The Federal Communications Commission, or FCC, established Electronic Serial Numbers (ESNs) to identify unique mobile devices. This was mainly used for Advanced Mobile Phone System (AMPS), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) phones all throughout the US. This is equivalent to the International Mobile Equipment Identity (IMEI) numbers for all Global System for Mobile communications, or GSM networks.

The ESN serves as the wireless phone identification number and is readily embedded in its microchip. Every time a call is placed, the ESN is automatically registered at the base station so the wireless phone carrier’s main office can verify the validity of the call. It is different from the Mobile Identification number (MIN), which serves to identify the phone within a network. Both ESN and MIN are used electronically to prevent possible fraud.

The ESN is 32 bits in length and is composed of three parts:

  1. 8-bit producer code
  2. 18-bit unique serial number
  3. The other 6 bits are set aside for use later on, but recently this has been combined with the 18-bit serial number to form a 24-bit serial number field.

Its particular format can only recognize up to 256 individual manufacturers. However, it became apparent early on that this was going to be insufficient, so a new system was devised. The new format now consists of only the 18-bit manufacturer code and the serial number, which is also 18 bits.

Newly assigned ESNs now follow this particular format. Meanwhile, manufacturer code 0x80 might also serve as pseudo-ESNs when reserved. It can be calculated through a EUIMID or MEID. Nevertheless, these so-called pseudo-ESNs cannot be assured of uniqueness or individuality.

ESN Usage

The main purpose of the ESN is to provide unique identities to mobile phones. ESNs perform the following functions:

  1. Since the ESN was designed (unlike subscription identifiers) not to be changed, this helps to uniquely identify a cellular phone. This particular use encompasses the following:
    • For identification of stolen phones;
    • For law interception;
    • For inventory purposes;
    • For warranty;
    • For provisioning.

  2. To address the radio interface of a mobile phone, especially with the lack of subscription identifier.
  3. For CAVE authentication.
  4. For registration timing.
  5. Validation for ANSI-41, also with the MIN and IMSI.
  6. For ‘back office’ systems, this is used as a unique key.

Today, ESNs are slowly running out and are being replaced by the Mobile Equipment ID (MEID). The MEIDs are 56 bits in length.