Transmission and Receiving Chain

In the last few posts i wrote about the Speech Transmission in GSM, later i was reading more on it then i found this brief from a book. Flow Diagram below shows the transmitting and receiving chain of a GSM receiver. Several successive operations have to be performed to convert a speech signal into a radio signal and back. 

The following operations take place on the transmitting side:

• Source coding: Converts the analogue speech signal into a digital equivalent.
• Channel coding: Adds extra bits to the data flow. This way redundancy is introduced into the data flow, increasing its rate by adding information calculated from the source data, in order to allow detection or even correction of bit errors that might be introduced during transmission. This is described in more detail below.
• Interleaving: Consists of mixing up the bits of the coded data blocks. The goal is to have adjacent bits in the modulated signal spread out over several data blocks. The error probability of successive bits in the modulated stream is typically highly correlated, and the channel coding performance is better when errors are decorrelated. Therefore, interleaving improves the coding performance by decorrelating errors and their position in the coded blocks.
• Ciphering: Modifies the contents of these blocks through a secret code known only by the mobile station and the base station.
• Burst formatting: Adds synchronisation and equalisation information to the ciphered data. Part of this is the addition of a training sequence.
• Modulation: Transforms the binary signal into an analogue signal at the right frequency. Thereby the signal can be transmitted as radio waves.

The receiver side performs the reverse operations as follows:

• Demodulation: Transforms the radio signal received at the antenna into a binary signal. Today most demodulators also deliver an estimated probability of correctness for each bit. This extra information is referred to as soft decision or soft information.
• Deciphering: Modifies the bits by reversing the ciphering code.
• Deinterleaving: Puts the bits of the different bursts back in order to rebuild the original code words.
• Channel decoding: Tries to reconstruct the source information from the output of the demodulator, using the added coding bits to detect or correct possible errors, caused between the coding and the decoding.
• Source decoding: Converts the digitally decoded source information into an analogue signal to produce the speech.

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Satya Sravan

Ciphering & Modulation - Speech Transmission in GSM

Ciphering 

Ciphering is used to protect signaling and user data from intruder. First of all, a ciphering key (Kc) is computed using the algorithm A8 stored on the SIM card, the subscriber key (Ki, also stored in the SIM card) and a random number (RAND) delivered by the network (this random number is the same as the one used for the authentication procedure). Secondly, a 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the TDMA frame number (Fn) number provided by the network. This bit sequence is then XORed (TRUE if only one of the inputs is TRUE) with the two 57 bit blocks of data included in a normal burst.

Modulation

The burst formatted bits are then modulated using GMSK which is nothing but 8PSK modulation technique and transmitted over a carrier. In GMSK the bit stream is first filtered using a Gaussian filter so as to remove the higher harmonics and round off the corners of the bit pulses. Then they are used for modulating the frequency of the carrier using MSK modulation.

That concludes one side of transmission; the decoding part happens in the other side. This is all together how speech transmission occurs in GSM

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Satya Sravan 

Burst Formatting - Speech Transmission in GSM

The GSM burst, or transmission can fulfill a variety of functions. Some GSM bursts are used for carrying data while others are used for control information.

Bursts and Frames

The information contained in one time slot on the TDMA frame is called a burst. The bit rate over the air interface is 270.8 kbps. This gives a bit time of 3.692 ms (48/13 ms). The time interval of TS thus corresponds to 156.25 bits. The physical content of TS is called a burst.

There are five different types of bursts:

• Normal Burst (NB): used to carry information on traffic and control channels.

• Frequency Correction Burst (FB): used for frequency synchronization of the mobile.

• Synchronization Burst (SB): used for frame synchronization of the mobile.

• Access Burst (AB): used for random access and handover access.

• Dummy Burst: used when no other type of burst is to be sent.

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Satya Sravan

Interleaving - Speech Transmission in GSM

Interleaving is a process of dispersing the bits of a data burst over multiple bursts in a systematic way. Benefit of this technique: when a data-burst is lost (due to burst error in the radio interface) it does not mean a 100% loss of a single burst rather a partial loss of many bursts

First level of interleaving

The channel coder provides 456 bits for every 20 ms of speech which are interleaved in eight blocks of 57 bits shown below.

In a normal burst, there is space for two of these speech blocks (Figure). Thus, if one burst transmission is lost, there is a 25% BER for the entire 20 ms of speech (2/8= 25%).

