TCP

GSM/GPRS modems are getting very common these days, as prices are getting cheaper and cheaper. Apart from providing SMS and call functions to my projects I also wanted to communicate via TCP.

Although there are many documents and blog posts to help but I have always found that they either are answers to specific problem faced by someone or not providing complete details. In this post I would first explain the AT commands used in brief. You may connect your SIM900 to your computer via a serial/usb and test these commands. In the later part of this post I would include arduino example code.

AT commands for TCP/UDP Connection with example response and a brief description are given in the table below. Refer to the AT commands manual of your modem for details

AT command	Response	Description
AT	OK	test command. reply is OK
AT+CGATT?	+CGATT:n	checks if GPRS is attached? n=1 if attached
AT+CIPMUX=n	OK	use n as 0 for single connection or use 1 for multiple connections
AT+CSTT=”apn”,”username”,”pass”	OK	Sets APN, user name and password
AT+CIICR	OK	Brings up wireless connection
AT+CIFSR	ip address	Get local IP address if connected
AT+CIPSTART=“TYPE” , “domain”, “port”	Connected	Establishes a connection with a server. Type can be UDP or TCP
AT+CIPSEND	>	Sends data when the a connection is established.
AT+CIPCLOSE	OK	Closes the connection
AT+CIPSHUT	SHUT OK	resets IP session if any

how to make a connection:

Send ATr and wait for a response from the modem. You should recieve OK
if everything is set.
Make sure that the Modem has registered to network and that PIN code is disabled on the SIM. Send AT+CGATT?r to check if GPRS is attached or not. +CGATT: 1 indicates that GPRS is attached.
Send AT+CIPSHUTr . Although its optional this will be helpful as it resets IP session if any. you will get a response SHUT OK .
Send AT+CIPMUX=0 to set a single connection mode, response would be OK
Now set APN settings by AT+CSTT= “apn ”, “username”, “password”r . replace apn, username and password to match APN (Access Point Name) ,username and password for your service provider.
Now send AT+CIICRr , this will bring up the wireless connection. OK is received on successful connection
Send AT+CIFSRr , this will reply with the IP address the modem has been assigned.
Send AT+CIPSTART=”TCP”,”server domain name or ip”,”port”r, replace the domain name/ip and port with appropriate values, on connection modem will reply with CONNECT OK
Now you can send your data using AT+CIPSENDr AT command. modem will respond with > indicating it is ready to receive data to be sent. Type in your data.
Now the modem is waiting for the ASCII 26 that is control+z on keyboard. Depending on the terminal software used you can either press control and Z together on keyboard or send hex value 0x1A. The modem will then send the response from server.
Now send AT+CIPSHUT to shut down the connection. Modem will reply with SHUT OK
cheers 🙂

ARDUINO CODE :

Below is example code for single and multiple connection using arduino and sim900

int8_t answer;
int onModulePin= 2;
char aux_str[50];
char ip_data[40]="Test string from GPRS shieldrn";
void setup(){
    pinMode(onModulePin, OUTPUT);
    Serial.begin(115200);
    Serial.println("Starting...");
    power_on();
    delay(3000);
    // sets the PIN code
    sendATcommand2("AT+CPIN=****", "OK", "ERROR", 2000);
    delay(3000);
    Serial.println("Connecting to the network...");
    while( sendATcommand2("AT+CREG?", "+CREG: 0,1", "+CREG: 0,5", 1000)== 0 );
}
void loop(){
    // Selects Single-connection mode
    if (sendATcommand2("AT+CIPMUX=0", "OK", "ERROR", 1000) == 1)
    {
        // Waits for status IP INITIAL
        while(sendATcommand2("AT+CIPSTATUS", "INITIAL", "", 500)  == 0 );
        delay(5000);

        // Sets the APN, user name and password
        if (sendATcommand2("AT+CSTT="APN","user_name","password"", "OK",  "ERROR", 30000) == 1)
        {
            // Waits for status IP START
            while(sendATcommand2("AT+CIPSTATUS", "START", "", 500)  == 0 );
            delay(5000);

