages to be exchanged

transparently between MS and MSC

Fig. 10




TM5108-02AEN01GLA01


4.2 Mobile Application Part

The example of a location update procedure mentioned previously is not confinedonly to the MSC-BSC section, it spans multiple PLMNs. In case of a first time locationupdate by an international roaming subscriber (where he is not in his home network),the VLR has to get the data from the subscriber’s HLR via the gateway MSC of thesubscriber’s home network.

While a mobile terminated call is being handled, the MSRN has to be requested fromthe HLR without routing the call to the HLR. Therefore, for these cases anotherprotocol layer was added to the SS#7 called the Mobile Application Part (MAP). MAPis used for signaling communication between NSS elements.

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

related signaling. For routing a call from one MSC to another MSC, TUP or ISUP isstill used.

4.3 Transaction Capabilities Application Part (TCAP)

In MAP signaling, 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 finalresult or some other messages, might also follow (or it might not be the last result).These invocations and results that are sent back and forth between multiple elementsusing MAP need some sort of secretary to manage the transactions. This secretary iscalled the Transaction Capabilities Application Part (TCAP). This completes the SS#7protocol stack in the GSM network and its functions.

The SS#7 picture is now complete.





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SS7 – MAP and TCAP


Transaction Capabilities Application PartTUP

NUP

ISUP

Physical connections

Data link control

MTP

TUP

NUP

ISUPSCCP

TCAPSignalling Connection &

Control Part

MAPMobile Application Part

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

MTP

SCCP

TCAP

MAP

Non-call-related requests andresponses, e.g. GMSCinterrogation for the MSRN.

Fig. 11




TM5108-02AEN01GLA01


4.4 Summary

Protocol Name FunctionMTP Message Transfer Part Responsible for transferring an

SS#7 message from one networkelement to another within the samesignaling network.

TUP NUP ISUP Telephone User Part

National User Part

ISDN User Part

User parts of MTP. They send,receive, analyze and act on themessages delivered by the MTP.All of these are Call ControlMessages that help in setting up,

supervising and clearing a call.

SCCP Signaling Connection andControl Part

Protocol layer responsible formaking virtual connections andmaking connectionless signalingacross multiple signaling networks.

BSSAP Base Station SubsystemApplication Part

Protocol layer responsible forcommunicating GSM specificmessages between the MSC andthe BSC, and the MSC and theMS.

MAP Mobile Application Part A GSM specific protocol for non-call-related applications betweenNSS elements.

TCAP Transaction CapabilitiesApplication Part

Protocol layer responsible forproviding service to MAP byhandling the MAP transactionmessages between multipleelements.





35

5 SS#7 Layers in GSM Elements




TM5108-02AEN01GLA01


In this section, the SS#7 requirements for individual GSM elements will be shown.The previous sections explained why SS#7 was needed in GSM and what protocollayers are used. It is useful to note that not all GSM elements have all the protocols in

the SS#7 stack. For example, a BSC would never need TUP because call control isnot the task of the BSC.

5.1 Protocol Stack in the MSC

Since the MTP is the foundation on which SS#7 is built, this will be required in everyelement that is capable of processing SS#7. The MSC is the element in GSMnetworks that is responsible for call control; therefore, TUP/ISUP sits on top of theMTP. The MSC/VLR is also responsible for location updates and communication withthe BSC and the HLR. For this reason it also needs to have BSSAP and MAP that siton top of the SCCP. The MSC also has the TCAP to provide services for the MAP. Itcan thus be seen that the MSC/VLR has all the SS#7 protocol stacks implemented init.





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Protocol Stack in the MSC

MTP

TUPNUPISUP

SCCP

BSSAP

MAP

TCAP

Fig. 12




TM5108-02AEN01GLA01


5.2 Protocol Stack in the HLR, VLR, and EIR

The registers are not responsible for call control. Therefore, the TUP/ISUP is notnecessary. In addition, the registers do not communicate directly with the BSC andso the BSSAP is not needed either, which leaves the MTP, SCCP, TCAP and MAPas the signaling 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 theSCCP, which in turn needs the MTP, the BSC contains the MTP, the SCCP and theBSSAP.





