What is IMEI Number? Learn about IMEISV and IMEI Check?

The IMEI number is a globally unique identifier that is used by both the device manufacturer and network operators. This is useful for updating software, tacking, and blacklisting or whitelisting a phone.

What is SMS and how it works in telecom?

The IMEI number is a 15 digit unique identifier assigned to each device by its manufacturer. Its full form is International Mobile Equipment Identity. At any time a device manufacturer can obtain information about the device by looking at the IMEI number, such as the year, model number, and so on.

What is an IMEI Number
What is an IMEI Number

IMEI number and SIM card:

A cellphone is also an electronic device that has an IMEI number. A SIM card is required on the phone to access voice, data, and SMS services.

Each sim card in cellular communication is uniquely identified by an IMSI number, while the mobile device into which the sim is inserted is uniquely identified by an IMEI number.

When a mobile subscriber replaces a sim card, only the IMSI changes, the device’s IMEI remains unchanged.  In this tutorial, we will discuss the IMEI and how a cellular network uses this unique identity in 3G and 4G.

How to get IMEI information?

There are various ways to get IMI details. In a basic way, the number can be obtained from the device itself. E.g for a smartphone (Android), in the settings of the mobile phone,(Samsung Galaxy S8 Settings->About phone->Status->IMEI Information,) or by dialing a code (e.g *#06#). The code works on all devices.

Why IMEI is important?

A mobile device can only communicate with a mobile operator if it has a valid hardware identity. It is used to allow or disallow devices in the event of theft or for technical reasons. The information on the device is not supposed to change once it is embedded.

What is the structure of an IMEI number?

It is fifteen digit long value. The digits are segmented. Each group of digits in the IMEI structure represents unique information. The structure is constructed in such a way that the manufacturer of the device and other details can be determined by looking at the number.

IMEI Structure Format

The number is divided into the following parts:

What is Type Allocation Code (TAC code)?

This is a unique eight-digit code. The manufacturer of the device requests a TAC code for a new model of the device from the international GSM standard body. The TAC code is divided into two sub-sections, one is two digits long and another is six digits.  AA BBBBBB is the format,

  1. AA –  Allocation Body
  2. BBBBBB – Manufacture’s code e.g Apple Company for iPhone has its own code.

What is Serial Number (SNR)?

This is a six-digit number. Within a TAC, each serial number uniquely identifies a device.  The manufacturer controls the serial number range. When a new IMEI needs to assign a device, the manufacturer allocates and reserves a new serial number from the range. As an outcome, each device will get a unique identity globally.

What is the Check Digit for the IMEI number verification?

This is the final component of the IMEI with a single-digit value. A check digit is calculated from the values of other (TAC and SNR) digits using a formula. An IMEI number check process utilizes the check digit to ensure that the IMEI has not been altered in any way.

The Luhn Algorithm verifies the IMEI’s integrity on the network. The receiving entity recalculates the check digit and compares it against the check digit received.  The check is critical because it verifies that the IMEI number is valid and that the device has not been tampered with.

What is IMEIsv Meaning?

It is similar to the IMEI but contains additional information. With a 16-bit value, the first 14 digits are identical to those in the IMEI. The last two digits correspond to the software version. The first 14 digits contain information about the device, while the remaining two contain information about the software version.

How IMEIsv is useful over IMEI?

To function properly, a device requires regular updates from the manufacturer and other application providers. To ensure that software updates are applied correctly, one should be aware of the device’s current software version. Version information is required to set up the application and firmware accurately.

What are the possible uses for the IMEI number?

Tracing of a lost phone.

Following a report of a lost mobile device. The police provide the IMEI number to all mobile operators in a country/area in order to track the phone. Each mobile operator places the device under surveillance by using the IMEI number.

When a new or older SIM card is inserted into a mobile phone. The phone attempts to connect to the roaming network. The roaming network transmits IMEI, IMSI, and MSISDN information to the operator’s backend servers, to trace the location.

With MSISDN location and other activities (voice call, SMS, etc.) can be traced for a SIM card.

Over the Air(OTA) updates for a device:

Updates by manufacturer:

From time to time, a manufacturer may require to send firmware and other types of updates. While sending, device information is required to ensure the correct updates are received by the device. The IMEI is a unique identifier for each device, from which the sender gets the information (type, brand, model, and so on).

Updates by mobile network operator:

While roaming, the carrier may send an update to alter the phone’s settings. IMSI, APN, and other parameters can be modified as a part of the process. The most common example is an OTA update that allows users to switch the IMSI on a multi-IMSI sim card in order to save money on roaming charges.

An OTA may be triggered to update the IMSI based on the location of a subscriber roaming to ensure that the subscriber is using the most appropriate IMSI for that area. If the mobile operator is aware of the IMEI, the device receives the appropriate settings or updates.

How does a mobile operator allow and disallow a device?

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 query for the legality of a device. When a call starts, serving MSC or SGSN sends an IMEI check operation to the EIR, and EIR responds back to the MSC. If the response is successful then the call is completed else the 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 calls. This is achieved by using a gsm network node named  Equipment Identity Register (EIR). Represents is a central database of IMEIs in the roaming network. Upon receiving a check request, EIR may return any one of the following.

  • 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.

When a call is initiated, the MSC does the ss7 signaling using a gsm map protocol to check IMEI.

In the 4G network :

In a 4G network, the underlying protocol used is the Diameter protocol.  The S13 and S13′ are used in the 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, or messages are used. When a mobile phone tries to register on the 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 has ME Identity, and the User name (optional) has 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 the result code received DIAMETER SUCCESS (2001) from EIR, then Equipment Status AVP has the status (whitelisted, blacklisted, or greylisted) of ME identity status.

Device lock from a mobile operator:

There are phones which mobile operators provide along with their sim cards. These phones work only when a sim card from that mobile operator is used. This check is done by hardware information obtained from the IMEI number.  When a device switches on, the mobile network checks if the IMSI is the device that was given with the subscription.  Then the only the device is allowed to register on the network.

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