USSD Call Back

B y E n g i n e e rRAWAND DLSHAD ALIIREG ENG.SOLUTION/INTERNATIONAL ROAMING

USSD CALL BACK

Prepaid

• There are many ways to implement prepaid roaming. Currently, most of the prepaid roaming implementations utilize the capabilities provided by

• USSD(unstructured supplementary service data)

1

• CAMEL (customized application mobile enhanced logic)

2

USSD Call BackUSSD deploys a callback mechanism as you know ,For each outgoing call initiated by a roamer, there will be two call legs—an international call leg back home and a follow-on call. That make call cost v.high because it is surely not an efficient way to handle the call.For that you can use USSD call Back service,But the user needs to initiate the service by using a special service code. However, implementing prepaid roaming with USSD is rather simple, fast, cheaper, and supported by almost all existing networks, providing a global footprint.

HOW USSD CALL BACK WORK?

1. The roamer keys in 1XY 0770 XXX XXXX# and presses the ∗ ∗<SEND/OK> button to initiate a call to subscriber 0770 XXXX in another network.

2. The VPLMN MSC/VLR transfers this USSD string to the HPLMN HLR.

3. The HPLMN HLR passes this string to the prepaid roaming platform

4. After the required precall checks, the prepaid roaming platform initiates two outgoing calls (i.e., one to the roamer and one to the requested Called party) and connects

USSD CALL CALL FLOW

Romer

1.*1XY*0770 XXXX #

MSC/VLR HLRUSSD

Gateway & Call

progess

5.0770 XXX

2. MAP/SS7 USSD trigger

Call Reguest

3. Out bound call

4. Fixed or mobile net

USSD string coding.

• The length and content of a USSD string is very flexible. USSD utilizes the characters “ ,” “#,” and ∗all the digits, If the user keys in a string coding scheme the USSD handlers in the MS, the MSC, the VLR, and the HLR treat it as a USSD request. The character “ ” is used to indicate the beginning of a ∗USSD string. It is also used as a separator between two parameters. The character “#” is used to terminate a USSD string.

USSD string coding.

• The formatting rules to create a USSD string are summarized as follows.

* • An asterisk to

indicate beginning

1XY• A predefined

access/service code

One or more parameters needed

to invoke supplementary

service

A # to terminate request

Roamer initiated USSD operation

• When a roamer invokes a USSD request (e.g., a prepaid roamer initiates USSD callback) by keying in the appropriate code (containing the HPLMN service code), the USSD handler within the MS invokes the USSD request by sending a REGISTER message to the network.

• The REGISTER message contains a process unstructured SS request invoke component. The serving MSC, on receiving a USSD request containing an HPLMN service code, sets up a transaction to the VLR and forwards the request unchanged.


• MSC will act in a transparent mode to any further Requests/responses for this transaction, passing them between the MS and the VLR without taking any action.

• When a VLR receives a USSD request containing an HPLMN service code, it sets up a transaction to the HLR and forwards the request unchanged. The VLR then acts in a transparent mode to any further requests/responses for this transaction. Passing them between the MSC and the HLR without taking Any action.


• When the HLR receives the USSD request, it processes and invokes the appropriate application, i.e., a prepaid roaming platform. The application, as an option, may request further information in order to perform the requested operation or terminate the dialogue. If the application requests more information, it initiates a USSD request , using the ongoing transaction. If the application decides to terminate the dialogue, the network side sends a release complete message. The MS can also terminate a dialogue by sending a release complete message.


MSC HLR

USSD Gateway

& Call progess

VLR

International Gatway CCS7/MAP

VPLMN HPLMN

USSD

RequestProcess USSD request Process USSD

request Process USSD request Process USSD

request

Response USSD Response

USSD Response USSD Response

USSD Response USSD

Process unstructured SS request.

• The MAP process unstructured SS request procedure is used to relay USSD information between the:

• MSC and the VLR• VLR and the HLR• HLR and the gsmSCF


• The process unstructured SS request message contains the following parameters:

• USSD data coding scheme: This parameter contains the alphabet and the language information used for the unstructured information in a USSD operation. The coding of this parameter is according to the cell broadcast data coding scheme as specified in 3GPP TS 23.038.

