Call Setup in MSS System

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Transcript of Call Setup in MSS System

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Call Handing in Nokia MSC Server System - GSM Profile -

Call Setup in MSS System

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Table of Contents: 1 Call Handling in MSC .........................................................................................4

1.1 IAM.............................................................................................................4 1.2 ACM, CPG and ANM..................................................................................4 1.3 REL and RLC .............................................................................................5

2 Call Handling in MSC Server ..............................................................................7 2.1 IAM.............................................................................................................7 2.2 APM ...........................................................................................................7

3 Control and User Plane Routing .........................................................................9 3.1 Routing in an MSC Environment.................................................................9 3.2 Routing in an MSC Server Environment 1/2................................................9 3.3 Routing in an MSC Server Environment 2/2..............................................10 3.4 Exercise....................................................................................................11

4 MGW Selection ................................................................................................13 4.1 MGW Selection.........................................................................................13 4.2 Input for MGW Selection...........................................................................13 4.3 Example of MGW-A Selection - Selection of UPD.....................................14 4.4 Example of MGW-A Selection - Resource Reservation ............................15 4.5 Example of MGW-B Selection - Selection of UPD.....................................16 4.6 Example of MGW-B Selection - Resource Reservation ............................17

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1 Call Handling in MSC

1.1 IAM

Let us start this course with a simple example, presenting the most important ISUP signalling sequences in the Nokia MSC environment.

When subscriber A initiates a call via a mobile switching centre (MSC) to another subscriber B who is located under another MSC, the originating MSC transmits an ISUP Initial Address Message (IAM) to reserve a free circuit from the originating MSC to the destination MSC. The original ISUP IAM includes, for instance: the originating point code, the destination point code, circuit identification code and the called party number.

The circuit identification code identifies the circuit that is to be used for the call.

The IAM is routed via the home network of the originating MSC to the destination MSC.

1.2 ACM, CPG and ANM

The destination MSC examines the called party number and tries to establish the terminating side radio access channel for the call.

After the TCH is established, the destination MSC transmits an ISUP Address Complete Message (ACM) to the originating MSC to indicate that the remote end of the connection has been reserved.

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The destination MSC connects the ringback tone to the calling party and sends the Connection Progress (CPG) message to the originating MSC to inform it that the destination phone is now alerting.

When the called party picks up the phone, the destination MSC terminates the ringing tone and transmits an ISUP Answer Message (ANM) to the originating MSC.

If the call is chargeable, the charging starts upon receipt of the ANM message.

1.3 REL and RLC

If the calling party hangs up first, the originating MSC sends an ISUP release message (REL) to release the connection between the MSCs.

If the called party hangs up first, or if the line is busy, the destination MSC sends a REL to the originating MSC indicating the release cause (e.g., normal release or busy).

Upon receiving the REL, the destination MSC disconnects the connection to the called party and transmits an ISUP release complete message (RLC) to the originating MSC to acknowledge the release of the remote end of the connection.

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2 Call Handling in MSC Server

2.1 IAM

When subscriber A initiates a call via an MSC Server, the originating MSC Server transmits a BICC initial address message (IAM) to the destination MSC Server. The key content is the digits of the called party.

In contrast to traditional ISUP IAM, the BICC IAM message is sent without a circuit being hunted for the user plane. Therefore, the ISUP circuit identification code is renamed to Call Instant Code and used only for the identification of the signalling relation between the peer BICC entities – not as an indication of the physical bearer for the user plane.

This indication of the peer entities is required because the BICC IAM message does not include destination or originating point codes - as ISUP IAM does.

The ISUP ‘user service information’ and ‘transmission medium requirements’ tags are used both in ISUP and BICC to identify the bearer requirements.

In addition, the BICC IAM may include other user plane Bearer Network Characteristics in the BICC message section called APP container.

2.2 APM

Upon receipt of the IAM message, the destination MSS requests bearer information from the destination MGW and forwards this information to the originating MSS in the Application Transport Mechanism (APM) message.

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The APM message contains addressing and optional codec information from the destination MGW.

If the bearer is established in the backward direction and no codec negotiation is needed, the APM is not sent and the destination MGW starts the bearer establishment at this stage. In all other cases the APM is sent.

At this stage, the codec negotiation can be performed i.e., the interchanges of supported codec information between originating and destination ends.

Next, the bearer establishment takes place using AAL2 and Nb UP bearer establishment and initiation procedures.

In Nokia’s solution, the ACM is sent backward immediately after the TCH assignment is completed. This is followed by the Connection Progress (CPG) message containing alerting indication.

The answer message (ANM) flow is the same as for ISUP.

The release signalling sequences are also unchanged from ISUP, although BICC ”Release” and ”Release Complete” messages contain the indication of the target bearer.

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3 Control and User Plane Routing

3.1 Routing in an MSC Environment

Let us next look at the routing functionality in the MSC environment.

There are routes and circuits defined between MSCs. Both Control Plane and User Plane are routed via the same elements (i.e. MSCs). This means, that the goal of the routing is to find the next destination node and then reserve user plane resource towards the same node. The user plane resources are reserved in the same element through which the control plane was routed.

• The following routing steps are made in the MSC-A:

• Execute Routing Attribute Analysis to find the routing tree

• Execute Dialed Digit Analysis to find the destination node to where the call must be routed.

• Execute Routing Analysis to find out the next subdestination (in fact, the outgoing route).

After this point routing basically ends, because the route always contains local circuits i.e. a physical resource connected to the destination MSC.

In the IAM the CIC code is sent; in MSC-B a circuit is reserved based on the CIC reserved and therefore no routing analysis is executed in the destination MSC.

