a data communication method , a communication system , and related devices are configured to establish a transaction identifier in a user equipment . the data communication method includes the following steps . a mobility management entity receives a request message and obtains ability information of the ue . if the ue has an ability to access a universal terrestrial radio access network / gsm / edge radio access network , the mme generates the ti . a communication system and related devices are also provided . thus , the ti is effectively established in the ue , so as to ensure normal processing of the ue .