What is IMEI or IMEI Number:

IMEI number is International Mobile Equipment Identity, is a unique 15 digits number which is assigned to a mobile device by the manufacture of device. Each sim is identified by an IMSI , the mobile device in which sim is inserted have IMEI number. While change of sim , only IMSI changes not the hardware information. The IMEI information of a device can be obtained from device information . E.g in setting of mobile device (Samsung Galaxy S8 Settings->About phone->Status->IMEIInformation) or by dialing a code (e.g *#06#). A mobile device can only work if it is having a valid IMEI number. The use of IMEI number is to allow , disallow in case of stolen devices or allow/disallow a device due to technical reasons. The IMEI should not changed once a device is manufactured.

IMEI Structure:

The structure is composed in such a way that it can be traced that who is the manufacture of the device.Structure having following sections

Type Allocation Code (TAC)

This is 8 digit code. The manufacture of device requests from international GSM standard body the TAC code for new model of device. TAC Codes for IMEI

Serial Number (SNR)

Each serial number uniquely identifies a device with in a TAC. This is a 6 digits in length.

Check Digit or imei number check

This digit is derived from other digits from a formula. To imei number check, this digit is used.

The integrity of IMEI  on network is check by Luhn Algorithm. The check digit should be calculated by receiving entity for verification. The imei number check is important , this ensures that IMEI number is valid and the device is not tempered.

IMEISV :

It is a 16 bits value assigned to a device. First 14 digits are same as IMEI. Last 2 digits are for software version. A device manufacture gets details from IMEI number. But for software updates propagates correctly they should know the current used version of software on device. Version information sets correct application update and firmware updates.

How lost Phone is traced:

After reporting of a lost mobile device. Its IMEI number is provided to all mobile operators in a country/area. Mobile operator put the device on monitoring. When mobile phone is switched on with new SIM card. Phone tries to register on the network. The vising network sends the IMSI and MSISDN information to the monitoring system. From MSISDN location and other activities (voice call, sms etc.) can be traced.

OTA updates for device setting:

A device gets updates from the manufactures most of the time. Device updates can be for firmware etc. From imei , the full device information ( type, brand , model etc), can obtained. For sending updates device information is important, so that device gets the correct updates. A mobile operator may send updates for device setting for the network information (IMSI, apn etc.). In case of changes required by home network operator in device setting , an OTA is send from home network to the device. If imei is know to the mobile operator, device get correct settings or updates. A roaming sim card have multiple IMSI, an OTA may be used to update the IMSI, based on the location a subscriber is roaming.

EIR :

Eir is the central database of mobile phones , which have imei number status along with imei. There may be a central eir in a country where all roaming mobile operators queries for legality of a device. When a call starts , serving msc sends a IMEI check operation to the EIR , and EIR responds back to the MSC. If response is successful then call is completed else call is dropped.

IMEI-Check over network:

In 2G or 3G network:

IMEI check is the process of finding whether a device is allowed to register for data or voice call. This is achieved by a gsm node call EIR or Equipment Identity Register. This is a central database in roaming network. Upon receiving a  check request , EIR may return with following. When a call is initiated, the MSC do ss7 signaling using gsm map protocol to check IMEI.
The roaming MSC or SGSN sends CHECK_IMEI to the EIR and EIR responds with one of following equipment status.

  • Unknown Equipment,When the device is not registered in EIR.
  • White List,When Equipment number is allowed to operate.
  • Blacklist, When Equipment  number is not allowed to operate.
  • Grey List, When Equipment number is allowed to operate with observation.

Device lock from a mobile operator:

There are phones which mobile operators provides along with their sim card. These phones works only when sim card from that mobile operator is used. This check is done by hardware information obtained from imei number. When a device is switch on and imsi in sim card is only allowed on a device then mobile registration fails.

In 4G network :

In 4G network the underlying protocol used is Diameter protocol.  The S13 and S13′ is  used in 4G network over diameter signaling. S13 is used between MME and EIR and S13′ is used between SGSN and EIR. The 3GPP application id is 16777252 . 3GPP Spec for S13/S13′
ME identity check procedure is used for identity verification.  ME-Identity-Check-Request/Answer (ECR/ECA),  Commands ior messages are used. When a mobile phone try to register on network then a ME identity check procedure is used between MME/SGSN and EIR.

< ME-Identity-Check-Request > ::= < Diameter Header: 324, REQ, PXY, 16777252 >
< Session-Id >
[ Vendor-Specific-Application-Id ]
{ Auth-Session-State }
{ Origin-Host }
{ Origin-Realm }
[ Destination-Host ]
{ Destination-Realm }
{ Terminal-Information }
[ User-Name ]
*[ AVP ]
*[ Proxy-Info ]
*[ Route-Record ]

The Terminal Information AVP have ME Identity , and User name (optional) have IMSI.

< ME-Identity-Check-Answer> ::= < Diameter Header: 324, PXY, 16777252 >
< Session-Id >
[ Vendor-Specific-Application-Id ]
[ Result-Code ]
[ Experimental-Result ]
{ Auth-Session-State }
{ Origin-Host }
{ Origin-Realm }
[ Equipment-Status ]
*[ AVP ]
*[ Failed-AVP ]
*[ Proxy-Info ]
*[ Route-Record ]

If result code received DIAMETER SUCCESS (2001) from EIR, then Equipment Status AVP have the status (white listed, black listed or grey listed) of ME identity status.

 

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