This article includes the followin Tops: GSM system architecture The Base station controller (BSC) the mobile switching center (MSC): The Home location register (HLR); The visitor location register (VLR The authentication center (AuC): The mobile station (MS) The equipment identity register (EIR): The base station subsystem (BSS Operation and Maintenance Center (OMC) The base transceiver station (BTS) Interfaces and protocols GSM1900 GSM1800 GSM900 Logical channels on the air interface The air interface Um GSM Channels on Air Interface Traffic channels on the air interface Burst formats The Abis interface Protocols on the A interface The A interface MSC protocols MSC-based interfaces Alarm monitoring Test and measurement problems in GSM Call Forwarding (CF) Call Hold (CH) Calling Line ID uthentication and much more