多项选择题 SONET Path Terminating Equipment (PTE) requires which three line codes to be supported for DS1 and DS3 circuits?（） (Choose three.)

单项选择题 Which timing mode is appropriate for a SONET NE that is adjacent to a SONET NE using BITS-1 timing source?（）

单项选择题 If node 2 is accepting line timing from node 1, node 2 will send （）to node 1 as its SSM message.