|
|
6.2 | |
Ethernet Operation | |
| 6.2.9 | |
Ethernet auto-negotiation |
|
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.
10BASE-T transmits using signaling
between +1 and -1 volts, for a 2-volt peak-to-peak differential
signal. NLP signaling only uses the range from 0 to +1 volts. The
duration of a single NLP pulse is 100 ns. Figure
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.
An FLP burst consists of 33 pulse
positions. Data pulse positions are found between each clocking pulse. If a data pulse is present it is interpreted as a binary 1. The absence of a data pulse in the window between two clocking pulses is interpreted as a binary 0. When Auto-Negotiation is implemented, additional information can be added using the concept of “pages.” Pages are additional bits representing more sophisticated negotiation and link parameters. After a device has decoded the link code word offered by its link partner, it acknowledges receipt of the current word by sending at least three FLP bursts with the Acknowledge bit set. After both link partners acknowledge the current FLP link code word exchange in that manner, the link partners will either move on to the next page, or will enable the agreed configuration and attempt to link accordingly. Link partners may send any number of Next Pages following the initial configuration Base Page and any necessary Next Pages that are associated with the Base Page.
|
shows two
sample NLP pulses. The left pulse is very sharp and clean, and is from
an interface capable of 1000 Mbps operation. The right pulse is from
an older device capable of only 10/100 Mbps operation, which did not
require as precise of a signal. 
The 33 pulse positions represent a 16-bit link code word that is
framed by 17 clocking pulses. The 17 clocking pulses are always
present. The 16 data pulses are present only if they represent binary
1, and are absent if they represent binary 0 in the encoded 16-bit
data word. In Figure
Web Links