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Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance transmission mode they both support. In the OSI model, autonegotiation resides in the physical layer. For Ethernet over twisted pair it is defined in clause 28 of IEEE 802.3.〔 http://standards.ieee.org/getieee802/download/802.3-2012_section2.pdf IEEE 802.3, clause 28, page 221, "Physical Layer link signaling for Auto-Negotiation on twisted pair" Note: Download of PDF is free, but behind a Captive portal. 〕 Autonegotiation was originally defined as an optional component in the fast Ethernet standard. It is backwards compatible with 10BASE-T. The protocol was significantly extended in the gigabit Ethernet standard, and is mandatory for 1000BASE-T gigabit Ethernet over twisted pair. ==Overview== In 1995, a standard was released to allow connected network adapters to negotiate the best possible shared mode of operation. The initial autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation. Autonegotiation can be used by devices that are capable of different transmission rates, different duplex modes (half duplex and full duplex), and/or different standards at the same speed (though in practice only one standard at each speed is widely supported). Each device declares its ''technology abilities'', that is, its possible modes of operation, and the best mode is chosen from those shared by them, with higher speed preferred over lower, and full duplex preferred over half duplex at the same speed. Parallel detection is used when a device that is capable of autonegotiation is connected to one that is not. This happens if the other device does not support autonegotiation or autonegotiation is administratively disabled. In this condition, the device that is capable of autonegotiation can determine and match speed with the other device. This procedure cannot determine the presence of full duplex, so half duplex is always assumed. The standards for 1000BASE-T, 1000BASE-TX and 10GBASE-T require autonegotiation to be always present and enabled. Other than speed and duplex mode, autonegotiation is used to communicate the port type (single port or multiport) and the master-slave parameters (whether it is manually configured or not, whether the device is master or slave if this is the case, and the master-slave seed bit otherwise). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Autonegotiation」の詳細全文を読む スポンサード リンク
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