The USSD Standard
There are different standards
of USSD in different phases which are as follows:
USSD phase 1
It supports only mobile-initiated operations (not network-initiated
operations). There is no dialogue mechanism; a USSD Phase 1 session
comprises only one request and one answer, i.e. one USSD transaction.
USSD phase 2
USSD Phase 2 was implemented in the mid-90s. With USSD Phase 2,
a dialogue is established between the mobile and the network node.
Multiple USSD operations can be sent within this dialogue. The
dialogue can be network (application)-initiated or mobile station-initiated.
Most legacy handsets and GSM networks support USSD Phase 2.
USSD phase 2+ Enhanced USSD
The following USSD enhancements are considered by ETSI:
- Extending the usage of the DCS to distinguish between MMI-mode
(DCS= ‘0000 1111’) and bearer mode (DCS=’01xx
xxxx’).
- In bearer mode: the DCS may indicate message classes; Immediate
display, ME-specific, SIM specific and TE-specific.
- In bearer mode: replacing the alphanumerical Service Code with
a binary coded Network Element Identifier to be used when routing
USSD through the network.