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SS7 Protocols in Different Network Elements

MAP

TCAP

SCCP

MTP

BSSAPMAP

TCAP

SCCP

MTP

TUPNUPISUP

SCCP

MTP

BSSAP

SCCP

MTP

MSC

HLR

PSTNexchange

BSC

TUPNUPISUP

Fig. 13




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5.4 Protocol Stack in Release 4 Network Elements

The Release 4 architecture adds a few new signaling protocols to the already existingoptions. Some of the new protocols are:

• SIGTRAN

• BICC CS-2

• H.248/MEGACO/MGCP

• SIP (if IP based transport is used in the core network)

SIGTRAN is the definition of a suite of protocols to carry SS#7 and ISDN messagesover IP. This protocol suite is made up of a new transport layer–the Stream Control

Transmission Protocol (SCTP)–and a set of User Adaptation (UA) layers which mimicthe services of the lower layers of SS#7 and ISDN.

Stream Control Transmission Protocol (SCTP) is a new transport protocol, designedwith the transport of time-sensitive signaling data in mind. It is a Unicast protocolused as transport layer instead of the TCP protocol. SCTP has many features similarto TCP but is enhanced for signaling transport. Data exchange is between two knownendpoints and reliable transport of user data is provided. The major differencetowards TCP is that SCTP is message-oriented, defining structured frames of data.TCP, conversely, imposes no structure on the transmitted stream of bytes.Furthermore, the SCTP has multi-streaming capability. Data is split into multiple

streams, each with independent sequenced delivery. TCP has no such feature.The transmission of bounded, structured frames is useful since the transport protocoldoes all the work to split the stream of data into message segments, reducing theuser’s responsibility for interpreting a continuous stream of bytes. Multi-streamingisdesigned to allow users to partition a single IP connection between two endpointsinto separate logical streams of data and assign each stream to a particularapplication or resource. The purpose is that errors or delays on one stream will notinterfere with normal delivery on another.





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MSS Protocol Stack

MAPMAP CAPCAP INAPINAP

TCAPTCAP BSSAPBSSAP RANAPRANAP

SCCPSCCP BICCBICC ISUPISUP

M3UAM3UA H.248H.248 SIPSIP

SCTPSCTP TCPTCP UDPUDP

IPv4, IPv6IPv4, IPv6

EthernetEthernet

Fig. 14




TM5108-02AEN01GLA01


The SIGTRAN adaptation layers (UA) serve a number of common purposes. The UAlayers are named according to the service they replace, rather than the user of thatservice. For example, M3UA adapts SCTP to provide the services of MTP3. SCTP is

used to carry upper layer signaling protocols over a reliable IP-based transport. TheUA:s provide the same class of service offered at the interface of the PSTNequivalent. As an example, M3UA must provide the same look and feel to its usersas MTP3 in terms of services. The UA:s are also transparent. The user of the serviceshould be unaware that the adaptation layer has replaced the original protocol. Tosum up, the UA:s remove as much of the need for lower layers of the SS#7 stack aspossible.

Bearer Independent Call Control (BICC) protocol is a call control protocol usedbetween serving nodes. This protocol is based on the ISUP protocol, and wasadapted to support the ISDN services independent of the bearer technology andsignaling message transport technology used. The messages are of call set-up type.The BICC can be transported in several ways as shown in the MMS protocolarchitecture. Both traditional SS/ and SIGTRAN stacks are allowed for.

The Megaco protocol is used between multimedia gateways and their controllers.This protocol creates a general framework suitable for gateways, multipoint controlunits and interactive voice response units (IVRs). Packet network interfaces mayinclude IP, ATM or possibly others. The interfaces support a variety of SCN signalingsystems, including tone signaling, ISDN, ISUP, QSIG and GSM. National variants ofthese signaling systems are supported where applicable.

Megaco/H.248 addresses the relationship between the Media Gateway (MGW),

which converts circuit-switched voice to packet-based traffic, and the MSS whichcontrols the service logic of the user plane traffic. Megaco/H.248 instructs an MGW toconnect streams coming from outside a packet or cell data network onto a packet orcell stream such as the ATM AAL2 stream. Megaco/H.248 is essentially similar toMGCP from an architectural standpoint and the controller-to-gateway relationship,but Megaco/H.248supports a broader range of networks, for example ATM. Thereare two basic components in Megaco/H.248: terminations and contexts. Terminationsrepresent streams entering or leaving the MGW (for example telephone lines).Terminations have properties, such as the maximum size of a jitter buffer, which canbe inspected and modified by the MSS thus guaranteeing QoS.