Second level of interleaving

If only one level of interleaving is used, a loss of this burst results in a total loss of 25%. This is too much for the channel decoder to correct. A second level of interleaving can be introduced to further reduce the possible BER to 12.5%.

Instead of sending two blocks of 57 bits from the same 20 ms of speech within one burst, a block from one 20 ms and a block from next sample of 20 ms are sent together. A delay is introduced in the system when the MS must wait for the next 20 ms of speech. However, the system can now afford to lose a whole burst, out of eight, as the loss is only 12.5% of the total bits from each 20ms speech frame. 12.5% is the maximum loss level that channel decoder can correct.

Speech Frame

The bits must then be sent over the air using a carrier frequency. GSM uses the GMSK modulation technique. The bits are modulated onto a carrier frequency and transmitted.

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Satya Sravan

Channel Coding - Speech Transmission in GSM

Channel coding in GSM uses the 260 bits from speech coding as input to channel coding and outputs 456 encoded bits.

The 260 bits are split according to their relative importance:

• Block 1: 50 very important bits
• Block 2: 132 important bits and
• Block 3: 78 not so important bits

The first block of 50 bits is sent through a block coder, which adds three parity bits that will result in 53 bits. These three bits are used to detect errors in a received message.

The 53 bits from first block, the 132 bits from the second block and 4 tail bits (total = 189) are sent to a 1:2 convolutional coder which outputs 378 bits. Bits added by the convolutional coder enable the correction of errors when the message is received.

The bits of block 3 are not protected.

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Satya Sravan

Speech Coding & Segmentation - Speech Transmission in GSM

In GSM, the speech coding process analyzes speech samples and outputs parameters of what the speech consists of the tone, length of tone, pitch, etc. This is then transmitted through the network to another MS, which generates the speech based on these parameters.

The process of segmentation and speech coding is explained in more detail as follows:

The human speech process starts in the vocal chords or speech organs, where a tone is generated. The mouth, tongue, teeth, etc. act as a filter, changing the nature of this tone. The aim of speech coding in GSM is to send only information about the original tone itself and about the filter.

Segmentation:- Given that the speech organs are relatively slow in adapting to changes, the filter parameters representing the speech organs are approximately constant during 20 ms. For this reason, when coding speech in GSM, a block of 20 ms is coded into one set of bits. In effect, it is similar to sampling speech at a rate of 50 times per second instead of the 8,000 used by A/D conversion.

Speech Coding:- Instead of using 13 bits per sample as in A/D conversion, GSM speech coding uses 260 bits. This calculates as 50 x 260 = 13 Kbits/s. This provides a speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones.

Many types of speech coders are available. Some offer better speech quality, at the expense of a higher bit rate (waveform coders). Others use lower bit rates, at the expense of lower speech quality (vocoders). The hybrid coder which GSM uses provides good speech quality with a relatively low bit rate, at the expense of speech coder complexity.

The GSM speech coder produces a bit rate of 13 kbits/s per subscriber. When it is considered that 8 subscribers use one radio channel, the overall bit rate would be 8 x 13 kbits/s = 104 kbits/s. This compares favorably with the 832 Kbits/s from A/D conversion.

However, speech coding does not consider the problems which may be encountered on the radio transmission path. The next stages in the transmission process, channel coding and interleaving, help to overcome these problems.

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Satya Sravan

Speech Transmission in GSM [Intro]

In the series of posts, i wish to write about how the actual speech that you talk gets transmitted at the other end. The whole process is shown in this illustration along with the rate at which the transfer occurs. 

The steps involved at one end are 

• Speed Coding and Segmentation
• Channel Coding
• Interleaving
• Burst Formatting 
• Ciphering 
• Modulation

And at the end the counterpart is gonna happen. I will discuss each step in separate post. 

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Satya Sravan

Simplex, Half Duplex & Full Duplex

I was reading a book named Fundamentals of Telecommunications by John G Proakis and thought to share about the modes of channel operation.

Broadly there are three modes
1. Simplex
2. Half Duplex
3. Full Duplex or simply Duplex

Each suits to a particular purpose.

Simplex 

• One-way operation; there is no reply channel provided. 
• Ex. Radio and television broadcasting

Half Duplex 

• Two-way service. 
• Defined as transmission over a circuit capable of transmitting in either direction, but only in one direction at a time.
• Internet browsing is the best example where we send request first and then the content is loaded

Full Duplex/Duplex 

• Two-way service
• Defines simultaneous two-way independent transmission on a circuit in both directions. 
• Mobile communication is full duplex connection

An add-on illustration that i found somewhere on internet

GIF images credits - http://www.iec-usa.com/Browse05/DTHFDUP.html

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Satya Sravan

GPRS EDGE 3G HSPA HSPA+ LTE

We often get different kinds of indicators in the status bar (on top) when you browse data through your mobile internet. I listed down the different possible types of indicators.

