4 - Introduction to SS7 Signalling

2.5G SYSTRA

Introduction to SS7 Signalling Training Document

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© Nokia Networks Oy

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Introduction to SS7 Signalling

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia's customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia welcomes customer comments as part of the process of continuous development and improvement of the documentation.

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Contents

Contents

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

2 Introduction ............................................................................................5 2.1 Standard messages .................................................................................6 2.2 Implementation and evolution ..................................................................8 2.2.1 Drawbacks of the CAS system.................................................................8 2.2.2 Common Channel Signalling (CCS).........................................................8

3 Common Channel Signalling System No. 7.......................................10 3.1 Message Transfer Part (MTP)................................................................10 3.2 Telephone User Part (TUP)....................................................................11 3.3 Signalling Connection and Control Part (SCCP) ....................................12 3.4 Summary................................................................................................14

4 Additional SS7 protocols in GSM networks ......................................15 4.1 Base Station Subsystem Application Part (BSSAP)...............................15 4.2 Mobile Application Part...........................................................................16 4.3 Transaction Capabilities Application Part (TCAP)..................................17 4.4 Summary................................................................................................18

5 SS7 layers in GSM elements ...............................................................19 5.1 Protocol stack in the MSC......................................................................19 5.2 Protocol stack in the HLR, VLR, and EIR...............................................19 5.3 Protocol stack in the BSC.......................................................................20 5.4 Protocol stack in Release 4 network elements.......................................20

6 Other signalling protocols in GSM .....................................................23

7 Summary and Key Points ....................................................................25

8 Review questions.................................................................................27

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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 network element (BSC, MSC, and HLR)

• identify the protocols introduced in Release 4 core networks

without using any references.


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Introduction

2 Introduction Signalling in telecommunication networks has come a long way since the early days when a lady operator used to sit at the central exchange. Telecommunication networks were relatively simple and the general procedure of 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 in one 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 to your 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 the call, 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 a telephone exchange during the first half of this century.

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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 wireline phone 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 availability to subscriber A.

3. Subscriber A is dialling 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, the exchange can decide, how to serve subscriber A. If the subscriber is requesting a service, which he is not allowed to use, the service 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 by the 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 about the resources, which have be reserved on exchange 1’s side.

6. Also exchange 2 is performing a number analysis. In our example, the called subscriber’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 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 B’s telephone, and that telephone B is ringing.

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


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Introduction

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 request2. 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. ringing10. 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. They exchange messages to inform each other, which of the available transmission resources to allocate to a specific call. They also have to inform each other about abnormal endings of calls. If not, one exchange would release the transmission resources, while the other one is still blocking them. This already indicates the importance to exchange messages between network elements of a telecommunication network. These messages are called signalling and control messages. A wide range of signalling systems exist. A signalling system represents a specified set of rules on how network elements have to exchange signalling and control information. Each signalling system must support messages for

• call set-up,

• call supervision,

• call termination, and

• abnormal situation handling.

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In Europe, the signalling system European Digital Subscriber Signalling System No. 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 the most common protocol. It has also been 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 the call.

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 signalling implementation 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 Channel Associated Signalling System (CAS) as the standard. In CAS, signalling messages 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 good system that performed 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 for cases 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 a signalling system which was the Common Channel Signalling System Number 7. One of the main advantages of this system was that signalling did not have to go 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.

SS7 was developed in the beginning of the 1980s and is a Common Channel Signalling system (CCS) with a signalling path bandwidth of 64 Kbits/s. The term “Common Channel Signalling” indicates, that signalling information and user data are transmitted via separate resources. Signalling messages are transmitted via transmission resources, which are used for several hundreds up to thousand calls. This


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Introduction

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 termination phase. 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, but also 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 SS7 messages. 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, following rules 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 the transmission resources, while the green lines represent the independent packet oriented SS7 network. Exchange 1 can directly sent SS7 messages to exchange 2. If the signalling link between the two exchanges fails, then there is still an option 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, the signalling messages must be always routed via the Signalling Transfer Point.

exchange 3

SP

exchange 1

SP

exchange 1

SP

STP

SP Signalling PointSTP Signalling Transfer Point

Independent,packet

orientedsignallingnetwork

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 at this system in the following sections.