SIP (Session Initiation Protocol) is an application-layer control protocol that can

establish, modify, and terminate multimedia sessions such as Internet telephony calls(VOIP). SIP can also invite participants to already existing sessions, such asmulticast conferences. Media can be added to (and removed from) an existingsession. SIP transparently supports name mapping and redirection services, whichsupports personal mobility -users can maintain a single externally visible identifierregardless of their network location. Sip is however optional in the interfaces MSS-MSS but it may make sense to implement it since the future IMS will use SIP.





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SIGTRAN Protocols

TCAP

SCCP

MTP 3

SCTP

IP

M3UA M2PA SUA

ISDN

IUAM2UA

M2UA provides services of MTP2 in aclient-server situation, such as SG toMGC. Its user would be MTP3.

M2PA provides the services of MTP2 ina peer-to-peer situation, such as SG-to-SG connections. Its user would beMTP3.

M3UA provides the services of MTP3 inboth a client-server (SG to MGC) andpeer-to-peer architecture. Its userswould be SCCP and/or ISUP.

SUA provides services of SCCP in apeer-to-peer architecture, such as SGto IP SCP. Its user would be TCAP, oranother transaction-based applicationpart.

IUA provides services of the ISDNData Link layer (LAPD). Its user wouldbe an ISDN layer 3 (Q.931) entity.

Fig. 15




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45

6 Other Signaling Protocols in GSM




TM5108-02AEN01GLA01


As we have already seen, the GSM core network elements use SS#7 (SignalingSystem No. 7) to pass signaling messages between them.

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

Protocol for the ISDN "D" channel) is used. This is the same protocol that is used inISDN networks between the customer and the network. This protocol is also used inthe exchange requests and responses between the BSC and the TRAU.

Between the mobile station and the BTS, the LAPD is used with small modificationsto cope with the characteristics of the radio transmission medium. This protocol isknown as LAPDm where the "m" denotes modified.

The LAP-D message structure is similar to SS#7, but it does not support networkingcapabilities, therefore, it is used for point to point connections.

A signaling protocol is required to negotiate to radio resources to be used for

dedicated signaling and user data transport. The protocol is called Radio Resource(RR) management protocol, and its messages are exchanged between the MS andthe BSC via LAPDm and LAPD. RR’ indicates that some radio resource managementtasks can be directly performed between the MS and the BTS.

The BTS determines which radio interface resources are allocated to the MS fordedicated traffic. But the BTS must also be informed about it. This information is sentto BTS from the BSC on BTSM (BTS Management) protocol.

Radio Interface Signaling

Signaling on the radio interface is performed using a 3-Layer protocol. It consists ofLayer 1 (Physical Layer), Layer 2 (LAPDm) and Layer 3 (RR, MM & CM).

L1: (Physical Layer)

The radio transmission (L1 physics & logic) is described in detail in the next chapter.

L2: Link Access Protocol on the D-Channel m (modified) –LAPDm

The link layer on Um is handled in Rec. 04.06. It is responsible for reliabletransmission of L3 units. LAPDm has been derived from fixed network LAPD protocoland optimized for radio interface transmission. Different to MTP L2 / the LAPDprotocol, no Frame Check Sequences (FCS) are necessary. This function and someothers are taken over by Um L1 functions (Channel Coding / Decoding).





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Signalling in GSM

ExternalExternalExternalExternal

ExchangeExchangeExchangeExchangeTDMA

FDMA

LAPDm

RR

MM

CM

TDMA

FDMA

LAPDm

RR‘

L1

LAPD

BTSM

LAPD

BTSM

RR

MTP

SCCP

BSSAPBSSAPBSSAPBSSAPPPPP

MM

CM

BSSAPBSSAPBSSAPBSSAP

TCAPTCAPTCAPTCAP

SCCP

MTP‘

MS MSCBSCBTS

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

TUPTUPTUPTUPNUPNUPNUPNUPISUSISUSISUSISUS

MAPMAPMAPMAP

HLR

Fig. 16




TM5108-02AEN01GLA01


L3: CM / MM / RR

Radio Resource management RR

RR (Rec. 04.08) is responsible for management of frequency resources. It handlesthe paging for MTCs, request for and assignment of dedicated channels, Intra-&Inter-cell handover, transmission of measurement reports, cipher mode setting,channel mode modification (e.g. from speech to data) and channel release.

Mobility Management MM

MM (Rec. 04.08) supports all mobility aspects. Different MM procedures allow tokeep track of the MS, e.g. IMSI Attach / Detach, (periodic) Location Update,Authentication, TMSI-Reallocation, Identification and CM Service Request.

