ntity. Certain subscriber parameters are stored on the SIM card, together with personal data used by the subscriber. The SIM card identifies the subscriber to the network. To protect the SIM card from improper use, the subscribers have to enter a 4-bit Personal Identification Number (PIN) before using the mobile. The PIN is stored on the card. If the wrong PIN is entered three times in a row, the card blocks itself and may only be unblocked with an 8-bit personal blocking key (PUK), also stored in the card. International Mobile Station Equipment Identity (IMEI) This serial number uniquely identifies mobile stations internationally. It is allocated by the equipment manufacturer and registered by the network operators who store them in the Equipment Identity Register (EIR). IMEI is a hierarchical address, containing the following parts: Type Approval Code (TAC): 6 decimal places, centrally assigned; Find Assembly Code (FAC): 6 decimal places, assigned by the manufacturer; Serial Number (SNR): 6 decimal places, assigned by the manufacturer; Spare (SP): 1 decimal place. Hence, IMEI = TAC + FAC + SNR + SP. International Mobile Subscriber Identity (IMSI) While registering for service with a network operator, each subscriber receives a unique identifier, the International Mobile Subscriber Identity (IMSI), which is stored in the SIM. A mobile station can be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI. The IMSI also consists of the following parts: Mobile Country Code (MCC): 3 decimal places, internationally standardised; Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile networks across the country; Mobile Subscriber Identification Number (MSIN): maximum 10 places, identification number of the subscriber in his/her mobile home network. Thus IMSI = MCC + MNC + MSIN and a maximum of 15 digits is used. Mobile Subscriber ISDN Number (MSISDN) The real telephone number of the MS is the Mobile Subscriber ISDN Number. It is assigned to the subscriber, such that 