Link between Asterisk and Avaya IPO 500

Hello everybody,these two PBX are on the same network,Asterisk and Avaya IPO 500,users registered on Avaya IPO can easily communicate togheter and it’s the same for users registered on Asterisk,
on Avaya IPO these users use H.323 Protocol,on Asterisk they use SIP Protocol. Now i want to know how to interconnect these two PBX system togheter that allow users on Avaya communicate with users on Asterisk. If someone has an idea or any link about this configuration,it will help me.