NSS Interfaces

8/7/2019 NSS Interfaces

1/26

NSS Interfaces


2/26

Outlines

5.1SS7 Signalling and NSS Interfaces5.2The OSI Reference Model

5.3The SS7 System5.3.1Message Transfer Part (MTP)5.3.2Telephone User Part (TUP)5.3.3Signalling Connection and Control Part (SCCP)

5.4 SS7 Applications in GSM Networks

5.4.1Base Station Subsystem Application Part (BSSAP)5.4.2Transaction Capabilities Application Part (TCAP)5.4.3Mobile Application Part (MAP)

5.5Protocols of the GSM Elements

5.5.1Protocol Structure in the MSC5.5.2Protocol Structure in the HLR and the BSC5.5.3Communication between Network Elements5.5.4Other GSM Signalling Protocols

5.6Overview of the NSS Interfaces


3/26

To enable GSM customers to use speech-, fax- and data services whereverthey are within the network, information and data must be exchanged within aswell as between different networks

This process is called signalling. To set up a call, signalling takes placebetween subscriber interface points, that is between the user and the network,

as well as between different network elements To make sure that the individual elements through which the information travelscan understand each other, they must, as it were, agree on a common officiallanguage

This language is specified by protocols. The protocol used in the Network

Subsystem NSS is called Signalling System No 7 - or SS7. SS7 is based on theOpen System Interconnection model, also called the OSI reference model


4/26

We will now illustrate in general terms the OSI reference model with an examplefrom business 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 of this business deal is important. Theprocess that takes place in the lower hierarchy to get the tyres from the supplierto the car manufacturer does not interest them. The managers rely on theirpurchasing- and sales departments, which will deal with practical details

The car manufacturer's purchasing department, however, only communicateswith the supplier's sales department. As soon as the financial transactions areconcluded, the goods can be delivered from A to B.


5/26

The purchasing and sales departments are not interested in the practical detailsof delivery

At the supplier, the warehouse workers must pack the tyres and load them on

trucks, to get them ready for transport As soon as the tyres arrive at the manufacturer, the warehouse workers will

unpack the tyres and store them. In summary, we can say: It's always severallevels of a company that collaborate in a business transaction

The higher levels give the lower levels instructions, without paying attention to

the details of the processes. Communication between the two companies takesplace only between peer levels. With the OSI model, it's similar.


6/26

OSI is a reference model consisting of 7 layers that arebased on each other. Each layer has its own tasks

The lower layer always provides support functions for thelayer above. For a layer, the data transported in the layersunderneath is irrelevant. Communication only takes placebetween the elements of the same layer

This type of communication between elements belonging tothe same layer in different systems is known as peer-to-peercommunication.


7/26

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 data

security. 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 as

charging. Layer 6 is the Presentation layer. It takes over the transfer of application-oriented formats, as well

as encryption and translation. At the top resides layer No 7, the Application layer. It is responsible for the application protocols

and the user interfaces.


8/26


9/26

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 user's side, which

receives, sends, and acts on these messages. Let's turn our attention to MTP first.


10/26

The Message Transfer Part (MTP), represents the basis for the entire

SS7 system. It transmits messages between network elements MTP is composed of three layers MTP layer 1 defines the physical and electrical characteristics of the

connection MTP layer 2 supports the error free transmission of signalling messages

between neighboring network elements MTP layer 3 is responsible for taking the message from any element in a

signalling network to any other element within the same network.


11/26

While MTP is responsible for message transfer, the Telephone UserPart (TUP) represents the protocol used for sending, receiving, andacting on these messages from the user's point of view

TUP handles call set-up, call supervision and clear down, and exists fornormal public fixed networks, which are also known as Public SwitchedTelephone Networks, or PSTN

With the introduction of the more capable ISDN network, some extra

sets of messages became necessary. These features are contained inthe ISUP which replaces the TUP.


12/26

To guarantee virtual connections and connectionless signalling, that is signallingwhich is not bound to a call, another protocol layer on top of MTP is required,parallel to TUP. This is the Signalling Connection and Control Part, SCCP

TUP and SCCP take over different tasks, but both make use of the servicesprovided by MTP. In contrast to MTP, SCCP uses sequence numbers to makesure that messages arrive at the receiver in a determined order, so a virtualconnection can be guaranteed

SCCP also enables the routing of signalling messages across multiple networksin the absence of a call.

This layer structure, consisting of MTP and TUP/ISUP, as well as SSCP,represents the SS7 protocol, which is the protocol used for signalling withinPublic Switched Telephone Networks and ISDN networks.


