Question

I’m coding an Android APK to manage GPS trackers by SMS. My software has a special notification part that decodes all alarms that device sends via SMS messages. Problem is that some phone companies use different limits for SMS length, so my code fails due to truncated messages. I’ve tested three different local companies and some of them use 140 chars while others use 70 chars. My question… is there any reliable way of getting that parameter? I’ve read all SDK docs and found nothing like that but the obvious constant MAX_USER_DATA_BYTES.

Answer 1

As this is a very convoluted matter and requires piecing together different parts of standards and protocols, I’ve decided to post all the info I’ve found for whoever needs to solve this issue like me.

All checks should be done in PDU data directly, accessing bits and bytes, either within SmsMessage with getPdu() or directly accessing the PDU array from Bundle in onReceive() of your SMS BroadcastReceiver.

First of all, we need to check two things for detecting a multi-part SMS:

1. Presence of IDH, bit 6 in PDU_TYPE
2. Presence of multipart indicator in IDH, second byte of UDH info

After detecting a multi-part message we need to identify messages belonging to same multi-part by checking CSMS identifier, number of parts and part index.

/**
 * EXAMPLE OF A PDU 
 *
 * 07 length of SMSC info in bytes
 * 91 type of address
 * 551218317600 SMSC address (variable size)
 * 04 PDU_TYPE to check for IDH embedded test bit 6  0=not UDH  1=has UDH
 * 0B address length in nibbles, here is 11 nibbles so its 6 bytes
 * 81 address type
 * 1089920478F1 sender number (variable size)
 * 00 TP-PID
 * 08 TP-DCS
 * 31105161225188  TP-SCTS  timestamp
 * 38  TP-UDL user data length
 * [USER DATA STARTS HERE] When UDH info is embedded, starts in first
 *                         bytes of user data
 * 
 * EXAMPLE OF UDH for 8 bit CSMS reference (unique number, indicates
 * parts belong to same message)
 *
 * 05 UDH length in bytes
 * 00 For multipart 8 bits CSMS it is 00, for 16bits CSMS it is 08
 * 03 length of header
 * 48 CSMS reference number, this identify SMSs that belong
 *                           to same multi-part
 * 02 total parts of this sequence (in this case, 2 parts)
 * 02 sequence number (in this case, 2 of 2)
 *
 *
 * EXAMPLE OF UDH for 16 bit CSMS reference (unique number, indicates
 * parts belong to same message)
 *
 * 06 UDH length in bytes
 * 00 For multipart 8 bits CSMS it is 00, for 16bits CSMS it is 08
 * 04 length of header
 * 4823 16 bits CSMS reference number, this identify SMSs that belong
 *                                     to the same multi-part
 * 02 total parts of this sequence (in this case, 2 parts)
 * 01 sequence number (in this case, 1 of 2)
 * 
 */

// SOME HELPERS TO CHECK FOR MULTIPART INSIDE PDU

private final byte UDH_BITMASK = 0x60;
private final byte MULTIPART_8BITSEQ = 0x00;
private final byte MULTIPART_16BITSEQ = 0x08;

private final int TP_ADDLENGTH_LEN=1;
private final int TP_ADDTYPE_LEN=1;
private final int TP_PID_LEN=1;
private final int TP_DCS_LEN=1;
private final int TP_SCTS_LEN=7;
private final int TP_UDL_LEN=1;

/**
 * Check if this PDU message is multi part
 * @param pdu
 * @return
 */
private boolean isMultiPart(byte[] pdu)
{
    // get length of first part+1 SMSC index to PDU type
    int idx= pdu[0]+TP_ADDLENGTH_LEN;

    if((pdu[idx] & UDH_BITMASK) != 0) // has UDH ??
    {
        // 00=length of UDH  01=type
        int udataidx = getUserDataIndex(pdu);
        if( (pdu[udataidx+1] == MULTIPART_8BITSEQ)
         || (pdu[udataidx+1] == MULTIPART_16BITSEQ))
            return true;
    }

    return false;
}


/**
 * Return index for user data part. Used to retrieve UDH info
 * @param pdu
 * @return
 */
private int getUserDataIndex(byte[] pdu)
{
    // get index to  ADDRESS length
    int idx=pdu[0]+ TP_ADDLENGTH_LEN + TP_ADDTYPE_LEN;

    // get length in nibbles
    int oalength = pdu[idx];

    // convert to bytes with padding for odd values
    int oalengthinbytes = (oalength + 1) / 2;
    return(idx + TP_ADDLENGTH_LEN + TP_ADDTYPE_LEN + oalengthinbytes +
            TP_PID_LEN + TP_DCS_LEN +  TP_SCTS_LEN + TP_UDL_LEN);
}