| 摘要 |
A copper based Gigabit Ethernet interface also called 1000BaseT, as specified by IEEE802.3 Clause 40, complies with the synchronous Ethernet method standardized in ITU-T G.8261 to convey synchronization information needed for end-to-end recovering of TDM flows or for other purposes. Accordingly, at the two ends of the four couples of balanced twisted pairs each bearing full-duplex transmissions, the transceivers share a reference clock. The two interfaces have direct access to the reference clock, nevertheless the negotiation is carried out the same at start up, and at the end the master role is assigned to one only interface, the other is slave and it loopback to the transmitter the reference clock recovered from the reception signal. In case of subsequent defects of synchronization, the two interfaces depart from negotiation and nearly simultaneously commutate their role in few microseconds. According to a first method, a TMN interface generates an out-of-band switching command towards additional clock switches. Following a second method, the interface in defect of synchronization transmits a signaling in-band of having commutated its role autonomously, the companion interface which receives this signaling switches immediately its role too, without sending back nothing to prevent endless cycles.
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