A data communication method, a communication system, and related devices are configured to establish a transaction identifier (TI) in a user equipment (UE). The data communication method includes the following steps. A mobility management entity (MME) 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 (UTRAN/GERAN), 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.