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3 Common Channel Signalling System No. 7 Originally, the Common Channel Signalling System No. 7 (hereafter referred to as SS7) consisted of two parts. The first part was responsible for transferring the message within a signalling 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 the 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 receiving manager 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 help facilitate this. The part responsible for taking these messages from one network element to another network element is known as the Message Transfer Part (MTP). The entire SS7 is built on the foundation of this MTP, which consists of three sub layers as shown in the figure below.


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Common Channel Signalling System No. 7

Layer 1 Physical Connections

MessageTransfer

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 physical and electrical characteristics. MTP layer 2 (data link control) helps in error free transmission of the signalling messages between adjacent elements. MTP layer 3 (network layer) is responsible 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 the standard 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 the National User Part (NUP).

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Transport of signallingmessages within one network

Call Control messages

Layer 1

Layer 2

Layer 3

TUP

NUP

ISUP

MTP

MSCSP

Layer 1‘

Layer 2‘

Layer 3

TUP

NUP

ISUP

MTP

PSTN exchangeSP

Physicalconnections Layer 1

Layer 2

Layer 1‘

Layer 2‘Data 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 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.

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 requirements also 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 of messages 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 situations where the order of messages that are received, are different from the original sequence. When this order is important, there is need for establishing a "virtual connection".

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


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Common Channel Signalling System No. 7

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

B

A

VirtualConnection using

“Connection Oriented”SCCP

SignallingPoint

SignallingPoint

SignallingPoint Destination

SignallingPoint

MTP

MTP

MTP

Figure 7. Virtual connections

The solution to these two problems was the creation of another protocol layer on top of the MTP that was called the Signalling Connection and Control Part (SCCP). The SCCP takes care of virtual connections and connectionless signalling. Note that the tasks of TUP and SCCP are different, and thus they are parallel to each other, but both use the services of the MTP.

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Call Control messages

Layer 1

Layer 2

Layer 3

MTP

ExchangeSP

Layer 1‘

Layer 2‘

Layer 3

MTP

ExchangeSP

Physicalconnections Layer 1

Layer 2

Layer 1‘

Layer 2‘Data 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 serve their 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. It consists of three sublayers.

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

The SCCP is the signalling connection and control part. Its main function is to provide virtual connections and connectionless signalling.


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Additional SS7 protocols in GSM networks

4 Additional SS7 protocols in GSM networks In GSM networks, signalling is not as simple as in the PSTN. There are extra signalling requirements in GSM due to the different architecture of the network that requires a large amount of non-call-related signalling. 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 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 network are 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 Subsystem Application 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 the service of the SCCP is also needed.

The authentication verification procedure and assigning a new TMSI all take place 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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Control messages

MTP

BSCM

TPMSC

Physicalconnections

Data linkcontrol

Signalling Connection &Control Part

Used to• exchange requests and responses between

BSC and MSC• encapsulate messages to be exchanged

transparently between MS and MSC

Layer 1

Layer 2

Layer 3

SCCP

BSSAP

Layer 1

Layer 2

Layer 3

SCCP

BSSAP

BSSAP Base 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 is not in his home network), the VLR has to get the data from the subscriber’s HLR via the gateway MSC of the subscriber’s home network.

While a mobile terminated call is being handled, the MSRN has to be requested from the HLR without routing the call to the HLR. Therefore, for these cases another protocol layer was added to the SS7 called the Mobile Application Part (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 or ISUP is still used.


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Additional SS7 protocols in GSM networks

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 might not 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 manage the transactions. This secretary is called the Transaction Capabilities Application Part (TCAP). This completes the SS7 protocol stack in the GSM network and its functions.

The SS7 picture is now complete.


Transaction CapabilitiesApplication Part

MTP

GMSC

MTP

HLR

Physicalconnections

Data linkcontrol

Signalling Connection &Control Part

Secretary function for higher layer, e.g. when the higher layer transaction consists of a set of messages, which have to be exchanged between the peer entities in an orderly way.

Layer 1

Layer 2

Layer 3

SCCP

TCAP

Layer 1

Layer 2

Layer 3

SCCP

TCAP

MAP MAPMobileApplication Part

Non-call-related requests and responses, e.g. GMSC interrogation for 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 message from one network element to another within the 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 delivered by the MTP. All of these are Call Control Messages that help in setting up, supervising and clearing a call.

SCCP Signalling Connection and Control Part

Protocol layer responsible for making virtual connections and making connectionless signalling across multiple signalling networks.

