2G 3G 4G 5G attach cell identifier CG charging gateway ECGI ECI EPS Bearer Ga interface Gb interface GGSN Gn interface GPRS GPRS attach GPRS mobility management Gs interface GTP Gy interface LTE MCC MM state MNC mobile network MSC association OCS online charging online charging system PCF PDP PDP context PDP context activation PLMN PS signaling connection S1-MME S1-U service request SGSN TA TAC TAI tracking area user plane