<P> Electronic serial numbers (ESNs) were created by the U.S. Federal Communications Commission (FCC) to uniquely identify mobile devices, from the days of AMPS in the United States starting in the early 1980s . The administrative role was taken over by the Telecommunications Industry Association in 1997 and is still maintained by them . ESNs are currently mainly used with CDMA phones (and were previously used by AMPS and TDMA phones), compared to International Mobile Equipment Identity (IMEI) numbers used by all GSM phones . </P> <P> The first 8 bits of the ESN was originally the manufacturer code, leaving 24 bits for the manufacturer to assign up to 16,777,215 codes to mobiles . To allow more than 256 manufacturers to be identified the manufacturer code was extended to 14 bits, leaving 18 bits for the manufacturer to assign up to 262,144 codes . Manufacturer code 0x80 is reserved from assignment and is used instead as an 8 - bit prefix for pseudo-ESNs (pESN). The remaining 24 bits are the least significant bits of the SHA - 1 hash of a mobile equipment identifier (MEID). Pseudo-ESNs are not guaranteed to be unique (the MEID is the unique identifier if the phone has a pseudo-ESN). </P>

What's the esn number on a cell phone
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