You have just obtained a modular Cisco router with 2 serial connections and a BRI/U interface.What other physical piece of hardware is required in order to establish an ISDN connection?

• 你获得了一个带有2个串口和一个BRI/U接口的模块化的思科路由器，如果想建立一个ISDN连接，还需要那些硬件设备？