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(C) Herbert Haas
2005/03/11
Auto Negotiation
Enables each two Ethernet devices to
exchange information about their
capabilities
Signal rate, CSMA/CD, half- or full-duplex
Using Link-Integrity-Test-Pulse-Sequence
Normal-Link-Pulse (NLP) technique is used
in 10BaseT to check the link state (green LED)
10 Mbit/s LAN devices send every 16.8 ms a
100ns lasting NLP, no signal on the wire
means disconnected
Several Ethernet operating modes had been defined, which are incompatible to
each other, including different data rates (10, 100, 1000 Mbit/s), half or full
duplex operation, MAC control frames capabilities, etc.
Original Ethernet utilized so-called Normal Link Pulses (NLPs) to verify layer 2
connectivity. NLPs are single pulses which must be received periodically
between regular frames. If NLPs are received, the green LED on the NIC is
turned on.
Newer Ethernet cards realize auto negotiation by sending a sequence of NLPs,
which is called a Fast Link Pulse (FLP) sequence.