ase. The 2. 935MHz to 960MHz for the base station to mobile (downlink) HLR is the master subscriber database and carries information about direction. individual subscribers numbers, subscription levels, call restriction- S, supplementary services and the current location (or most recent These 25MHz bands are divided into 124 pairs of carriers spaced location) of subscribers. The VLR acts as a temporary subscriber by 200kHz. In addition, consideration is now being given to specif- database for all subscribers within its coverage area, and contains ying additional carriers in a pair of extension bands 872MHz to similar information to that in the HLR. The provision of a VLR 888MHz and 917MHz to 933MHz. Each of the carriers is divided means that the MSC does not need to access the HLR for every up into eight TDMA timeslots of length 0.577ms such that the frame transaction. length is 4.615ms. The recurrence of each timeslot makes up one The authentication centre (AUC) works closely with the HLR and provides information to authenticate all calls in order to guard against fraud. The equipment identity register (EIR) is used for Table 47.3 GSM air interface parameters equipment security and validation of different types of mobile equipment. This information can be used to screen mobile types from accessing the system, for example if a mobile equipment is Frequency band mobile - base 890 915MHz stolen, not type approved, or has a fault which could disturb the network. Frequency band base - mobile 935 - 960MHz Network management is used to monitor and control the major 124 radio carriers spaced by 200kHz elements of the GSM network. In particular, it monitors and reports faults and performance data. It can also be used to re-configure the TDMA structure with 8 timeslots per radio carrier network. Gaussian Minimum Shift Keying (GMSK) Modulation with BT = 0.3 47.5.2 Air interface Slow frequency hopping at 217 hops per second Block and convolutional channel coding with interleaving The GSM Air Interface (Um) provides the phys