Starting from first-born to recent trending service 

GSM 

• Global System for Mobile Communications
• 1G, first generation of mobiles
• Primarily used for voice but can be used for accessing internet via GPRS
• Majority of networks work on 900 MHz and 1800 MHz 

G

• Network Type – GPRS (General Packet Radio Service)
• 2G, second generation of mobiles
• We can get maximum speed of 160 Kbps
• It is slow but doesn't consume much of battery

E

• Network Type – EDGE (Enhanced Data Rates for GSM Evolution)
• It is developed based on GPRS (coding schemes are changed i.e., CS1 to CS4 = MCS1 to MCS9) but it was not famous due to the modulation scheme that was used (8-PSK instead of GMSK) 
• We can get maximum speed of 473.6 Kbps which is more similar to that we get in 3G

3G 

• Network Type – UMTS (Universal Mobile Telecommunications System)
• 3G, third generation of mobiles came into existence mainly for the purpose of video calling
• Can get max speed similar to that in EDGE

H

• Network Type – HSPA (High Speed Packet Access) 
• Precisely HSDPA (downlink only)
• Developed on basis of 3G network and can support up to 7.2 Mbps
• HSUPA (uplink only) derives speed up to 5 Mbps 

H+

• Network Type – HSPAP (HSPA Plus/ HSPA+)
• Evolution of HSPA that allows speed up to 21 Mbps 
• It is start to the fourth generation network, 4G devices are compatible to use H+

4G LTE

• Network Type – LTE (Long Term Evolution)
• It is an upgrade to HSPAP (P for Plus), starting phase of 4G technology. 
• It is mostly observed in foreign countries and in limited cities in India. 
• With LTE, we can get speeds varying from 100 Mbps to 1 Gbps. But in reality we acquire speed in between 50 Mbps to 100 Mbps (that is a lot what else do u expect). 
• For LTE, devices has to be compatible and changes has to be made in the network setup

With the increasing speeds day by day reaching to the max speed that LAN can provide, we can’t predict what’s coming next. Newer technologies LTE-Advanced, Mobile WiMax are arising. 

To conclude I wish to recommend using the data speed that serves your purpose well in order to get better battery life and also cost effective. 

P.S. Getting 'G' become difficult, collected '4G LTE' from my friend at San Jose

Addon - Check out the list of data bit rates here

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Satya Sravan

Rx Level - Why is it negative ??

Before i tell you why Rx level is negative, you must know about 'dBm'

dbm : decibel-milliwatt

• It is measured power referenced to one milliwatt (mW).
• Its value is 10 * log[ P(W) / 1mW ]

Rx level is the strength of the signal that our mobile receives from a BTS (mobile tower). Here Rx level stands for Receiver Level

Mathematical formula to calculate the Rx level is 

RxLev (dBm) = EIRP (dBm) - Path Loss (dB)

EIRP - Equivalent/Effective Isotropically Radiated Power is the amount of power that an antenna which evenly distributes power in all directions would emit to produce the peak power density observed in the direction of maximum antenna gain.

EIRP (dBm) = Pt (dBm) + Ga (dBi)

i - antenna gain in reference to isotropic antenna

t - output power transmitted 

Path Loss - For a given antenna, the received power is inversely proportional to distance between the transmit and receive antennas & also to the square of frequency.

Ls ~ d^2 f^2

Path loss - Ls(dB) = 32.4(dB) + 20*log [ f (MHz) ] + 20*log [ d (km)]

where 32.4 is a constant of proportionality

The value of Path loss is always greater than EIRP, hence Rx level is negative

Example:

If we take power as 2 W i.e., maximum output from a UMTS/3G mobile phone of Power class 1

Pt will be 33 dbm (calculated using dbm formula).

Let gain Ga be 9 dBi => EIRP = 33 + 9 = 42 dBm

Frequency be 1800 MHz; Distance be 2 Km

Path loss = 32.4 + 65.1 + 6 = 103.5 (calculated using path loss formula)

Rx level = 42 - 103.5 = - 62 dBm ** negative value **

RX level Range 

In few websites, i found Rx level mentioned in a range of 0 to 63 (positive value). Just subtract it from 110 dBm to get the value of Rx level. 0 to 63 equals  -110 dBm to -47 dBm.

