EDI from legacy systems......

Hi frenz,

I've got an object in which we get an incoming EDI (856) from our warehouse for updating picked quantity in delivery document.

Here i got some doubts.

1) Its not an IDoc we gotto recive, So how do i configure my XI to receive and recognise the EDI message?

2) Shall i call a RFC enabled function module to update the picked quantity, from XI or shall post the EDI message as IDoc in our R/3 and update it there?

Which will be the optimized way?

3) For this scenario, i'm not sure whether i would require just a message mapping or XSLT or Java Mapping.

If someone had done this scenario already, help me how to approach this. I'm expectin a reply as early as possible, frenz.

Thanks in advance.

---praveen.