TABLE OF CONTENTS But why, father?
Not expensive enough, my son.
Renault publicity for the R5
Unstructured supplementary service data (USSD) services allow operators to offer conversational text services to 2G (or more) subscribers. In contrast, SMS is not conversational: It takes a few seconds to send an SMS-MO and 5 to 7 seconds to receive an answer. "Conversational" means that the cell phone can initiate a session, get back and answer with a possible choice, reply to that answer, and so on. Most GSM networks can offer this service but have chosen not to because it was not as glamorous as WAP, although for many things, such as customer care, it is faster and simpler. It also has some very particular advantages:
Unlike SMS-MO, there are no charges for roaming from roaming partners when one of your subscribers is using your USSD services in another country. This is the call-back application described later in Section 12.4.
Your USSD services are accessible by your prepaid subscribers when roaming (receiving calls only) in another country (unlike with SMS-MO), which means that they can initiate USSD sessions even if the do not have SMS-MO service.
The bandwidth requirement is much lower than that required for WAP services, but much higher than for SMS service. It is like a voice connection that is maintained for the duration of a session on the air interface. However, in the PLMN, it does not occupy a circuit, it only uses MAP messages.
