5 NSS Interfaces

5/22/2018 5 NSS Interfaces

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Version 2.0 T.O.P. BusinessInteractive GmbH Page 1 of 23

5 NSS Interfaces


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5 NSS Interfaces......................................................................15.1 SS7 Signalling and NSS Interfaces..................................35.2 The OSI Reference Model (1/4).........................................45.3 The SS7 System ................................................................8

5.3.1 Message Transfer Part MTP .......................................................... 95.3.2 Telephone User Part TUP ............................................................105.3.3 Signalling Connection and Control Part SCCP...........................11

5.4 SS7 Applications in GSM Networks...............................125.4.1 Base Station Subsystem Application Part (BSSAP)...................135.4.2 Transaction Capabilities Application Part (TCAP).......................145.4.3 Mobile Application Part (MAP) .....................................................15

5.5 Protocols of the GSM Elements .....................................165.5.1 Protocol Structure in the MSC......................................................175.5.2 Protocol Structure in the HLR and the BSC................................185.5.3 Communication between Different Network Elements................195.5.4 Other GSM Signalling Protocols...................................................20

5.6 Overview of the NSS-Interfaces (1/3).............................21


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5.1 SS7 Signalling and NSS Interfaces

To enable GSM customers to use speech-, fax- and data services wherever they are withinthe network, information and data must be exchanged within as well as between differentnetworks. This process is called signalling. To set up a call, signalling takes place betweensubscriber interface points, that is between the user and the network, as well as betweendifferent network elements. To make sure that the individual elements through which theinformation travels can understand each other, they must, as it were, agree on a commonofficial language. This language is specified by protocols. The protocol used in the NetworkSubsystem NSS is called Signalling System No 7 or SS7. SS7 is based on the OpenSystem Interconnection model, also called the OSI reference model.


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5.2 The OSI Reference Model (1/4)

We will now illustrate in general terms the OSI reference model with an example frombusiness life. A car manufacturer B orders 1000 tyres from supplier A. This deal is concludedand signed by two managers at the highest level. For the two managers, only the outcome ofthis business deal is important. The process that takes place in the lower hierarchy to get thetyres from the supplier to the car manufacturer does not interest them. The managers rely ontheir purchasing- and sales departments, which will deal with practical details. The carmanufacturers purchasing department, however, only communicates with the supplierssales department. As soon as the financial transactions are concluded, the goods can bedelivered from A to B.


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The purchasing and sales departments are not interested in the practical details of delivery.At the supplier, the warehouse workers must pack the tyres and load them on trucks, to getthem ready for transport. As soon as the tyres arrive at the manufacturer, the warehouseworkers will unpack the tyres and store them.

In summary, we can say: Its always several levels of a company that collaborate in abusiness transaction. The higher levels give the lower levels instructions, without payingattention to the details of the processes. Communication between the two companies takesplace only between peer levels. With the OSI model, its similar.


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OSI is a reference model consisting of 7 layers that are based on each other. Each layer hasits own tasks. The lower layer always provides support functions for the layer above. For alayer, the data transported in the layers underneath is irrelevant. Communication only takesplace between the elements of the same layer. This type of communication betweenelements belonging to the same layer in different systems is known as peer-to-peercommunication.


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The layers take over the following tasks:

The lowest layer, layer No 1, is the Physical layer. It is responsible for transmission,encoding, and modulation.

Layer 2 is the Data Link layer. It is responsible for the signalling link management and datasecurity.

Layer 3 is the Network layer. It contains the information needed for switching and routing,

and handles call set-up, -supervision and -clear down.

Layer 4 is the Transport layer. Here, the peer-to-peer connections dataflow is controlled.Layer 5 is the Session layer. It handles the connections for application processes as well ascharging.

Layer 6 is the Presentation layer. It takes over the transfer of application-oriented formats, aswell as encryption and translation.

At the top resides layer No 7, the Application layer. It is responsible for the applicationprotocols and the user interfaces.


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5.3 The SS7 System

The basic SS7 version consists of two parts:

The Message Transfer Part (MTP), which is responsible for message transfer

and

The Telephone User Part (TUP) on the users side, which receives, sends andacts on these messages.

Lets turn our attention to MTP first.


