13 IJAEST Volume No 2 Design and Development of Micro Controller Based SMS Gateway for GSM Mobile...

Design and Development of Microcontroller Based

SMS Gateway for GSM Mobile

Susmita Ghose,Md. Shafiqur Rahman, Dilruba Sharmin,Istiak Hussain,T.K.Yousufzai

Dept.of Applied Physics Electronics & Communication Engineering

University of Dhaka

Bangladesh, Dhaka

Email:shafiqrahman50&yahoo.com

Abstract--In this work, a microcontroller based SMS gateway

for GSM mobile has been designed and developed. Most of the

SMS gateway system was controlled by PC based software

where microcontroller only used for controlling and sending

status of devices or any appliances connected with the system.

An Ericsson T68i, one of the cheapest GSM mobile phone sets

available with most of the advanced features, has been

interfaced with a PC via RS232 serial port. The SMS packet

has been analyzed and its different fields have been identified

for the Grameen Phone, the largest GSM operator in

Bangladesh. Then the PC has been removed from the system

and the transmission and reception technique of SMS have

been implemented into the PIC microcontroller. The

developed system has been tested successfully. The system is

also simple, smarter, portable, cost effective (as the PC has

been removed) and low power consuming.

Index term—Microcontroller, SMS, LCD, PICmicro

I. INTRODUCTION

People now in the age of modern science need the real-

time information whenever they desire. And this can be

achieved by the various technological advancement of

communication system. Introduction of GSM mobile phone

is one of them which are no longer a luxurious item, easily

available, accessible, portable, cost-effective and have

device availability throughout the country and the world

even. So, the idea of introducing SMS should be an

efficient real-time approach in any kind of appliance

controlling.

SMS encoding and decoding for sending and receiving

in mobile communication is usually done by PC based

software where PC is used as an SMS gateway [1]. In that

case microcontroller IC plays an important role in

controlling any devices or appliances. Here microcontroller

also gets instruction through SMS and then goes for the

next step. The overall system then becomes both costly and

power consuming as the PC works round the clock. If that

PC is a clone then may not run for all the time and power is

a matter of availability and cost also.

The serial communication between PC and Mobile uses

RS232 protocol. Data in digital form i.e. „1‟ and „0‟ for

RS232 port are represented by the voltage level of 3V to

25V and -3V to -25V respectively. But Mobile phones in

general use only 3V or 1.8V for its internal communication.

Therefore, to make communication between the GSM

Mobile and PC a voltage conversion device called

MAX232 plays an important role here [2].

In most of the centralized device-controlling system, PC

is interfaced to mobile and then used as a server to control

the devices through the developed software. PC-based-

software is easy to develop and control, so people usually

depends on PC. PIC Single-Chip-Microcontrollers are

sufficient enough to encode and decode the sending and

receiving message through GSM mobile and control the

devices according to the instruction given by the SMS. PIC

microcontroller can also be used as a web-server by

removing the mobile device at the end used part so that user

can also control the appliances through the Internet. The

PIC are also less-expensive as well as they need not much

energy and have a very good sleep mode. The development

tools (simulator, assembler, linker compiler) for PIC are

also very good, available for free and can be downloaded

from Microchip [3].

(a)

Data Cable MAX232

USB Serial

Susmita Ghose et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 090 - 098

ISSN: 2230-7818 @ 2011 http://www.ijaest.iserp.org. All rights Reserved. Page 90

IJAEST

(b)

Figure 1: Block diagram of (a) PC based (b) microcontroller based SMS Gateway

But the resources, documentation and procedure are not

available to interface the PIC with GSM mobile. Therefore

in this present work, an attempt has been taken to design a

PIC microcontroller based SMS gateway for GSM mobile

system in a well-organized and systematic way so that any

one can develop the interfacing part and design the PIC for

SMS gateway to control any appliances or for any other

security purpose. A system has been developed and tested

as a prototype for either remote or accessible appliance

controlling. Figure 1 shows the block diagram of (b) the

developed PIC based SMS Gateway which replaces the

existing (a) PC based SMS Gateway system for GSM

Mobile.

