Electronic Serial Number (ESN) is a 32-bit serial number assigned to each mobile device to identify each one of them, uniquely, by the manufacturers. ESN is typically found on label under cellular phone's battery. ESN is automatically transmitted to a base station when user makes a call. This number is used to prevent fraud and authenticate the validity of the phone. ECNS were mainly used with CDMA phone. On sealed devices, like the today's smart phone, you can find the identification number in the system menu.
Initially, First 8 Bits were Manufacturer's code and 24 bits to assign codes to mobile device by the manufacturer. This allowed up to 256 manufacturers and up to 16,777,215 unique coded. But later, to allow for more manufacturers, manufacturer's code was extended up to 14 bits, leaving 18 bits for mobile codes. However, in 2006, manufactures started running out of unique ESNs, so a whole new transition was needed to replace ESN and then MEID (Mobile Equipment Identifier) solved that problem. MEIDs are 56 bit long, the same length as IEMI. The main difference between IMEI is that MEID allows hexadecimal digits while IMEI allows only decimal digits.
Now, let's see how does the ESN matters to User?
ESNs are used as a tool for tracking the phones and when need be, banning phones from network. This is very useful in case of theft. It your phone is stolen, you can track phone with ESN and can get it blocked if needed. A network service provider company maintains the list of ESNs of stolen phones. Such phones are said to be bad ESN phone.
ESN was assigned by U.S. Federal Communications Commission (FCC) in the United States in early 1980s, and originally used for the very first analog mobile phone technology, the AMPS. Although ESN assignments may still occur in the future based on applications received before June 30, 2010, there have not been any assignments made since December 31, 2010. Administration was afterward taken over by the Telecommunications Industry Association in 1997.
Advantages of the ESN system:
a) The ESN is particularly useful for identifying stolen cellular phones even if the SIM card is changed.
b) It can be used for national security by maintaining records of bad ESN cell phones and blocking such users if needed.
c) An ESN can also be used to prove that a certain mobile device was used to make or receive a call.
d) ESN can also be used for manufacturer tracking for inventory and warranty claims.