Compiled by Abdul Basit Qureshi
GSM RF INTERVIEW FAQs 1)
GSM PLMN Services
Bearer Services Thse services give the subscriber the capacity required to transmit appropriate signals b/w certain access points(i.e., user-network interfaces).
Teleservices These services provide the subscriber with necessary capabilities including terminal equipment functions to communicate with other subscribers.
Supplementary Services These services modify or supplement basic telecommunications services. These services are offered together or in association with basic telecommunications services.
2)
GSM 900 Uplink :890 – 915 MHz Downlink : 935 - 960 MHz
3)
DCS 1800 Uplink : 1710 - 1785 MHz Downlink :1805 – 1885 MHz
4)
Bands Carrier frequencies GSM 900 (915-890) MHz / 200 kHz = 125, (1 – 124) ARFCNs DCS 1800 (1785-1710) MHz / 200 kHz = 375, (1 – 374) ARFCNs each band having a carrier separation of 200 kHz.
5)
Ciphering The purpose of ciphering id to encode the transmitted burst so that it cannot be hacked or tapped by any other device before reaching the receiver. The algorithm used for ciphering is the A5 algorithm
6)
Authentication normally takes place when the MS is turned on with each incoming call and outgoing call. A verification that the »Ki« (security code) stored in the AuC matches the »Ki« stored in SIM card of the MS completes this process.
7)
Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.