BSSAP Base Station Subsystem Application Part

Protocol layer responsible for communicating GSM specific messages between the MSC and the BSC, and the MSC and the MS.

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

TCAP Transaction Capabilities Application Part

Protocol layer responsible for providing service to MAP by handling the MAP transaction messages between multiple elements.


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SS7 layers in GSM elements

5 SS7 layers in GSM elements In this section, the SS7 requirements for individual GSM elements will be shown. The previous sections explained why SS7 was needed in GSM and what protocol layers are used. It is useful to note that not all the GSM elements have all the protocols in the SS7 stack. For example, a BSC would never need TUP because call control is not the task of the BSC.

5.1 Protocol stack in the MSC

Since the MTP is the foundation on which SS7 is built, this will be required in every element that is capable of processing SS7. The MSC is the element in GSM networks that is responsible for call control; therefore, TUP/ISUP sits on top of the MTP. The MSC/VLR is also responsible for location updates and communication with the BSC and the HLR. For this reason it also needs to have BSSAP and MAP that sit on top of the SCCP. The MSC also has the TCAP to provide services for the MAP. It can thus be seen that the MSC/VLR has all the SS7 protocol stacks implemented in it.

MTP

TUP NUPISUP

SCCP

BSSAPMAP

TCAP

Figure 11. Protocol stack in the MSC

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

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

Error! Objects cannot be created from editing field codes.

Figure 12 SS7 in network elements

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.

5.4 Protocol stack in Release 4 network elements

The Release 4 architecture adds a few new signalling protocols to the already existing options. 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)


Figure 13 MSS protocol stack example (Nokia MSS)

SIGTRAN is the definition of a suite of protocols to carry SS7 and ISDN messages over 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 mimic the services of the lower layers of SS7 and ISDN.


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SS7 layers in GSM elements


Figure 14 SIGTRAN protocol stack

Stream Control Transmission Protocol (SCTP) is a new transport protocol, designed with the transport of time-sensitive signaling data in mind. It is a Unicast protocol that is used as transport layer instead of the TCP protocol. SCTP hs many features similar to TCP but is enhanced for signaling transport Data exchange is between two known endpoints and reliable transport of user data is provided. The major difference towards TCP is that SCTP is message-oriented, defining structured frames of data. TCP, conversely, imposes no structure on the transmitted stream of bytes. Furhtermore 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 protocol does all the work to split the stream of data into message segments, reducing the user’s responsibility for interpreting a continuous stream of bytes. Multi-streaming is designed to allow users to partition a single IP connection between two endpoints into separate logical streams of data and assign each stream to a particular application or resource. The purpose is that errors or delays on one stream will not interfere with normal delivery on another.

The Sigtran adaptation layers(UA) serve a number of common purposes. The UA layers are named according to the service they replace, rather than the user of that service. For example, M3UA adapts SCTP to provide the services of MTP3 UA:s carry upper layer signaling protocols over a reliable IP-based transport. The UA:sprovide the same class of service offered at the interface of the PSTN equivalent. As an example, M3UA must provide the same look and feel to its users as MTP3 in terms of services. The UA:s are also transparent. The user of the service should be unaware that the adaptation layer has replaced the original protocol. To sum up the UA:s remove as much of the need for lower layers of the SS7 stack as possible.

Bearer Independent Call Control (BICC) protocol is a call control protocol used between serving nodes. This protocol is based on the ISUP protocol, and was adapted to support the ISDN services independent of the bearer technology and signalling message transport technology used. The messages are of call set-up type. The BICC can be transported in several ways as shown on the MMS protocol architecture. 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 control units and interactive voice response units (IVRs). Packet network interfaces may include IP, ATM or possibly others. The interfaces support a variety of SCN signalling systems, including tone signalling, ISDN, ISUP, QSIG and GSM. National variants of these signalling systems are supported where applicable.

