(ME)ID Required
In the mid-1980s when cellphones were still analog, required battery packs and looked like more like briefcases than phones, no one could imagine the day when more than 4 billion cell phones were manufactured. In order to identify phones on a network, engineers developed a 32-bit code called the Electronic Serial Number (ESN). Initially this code was used for billing and to make sure the right call went to the right phone. Although 32 bits only allowed 4 billion unique numbers, engineers probably didn't consider ESN a short-term solution, but certainly they believed that technology would change before ESNs were exhausted.
However, as technology advanced from analog to TDMA or CDMA and then CDMA2000, the networks continued to use ESNs to identify phones. It made sense to use them in order to maintain compatibility with older networks, since many carriers upgraded their systems piece by piece. We still have dual- or tri-mode phones today. Once they upgraded to digital networks, carriers also started using ESNs to secure phone calls and eventually prevent fraud.
Unfortunately, the rapid growth of TDMA and CDMA outside the US, as well as short handset replacement cycles have nearly exhausted the supply of ESNs. In addition, early on in cell phone history, large blocks of serial numbers were distributed to manufacturers rather liberally, speeding the depletion of a limited supply.
The fact that it only took 20 years to use 4 billion serial numbers is even more amazing when you consider that GSM phones don't use ESNs. Since GSM was launched as an all-digital system with no need for backward compatibility, they use a different numbering system called IMEI. Probably since GSM came along later, and could learn from the mistakes of ESN, IMEI is almost twice as long, providing a significantly higher limit of unique codes.
As recently as late 2004, it was estimated ESNs would run out by mid-2005. Thanks to the efforts of the organization in charge of dispensing ESNs (the TIA) to recover issued but unused codes, most manufacturers now won't run out of them until sometime in 2006 or early 2007. Before the supply of ESNs is exhausted, manufacturers and carriers will need to be ready for ESN's successor, Mobile Equipment ID (MEID). But they will be rushing to do this much sooner than they expected.
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