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5.3.1 Message Transfer Part (MTP)

The Message Transfer Part (MTP), represents the basis for the entire SS7 system. Ittransmits messages between network elements. MTP is composed of three layers. MTPlayer 1 defines the physical and electrical characteristics of the connection. MTP layer 2supports the error free transmission of signalling messages between neighboring networkelements. MTP layer 3 is responsible for taking the message from any element in a signallingnetwork to any other element within the same network.


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5.3.2 Telephone User Part (TUP)

While MTP is responsible for message transfer, the Telephone User Part (TUP) representsthe protocol used for sending, receiving, and acting on these messages from the users pointof view. TUP handles call set-up, call supervision and clear down, and exists for normalpublic fixed networks, which are also known as Public Switched Telephone Networks, orPSTN. With the introduction of the more capable ISDN network, some extra sets ofmessages became necessary. These features are contained in the ISUP which replaces theTUP.


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5.3.3 Signalling Connection and Control Part (SCCP)

To guarantee virtual connections and connectionless signalling, that is signalling which is notbound to a call, another protocol layer on top of MTP is required, parallel to TUP.This is theSignalling Connection and Control Part, SCCP. TUP and SCCP take over different tasks, butboth make use of the services provided by MTP.

In contrast to MTP, SCCP uses sequence numbers to make sure that messages arrive at thereceiver in a determined order, so a virtual connection can be guaranteed. SCCP alsoenables the routing of signalling messages across multiple networks in the absence of a call.

This layer structure, consisting of MTP and TUP/ISUP, as well as SSCP, represents the SS7protocol, which is the protocol used for signalling within Public Switched Telephone Networks

and ISDN networks.


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5.4 SS7 Applications in GSM Networks

In GSM networks, signalling is not as easy as in a fixed network. This is because, due to thenetwork architecture, a digital mobile radio network makes much higher demands onsignalling. GSM requires a considerably higher amount of non-call-related signallinginformation. After all, it must be considered that the GSM customer is mobile, in contrast tothe user of a fixed network, who telephones from a fixed device. Therefore, the mobilestation must continuously be provided with localization signals,to enable the LocationUpdate. The Location Update is an example of a non-call-related communication betweenthe phone and the network.To guarantee that the signalling demands in GSM networks aremet, additional standard sets of messages are required. The following protocol layers are

necessary:

The Base Station Subsystem Application Part (BSSAP) The Transaction Capabilities Application Part (TCAP)

and

The Mobile Application Part (MAP)


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5.4.1 Base Station Subsystem Application Part (BSSAP)

The Base Station Subsystem Application Part (BSSAP) is a protocol layer responsible forcommunication between the MSC and the BSC in GSM. BSSAP is responsible for the entiremanagement and control of the radio resources in the BSS. It resides on top of the SignallingConnection and Control Part, SCCP.


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5.4.2 Transaction Capabilities Application Part (TCAP)

The Transaction Capabilities Application Part (TCAP) is a protocol layer which residesdirectly on top of SCCP.

TCAP is able, for example, to organize a complex dialogue between an MSC and an HLR,including a sequence of successive requests and replies. TCAP functions like a secretarysoffice, where many different requests are brought into the correct sequence and distributed.TCAP handles the access to data bases like the HLR or the VLR. It must exist so that ahigher protocol the Mobile Application Part (MAP) can be used.


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5.4.3 Mobile Application Part (MAP)

The Mobile Application Part (MAP) is a GSM specific protocol for non-call-relatedapplications between elements in the NSS. MAP resides directly on top of TCAP, which canbe used as a secretarys office by the MAP, and which coordinates and guarantees asmooth MAP communication.

A MAP-based communication takes place when data is exchanged between NSS elementsin the absence of a call.

This is the case for example with normal call set-up. To put a call through to the subscriber,the Gateway MSC must request necessary routing data from the HLR. Thus, there is no dataexchange between the GMSC and the HLR, without the actual call being routed to the HLR.

In such cases, the network relies on MAP, which is used for signalling communicationbetween NSS elements. Please note: in the MSC-MSC communication, MAP is only used fornon-call-related signalling. To forward a call from an MSC to another MSC, TUP or ISUP isused.


