单项选择题 Which WAN technology was designed to give homes and small business high speed digital dial-up service over already existing communication infrastructure?（）

单项选择题 You are preparing to create a point to point WAN by connecting a Cisco router with a Juniper router by way of their serial links. Which one of the following commands would you use?（）

单项选择题 Which one of the WAN data-link protocols below is used for out-of-band signaling?（）