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Megaco/H.248 addresses the relationship between the Media Gateway (MGW), which converts circuit-switched voice to packet-based traffic, and the MSS which controls the service logic the user plane traffic. Megaco/H.248 instructs an MGW to connect streams coming from outside a packet or cell data network onto a packet or cell stream such as the ATM AAL2 stream. Megaco/H.248 is essentially similar to MGCP from an architectural standpoint and the controller-to-gateway relationship, but Megaco/H.248 supports a broader range of networks, for example ATM. There are two basic components in Megaco/H.248: terminations and contexts. Terminations represent streams entering or leaving the MGW (for example telephone lines. Terminations have properties, such as the maximum size of a jitter buffer, which can be 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 as multicast conferences. Media can be added to (and removed from) an existing session. SIP transparently supports name mapping and redirection services, which supports personal mobility - users can maintain a single externally visible identifier regardless 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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Other signalling protocols in GSM

6 Other signalling protocols in GSM As we have already seen, the GSM core network elements use SS7 (Signalling System No. 7) to pass signalling messages between them.

TDMAFDMA

LAPDm

RR

MM

CM

TDMAFDMA

LAPDm

RR‘

L1

LAPD

BTSM

LAPD

BTSM

RR

MTP‘

SCCP

BSSAPP

MM

CM

BSSAP

TCAP

SCCP

MTP‘

MS MSCBSCBTS

CM Connection Management BTSM Base Transceiver Station ManagementMM Mobility Management BSSAP BSS Application PartRR Radio Resource Management SCCP Signalling Connection Control PartLAPDm Link Protocol MTP Message Transfer Part

L1Um Abis A

TUPNUPISUS

MAP

HLR

externalex-

change

Figure 15. 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 protocol that is used in ISDN networks between the customer and the network. This protocol is also used the exchange requests and responses between the BSC and the TRAU. Between the mobile station and the BTS, the LAPD is used with small modifications to cope with the characteristics of the radio transmission medium. This protocol is known as LAPDm where the "m" denotes modified. The LAP-D message structure is similar to SS7, but it does not support networking capabilities, therefore, it is used for point to point connections.

As can be seen with the figure above, a signalling protocol is required to negotiate to radio resources to be used for dedicated signalling and user data transport. The protocol is called Radio Resource (RR) management protocol, and its messages are exchanged between the MS and the BSC via LAPDm and LAPD. RR’ in the above figure indicates that some radio resource management tasks can be directly performed between the MS and the BTS.

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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 one cause for the existence of the BTSM (BTS Management) protocol.

What about mobility management? Mobility Management (MM) comprises tasks such as location update and authentication. These are messages which are exchanged 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 network elements 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 peer entities 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. messages necessary for call control. Also SMS and supplementary services are managed with the help of the connection management.


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Summary and Key Points

7 Summary and Key Points The following table highlights the function of the SS7 protocol in every GSM network element capable of processing SS7.

MSC BSC HLR

MTP Transfer of SS7 messages between different network elements.

Transfer of SS7 messages between different network elements.

Transfer of SS7 messages between different network elements.

TUP/ISUP Setting up, supervising, and clearing call connections.

Unavailable. Unavailable.

SCCP Connectionless signalling and virtual connections.

Virtual connection between MSC and MS.

Connectionless signalling.

BSSAP GSM signalling with BSC and MS.

GSM signalling with MSC.

Unavailable.

MAP GSM specific signalling with HLR and other MSC.

Unavailable. GSM specific signalling with MSCs and other HLRs.

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

A virtual connection uses packet type switching principles and the connection only exists when packets or messages are being transferred. In the simplest form of 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 connection being set up before communication begins, and the network treats each packet independently. 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, each packet 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.

• GSM networks need non-call related signalling, which is possible with SS7.

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• 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 same signalling 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 a modified version between the MS and the BTS (LAP-Dm).

• Release 4 introduces several new protocols for signaling


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Review questions

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

1. Which of the following is not a 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 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.

3. Which of the following signalling requirements is specific to GSM networks 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 across switching exchanges.

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

4. 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.

5. Which pair of network elements in the GSM network does not both have SS7?

a. MSC, HLR.

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b. BSC, HLR.

c. MSC, BSC.

d. BTS, HLR.

6. Which of the following pictures is correct?

a.

MTP

SCCP

ISUPMAP

TCAP BSSAP

b.

MTP

TCAP

BSSAPMAP

SCCP TUPNUPISUP

c.

MTP

SCCP

BSSAPMAP

TCAP TUPNUPISUP

d.


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Review questions

MTP

SCCP

MAPBSSAPTCAP

TUPNUPISUP

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4 - Introduction to SS7 Signalling

Documents

Transcript of 4 - Introduction to SS7 Signalling