With the autonegotiation function, Ethernet devices can determine the functionality of the opposite side and configure themselves automatically. Because Ethernet has many megabit and gigabit variants, Ethernet configurations can have different components and end devices.
The autonegotiation protocol, formerly referred to as Nway, addresses compatibility issues by automatically detecting the far end. Autonegotiation is used equally for Ethernet with comparatively low transmission rates of 10 Mbit/s and 100 Mbit/s to achieve the highest possible speed, but also for Gigabit Ethernet, 10 Gig abit Ethernet and 100 Gigabit Ethernet.
The method extends the Normal Link Pulse( NLP) known from 10Base-T, which is sent every 16 ms for line monitoring, to a Fast Link Pulse( FLP), which transmits further information about the capabilities of the end device.
For Fast Ethernet, for example, the 100Base-T autonegotiationframe consists of a 32-bitpreamble followed by a 2-bit ST field(Start of Frame), another 2-bit OP field(OperationCode), the 48-bit PHYAD field (Physical Address), the 512-bit Register Address (REGAD), the 2-bit TA field(Turn Around), and 16 bits of register content. The register address consists of 32 negotiation registers of 16 bits each. Of these 32 registers, the first eight are used by 100Base-T.