• USSD string :This parameter contains a string of unstructured information The string is sent either by the mobile user or the network.

• MSISDN :Originating subscriber international number. The MSISDN is an optional parameter.

Process unstructured SS request.• The receiving entity, on unsuccessful outcome of the service,

returns a user error. The possible error types are as follows:

• System failure: This indicates that the requested task could not be completed because of a problem in another entity.

• Data missing: This indicates that the context is missing in the received message.

• Unexpected data value: This error is returned if the receiver is not able to deal with the contents of the USSD string.

• Call barred: This indicates that the receiving entities cannot process the request because of barring of the initiated service.

• Unknown alphabet: This indicates that the receiving entity does not support the alphabet indicated in the USSD operation.


• Illegal subscriber: The receiving entity indicates that the delivery of the USSD failed because the destination MS failed authentication.

• Illegal equipment: The receiving entity indicates that the delivery of the USSD failed because the destination MS failed the IMEI check.

• Illegal equipment: The receiving entity indicates that the delivery of the USSD failed because the destination MS failed the IMEI check.

Prepaid roaming—USSD callback scenario

.

How the USSD call back work when a roamer from B PLMN visit APLMN make a USSD call back to call C number (fixed line belong to C network?

Ans:See the next call flow

MSC/VLR HLRUSSD

GatewayPrP Roaming

PlatformSSP Local

Switch C net

USSD *155*C# Process USS

request *155*C# USSD *155*C#

USSD ACkProcess USS responseUSSD ACK Send rout info

MSISDNProvide Roaming Number(IMSI)

MSRNSRI ack(MSRN)

Initiate Call CgP=A CdP=MSRN

IAM (MSRN)

Setup

AlertANM

Initiate Call CgP=A CdP=C

ACM

ANMSpeech

VPLMNHPLMN Application Server

IAM (MSRN)

Call Flow Description• The USSD handler at the MS, on recognizing a valid USSD string,sends a

register message with an invoke process unstructured SS request to the serving MSC. This message contains the USSD data coding scheme and USSD string.

• The USSD handler within MSC analyzes the service code on realising that the service code is not meant for its own applications, passes the USSD request to the VLR, using the MAP process unstructured SS request procedure.

• As the service code 111 is reserved for the HPLMN, the VLR invokes the MAP process unstructured SS request procedure toward the HPLMN HLR. If the alphabet used for the message is understood by the HLR, then the message is fed to an application contained locally in the HLR, or to the gsmSCF, or to a secondary HLR where the USSD application is located. If the alphabet is not understood. then the error message unknown alphabet is returned.

• A USSD acknowledgment is sent by the HLR to the roamer by the same transaction.

• The application performs precall checks. For example, it checks if the caller is authorized to make such a call and if there is enough credit balance in the caller’s account. If the caller does not qualify, an appropriate notification is sent by using the unstructured SS notify procedure. If the caller qualifies, the application initiates a mobile terminating call to the calling party first. The MAP send routing information (SRI) procedure is invoked toward the HLR to get the routing information necessary to establish the call. The HLR, on receiving the SRI message, invokes the provide roaming number (PRN) procedure toward the serving MSC/VLR to get the MSRN assigned to the roamer.

• Once the MSRN is known, the application using the ISUP procedure initiates an outgoing call. (The application server implementation is vendor dependent. As a minimum, it should have call processing capabilities and should be connected to a MSC/STP by CCS7 ISUP links. It should also have the capability to send SMS notification to a roamer via SMSC.)

Call Flow Description• On answer from the roamer, a suitable announcement or tone is

fed to the roamer indicating the call progress.• The prepaid roaming platform then initiates a second outgoing

call toward called party C, using ISUP procedures.• On answer from called party C, a circuit-switched call is

established between the roamer and the called party.• The application server monitors the call. On disconnection from

any• of the parties, it releases the call and frees up resources. In cases

where the credit balance is exhausted during the call, an appropriate notification is sent to the roamer before call disconnection.

QUESTIONS?For questions or more details or any idea to make this presentation more useful for all.Send emial to:

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Documents

USSD Call Back