In routers, for serial interfaces, we need to manually set the clock at the DCE router so that the ends of the connection can communicate. I know that in this way we are using time-division multiplexing (TDM). In FastEthernet interfaces, we don't need to set the clock and it's auto negotiated. I was looking for the protocol name for finding the clock in a FastEthernet connection and further information about how this is done. Thanks.
1 Answer
With Fast Ethernet, the clock is embedded in a 4b/5b line code. More or less, the line code inserts additional bits into the data stream that enable the receiver to recover the transmitter's clock. Initial clock synchronization is also achieved by each frame's preamble/start of frame (syncword) that sends 63 alternating 0 and 1 bits, followed by an out-of-clock 1 before actual transmission.
You can find the details in IEEE 802.3 Clauses 24 and 3.2.
Note that 1000BASE-T's (and faster) duplex transmission requires both directions to be in sync. 1000BASE-T link partners negotiate a clock master that the slave synchronizes to, making Auto Negotiation (also used for speed and duplex mode) mandatory.
