Editing Ethernet over twisted pair (section)

== Autonegotiation and duplex ==
Ethernet over twisted-pair standards up through Gigabit Ethernet define both [[full-duplex]] and [[half-duplex]] communication. However, half-duplex operation for gigabit speed is not supported by any existing hardware.<ref>{{Cite book|title=Gigabit Ethernet: Technology and Applications for High-Speed LANs |chapter=10 |last=Seifert |first=Rich |publisher=Addison Wesley |year=1998 |isbn=0-201-18553-9}}</ref><ref>{{Cite web|url=http://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/10561-3.html#gb |title=Configuring and Troubleshooting Ethernet 10/100/1000Mb Half/Full Duplex Auto-Negotiation |publisher=Cisco |date=2009-10-28 |access-date=2015-02-15}}</ref> Higher speed standards, [[2.5GBASE-T]] up to [[40GBASE-T]]<ref name=bq>{{cite web|title=IEEE P802.3bq 40GBASE-T Task Force|url=http://www.ieee802.org/3/bq/|publisher=IEEE 802.3}}</ref> running at 2.5 to {{nowrap|40 Gbit/s}}, consequently define only full-duplex point-to-point links which are generally connected by [[network switch]]es, and do not support the traditional shared-medium [[CSMA/CD]] operation.<ref name="Palmer">{{cite book|author=Michael Palmer|title=Hands-On Networking Fundamentals, 2nd ed|url=https://books.google.com/books?id=-QELAAAAQBAJ&pg=PA180|publisher=Cengage Learning|isbn=978-1-285-40275-8|page=180|date=2012-06-21}}</ref>

Many different modes of operations (10BASE-T half-duplex, 10BASE-T full-duplex, 100BASE-TX half-duplex, etc.) exist for Ethernet over [[twisted pair]], and most [[network adapter]]s are capable of different modes of operation. [[Autonegotiation]] is required in order to make a working 1000BASE-T connection.

When two linked interfaces are set to different [[Duplex (telecommunications)|duplex]] modes, the effect of this [[duplex mismatch]] is a network that functions much more slowly than its nominal speed. Duplex mismatch may be inadvertently caused when an administrator configures an interface to a fixed mode (e.g. {{nowrap|100 Mbit/s}} full-duplex) and fails to configure the remote interface, leaving it set to autonegotiate. Then, when the auto-negotiation process fails, half-duplex is assumed by the autonegotiating side of the link.