With reference to FIG. 4, the arbitration apparatus in FIG. 4 and each MODEM each may be an independent hardware device, and each perform a respective function. Alternatively, the arbitration apparatus and each MODEM may be software modules running on the processor, that is, exist in a form of software code. Alternatively, the arbitration apparatus and each MODEM may be implemented by a combination of software and hardware. The software is a functional module formed by software code run by the processor, and the hardware is a dedicated hardware circuit, including but not limited to a dedicated integrated circuit, a logic gate, an algorithm circuit, or an FPGA (field programmable gate array) circuit.