II. SYSTEM ARCHITECTURE, INTERFACING

& HARDWARE CONFIGURATION

This system runs on a 28 pins flash-based 8-bit CMOS

microcontroller, PIC 16F876 with a 4 MHz external clock.

The chip has up to 8K x 14 words of FLASH Program

Memory, 368 x 8 bytes of Data Memory (RAM) and 256 x

8 bytes of EEPROM Data Memory. It has more I/O

capabilities than the other PIC of this series and also has

two PWM which are useful to control any kind of motors

speed [3].

The serial RS232 connection is driven by the PIC

USART with a high data transmission rate. Data in the

digital form i.e. „1‟ and „0‟ for PIC USART are represented

by the voltage level of 5V and 0V respectively. Therefore,

to make communication between the GSM Mobile and

Microcontroller, no voltage conversion device is needed.

A. RS232 serial communication

RS232 is the most known serial port used to interface

and transmit the desired data in communication. Though

the serial port is hard enough to program than the parallel

port, this is the most effective method in which the data

transmission requires less wire (only three links – transmit,

receive and common ground) that yields the low system

cost. The two pins TxD & RxD are used for transmit and

receive data between the communication devices. There are

some other lines in this port which are set as default [4].

The data format frame for both the PIC and RS232

protocol has been shown in figure 2. For checking the

RS232 communication between microcontroller and mobile

phone, another PC with running hyper terminal (the Micro

Soft standard terminal program) has been used and Mobile

has been connected to the GND, RxD and TxD to the serial

port of that PC via a MAX232 voltage conversion IC.

(a)

(b)

Figure 2: Format of data frame for serial communication (a) for PIC (b) for RS-232 I/O.

B. PIC USART Configuration

USART, Universal Synchronous Asynchronous Receiver

Transmitter, also known as Serial Communications

Interface can be configured into two operating mode called

synchronous and asynchronous. In this present work the

later one has been used which is accessed through the pins

RC6 and RC7 for the PIC 16F876 and RC6 & RC7 act as

data transmitter (Tx) and receiver (Rx) respectively

[3].Data is usually transmitted in 8-bit words (9 is an

option), with the least significant bit sent first. Standard

clock (baud) rate is used so that the receiver can sample the

Data cable

Serial



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Input at the same rate as the data is sent. 9600 baud is used

in this work, i.e., the bits are transmitted at about 10kbps.

In this mode, the USART uses standard non-return-to

zero (NRZ) format (one START bit, eight or nine data bits,

and one STOP bit). The most common data format is 8-bits.

An on-chip, dedicated, 8-bit baud rate generator can be

used to derive standard baud rate frequencies from the

oscillator. The transmitter and receiver are functionally

independent, but use the same data format and baud rate.

To configure a USART of the PIC; INTCON, PIR1,

RCSTA, RCREG, PIE1, TXSTA, TXREG and SPBRG

registers are needed to configure.

C. Transmitter Configuration

The data is transmitted through the RC6/TX/CK pin from

the transmit shift register (TSR) which obtains its data from

the read/write transmit buffer, TXREG. The TXREG

register is loaded with data in software. The TSR register is

not loaded until the STOP bit has been transmitted from the

previous load. As soon as the STOP bit is transmitted, the

TSR is loaded with new data from the TXREG register (if

available). Once the TXREG register transfers the data to

the TSR register, the TXREG register is empty and flag bit

TXIF (PIR1<4>) is

D. Transmitter Configuration

The data is transmitted through the RC6/TX/CK pin

from the transmit shift register (TSR) which obtains its data

from the read/write transmit buffer, TXREG. The TXREG

register is loaded with data in software. The TSR register is

not loaded until the STOP bit has been transmitted from the

previous load. As soon as the STOP bit is transmitted, the




TXIF (PIR1<4>) is




TXIF (PIR1<4>) is set. This interrupt can be

enabled/disabled by setting/clearing enable bit TXIE

(PIE1<4>). Flag bit TXIF will be set, regardless of the state

of enable bit TXIE and cannot be cleared in software. It

will reset only when new data is loaded into the TXREG

register. While flag bit TXIF indicates the status of the

TXREG register, another bit TRMT (TXSTA<1>) shows

the status of the TSR register. Status bit TRMT is a read

only bit, which is set when the TSR register is empty [3].