• Upto - 65 dbm : Excellent  
• -65 to -75         : Good 
• -75 to -95         : Normal
• < -95                : Weak 

Addon -  Check the table in the Unit conversions of this link

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Satya Sravan

GPRS Coding Schemes

In the present data speeds (3G) discussing about GPRS sounds low, but i wish to write about the four coding schemes that are used in GPRS

The channel encoding process in GPRS consists of two steps: first, a cyclic code is used to add parity bits, followed by coding with a possibly punctured convolution code. Here, the Coding Schemes CS-1 to CS-4 specifies the number of parity bits generated by the cyclic code and the puncturing rate of the convolution code.

In Coding Schemes CS-1 through CS-3, the convolution code is of rate 1/2, i.e. each input bit is converted into two coded bits. In Coding Schemes CS-2 and CS-3, the output of the convolution code is punctured to achieve the desired code rate i.e., 1/2. In Coding Scheme CS-4, no convolution coding is applied. Modulation technique used in these coding schemes is GMSK. 

  

Overview of all the four coding schemes

Channel Coding Scheme	CS-1	CS-2	CS-3	CS-4
Pre-cod. USF	3	6	6	12
Info bits without USF	181	268	312	428
Parity bits BC	40	16	16	16
Tail bits	4	4	4	-
Output convolution encoder	456	588	676	456
Punctured bits	0	132	220	-
Code rate	1/2	~2/3	~3/4	1
Data rate kbit/s	9.05	13.4	15.6	21.4
Maximum data speed with 8 time-slots	72.4 kb/s	107.2 kb/s	124.8 kb/s	171.2 kb/s

Utilization

• CS - 4 which is fastest is used near a BTS while CS - 1 is used when the MS is far away from BTS. 
• CS - 4 can achieve a user speed of 20.0 Kbit/s per time slot, but has 25% of normal cell coverage whereas CS - 1 gives 98% of normal cell coverage with 8 kbps speed.

Which coding scheme to choose ?

• The choice of coding scheme depends on the condition of the channel provided by the mobile network.
• If the channel is very noisy, the network may use CS-1 with only 8 kbps speed to ensure higher reliability while if the channel is providing a good condition, the network could use CS-3 or CS-4 to obtain optimum speed.

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Satya Sravan

GSM Security - Authentication & Confidentiality

In general, the two common weakness in a mobile network are false usage of service and interception of voice and data. This post briefs you how secure is your mobile network & what plays major role in making it so.

Before i write, i will introduce to the terms that are needed to known

Ki, RAND, SRES, Kc

Ki

• Authentication/Secret Key
• It is stored in SIM and AuC 
• It is based on IMSI
• 128 bit

RAND

• Random Number
• It is generated by AuC
• 128 bit

SRES

• Signed Response
• calculated at AuC and SIM
• generated from RAND and Ki using A3 algorithm
• 32 bit

Kc

• Cipher/Session Key
• generated from RAND and Ki using A8 algorithm
• 64 bit

RAND, SRES & Kc are known to be triplets

A3, A5 & A8 Security Algorithms

• A3
• authentication algorithm
• used to generate SRES 
• operator specific
• stored in SIM and AuC of HLR
• A8
• key generation algorithm
• used to generate Kc
• operator specific
• stored in SIM and AuC of HLR
• A5
• ciphering/encryption algorithm
• stored in the mobile device itself
• common to all network providers
• defined for data encryption and decryption over air interface
• types - A5/0(no encryption), A5/1 & A5/2

AuC - Authentication Center

HLR - Home Location Register

MS - Mobile Station - Mobile phone + SIM

VLR - Visitor Location Register

BSS - Base Station Subsystem - BTS + BSC 

BTS -  Base Trans-receiver Station

BSC - Base Station Controller

MSC - Mobile Switching Center

Authentication

They say picture speaks it better, here this flowchart & illustration will describe how authentication procedure is done.

Sending SRES through air interface between MS to BSS is not encrypted. Encryption is not necessary too because Ki is never transmitted over the radio channel, two copies of the value are stored in SIM and AuC. Moreover if eavesdroppers attack using the SRES & RAND, they will get infinite combinations of RAND for a particular noted SRES.

Authentication is referred to as challenge-response process.

Authentication process takes place for the first time when a subscriber attempts to make a call or location update. Later, it may not be necessary as the data generated earlier is available for a certain time span.

