- Mobile equipment identifier
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A mobile equipment identifier (MEID) is a globally unique number identifying a physical piece of CDMA mobile station equipment. The number format is defined by the 3GPP2 report S.R0048 but in practical terms it can be seen as an IMEI but with hexadecimal digits.
Regional code Manufacturer code Serial number CD R R X X X X X X Z Z Z Z Z Z C An MEID is 56 bits long (14 hex digits). It consists of three fields, including an 8-bit regional code (RR), a 24-bit manufacturer code, and a 24-bit manufacturer-assigned serial number. The check digit (CD) is not considered part of the MEID.
The MEID was created to replace ESNs, whose virgin form was exhausted in November 2008 [1]. As of TIA/EIA/IS-41 Revision D and TIA/EIA/IS-2000 Rev C, the ESN is still a required field in many messages—for compatibility, devices with an MEID can use a pseudo ESN (pESN), which is a manufacturer code of 0x80 (formerly reserved) followed by the least significant 24 bits of the SHA-1 hash of the MEID.[2]
Contents
Administration
The separation between international mobile equipment identifiers (IMEIs) used by GSM/UTMS and MEIDs is based on the number ranges. There are two administrators: the global decimal administrator (GDA) for IMEIs and the global hexadecimal administrator (GHA).
As of August 2006, the TIA acts as the GHA to assign MEID code prefixes (0xA0 and up), and the GSM Association acts as the global decimal administrator. http://www.babt.com/gsm-imei-number-allocation.asp
The middle ground between IMEIs and MEIDs is for inter-standard "worldphone" devices. These devices will have all decimal digits and can be allocated by any administration. Since they must have all decimal digits the GHA has opened up the range '99' for these assignments. IMEI administrators can just assign numbers for dual-technology phones out of their existing ranges.
Display formats
There are two standard formats for MEIDs, and both can include an optional check-digit. This is defined by 3GPP2 standard X.S0008.
The hexadecimal form is specified to be 14 digits grouped together where at least one of the first two ('RR') digits is in the range 'A' through 'F'. In this case, the check-digit is calculated using a modified (base 16) Luhn algorithm. The check-digit is never transmitted or stored. It is intended to detect most (but not all) input errors, it is not intended to be a checksum or CRC to detect transmission errors. Consequently it may be printed on phones or their packaging in case of manual entry of an MEID (e.g. because there is no bar code or the bar code is unreadable). If both the first two digits are in the range '0' through '9' then the check digit is calculated using the standard base 10 Luhn algorithm as the number meets the definition of IMEI.
The decimal form is specified to be 18 digits grouped in a 5 5 4 4 pattern and is calculated by converting the manufacturer code portion (32 bits) to decimal and padding on the left with '0' digits to 10 digits and separately converting the serial number portion to decimal and padding on the left to 8 digits. A check-digit can be calculated from the 18 digit result using the standard base 10 Luhn algorithm and appended to the end.
pESN conflicts
Because the pESN is formed by a hash on the MEID there is the potential for hash collisions. These will cause an extremely rare condition known as a 'collision' on a pure ESN-only network as the ESN is used for the calculation of the Public Long Code Mask (PLCM) used for communication with the base-station. Two mobiles using the same pESN within the same base-station area (operating on the same frequency) can result in call setup and page failures.
The probability of a collision has been carefully examined [3]. Roughly, it is estimated that even on a heavily loaded network the frequency of this situation is closer to 1 out of 1 million calls than to 1 out of 100 000.
3GPP2 specification C.S0072 provides a solution to this problem by allowing the PLCM to be established by the base station. It is easy for the base station to ensure that all PLCM codes are unique when this is done. This specification also allows the PLCM to be based on the MEID or IMSI.
A different problem occurs when ESN codes are stored in a database (such as for OTASP). In this situation, the risk of at least two phones having the same pseudo-ESN can be calculated using the birthday paradox and works out to about a 50 per cent probability in a database with 4,800 pseudo-ESN entries. 3GPP2 specifications C.S0016 (Revision C or higher) and C.S0066 have been modified to allow the replacement MEID identifier to be transmitted, resolving this problem.
Another problem is that messages delivered on the forward paging channel using the pESN as an address could be delivered to multiple mobiles seemingly randomly. This problem can be avoided by using MIN or IMSI based addressing instead.
Code to convert
This short Python script will convert an MEID to a pESN.
import hashlib meid = raw_input("Enter an MEID: ").upper() s = hashlib.sha1(meid.decode('hex')) #decode the hex MEID (convert it to binary!) pesn = "80" + s.hexdigest()[-6:].upper() #put the last 6 digits of the hash after 80 print "pESN: " + pesn
References
- ^ http://www.tiaonline.org/standards/resources/esn/documents/esn_meid_milestones_timeline.pdf
- ^ CDG Resources on MEID and EUIMID
- ^ Pellegrino G, Quick F. White Paper on Pseudo-ESN Collisions. TIA. 26 May 2005.
External links
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