“LLC frame” refers to the frames sent from SGSN to MFS (DL) and from MFS to SGSN (UL). This is the b interface.
Those frames can contain data (user data) or signalling (usually GMM)
The LLC DATA VOLUME refers to the amount of data travelling on the Gb interface. Now, check the indicator definition to know if this “volume” includes User Data + Signalling or only “User Data”.