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Gigabit Ethernet Auto-Negotiation
By Rich Hernandez (Issue 1 2001)
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.
Today a number of technologies, such as 10BaseT, 100BaseTX, and 1000BaseT, use the same RJ-
45 connector, creating the potential for connecting electrically incompatible 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 motherboard
(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 automatically 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:
Mandatory auto-negotiation for 1000BaseT
Configure master and slave modes for the PHY
The Auto-Negotiation specification includes reception, arbitration, and transmission of normal link
pulses (NLPs). It also defines a receive LIT function for backward compatibility with 10BaseT
devices. All of these functions are implemented as part of the physical layer transceiver as shown
in Figure 1 . The exchange of link information occurs between the PHY and the Medium
Dependent Interface (MDI) or RJ-45 connector.
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