GSM和GPRS信令详解 ABIS BTS BSC (PCU) Gb A Gs SGSN Gr HLR VLR MSC MAP-D MAP-C MAP-E Gd Gc Gn Gi Gn Gp GGSN GGSN GGSN PDN PLMN EIR GPRS SM-SC SMS-GMSC SMS-IWMSC PSTN Um