Transmission is enabled by setting enable bit TXEN

(TXSTA<5>). The transmission can be started by first

loading the TXREG register and then setting enable bit

TXEN. Normally, when transmission is first started, the

TSR register is empty. At that point, transfer to the TXREG

register will result in an immediate transfer to TSR,

resulting in an empty TXREG. A back-to-back transfer is

thus possible. CLRC bit (TXSTA<7>) is ignored in

asynchronous mode. The baud rate generator produces a

clock, either x16 or x64 of the bit shift rate, depending on

bit BRGH (TXSTA<2>).If clock frequency is 4MHz &

BRGH is set and then generated baud rate is less deviated

from the required data rate. Parity is not supported by the

hardware, but can be implemented in software (and stored

as the ninth data bit) [3].

E. Receiver Configuration

The data is received on the RC7/RX/DT pin and drives

the data recovery block. Once Asynchronous mode is

selected, reception is enabled by setting bit SPEN

(RCSTA<7>) & CREN (RCSTA<4>). After sampling the

STOP bit, the received data in the receive shift register

(RSR) is transferred to the RCREG register (if it is empty).

If the transfer is complete, flag bit RCIF (PIR1<5>) is set.

The actual interrupt can be enabled/disabled by

setting/clearing enable bit RCIE (PIE1<5>). Flag bit RCIF

is a read only bit, which is cleared by the hardware. It is

cleared when the RCREG register has been read and is

empty. The RCREG is a double buffered register (i.e., it is

a two deep FIFO). It is possible for two bytes of data to be

received and transferred to the RCREG FIFO and a third

byte to begin shifting to the RSR register. On the detection

of the STOP bit of the third byte, if the RCREG register is

still full, the overrun error bit OERR (RCSTA<1>) will be

set. The word in the RSR will be lost. The RCREG register

can be read twice to retrieve the two bytes in the FIFO.

Then the Overrun bit OERR has to be cleared in software.

This is done by resetting the receive logic (CREN is cleared

and then set). If bit OERR is set, transfers from the RSR

register to the RCREG register are inhibited, and no further

data will be received. It is, therefore, essential to clear error

bit OERR if it is set. Framing error bit FERR (RCSTA<2>)

is set if a STOP bit is detected as clear [3]

F. LCD interfacing

The display is a standard LM020a which displays 4 lines

of 20 characters (20×4). Each character is of 5×10 pixels.

The display receives ASCII codes for each character at the

data inputs (D0–D7). The data is presented to the display

inputs by the MCU, and latched in by the pulsation of the E

(Enable) input. The RW (Read/Write) line can be tied low

(write mode), as the LCD is receiving data only. The RS

(Register Select) input allows commands to be sent to the

display. RS = 0 selects command mode, RS = 1 data mode.

The display itself contains a microcontroller; the standard

chip in this type of display is the Hitachi HD44780. It must

be initialized according to the data and display options

required. More details can be found in the web-site [5].



IJAEST

Figure 3: Schematic diagram of SMS Gateway System

G. LCD interfacing

The display is a standard LM020a which displays 4 lines

of 20 characters (20×4). Each character is of 5×10 pixels.

The display receives ASCII codes for each character at the

data inputs (D0–D7). The data is presented to the display

inputs by the MCU, and latched in by the pulsation of the E

(Enable) input. The RW (Read/Write) line can be tied low

(write mode), as the LCD is receiving data only. The RS

(Register Select) input allows commands to be sent to the

display. RS = 0 selects command mode, RS = 1 data mode.

The display itself contains a microcontroller; the standard

chip in this type of display is the Hitachi HD44780. It must

be initialized according to the data and display options

required. More details can be found in the web-site [5].

