GSM 7bit PDU encoding algorithm in Java

Text to 7-bit PDU string converter

Being honest I forgot why I had written this code some time ago. I put it here
from my another small blog page hosted on blogpost
(engnick.blogpost.com).

Now I remember how was it and decided to bring this piece of code, as maybe
it might be useful for somebody. I wrote this code when I was working in
MegaFon as part of automated testing tool for
doing some tests against SMS handling billing solutions.

Using this you can encode messages for variety of modem devices working in
PDU mode for sending SMS and USSD requests to GSM network
(PDU mode).

As example, this code coverts text hellohello into E8329BFD4697D9EC37.
Or this can be used even for converting of MSISDN numbers.

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public static String stringTo7bitHex(String text) {
    StringBuilder output = new StringBuilder();
    byte[] bytes = text.getBytes();
    int f = 0;
    while (f < bytes.length - 1) {
        int t = (f % 8) + 1;
        if (t < 8) {
            byte b = (byte) (((bytes[f] >>> (t - 1)) | (bytes[f + 1] << (8 - t))) & 0x000000FF);
            output.append(intToHex(b & 0x000000FF));
        }
        f++;
    }
    if ((f % 8) + 1 < 8) {
        output.append(intToHex((bytes[f] >>> (f % 8)) & 0x000000FF));
    }
    return output.toString();
}

private static String intToHex(int i) {
    String hex = Integer.toHexString(i);
    if (hex.length() % 2 != 0) hex = 0 + hex;
    return hex.toUpperCase();
}

GSM 7bit PDU encoding algorithm in Java

Text to 7-bit PDU string converter

See Also