Autonegotiation is a mechanism defined in the IEEE 802.3u
specification whereby devices sharing a link segment can exchange
information and automatically configure themselves to operate at
the highest capability mode shared between them.
Autonegotiation is like a rotary switch that automatically
switches to the correct technology such as 10Base-T or 100Base-TX
or between half- and full-duplex modes. Once the highest performance
common mode is determined, auto-negotiation passes control of the
link to the appropriate technology, sets the appropriate duplex
mode, and then becomes transparent until the link is broken.
Following is the IEEE 802.3u-defined hierarchy for resolving
multiple common abilities for a 10/100Base-TX card. The PCI 10/100Base-TX/9000
product provides the means for interfacing various types of HP 9000
workstations to either a 10Base-T or 100Base-TX network. 100Base-TX
is a subset of 100Base-T networking defined by the IEEE 802.3u-1995
standard. 100Base-TX provides 100 Mbit/s data transmission over
category 5 unshielded twisted-par (UTP) cable. Two pairs of wires
in the cable are used--one wire pair is for receiving data, and
one wire pair is for transmitting data. The same card port that
supports 100Base-TX operation can also support 10Base-T operation.
For example, if both devices on the link support 10Base-T
(half duplex) and 100Base-TX (half duplex), autonegotiation at both
ends will connect the 100Base-TX (half duplex) instead of the 10Base-T
(half duplex).
Most Fast Ethernet devices on the market today such as hubs
and switches do not support autonegotiation. Either the speed and
duplex mode of the device are fixed (as is usually the case with
hubs), or they are often manually configured at the desired speed
and duplex (as is often the case for switches). However, switches
that support autonegotiation are starting to be offered.
The PCI 10/100Base-TX/9000 card supports autonegotiation with
other devices. The interface card is also capable of automatically
sensing a statically configured speed of a Fast Ethernet hub or
switch and configuring itself accordingly. This is called autosensing
though the term is not strictly defined by the IEEE.
Usually, a Fast Ethernet hub will operate at one fixed speed,
100 Mbit/s, and in half-duplex mode. When using the PCI 10/100Base-TX
card in conjunction with either a Fast Ethernet or 10Base-T switch,
the switch port(s) on the link should have their speeds and duplex
modes either fixed or manually configured. If autonegotiation is
offered on the switch, it should be enabled for correct operation
of the PCI 10/100Base-TX card.
The PCI 10/100Base-TX card will sense when the connection
between itself and a hub or switch on the other end of a link has
been broken. If a connection is made to another (or the same) device,
the autosensing process will be done again automatically. Autosensing
is also done whenever the interface is reset.
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