Now what about the cipher key Kc that is generated through A8 algorithm ??

Confidentiality

Encryption and decryption of voice and data between the MS and the network is accomplished by the use of the A5 algorithm, Kc and the GSM hyper-frame number.

In order to discuss further, a little background is necessary. 

GSM uses time division multiplexing i.e., each subscriber takes turns to use the common radio channel for sending and receiving information only during one of the eight available time slots. (a channel can be shared by 8 users). Each time slot lasts for only 4.6 milliseconds and is identified by a frame number. A GSM conversation uses two such frames, one BSS to MS and another from MS back to BSS. Each of these frames contains 114 bits of user information. So, every 4.6 milliseconds, MS receives 114 bits of information from BSS and transmits another 114 bits to BSS. It is these 228 bits that require encryption to protect them from getting attacked.

Using the RAND and Ki, the SIM produces Kc through A8 algorithm. This Kc together with the current frame number generates a key stream of 228 bits through A5 algorithm. These 228 bits are transferred on air interface implying increased confidentiality.

Notes:

• A5 generates new 228 bits every 4.6 milliseconds
• COMP 128 is combined A3/A8 algorithm which is used.

We reviewed about the data and signalling confidentiality in the above discussion but what about the subscriber data confidentiality ? To ensure this, the Temporary Mobile Subscriber Identity (TMSI) is used. It is a temporary number that is MS after the authentication and encryption procedures have taken place. It is allocated after certain intervals to make it more secured.

Here i discussed about GSM security, for GPRS there is no much difference; you can read this link for reference. 

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Satya Sravan

APN - Access Point Name

• It is a setting that enables you to access data (internet service) on your mobile [common man meaning]
• It identifies an access point on the far side of a GGSN, it acts as a reference to GGSN [technical meaning]

In addition to identifying a Packet Data Network, an APN may also be used to define the type of service (e.g. connection to WAP server, MMS) that is provided by the PDN

An APN resembles to a DNS name of a GGSN and has a maximum length of 100 octets. An APN consists of one more labels. 

Structure of APN

Network Identifier 

• Defines to which external network the GGSN is connected to. [required field]
• It will contain atleast one label and have a maximum length of 63 octets. 
• It will not start with the strings 'rac' 'lac' or 'sgsn'; not end in '.gprs' and not take the value '*'.

Operator Identifier 

• Defines in which PLMN GPRS backbone the GGSN is located
• It is composed of three labels. The last label be 'gprs'. The first two labels together identify the GPRS PLMN. 
• For each service provider (operator), there is a default Operator Identifier that is derived from IMSI 
• mnc<MNC>.mcc<MCC>.gprs
• 
  

NOTE - To support inter-PLMN roaming, the internal GPRS DNS functionality is used to translate the APN into the IP address of the GGSN.

How to configure APN manually ?? [DIY]

Go to Settings --> Wireless & Networks --> Mobile Networks --> Access Point Names --> Create New APN and Edit the fields you get from the service provider

Look !! how i did in my mobile

** You just need to set Name and APN fields only **

• Proxy, Port, Username, Password & Server not required to be set
• MMSC, MMS Proxy and MMS Port are related to MMS functionality
• MCC & MNC will already be set when you create a new APN (depending on your operator)
• Authentication Type is not needed to be set, but you can check about the two types here
• APN Type can be set as default or not needed to be set
• APN & APN roaming protocol is also set as IPv4 once you create a new APN
• Bearer field must by set Unspecified
• MVNO - Mobile Virtual Network Operator 
• It is the network operator who doesn't have the frequency spectrum or infrastructure to provide internet service. 
• Three types are SPN (Service Provider Name); IMSI (International Mobile Subscriber Identity & GID (Group Identifier).
• So, MVNO type and value are not needed to be set. You can try if you wish to know the SPN, IMSI and GID of your mobile

After you set the APN, just restart your phone and surf internet.

