3)transmission link a)It performs transmit information between adjacent nodes. Equally,a pair of two nodes or more nodes are connected physically by it in the forms of either a single channel or a broadcast one b)It usually includes the various transmission facilities(设施)， equipment or devices and their combinations.More specially,it maybe used by physical transmission network like Synchronous Digital Hierarch(SDH#,同步数字体系)link,( or even Asynchronous Transfer Mode(TM#,异步转移模式)link More details in both Chapter three of Access network and broadband ATM techniques PPT. 3) transmission link a) It performs transmit information between adjacent nodes. Equally, a pair of two nodes or more nodes are connected physically by it in the forms of either a single channel or a broadcast one . b) It usually includes the various transmission facilities (设施), equipment or devices and their combinations. More specially, it maybe used by physical transmission network like Synchronous Digital Hierarch (SDH#，同步数字体系) link , or even Asynchronous Transfer Mode (ATM#，异步转移模式) link . More details in both Chapter three of Access network and broadband ATM techniques PPT