VoLTE Online Charging – SMS

Today we will start with Online Charging, or Ro interface in our terminology. And because Online Charging is a bit more complex than Offline Billing, we’ll split it into several posts. In this first one we will take a look at so-call Direct-Debiting, which is used mainly for charging of SMS Service.

Perhaps from our Charging Overview you remember that we can use two different approaches to the Online Charging – Session based or Event based. SMS Ro Charging is using Event based mechanism. It is easier than the Session based and we will also need it later to complement the Session based Charging for the Voice service.

The SMS Charging architecture is standardized in 3GPP 32.274, the SMS online charging then uses the Credit-Control application as specified in TS 32.299.

Ro Reference Point for SMS

The Online Charging is triggered over Ro Reference point. It is up to a Mobile Operator if they will signal charging operations from IP-SM-GW, SMSC, SMS Router or any combination of these. Typically the charging requests are sent from SMSC only. For simplicity we will refer only to the SMSC in the following text. SMSC may either directly support Ro interface, or it can use CAP (CAMEL) protocol which is then translated to Ro by an Interworking Function (IWF) as described in 3GPP 32.293.

The SMS charging may use two different models:

• Immediate Event Charging (IEC)

or

• Event Charging with Unit Reservation (ECUR)

In this post we will rather focus on IEC, as the ECUR is more complex and in my personal experience it is less common than IEC. The ECUR principle is specified in TS 32.299.

IP-SM-GW Transport Level Interworking

Very funny. Even in 2015 we still rely in many ways on the old good SMS. No, operators are not making the money on the service as they were used to in the past. However it is not easy for them to fully replace the service (e.g. by RCS). That’s why the VoLTE standard GSMA IR.92 says:

The UE must implement the roles of an SM-over-IP sender and an SM-over-IP receiver, the IMS core network must take the role of an IP-SM-GW.

In other words, the VoLTE network has to support the (legacy) SMS sent over SIP. The VoLTE phone will receive a common (binary) SMS and the native client will display this message as any other. The only difference is that this time the SMS is sent from an IMS network over SIP protocol. Mind the purpose is not just to support common text messages, but (more importantly) to support OTA messaging for (U)SIM provisioning, SMS ‘non-text’ applications or Message Waiting Indication for Voice Mail.

SMS over IP

The network functionality which provides messaging service in the IMS network is called IP-SM Gateway (IP-SM-GW) and from the IMS point of view it is an Application Server.