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A SIM lock, simlock, network lock, carrier lock or (master) subsidy lock is a technical restriction built into GSM and CDMA [1] mobile phones by mobile phone manufacturers for use by service providers to restrict the use of these phones to specific countries and/or networks.
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation digital cellular networks used by mobile devices such as mobile phones and tablets. GSM is also a trade mark owned by the GSM Association. [2] "
The Type Allocation Code (TAC) is the initial eight-digit portion of the 15-digit IMEI and 16-digit IMEISV codes used to uniquely identify wireless devices.. The Type Allocation Code identifies a particular model (and often revision) of wireless telephone for use on a GSM, UMTS, LTE, 5G NR, iDEN, Iridium or other IMEI-employing wireless network.
Phone unlocking, the removal of a SIM lock on a mobile phone iOS jailbreaking , removing the limitations imposed by Apple, Inc. on iOS devices Rooting (Android) , allowing users of Android devices to obtain root access within Android's subsystem (similar to iOS jailbreaking)
Talktime is generally higher in GSM phones due to the pulse nature of transmission. The availability of Subscriber Identity Modules allows users to switch networks and handsets at will, aside from a subsidy lock. GSM covers virtually all parts of the world so international roaming is not a problem.
Dual SIM phones are mainstream in many countries where phones are normally sold unlocked. Dual SIMs are popular for separating personal and business calls, in locations where lower prices apply to calls between clients of the same provider, where a single network may lack comprehensive coverage, and for travel across national and regional borders.
An interesting special case of multi-mode phones is the WCDMA/GSM phone. The radio interfaces are very different from each other, but mobile to core network messaging has strong similarities, meaning that software sharing is quite easy. Probably more importantly, the WCDMA air interface has been designed with GSM compatibility in mind.
The current cellular location of the phone (i.e., which BTS it is at) is entered into the VLR record and will be used during a process called paging when the GSM network wishes to locate the mobile phone. Every SIM card contains a secret key, called the Ki, which is used to provide authentication and encryption services.