WSP Capabilities: What exactly is SDU size and what is Message si ze

Hi all,
WSP spezifies two size capabilities: the SDU size and the Message size.
Unfortunately I could not find an exact definition of what these sizes
are actually refering to.

My problem: As you know by now I'm working on a MMS client. To 
announce that I'm going to send a huge payload (e.g. a picture) 
I send a server-sdu-size of 128000 to the server. The Vodafone 
Gateway answers with a server-sdu-size of 32200. Despite of this 
claimed limitation I'm able to send messages of up to 100K.

I'm puzzled by this. Can anybody shed some light on this issue?

Thanks in advance
  Joerg