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6.2 | |
Ethernet Operation | |
| 6.2.9 | |
Ethernet auto-negotiation |
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As Ethernet grew from 10 to 100 and 1000
Mbps, one requirement was to make each technology interoperable, even
to the point that 10, 100, and 1000 interfaces could be directly
connected. A process called Auto-Negotiation of speeds at half or full
duplex was developed. Specifically, at the time that Fast
Ethernet was introduced, the standard included a method of
automatically configuring a given interface to match the speed and
capabilities of the link partner. This process defines how two link
partners may automatically negotiate a configuration offering the best
common performance level. It has the additional advantage of only
involving the lowest part of the physical layer.
10BASE-T required each station to
transmit a link pulse about every 16 milliseconds, whenever the
station was not engaged in transmitting a message. Auto-Negotiation
adopted this signal and renamed it a Normal Link Pulse (NLP). When a
series of NLPs are sent in a group for the purpose of
Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst.
Each FLP burst is sent at the same timing interval as an NLP, and is
intended to allow older 10BASE-T devices to operate normally in the
event they should receive an FLP burst.
Auto-Negotiation is accomplished by
transmitting a burst of 10BASE-T Link Pulses from each of the two link
partners. The burst communicates the capabilities of the transmitting
station to its link partner. After both stations have interpreted what
the other partner is offering, both switch to the highest performance
common configuration and establish a link at that speed. If anything
interrupts communications and the link is lost, the two link partners
first attempt to link again at the last negotiated speed. If that
fails, or if it has been too long since the link was lost, the
Auto-Negotiation process starts over. The link may be lost due to external
influences, such as a cable fault, or due to one of the partners
issuing a reset.
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