3.2 Routing in an MSC Server Environment 1/2

In the MSC Server environment, the Control Plane and User Plane no longer pass through the same element, so additional routing steps are necessary in order to find

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on adequate element for the user plane. The old analysis steps used in the earlier releases remain unchanged to ensure backward compatibility.

New terms have been introduced for the user plane routing: User Plane Destination Reference, User Plane Destination, Bearer Establishment Action Indicator and BNC characteristics.

New routing analysis steps have been defined to determine the network element through which the call is routed. These are: Succeeding BNC characteristics determination, Succeeding UPD determination and Succeeding Action Indicator determination.

The MSC Servers maintain a new User Plane Topology database to keep record of the routing elements.

3.3 Routing in an MSC Server Environment 2/2

The following routing steps are made in MSS-A:

The same control plane analyses as in the case of MSC are carried out to determine the subdestination. There is a new information element called User Plane Destination Reference (UPDR) attached to the route data. This information element will represent control plane destination in User Plane Routing.

Succeeding BNC characteristic determination is executed to determine the bearer type to use. The BICC can use ATM AAL2, IPv4, IPv6 or IP (without the version). If MSS does not select the IP version, the MGW then decides which version it uses.

SIP can use IPv4 or IPv6 bearers only.

Succeeding User Plane Destination (UPD) determination is executed next to find the succeeding UPD. This will contain a set of Multimedia Gateways.

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A Multimedia Gateway is then selected from this set, and finally the actual user plane resources can be reserved.

The following routing steps are performed in MSS-B:

Based on the incoming CGR, the UPDR information is sent to the User Plane Analysis.

A preceding UPD determination is carried out.

A MGW is selected from this set and then resources are reserved.

3.4 Exercise

Please read carefully the questions presented by the tutor and try to compose your own answers. Then look at the statements presented and select the correct one!

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4 MGW Selection

4.1 MGW Selection

As the name suggests, the MGW Selection procedure is responsible for selecting MGWs for the call. The basic idea is that the MSS collects the data of available MGWs for incoming and outgoing sides, and tries to select a common MGW.

Within optimal circumstances one MGW is selected for a call and it is also maintained during handovers.

If there is no single MGW connecting the incoming and outgoing sides, two MGWs will be selected for the basic call and interconnection will be created between them.

In addition, there are certain special cases when there can be even more MGWs within one call, especially in the context of handovers.

When a TDM circuit is being reserved, there is no choice of MGW: in this case only that particular MGW can be selected to which the TDM trunk is connected.

The MGW selection can be performed several times per call. It is always executed during the basic call setup if MGW is involved in the call. The MGW selection can also be performed during handovers and relocations.

4.2 Input for MGW Selection

The MGW selection process depends on the interface used for the call. If ATM/AAL2 or IP bearers are to be used, the MGW is selected from the UPD.

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The MGW Selection is a load-sharing-based selection. The operator can define load sharing values to MGWs on a per UPD basis.

These load sharing values will govern how frequently a MGW is selected from a given UPD.

The MGW Reselection is used when MGW has already been selected, but the resource reservation to it has failed (e.g. due to overloading). When this happens, the MSS selects another MGW if there is one available in the UPD.

If the TDM bearer is to be used for the call, the circuit is hunted by the MSS; this explicitly defines the MGW to be used.

4.3 Example of MGW-A Selection - Selection of UPD

In this example, a call from MS-A is routed to the MS-B through the ATM Backbone. The example concentrates on the Multimedia Gateway selection and user plane analysis parts of the call setup. Full message sequences are available in the final two modules.

The MS makes a call.

The MSS knows that that the circuit is located in MGW-1, and that MGW-1 is, therefore, automatically selected for the incoming side.

Then the outgoing route is determined - including the outgoing UPDR. All control plane information such as signaling types, UP bearer requirement, emergency call indicator and original dial class, is sent to the User Plane Analysis.

The succeeding BNC determination is executed, with the result that the outgoing bearer will be ATM AAL2.

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The succeeding UPD Determination is executed, with the result that the outgoing UPD will be UPD-A1.

4.4 Example of MGW-A Selection - Resource Reservation

The MSS selects the outgoing MGW from the UPD-A1. The MSS always tries to optimise the call path to have as few MGWs as possible. The MSS knows that MGW-1 has been selected for the incoming side. MGW-1 is also present in the UPD-A1, so the MSS selects MGW-1 from the UPD-A1. In this case load sharing is unnecessary.

The next step is that Succeeding Action Indicator Determination is carried out. The result is that the outgoing action indicator is “backward”. This means that Backward Bearer establishment method is used during the resource reservation and bearer establishment.

The MSS reserves outgoing ATM AAL2 resource to the MGW-1.

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4.5 Example of MGW-B Selection - Selection of UPD

The IAM is received in the succeeding MSS.

The MSS determines the incoming UPDR from the incoming CGR. The UPDR and other control plane data such as signalling type, UP bearer requirement, emergency call indicator and original dial class are sent to User Plane Analysis for Preceding UPD Determination.

The MSS determines the direction of the incoming call to find the MGW having a connection towards that particular network.

MSS-B executes Preceding UPD Determination and selects UPD-B1.

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4.6 Example of MGW-B Selection - Resource Reservation

The MSS selects the MGW for the call but cannot optimise the speech path, because the outgoing side location is not yet known.

Since the UPD-B1 contains only one MGW, MGW-3 will be used as the incoming MGW.

The MSS reserves the incoming ATM AAL2 resource to MGW-3.

When the outgoing TDM reservation request is issued, the MSS sees that the TDM is located in the same MGW-3.

The MSS reserves the outgoing TDM resource to MGW-3.

This is the end of user plane routing during the 2G call establishment.