In a BMT (Bean Managed Trandaction) you need to mark the boundaries
of the transaction in your code.

In a CMT (Container Managed Transaction) the EJB container (JBoss :-))
sets the boundaries of the transactions. You can set the transaction attribute of a bean method in ejb-jar.xml:

<assembly-descriptor>
 <container-transaction>
 <method>
 <ejb-name>SampleBean</ejb-name>
 <method-name>*</method-name>
 </method>
 <trans-attribute>Required</trans-attribute>
 </container-transaction>
 </assembly-descriptor>