Serial Serial port is used for AT command communication, data transmission, GNSS NMEA sentences output, software, debugging, firmware upgrade and voice over USB. Power Power design for the module is very important, as the performance of the module largely depends on the power source. RF When the receiver and transmitter in cellular terminal is ON, it receives and transmits radio frequency signals. Call In order to make or receive a call, the cellular terminal must be switched on in a service area with adequate cellular signal strength. SIM Card In order to enhance the reliability and availability of the (U)SIM card in customers’ applications, please follow the criteria from our hardware design note in (U)SIM circuit design. PHB Read/write/search the phonebook entries from the current phonebook memory storage. TCP/UDP Our modules provide the following socket services: TCP client, UDP client, TCP server and UDP server. Audio PCM interface is used for audio function with external codec, supports echo cancellation and noise suppression. SMS The format of messages can be either PDU mode (entire TP data units used) or text mode (headers and body of the messages given as separate parameters). PPP / MMS / FLASH All subjects related to procedures for PPP setup and termination, modes for PPP connection, MMS application interface for sending messages including multimedia objects (image, audio, rich text, etc.), internal flash.