You use + which is Java's string concatenation. If you have non-string in the expression, they are converted to string using default rules (or toString() mehtod).

So you don't send binary data but textual data, probably "DSEL0101.0" which is indeed 10 bytes long.

A quick look at the UDP reference shows there is a setBuffer() method to define the size of the buffer, and send() accepts a buffer argument, although it fails to indicate what is the type of this buffer. I guess it can be a byte[]. You need to fill the array with the Ascii codes of the string and the bytes of the integer and float (perhaps using bit arithmetic beware of byte order!).

If you code the server as well, you might want to use a simpler protocol. If it is out of your control, I fear the buffer building is the way to go.

Ah, a quick Google search shown a page pointing to ByteBuffer class. Looks like what you need (without reading the full page!).
You can use wrap() to get a ByteBuffer out of a byte array, and putChar, putInt and putFloat looks promising!