            // Brings Up Wireless Connection
            if (sendATcommand2("AT+CIICR", "OK", "ERROR", 30000) == 1)
            {
                // Waits for status IP GPRSACT
                while(sendATcommand2("AT+CIPSTATUS", "GPRSACT", "", 500)  == 0 );
                delay(5000);

                // Gets Local IP Address
                if (sendATcommand2("AT+CIFSR", ".", "ERROR", 10000) == 1)
                {
                    // Waits for status IP STATUS
                    while(sendATcommand2("AT+CIPSTATUS", "IP STATUS", "", 500)  == 0 );
                    delay(5000);
                    Serial.println("Openning TCP");

                    // Opens a TCP socket
                    if (sendATcommand2("AT+CIPSTART="TCP","IP_address","port"",
                            "CONNECT OK", "CONNECT FAIL", 30000) == 1)
                    {
                        Serial.println("Connected");

                        // Sends some data to the TCP socket
                        sprintf(aux_str,"AT+CIPSEND=%d", strlen(ip_data));
                        if (sendATcommand2(aux_str, ">", "ERROR", 10000) == 1)
                        {
                            sendATcommand2(ip_data, "SEND OK", "ERROR", 10000);
                        }

                        // Closes the socket
                        sendATcommand2("AT+CIPCLOSE", "CLOSE OK", "ERROR", 10000);
                    }
                    else
                    {
                        Serial.println("Error openning the connection");
                    }
                }
                else
                {
                    Serial.println("Error getting the IP address");
                }
            }
            else
            {
                Serial.println("Error bring up wireless connection");
            }
        }
        else
        {
            Serial.println("Error setting the APN");
        }
    }
    else
    {
        Serial.println("Error setting the single connection");
    }

    sendATcommand2("AT+CIPSHUT", "OK", "ERROR", 10000);
    delay(10000);
}

void power_on(){

    uint8_t answer=0;

    // checks if the module is started
    answer = sendATcommand2("AT", "OK", "OK", 2000);
    if (answer == 0)
    {
        // power on pulse
        digitalWrite(onModulePin,HIGH);
        delay(3000);
        digitalWrite(onModulePin,LOW);

        // waits for an answer from the module
        while(answer == 0){     // Send AT every two seconds and wait for the answer
            answer = sendATcommand2("AT", "OK", "OK", 2000);
        }
    }

}

int8_t sendATcommand2(char* ATcommand, char* expected_answer1,
        char* expected_answer2, unsigned int timeout){

    uint8_t x=0,  answer=0;
    char response[100];
    unsigned long previous;

    memset(response, '', 100);    // Initialize the string

    delay(100);

    while( Serial.available() > 0) Serial.read();    // Clean the input buffer

    Serial.println(ATcommand);    // Send the AT command

    x = 0;
    previous = millis();

    // this loop waits for the answer
    do{
        // if there are data in the UART input buffer, reads it and checks for the asnwer
        if(Serial.available() != 0){
            response[x] = Serial.read();
            x++;
            // check if the desired answer 1  is in the response of the module
            if (strstr(response, expected_answer1) != NULL)
            {
                answer = 1;
            }
            // check if the desired answer 2 is in the response of the module
            else if (strstr(response, expected_answer2) != NULL)
            {
                answer = 2;
            }
        }
    }
    // Waits for the asnwer with time out
    while((answer == 0) && ((millis() - previous) < timeout));

    return answer;
}

int8_t answer;
int onModulePin= 2;
char aux_str[50];

char ip_data[40]="Test string from GPRS shieldrn";

void setup(){

    pinMode(onModulePin, OUTPUT);
    Serial.begin(115200);

    Serial.println("Starting...");
    power_on();

    delay(3000);

    // sets the PIN code
    sendATcommand2("AT+CPIN=****", "OK", "ERROR", 2000);

    delay(3000);

    Serial.println("Connecting to the network...");

    while( sendATcommand2("AT+CREG?", "+CREG: 0,1", "+CREG: 0,5", 1000) == 0 );

}


void loop(){


    // Selects Multi-connection mode
    if (sendATcommand2("AT+CIPMUX=1", "OK", "ERROR", 1000) == 1)
    {
        // Waits for status IP INITIAL
        while(sendATcommand2("AT+CIPSTATUS", "INITIAL", "", 500)  == 0 );
        delay(5000);

