Electrical Signals
Auto-negotiation (i.e. NWay) is based on pulses similar to those used by 10BASE-T devices to detect the presence of a connection to another device. These connection present pulses are sent by Ethernet devices when they are not sending or receiving any frames. They are unipolar positive-only electrical pulses of a nominal duration of 100 ns, with a maximum pulse width of 200 ns, generated at a 16 ms time interval (with a timing variation tolerance of 8 ms). These pulses are called link integrity test (LIT) pulses in the 10BASE-T terminology, and are referred to as normal link pulses (NLP) in the auto-negotiation specification.
A device detects the failure of a link if neither a frame nor two of the LIT pulses is received for 50-150 ms. For this scheme to work, devices must send LIT pulses regardless of receiving any.
Auto-negotiation uses similar pulses labeled as NLP. NLP are still unipolar, positive-only, and of the nominal duration of 100 ns; but each LIT is replaced by a pulse burst consisting of 17 to 33 pulses sent 125 uS apart. Each such pulse burst is called a fast link pulse (FLP) burst. The time interval between the start of each FLP burst is the same 16 milliseconds as between normal link pulses (variation tolerance of 8 ms).
The FLP burst consists of 17 NLP at a 125 µs time interval (with a tolerance of 14 µs). Between each pair of two consecutive NLP (i.e. at 62.5 us after first NLP of the pulse pair) an additional positive pulse may be present. The presence of this additional pulse indicates a logical 1, its absence a logical 0. As a result, every FLP contains a data word of 16 bits. This data word is called a link code word (LCW). The bits of the link code word are numbered from 0 to 15, where bit 0 corresponds to the first possible pulse in time and bit 15 to the last.
Read more about this topic: Autonegotiation
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