OR

Call your operator's customer care and tell him to send internet settings for your phone. Once received, just save it and make it as default settings

Example:

internet.mnc852.mcc405.gprs ; Set APN as internet and Name as specified by your operator

** Now-a-days, service providers are using star APN or default APN, you must save the settings sent by them. Rest you can make any changes in name or other fields but data surfing wont get affected **

Add-ons

• Watch this 2 minute video if you face any difficulty in setting APN
• List of APNs of leading service operators in India - Check this link
• Find APN list for US, Canada, UK & Australia here

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Satya Sravan

All about IMEI

International Mobile Station Equipment Identity

(Here Mobile Station means the handset + SIM) 

• It is a unique number to identify mobile phones.
• mobile phones  i.e.,  3GPP (i.e., GSM, UMTS and LTE) and iDEN mobile phones, as well as some satellite phones.
• One can find the IMEI number in different ways, this link here provides you with the ways to find the IMEI number for different handsets. (I dialed *#06# and checked) 
• It is related to the device only, not to the subscriber who is using it. 
• These IMEIs are allocated by BABT
• It includes information on the origin, model and serial number of the device (note - this serial no. is different to the one which we get while we buy a phone)

Structure of IMEI Number

AA	-	BB	BB	BB	-	CC	CC	CC	D or EE

AA - BB BB BB : TAC (Type Allocation Code) 

• These represent the origin and the particular model of the device
• The first two digits are the RBI and the rest makes up the allocation number
• Reporting Body Identifier - indicates the GSMA-approved organization that allocated TAC 
• find the list of RBIs in the Annex A (page 19) of the doc stated at the end
• Earlier TAC used to be only six digits followed by FAC
• Final Assembly Code 
• was a manufacturer-specific code indicating the location of the device's construction (prior 2002) 
• was 00 (January 1, 2003 until that April 1, 2004 = transition period)
• are terminated to make TAC a 8 digit code (after April 1, 2004)
• Go through few TAC examples given here

CC CC CC : SNR

• Serial Number that distinctively identifies the unit of a model 

D or EE : Luhn check digit / checksum

• For the meaning of checksum, follow this link
• used to validate the IMEI number. 

Luhn Algorithm:

• It is used to validate a variety of identification numbers, such as bank account numbers, credit and debit card numbers, IMEI numbers etc. 
• Steps to calculate the number
• Moving from rightmost digit to leftmost, double the value of every second digit; if the product of this doubling operation is greater than 9 (e.g., 6 × 3 = 18), then sum the digits of the products (i.e., 1 + 8 = 9)
• Compute the sum of the digits
• Take the units digit
• Subtracting it from 10 results in the check digit.
• The above mentioned method is the easiest way, one can also verify the checksum by checking whether the total modulo 10 of the sum of the digits is equal to 0 or not. 

EE represents the Software Version Number identifying the revision of the software installed on the phone. The number sounds IMEISV in this case.

Theory done !! Practical part now !! 

I dialed *#06# on my phone

I will take the first one

352251065012588
TAC	35 - 22 51 06
RBI	35
SNR	50 12 58
Checksum	8

  

Now the verification of the checksum (Luhn algorithm)

Taking the first 14 digits = 3522510651258

3	5	2	2	5	1	0	6	5	0	1	2	5	8
3	10	2	4	5	2	0	12	5	0	1	4	5	16
3	1	2	4	5	2	0	3	5	0	1	4	5	7

The sum of digits equals 3 + 1 + 2 + 4 + 5 + 2 + 0 + 3 + 5 + 0 + 1 + 4 + 5 + 7 = 42

Unit digit number - 2 

So, the check digit is 10 - 2 = 8 (here it is) 

*** Interesting thing ***

Know the quality of your phone using the IMEI number

• Look at 7th and 8th digit, these will tell your handset quality
• Check this table

7th & 8th Digit	Manufactured at	Quality
00	Original Factory	Best
01/10	Finland	High
13	Azerbaijan	Too bad
02/20	UAE	Very Poor
03/30 & 04/40	China	Better
05/50	Brazil/Finland/USA	Good
06/60	China/Hong Kong/Mexico	OK
08/80	Germany	Fair

I am not sure about how much true it is but found this information in many websites so i included it.

Blocking IMEI Number

People ask the operator to block the SIM card once their mobile is stolen. What do they do exactly is blacklist the IMEI number. Before i write further i wish to say one thing i.e., IMEI is handset specific it becomes active once SIM is inserted.

Blocking of IMEI number makes the handset unfit to use particular operator's SIM which means one cannot put the same operator's SIM, they can try using other operator's SIM card in same state or other state also.

So when a person lodges a complaint that his mobile is stolen, what police does is that he provides the IMEI number to all the service providers throughout the nation and tell to check to which VLR is it latched to, from this he can identify under which BTS he is and go and catch him. It is also used to trace a particular MS.

NOTE: For CDMA handsets the counterpart is MEID

That is it, it sums up all the info i gathered about IMEI. Please refer this doc to read more

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Satya Sravan