The present document defines the principal purpose and use of International Mobile station Equipment Identities (IMEI). 3GPP TS 23.003 describes the technical manner of numbering, addressing and identification.

An MS can only be operated if a valid "International Mobile Subscriber Identity" (IMSI) is present. An IMSI is primarily intended for obtaining information on the use of the PLMN by subscribers for individual charging purposes. Besides the IMSI, the implementation of IMEI is found necessary in order to obtain knowledge about the presence of specific mobile station equipment in the network, disregarding whatever subscribers are making use of these equipment. The main objective is to be able to take measures against the use of stolen equipment or against equipment of which the use in the PLMN can not or no longer be tolerated for technical reasons. The IMEI is incorporated in an UE module which is contained within the UE. The IMEI shall be unique and shall not be changed after the ME's final production process. It shall resist tampering, i.e. manipulation and change, by any means (e.g. physical, electrical and software). The manufacturer implementing the IMEI module in the ME is responsible for ensuring that each IMEI within the allocated range is unique to the ME in which it resides, and is also responsible for keeping detailed records of produced and delivered MEs.

The composition of the IMEI shall be such that each individual mobile station equipment can be separately identified. Information is contained in the IMEI by which the PLMN, after requesting it, can immediately decide whether or not to accept calls made by means of this equipment. Secondly, the IMEI shall directly or indirectly contain all information which is necessary for the network operator to make relations through its administrative system to trace the equipment to its origin of production. TS 23.003 describes the structure of the IMEI in detail. The IMEI is complemented by a check digit. The check digit is not part of the digits transmitted at IMEI check occasions, as described below. The Check Digit shall avoid manual transmission errors, e.g. when customers register stolen MEs at the operators' customer care desk.

A network operator can make administrative use of the IMEI in the following manner:

It shall be possible to perform the IMEI check at any access attempt, except IMSI detach, and during an established call at any time when a dedicated radio resource is available, in accordance with the security policy of the PLMN operator. It shall also be possible to perform the IMEI check when a UE is IMS registered. The network shall terminate any access attempt or ongoing call when receiving indication from the EIR that the UE is on the prohibited list or the UE is not on the allowed list). An indication of "illegal ME" shall in these cases be given to the user. Furthermore this is equivalent to an authentication failure hence any call or IMS session establishment or any location updating is forbidden for the MS, it cannot answer to paging, it is just allowed to perform Emergency Calls. Emergency calls must never be terminated as a result of the IMEI check procedure.

Emergency calls can in some PLMNs be made without having to send the subscriber identity (IMSI) to the network. In this case the misuse of MS equipments after placing invalid emergency calls can be restrained by using the equipment identity. The network request for the equipment identity is sent to the MS after the emergency call has been set-up. The procedure is the same as for normal call set-up.

A Software Version Number (SVN) field shall be provided. This allows the ME manufacturer to identify different software versions of a given mobile. The SVN is a separate field from the IMEI, although it is associated with the IMEI. When the network requests the IMEI from the MS, it may also request that the SVN is also sent towards the network. The allowed list shall use the IMEI, The prohibited and tracked lists may also use the SVN.