The Service Indicator (SI) is a service element ofISDN with which various services can be identified.
The Service Indicator can be used to indicate whether the transmitted information is voice or data. Unfortunately, the Service Indicator does not provide any marking for different voice quality or the selection of voice codecs.
Indeed, by negotiating voice codecs, it would be possible to choose between normal voice transmission and HD telephony. The Service Indicator is part of the Service Information Octet( SIO), a data field in the Message Signal Unit( MSU) of the SS7 signaling.