Gigabit Ethernet Auto-Negotiation
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2010-04-06
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T
oday a number of technologies, such as 10BaseT,
100BaseTX, and 1000BaseT, use the same RJ-45 connector,
creating the potential for connecting electrically incom-
patible components together and causing network disruption.
In addition, with the advent of Gigabit Ethernet over copper,
three-speed devices now support 10 Mbps, 100 Mbps, and
1000 Mbps operation. The Institute of Electrical and
Electronics Engineers (IEEE
®
) developed a method known
as auto-negotiation to eliminate the possibility of dissimilar
technologies interfering with each other.
Gigabit transceivers at the physical layer (PHY) of the Open
Systems Interconnection (OSI) model use auto-negotiation to
advertise the following modes of operation: 1000BaseT in full or
half duplex, 100BaseTX in full or half duplex, and 10BaseT in full
or half duplex. Although auto-negotiation can be disabled for
100BaseTX or 10BaseT connectivity, it is always required for normal
1000BaseT operation.
Auto-negotiation enables an easy upgrade path to gigabit
speeds by future proofing the server network connectivity with a
three-speed network interface card (NIC) or LAN on mother-
board (LOM). A server connected to a Fast Ethernet switch or
hub can easily be upgraded to Gigabit Ethernet by connecting
the NIC to a Gigabit Ethernet switch. If both the NIC and the
switch are set to auto-negotiate, the interface will be automati-
cally configured to run at 1000 Mbps.
The auto-negotiation algorithm (known as NWay) allows two
devices at either end of a 10 Mbps, 100 Mbps, or 1000 Mbps link to
advertise and negotiate the link operational mode—such as the
speed of the link and the duplex configuration of half or full
duplex—to the highest common denominator.
In addition, for 1000BaseT, NWay determines the master-slave
interlock between the PHYs at the ends of the link. This mode is
necessary to establish the source of the timing control of each
PHY. NWay is an enhancement of the 10BaseT link integrity test
(LIT) signaling method and provides backward compatibility with
link integrity.
Auto-negotiation is defined in Clause 28 of the 1998 edition
of the IEEE Standard (Std) 802.3. Clause 28 defines a standard to
address the following goals:
Provide easy, plug-and-play upgrades from 10 Mbps,
100 Mbps, and 1000 Mbps as the network infrastructure
is upgraded
Prevent network disruptions when connecting mixed
technologies such as 10BaseT, 100BaseTX, and
1000BaseT
Accommodate future PHY (transceiver) solutions
Allow manual override of auto-negotiation
Support backward compatibility with 10BaseT
Provide a parallel detection function to recognize 10BaseT,
100BaseTX, and 100BaseT4 non-NWay devices
In addition, the 1999 standard for Gigabit over copper cabling,
IEEE Std 802.3ab, added the following enhancements to the Auto-
Negotiation standard:
NETWORK ENVIRONMENT
Gigabit Ethernet
Auto-Negotiation
The Auto-Negotiation standard allows devices based on several Ethernet standards, from
10BaseT to 1000BaseT, to coexist in the network by mitigating the risks of network disruption
arising from incompatible technologies. This capability helps ensure a smooth migration path
from Ethernet to Fast Ethernet and Gigabit Ethernet. This article provides an in-depth explanation
of auto-negotiation and its functioning and also discusses special cases that may be encountered.
By Rich Hernandez
www.dell.com/powersolutions
PowerSolutions
117
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