III. MICROCONTROLLER SMS GATEWAY

Modern mobile phones are able to send & receive SMS

with appropriate AT command originated from the

microcontroller. The microcontroller circuit is used to

control and interface hardware devices and the SMS is

generated, received, decoded and displayed through it. The

complete system for SMS Gateway can be setup for many

applications. Some of the examples are Smart Home

System and Remote Data Collection System. The SMS

Gateway main program is written using PIC Assembly

Language which is assembled using MPLAB 7.60 [3]. The

main program communicates to mobile equipment (ME)

via GSM 07.07 protocol [6] is applied to send and receive

SMS. Typical ME can be a mobile phone or a GSM modem

with the capability to interface with PIC. In this work, a

low cost mobile phone, Ericsson T68i has been connected

to the PIC RS232 serial port via its data cable and a

MAX232 voltage converter IC has been used to have an

interface with RS232 serial port of PC for the checking

purpose. The system is initiated by applying a start pulse.

An SMS is automatically generated by the system which

forwards to a default number. The main program

continuously looks for the arrival of any SMS that is

needed to process. The received SMS is decoded and also

displayed to the LCD panel.

IV. ENCODING DECODING TECHNIQUE OF

SMS PACKET

The SMS message can be up to 160 characters long,

where each character is 7 bits according to the 7-bit default

alphabet. There are two ways of sending and receiving

SMS messages: Text mode and PDU (protocol description

unit) mode. As text mode is unavailable on some phones,

the PDU mode is used in this work. The PDU string

contains not only the message, but also a lot of meta-

information about the sender, SMS service centre, the time

stamp etc. It is all in the form of hexa-decimal octets or

decimal semi-octets [7, 8]. Figure 4 shows details data

format, frame and instruction uses within an SMS packet.



IJAEST

Figure 4: Data format and frame of an SMS packet

All the octets in the format as shown in figure 4 are

hexa-decimal 8-bit octets, except the Service centre

number, the sender number and the timestamp; they are

decimal semi-octets. The message part in the end of the

PDU string consists of hexa-decimal 8-bit octets, but these

octets represent 7-bit data. The semi-octets are decimal, and

e.g. the sender number is obtained by performing internal

swapping within the semi-octets from "881007000006F0"

to "8801700000600F". The length of the phone number is

odd, so a proper octet sequence cannot be formed by this

number. This is the reason why the trailing F has been

added. The time stamp, when parsed, equals

"80506202913242", where the first six bytes represent date,

the following six bytes represent time, and the last two

represent time-zone related to GMT.

Following table1 shows an example of the technique for

encoding and decoding of user data in TPDU part. The

message "Testing SMS" consists of 11 characters, called

septets when represented by 7 bits each. These septets need

to be transformed into octets for the SMS transfer.

TABLE1 ENCODING AND DECODING TECHNIQUE OF USER DATA IN TPDU PART

Uncompressed message Compressed message

Byte No. ASCII DEC BINARY

(Septets)

BINARY

(Octets)

HEX

1 “T” 84 1010100 1010100 11010100 D4

2 “e” 101 1100101 1100101 11110010 F2

3 “s” 115 1110011 1110011 10011100 9C

4 “t” 116 1110100 1110100 10011110 9E

5 “i” 105 1101001 1101001 01110110 76

6 “n” 110 1101110 1101110 10011111 9F

7 “g” 103 1100111 1100111 01000001 41

8 “ ” 32 0100000 0100000 11010011 D3

9 “S” 83 1010011 1010011 11100110 E6

10 “M” 77 1001101 1001101 10100 14

11 “S” 83 1010011 1010011

The first septet (T) is turned into an octet by adding the

rightmost bit of the second septet. This bit is inserted to the

left which yields 1 + 1010100 = 11010100 ("D4"). The

rightmost bit of the second character is then consumed, so

the second character (septet) needs two bits (underlined

bold) of the third character to make an 8bit octet. This

process goes on and on yielding the following octets:

