Overview of GSM architecture- Subsystems

Mobile Station – MS
Subscriber Identity Module –SIM
Base transceiver station (BTS)
Base station controller (BSC)
Transcoding rate and adaptation unit (TRAU)
Mobile services switching center (MSC)
Home location register (HLR),
Visitor location register (VLR)
Equipment identity register (EIR).
Together, they form a
public land mobile network (PLMN).