Mobility Management are exchanged directly between the MS and the MSC/VLR. Asseen above, there are other network elements in the transmission path of the mobilitymanagement messages, such as the BTS and the BSC. But these network elementstransparently transmit the higher layer mobility management messages. Similar to theSTP in SS#7, they take the signaling message, and forward it to the next entity. TheMS and the MSC/VLR are the so-called peer entities of mobility managementmessages.

Connection Management CM

CM (Rec. 04.08) contains 3 sublayers: Call Control CC, SMS and SupplementaryServices SS support. CC handles procedures to setup, maintain and release calls

(e.g. "Setup", "Alert", "Connect", "Disconnect"). SMS support allows transmission ofSMS via signaling channels (SDCCH / SACCH; explained in the next chapter). SSallows to support Supplementary Services (e.g. Call Forwarding, Call back whenbusy, Advice of charge) related with or without calls. CM messages are alsotransparent for BTS and BSC.





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Layer 1Logical

Physical

Layer 2: LAPDm

Radio Resource managementRR

Mobility Management MM

Connection Management CM

CallControl CC

SupplementaryServices SS

Short MessageServices SMS

L3

Support of “connection types”:Call Control CC support, e.g.• “Setup”, “Alert”, “Connect”, “Disconnect”

Supplementary Services” support, e.g.• Call Forwarding, Call back when busy, Advice Of Charge,..SMS support

Support of mobility(Keep track of MS), e.g.:

• “IMSI attach / detach”• “Location Update request / accept”• “Authentication request /response”• “TMSI reallocation command / complete”

• “CM Service Request / accept” (activates: SS, MOC, SMS, Emergency Call)

Secure transfer ofsignaling data, e.g.:• Broadcast, Paging, Access Grant, dedicated signaling

Setup / Maintenance / Releaseof Dedicated Channels, e.g.:• “paging”

• “channel request”• “immediate assignment”• “cipher mode command / complete”• “channel release”

Radio Interface Signaling

Fig. 17




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51

7 Summary and Key Points




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The following table highlights the function of SS#7 protocol in every GSM networkelement capable of processing SS#7.

A virtual connection uses packet type switching principles and the connection only

exists when packets or messages are being transferred. In the simplest form ofpacket switching each packet is regarded as a complete transaction in itself. This isknown as the “connectionless” mode as there is no sense of a connection being setup before communication begins, and the network treats each packet independently.Some applications, however, involve the transfer of a sequence of packets, for whichthe “connection-oriented” approach is more appropriate. In this case, a virtualconnection is established by an initial exchange of "set-up" packets between thecommunicating terminals. During the data transfer, each packet associated with aconnection is passed over the same route through the network.

Key points

• Signaling is the transfer of information between subscriber interface points and thenetwork, and between different network elements to help establish a call.

• Signaling information is interchanged as standard sets of messages that wasdeveloped and standardized into the present SS#7 system.

• GSM networks need non-call related signaling, which is possible with SS#7.

• The SS#7 used in PSTN networks is not sufficient to fulfill the signalingrequirements of GSM networks, thus new protocols specific to GSM were

developed.

• The MTP is the basis of SS#7, and it is responsible for transferring signalingmessages from one element to another within the same signaling network.

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

• The SCCP is needed for virtual connections and connectionless signaling.

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

• The MAP is needed for signaling between MSC-HLR, MSC-VLR, HLR-VLR (and

MSC-MSC in the case of non-call related signaling).• The Link Access Protocol in D channel (LAP-D) provides a point-to-point signaling

capability. It is used between the BTS and BSC, and in a modified versionbetween the MS and the BTS (LAP-Dm).

• Release 4 introduces several new protocols for signaling.





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MSC BSC HLR

MTP Transfer of SS#7messages betweendifferent networkelements.

Transfer of SS#7messages betweendifferent networkelements.

Transfer of SS#7messages betweendifferent networkelements.

TUP/ISUP Setting up,supervising, andclearing callconnections.

Unavailable. Unavailable.

SCCP Connectionlesssignalling andvirtual connections.

Virtual connectionbetween MSC andMS.

Connectionlesssignalling.

BSSAP GSM signalling withBSC and MS.

GSM signalling withMSC.

Unavailable.

MAP GSM specificsignalling with HLRand other MSC.

Unavailable. GSM specificsignalling withMSCs and otherHLRs.

TCAP Service provider toMAP.

Unavailable. Service provider toMAP.

TM5108-02A4EN01GLA01 Introduction to GSM Signaling

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Transcript of TM5108-02A4EN01GLA01 Introduction to GSM Signaling