The 10 octets from "Testing SMS" are D4 F2 9C 9E 76 9F

41 D3 E6 14

V. MESSAGE SENDING & RECEIVING

The AT (Attention) commands are the basic commands that

communicate with the GSM mobile phone. Table2

indicates some common AT commands necessary for SMS

transmission and reception. Detailed format of AT

SMS packet

The SMSC Part TPDU Part

08 91 88 10 07 00 00 06 F0

88 01 70 00 00 60 0F

8801700000600

11

Service Centre

Number

Length of SMSC

Information

Type of Address

of the SMSC

40 0D 91 88 10 17 43 88 39 F0 88 01 71 34 88 93 0F 8801713488930

00 00 A7 0B D4F29C9E769F41D3E614

Testing SMS

User data

length

First Octet

PDU

Address Length of

the destination

MSG Referen

ce

Destination

Number

Type of Address of the

destination

PID DCS Validity Period

User Data



IJAEST

commands are available in mobile phone, T68i developers‟

guidelines [9].

Table 2

SOME COMMON AT COMMANDS FOR SMS TRANSMISSION AND RECEPTION

Command Description

AT(Attention Command) Checks the communication between the phone and any accessory.

AT+CPMS (Preferred Message Storage) Selects memory storage spaces to be used for reading, writing, etc.

AT+CNMI (New Message Indication to

TE)

Selects the procedure how the reception of new messages from the network is indicated to the

TE when TE is active.

AT+CMGR (Read Message) Returns messages with location value <index> from preferred message storage <mem1> to the

TE. If the status of the message is „received unread‟, the status in the storage changes to

„received read‟.

AT+CMGS (Send Message) Sends message from a TE to the network.

AT+CMSS (Send From Storage) Sends message with location value <index> from message storage <mem2> to the network

AT+CMGW (Write Message To Memory) Stores a message to message storage <mem2>. The memory location

<index> of the stored message is returned.

AT+CMGD (Delete Message) Deletes message from preferred message <mem1> storage location

<index>.

V. TESTING & RESULT

The total system has been designed and a prototype has

also been developed based on the flowchart shown in figure

5. The designed system has been tested whether it

responses according to algorithm or not. A start pulse was

generated to initiate the system. The PIC microcontroller

issues an AT command to check whether the connection

with mobile is established or not. It executes AT+CPMS

command to select the preferred storage for SMS which is

chosen the phone memory. Then the microcontroller

generates the “Testing SMS” which it sends to the default

cell number. At the same time it displays the SMS in the

LCD panel. An SMS containing “OK” was then sent back

from the default number manually as an acknowledgement

which is read in the microcontroller as a TPDU format

“0891881007000006F0240D91881017257845F200008050

620291324202CF25”. Microcontroller then checks the

sender authentication by reading the encoded sender

number between the 13th and 19th octet of the TPDU part.

Message is then decoded from 30th octet where 29th

indicates the length of the PDU. Then microcontroller

decoded the PDU and shows it in LCD which is seen “OK”

as expected. The overall procedure indicates that the

developed SMS Gateway system can be used for any kind

of pre-defined SMS based controlling system. A five

seconds delay was introduced for successful transmission

and reception of SMS. After the reception of the message,

the PIC microcontroller executes it; delete it from the

phone memory, to release the 1st memory location.

VI. CONCLUSION

Successful completion of the design and testing of the

SMS Gateway indicates that the PC as an SMS gateway

can easily be replaced by a PIC microcontroller. Beside

this, the additional IC, MAX232, used for voltage

adjustment between the mobile and PC is no longer needed

in the proposed micro-controller based system. It also

reduces the complexity and the overall development cost of

such a system. Therefore the system becomes smarter,

efficient and portable. In addition, since the microcontroller

can also be configured as a web server, this system can be

accessed for controlling various devices in the remote place

through the Internet.



