Java 16bit CRC packet validation

Question

I have an Avl Packet that I'm recieving through GPRS from a Device.

The protocol manual says the packet has a 16bit CRC on the last 4 bytes and a source code for CRC calculation is given:

public static int getCrc16(Byte[] buffer) {
    return getCrc16(buffer, 0, buffer.length, 0xA001, 0);
    }

public synchronized static int getCrc16(Byte[] buffer, int offset, int bufLen, int polynom, int preset) {
    preset &= 0xFFFF;
    polynom &= 0xFFFF;
    int crc = preset;
    for (int i = 0; i < bufLen; i++) {
        int data = buffer[i + offset] & 0xFF;
        crc ^= data;
        for (int j = 0; j < 8; j++) {
            if ((crc & 0x0001) != 0) {
                crc = (crc >> 1) ^ polynom;
            } else {
                crc = crc >> 1;
            }
        }
    }
    return crc & 0xFFFF;
}

So I get the CRC number the packet sends me,then I call getCrc16 for the Byte array in which I have stored the packet and then compare the two numbers right??

Here is the code I use inside my program:

public static String toBinaryString(byte n) {
    StringBuilder sb = new StringBuilder("00000000");
    for (int bit = 0; bit < 8; bit++) {
        if (((n >> bit) & 1) > 0) {
            sb.setCharAt(7 - bit, '1');
        }
    }
    return sb.toString();
}

    int CalculatedCRC = getCrc16(AvlPacket);
    System.out.println("Calculated CRC= " + CalculatedCRC);

    int index = (AvlPacket.length)-4;
    String BinaryRecievedCRC = "";

    for (int j = 0; j < 4; j++) {
        BinaryRecievedCRC+= toBinaryString(AvlPacket[index]);
        index+=1;
    }

    int RecievedCRC = Integer.parseInt(BinaryRecievedCRC, 2);
    System.out.println("Recieved CRC= " + RecievedCRC);

toBinaryString() converts a byte to it's binary from and puts it into a string!

So I calculate the CRC through getCrc16() given to me from the manual.Then take an index 4 bytes before the end of the packet so I can read the last 4 bytes and get the CRC sent with the packet!

The for loop takes each of the last bytes and with toBinaryString() combines all them in binary form and into a String!So I got something like 0000000000000000101011011101001 (The manual states that first two bytes are always zeroes because its a 16bit CRC)

So I just parse the Binary String into a signed int and Compare the two CRCs...!

Yet I get Results like :

Calculated CRC= 21395 -----Recieved CRC= 30416

or

Calculated CRC= 56084 -----Recieved CRC= 10504

I've tested with many packets and they can't all have loss of data..And I'm parsing the data too so I know that the data I get is correct!

What am I missing in all this??

Durandal Durandal · Accepted Answer · 2012-09-24T13:28:36

There is probably something wrong with the documentation wording (or your understanding of it). If there is a 16-Bit CRC in a packet, its most likely occupying two bytes, not four (in binary form). If it were a decimal, even four bytes wouldn't suffice (you would need 5 didgts to store it as unsigned decimal string).

Your code shows you do conversions (but I can't see what kind of conversion its supposed to do):

BinaryRecievedCRC+= toBinaryString(AvlPacket[index]);

I would expect the CRC to be store in the data in binary form, so I assume the only thing you need to figure out are which endianess is used and where the CRC is stored in the data.

Edit: Judging from your comment you would need to extract the CRC like this:

public int getCRC(byte[] data, int index) {
    return ((data[index] & 0xFF) << 8)) | (data[index + 1] & 0xFF);
}

Java 16bit CRC packet validation

3 Answers