I hope we all know USSD. If not then hopefully my old blog post will help remind you of USSD. Apparently USSD is as popular as it was nearly a decade back since it is supported by 100% of the phones. As a result 3GPP have made sure that a USSD like service is available in LTE/SAE since USSD was designed for a CS domain and in SAE we have only the PS domain.
Picture Source: Aayush Weblog
The following is from the 3GPP document:
Today mobile initiated unstructured SS data in MMI mode are widely used to interact with proprietary home-network provided services, e.g. to activate or deactivate certain features or to interrogate some parameter settings.
The user dials a certain feature code, e.g. in the format “*# ”, this code is forwarded to the home network and answered with a text string providing the requested information. Unlike common SMS the string is displayed immediately and not stored on the UE.
A typical use case is the interrogation of the account balance in a prepaid service. The prepaid user e.g. dials "*101#", the message is forwarded to the HPLMN and further to the IN system where the account balance is checked and finally the current value is transferred to the user in a short answer string, e.g. "Balance: € 35,40". Another use case is controlling the active UE for incoming calls and messages in case of a hunting service / multi SIM service.
From a network perspective the functionality is as follows:
1. The user sends the request
2. USSD is sent as MAP message to the HPLMN
3. USSD is forwarded to a Service Node (SN) [non-standardized functionality]
4. USSD is answered
5. answer
6. answer
The mentioned functionality is not available in the EPS. So e.g. a business customer who is subscribed to a certain multi SIM service will use his UEs via CS and EPS/IMS. Dependent on the access he would have to use different mechanisms for controlling the active UE.
This problem can be avoided when introducing completely new services. Then mechanisms can be used that are available via all access networks, e.g. web interfaces via GPRS or EPS. However we are talking about existing services with a broad customer base that is accustomed to use USSD codes as they are fast and simple to use.
As USSD is widely used in CS domain, operators would benefit from re-using the already deployed servers also when the user accesses services that make use of USSD over IMS.
It is therefore desirable to create in 3GPP a service which provides the same capabilities for the user, like the well known "GSM Mobile User Initiated USSD" feature.
For the user, it is important that the user experience is transparent (I.e. the look and feel of the service is independent of the transport mechanism used to convey the USSD payload to the network).
Possible solutions
There are several possibilities to solve this issue. One would be to re-introduce USSD in EPS. This is not the intention as it creates too much overhead. The idea is to specify a light weight solution which provides the same look and feel for the user but uses existing network mechanisms, i.e. only to simulate the USSD service.
One variant could be that the UE when being attached via the EPS to the IMS encapsulates the USSD codes in IP messages and forwards them to the network. This could happen either via the Ut interface as XCAP data using http or in a SIP message.
It should be noted that there are also user initiated MMI mode USSDs for VPLMN use. The differentiation, if USSD are intended for HPLMN or VPLMN use, is done via the range of the feature code. If USSD for VPLMN use were to be supported / simulated this may prevent certain solutions (e.g. using the Ut) and have some architectural impact (considering all possible roaming scenarios for the IMS).
Proposal
To specify an easy solution having no architectural impact. Only the simulation of mobile initiated USSD – MMI mode for HPLMN use should be supported. The functionality should be available for Multimedia Telephony, i.e. it can be implemented with the MMTel UE client and USSD messages are sent to and answered by the MMTel AS.
Though there isn't much details on this feature available, Ayush's weblog has some more details on this feature here.
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