IJAEST

Figure 5: Flow chart of the developed system

REFERENCE

1. A.Y. Al-Zoubi, A.A. Tahat, and O.M. Hasan, “ Mobile

virtual experimentation utilizing SMS”, proceedings of the

Fourth IASTED International Conference Communication,

Internet, and Information Technology, October31-

November2, 2005, Cambridge, USA

2. Siang, B.K.; Bin Ramli, A.R.; Prakash, V.; Bin Syed

Mohamed, S.A.R., “SMS gateway interface remote

monitoring and controlling via GSM SMS”,

Telecommunication Technology, 2003. NCTT 2003

proceedings. 4th National Conference on Volume, Issue

14-15, Jan.2003 Page(s): 84 – 87

3. MPLAB IDE v7.60 and datasheet of PIC16f876

Power ON

Initialize Process

PORTB for LCD interface

PORTC<7:6> for USART

PORTC<2:1:0> for RS, R/W, E

BAUD rate = 9600

SPBRG = 25; BRGH = 1

8bit, No parity & 1stop bit for Tx/Rx

LCD initialization

Start pulse?

Save in EEPROM:

AT,CR,LF

AT+CPMS=“ME”,“ME”,“ME”

AT+CMGS=25,CR

AT+CNMI=3,3,0,0,0,CR,LF

AT+CMGD=1,CR,LF

PDU, ctrl-z

Start

NO

Yes

Read “AT+CMGS=25”,CR

& transmit byte wise

Receive

“<” ? NO

Yes

Read PDU,ctrl-z


Suppress answer from mobile

Display “Testing SMS”

5 sec Timer

Receive data

& save in GPR byte wise

Read

“AT+CNMI=3,3,0,0,0”,CR,LF


Read “AT”,CR,LR


Receive

“OK” ? NO

Yes

Receive

LF? NO

Yes

Authorized?

Display

“Unauthorized”

Display “SMS Gateway”

To start

NO Skip 28 byte & set data

length=29th byte

Decode

Complete?

Yes

NO

Display the

received SMS

To start

Yes

Read “AT+CMGD=1”,CR,LR

& transmit to delete SMS

Read “AT+CPMS=“ME”,“ME”,“ME” ”,CR,LF




IJAEST

http://www.microchipdirect.com/ProductSearch.aspx?Keywords=SW007002

http://ww1.microchip.com/downloads/en/DeviceDoc

accessed on 24 February, 2008

4. Martin P. Bates, “Interfacing PIC Microcontrollers:

Embedded Design by Interactive Simulation”, Publisher:

Butterworth-Heinemann, October 2006, ISBN-13:

9780750680288

5. Control a HD44780-based Character-LCD,

http://home.iae.nl/users/pouweha/lcd/lcd.shtml accessed

on 12 August, 2008

6. GSM 07.05 TECHNICAL SPECIFICATION, January

1998,Ver 5.5.0 www.ctiforum.com/standard/standard/etsi

accessed on 30 September, 2008

7. GSM SMS and the PDU format,

http://www.dreamfabric.com/sms accessed on 5 March,

2008

8. Michael Harrington, “Understanding SMS: Practitioner‟s

Basics”

http://mobileforensics.files.wordpress.com/2007/06/unders

tanding_sms.pdf accessed on 24 February, 2008

9. Mobile PhoneT68i Developers. Guidelines AT Commands

Online Reference http://pupius.co.uk/download/misc/t68i-

at-commands.pdf accessed on 30 June, 2008



IJAEST

http://ww1.microchip.com/downloads/en/DeviceDoc

http://search.barnesandnoble.com/booksearch/results.asp?ATH=Martin+P%2E+Bates

http://home.iae.nl/users/pouweha/lcd/lcd.shtml

http://www.ctiforum.com/standard/standard/etsi

GSM%20SMS%20and%20the%20PDU%20format,%20%20http:/www.dreamfabric.com/sms

GSM%20SMS%20and%20the%20PDU%20format,%20%20http:/www.dreamfabric.com/sms

http://mobileforensics.files.wordpress.com/2007/06/understanding_sms.pdf

http://mobileforensics.files.wordpress.com/2007/06/understanding_sms.pdf

http://pupius.co.uk/download/misc/t68i-at-commands.pdf

http://pupius.co.uk/download/misc/t68i-at-commands.pdf

13 IJAEST Volume No 2 Design and Development of Micro Controller Based SMS Gateway for GSM Mobile...

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