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5.5 Protocols of the GSM Elements

Not every GSM element must be able to understand every language. Consequently, onlythose protocol layers which the network element actually requires for carrying out its taskneed to be implemented. A BSC, for example, will never need the TUP protocol, because callsupervision which this layer supports is not its task. In the following lessons, the SS7requirements of the individual GSM elements will be introduced.


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5.5.1 Protocol Structure in the MSC

As in all the other elements, MTP is the basis protocol in the MSC/VLR. Without it, therewould be no SS7-based signalling. Furthermore, the MSC needs TUP/ISUP for callsupervision. Since the MSC communicates with the BSC and the HLR, it also requiresBSSAP and MAP, which are both based on SCCP. The use of MAP requires the presence ofTCAP. Thus the MSC, as the key element of the Network Subsystem NSS, must include thewhole range of SS7 protocols.


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5.5.2 Protocol Structure in the HLR and the BSC

The HLR is not responsible for call control, and therefore does not need TUP/ISUP.Furthermore, since it does not have to communicate directly with the Base Station Controller,BSSAP is not implemented in the HLR. Thus, only the four protocols MTP, SCCP, TCAP andMAP must be present in the HLR.

Normally, the BSC would manage on BSSAP. But since, in modern networks, BSSAP isbased on the SCCP protocol, whose functionality in turn requires the presence of MTP, theBSC contains MTP, SCCP and BSSAP.


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5.5.3 Communication between Different Network Elements

Communication between network elements always takes place using the relevantprotocols,each protocol relying on the protocols of the layers below. For example, the signalling fromthe MSC to a PSTN for call set-up is carried out by TUP, which is based on MTP in bothelements. Or, if an MSC wants to know the current location of a subscriber, it communicateswith the responsible HLR using MAP. In this communication, TCAP, SCCP and MTP arerequired. The BSSAP protocol, on the other hand, is only needed by the MSC when it wantsto communicate with the BSC.


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5.5.4 Other GSM Signalling Protocols

Whereas the elements in the Network Subsystem use SS7, further protocol types areneeded in the Base Station Subsystem.

The BSC and BTS communicate using the Link Access Protocol for the ISDN D channel, orLAP-D. This protocol is also used between the end user and the net in ISDN networks.

A slightly modified version of the same protocol also exists between the BTS and the mobilestation. Due to the modifications imposed by the characteristics of the air interface, theprotocol is called LAP-Dm. The message structure within LAP-D resembles that of SS7, butits limited to the support of point-to-point connections.


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5.6 Overview of the NSS Interfaces (1/3)

Between the NSS elements, data is either exchanged over copper cable or optical fiber, orvia microwave.

All NSS interfaces offer data rates of at least 64 kbps. 2 Mbit/s connections are the rule. Theprotocols are based on the SS7 standard.

Two kinds of information are transferred over the different interfaces.Signalling information such as addressing and mobility data, and user data, that meansspeech, fax- and data messages.


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Between the NSS elements we find the following interfaces. Note that the Mobile ServicesCenter (MSC) and the Visitor Location Register (VLR) form aspatial unit.

Between the MSC and the VLR we find the B-interface. This interface is used to transmitsignalling data.

The C-interface is located between the MSC and the Home Location Register (HLR). It isalso used exclusively for signalling data.

The D interface provides the connection between the VLR and the HLR. Like the interfaces B

and C, it transmits signalling data.


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The E interface is located between two MSCs. Apart from signalling data, user data andspeech can be transmitted as well.

The F interface is located between the MSC and the Equipment Identity Register (EIR).If an EIR exists, the interface is used exclusively for signalling data concerning the IMEIcontrol.

The MSCs which provide connections to another mobile radio network, that is a Public LandMobile Network (PLMN), or a Public Switched Telephone Network (PSTN), and which areconnected to the HLR, are also known as Gateway-MSCs, or GMSCs. The interfacebetween the visited network and a GMSC transmits user- and signalling data.

As a rule, every MSC can function as a GMSC nowadays. From the network operators pointof view, this is cost-efficient, because the more MSCs can function as Gateways to othernetworks, the longer a call can be routed within the own network before it is handed over to adifferent network.

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5 NSS Interfaces