To get customer number (KUNNR) from Bill of exchange number (BOENO/DISBN) or from document no (BELNR)

HI Experts ,

  Please let me know the relationship of tables to get customer number(KUNNR))

  from Bill of exchange number (BOENO/DISBN) or from document no (BELNR) ?

I have only the following fields  :

  Document Number : BELNR

  Bill of exchange number ; BOENO

  Bank Key : BANK

I need to get customer number under which Bill of exchange is posted using F-34 transaction.

I couldn't find the value of KUNNR through BSEG table.

Please help me in this .

Thanks and Regards

shahab