        // Sets the APN, user name and password
        if (sendATcommand2("AT+CSTT="APN","user_name","password"", "OK",  "ERROR", 30000) == 1)
        {
            // Waits for status IP START
            while(sendATcommand2("AT+CIPSTATUS", "START", "", 500)  == 0 );
            delay(5000);

            // Brings Up Wireless Connection
            if (sendATcommand2("AT+CIICR", "OK", "ERROR", 30000) == 1)
            {
                // Waits for status IP GPRSACT
                while(sendATcommand2("AT+CIPSTATUS", "GPRSACT", "", 500)  == 0 );
                delay(5000);

                // Gets Local IP Address
                if (sendATcommand2("AT+CIFSR", ".", "ERROR", 10000) == 1)
                {
                    // Waits for status IP STATUS
                    while(sendATcommand2("AT+CIPSTATUS", "IP STATUS", "", 500)  == 0 );
                    delay(5000);
                    Serial.println("Openning TCP");

                    // Opens a TCP socket with connection 1
                    if (sendATcommand2("AT+CIPSTART=1,"TCP","IP_address","port"",
                                    "CONNECT OK", "CONNECT FAIL", 30000) == 1)
                    {
                        Serial.println("Connected");

                        // Sends some data to the TCP socket
                        sprintf(aux_str,"AT+CIPSEND=1,%d", strlen(ip_data));
                        if (sendATcommand2(aux_str, ">", "ERROR", 10000) == 1)
                        {
                            delay(500);
                            sendATcommand2(ip_data, "SEND OK", "ERROR", 10000);
                        }

                        // Closes the socket
                        sendATcommand2("AT+CIPCLOSE=1", "CLOSE OK", "ERROR", 10000);
                    }
                    else
                    {
                        Serial.println("Error openning the connection 1");
                    }

                }
                else
                {
                    Serial.println("Error getting the IP address");
                }
            }
            else
            {
                Serial.println("Error bring up wireless connection");
            }
        }
        else
        {
            Serial.println("Error setting the APN");
        }
    }
    else
    {
        Serial.println("Error setting the multi-connection");
    }

    sendATcommand2("AT+CIPSHUT", "OK", "ERROR", 10000);
    delay(10000);
}

void power_on(){

    uint8_t answer=0;

    // checks if the module is started
    answer = sendATcommand2("AT", "OK", "OK", 2000);
    if (answer == 0)
    {
        // power on pulse
        digitalWrite(onModulePin,HIGH);
        delay(3000);
        digitalWrite(onModulePin,LOW);

        // waits for an answer from the module
        while(answer == 0){     // Send AT every two seconds and wait for the answer
            answer = sendATcommand2("AT", "OK", "OK", 2000);
        }
    }

}

int8_t sendATcommand2(char* ATcommand, char* expected_answer1,
        char* expected_answer2, unsigned int timeout){

    uint8_t x=0,  answer=0;
    char response[100];
    unsigned long previous;

    memset(response, '', 100);    // Initialize the string

    delay(100);

    while( Serial.available() > 0) Serial.read();    // Clean the input buffer

    Serial.println(ATcommand);    // Send the AT command

    x = 0;
    previous = millis();

    // this loop waits for the answer
    do{
        // if there are data in the UART input buffer, reads it and checks for the asnwer
        if(Serial.available() != 0){
            response[x] = Serial.read();
            x++;
            // check if the desired answer 1  is in the response of the module
            if (strstr(response, expected_answer1) != NULL)
            {
                answer = 1;
            }
            // check if the desired answer 2 is in the response of the module
            else if (strstr(response, expected_answer2) != NULL)
            {
                answer = 2;
            }
        }
    }
    // Waits for the asnwer with time out
    while((answer == 0) && ((millis() - previous) < timeout));

    return answer;
}

dmSherazi

Embedded Electronics HW/SW Engineer with expertise in Hardware & Software including PCB design and Analysis, firmware development and Android development apart from causual web-development (wordpress & Laravel) and to some extent photography.

TCP connection over GPRS using SIM900 and similar modems using AT commands