The PA-T3 and PA-2T3 serial port adapters provide one (PA-T3) or two (PA-2T3) high-speed full-duplex synchronous serial T3 interfaces and integrated data service unit (DSU) functionality. The T3 port adapters can transmit and receive data at T3 rates of up to 45 Mbps and use a 75-ohm coaxial cable to connect to a serial T3 network. Serial network interfaces reside on modular port adapters, which provide a direct connection between the high-speed bus in the router and the external networks. The PA-T3 serial port adapter provides a full-duplex synchronous serial PA-T3 interface for transmitting and receiving data at rates of up to 34 megabits per second (Mbps).