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2. Ethernet > Other Types of Signaling

Other Types of Signaling

In addition to the Ethernet frame, there are a couple of signals used to indicate health/status of the line or the capabilities of the link. This form of signaling is considered out of band because it occurs between the frames flowing across the network cabling.

What activates the link light? Answer: the link pulse. There are two types of link pulse signals—normal and fast. 10Base-T nodes support only the normal link pulse, or NLP. NLP indicates link status. The NLP is simply a blip sent every 16 ±8ms while the link is idle. Devices on either end of a link send each other these pulses to indicate that the link is up.

Autonegotiation

Autonegotiation uses the same NLP, but for a different purpose. In this chapter we have discussed full and half duplex and 10/100/1000Mbps systems. The question is, how do we determine the correct communication parameters? Autonegotiation replaces a single NLP blip with the fast link pulse (FLP) signal. There are 33 pulse positions in the time reserved for a NLP pulse, and these are divided into 17 odd and 16 even positions. The FLP blips are spaced every 62.5 ±7µsec. The odd positions are for link pulse and the clock. The even positions are used for autonegotiation data. If a pulse is present, it indicates a binary 1; if not, the position is a binary 0.


  

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