13/26

In GSM networks, signalling is not as easy as in a fixed network. This is because, due tothe network architecture, a digital mobile radio network makes much higher demands onsignalling

GSM requires a considerably higher amount of non-call-related signalling information.After all, it must be considered that the GSM customer is mobile, in contrast to the user of

a fixed network, who telephones from a fixed device. Therefore, the mobile station mustcontinuously be provided with localization signals, to enable the Location Update The Location Update is an example of a non-call-related communication between the

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 arenecessary:

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


14/26

The Base Station Subsystem Application Part(BSSAP) is a protocol layer responsible for

communication between the MSC and the BSC inGSM

BSSAP is responsible for the entire management

and control of the radio resources in the BSS. Itresides on top of the Signalling Connection andControl Part, SCCP


15/26

The Transaction Capabilities Application Part (TCAP) is a protocol layerwhich resides directly 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 andreplies

TCAP functions like a secretary's office, where many different requestsare brought into the correct sequence and distributed

TCAP handles the access to data bases like the HLR or the VLR. It mustexist so that a higher protocol - the Mobile Application Part (MAP) - canbe used.


16/26

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, whichcan be used as a "secretary's office" by the MAP, and which coordinates and guaranteesa smooth MAP communication.

A MAP-based communication takes place when data is exchanged between NSS

elements in 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 data exchange between the GMSC and the HLR, without the actual call beingrouted to the HLR. In such cases, the network relies on MAP, which is used for signallingcommunication between NSS elements. Please note: in the MSC-MSC communication,

MAP is only used for non-call-related signalling. To forward a call from an MSC to anotherMSC, TUP or ISUP is used.


17/26

Not every GSM element must be able to understand every language.Consequently, only those protocol layers which the network element

actually requires for carrying out its task need 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 followinglessons, the SS7 requirements of the individual GSM elements will beintroduced.


18/26

As in all the other elements, MTP is the basis protocol in the MSC/VLR.Without it, there would be no SS7-based signalling

Furthermore, the MSC needs TUP/ISUP for call supervision. Since the

MSC communicates with the BSC and the HLR, it also requires BSSAPand MAP, which are both based on SCCP The use of MAP requires the presence of TCAP. Thus the MSC, as the

key element of the Network Subsystem NSS, must include the wholerange of SS7 protocols.


19/26

The HLR is not responsible for call control, and therefore does not needTUP/ISUP

Furthermore, since it does not have to communicate directly with theBase Station Controller, BSSAP is not implemented in the HLR. Thus,only the four protocols MTP, SCCP, TCAP and MAP must be present inthe HLR

Normally, the BSC would manage on BSSAP. But since, in modernnetworks, BSSAP is based on the SCCP protocol, whose functionality inturn requires the presence of MTP, the BSC contains MTP, SCCP andBSSAP.


20/26

Communication between network elements always takes place usingthe relevant protocols, each protocol relying on the protocols of thelayers below

For example, the signalling from the MSC to a PSTN for call set-up is

carried out by TUP, which is based on MTP in both elements. Or, if anMSC wants to know the current location of a subscriber, itcommunicates with the responsible HLR using MAP

In this communication, TCAP, SCCP and MTP are required. TheBSSAP protocol, on the other hand, is only needed by the MSC when it

wants to communicate with the BSC.


21/26

Whereas the elements in the Network Subsystem use SS7, furtherprotocol types are needed in the Base Station Subsystem.

The BSC and BTS communicate using the Link Access Protocol for theISDN "D" channel, or LAP-D. This protocol is also used between the enduser and the net in ISDN networks.

A slightly modified version of the same protocol also exists between theBTS and the mobile station. Due to the modifications imposed by thecharacteristics of the air interface, the protocol is called LAP-Dm. The

message structure within LAP-D resembles that of SS7, but it's limited tothe support of point-to-point connections.


22/26


23/26

Between the NSS elements, data is either exchanged overcopper cable or optical fiber, or via microwave.

All NSS interfaces offer data rates of at least 64 kbps. 2

Mbit/s connections are the rule. The protocols are based onthe SS7 standard.

Two kinds of information are transferred over the differentinterfaces. Signalling information such as addressing and

"mobility data", and user data, that means speech, fax- anddata messages.


24/26

Between the NSS elements we find the following interfaces. Note thatthe Mobile Services Center (MSC) and the Visitor Location Register(VLR) form a spatial unit.

Between the MSC and the VLR we find the B-interface. This interfaceis used to transmit signalling data. The C-interface is located between the MSC and the Home Location

Register (HLR). It is also 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.


25/26

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 IMEI

control. The MSCs which provide connections to another mobile radio network, that is a Public

Land Mobile Network (PLMN), or a Public Switched Telephone Network (PSTN), andwhich are connected to the HLR, are also known as Gateway-MSCs, or GMSCs. Theinterface between 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'

point of view, this is cost-efficient, because the more MSCs can function as Gateways toother networks, the longer a call can be routed within the own network before it is handedover to a different network.


26/26

NSS Interfaces

Documents

Transcript of NSS Interfaces