Autonegotiation
Autonegotiation is a signaling mechanism and procedure used by Ethernet over twisted pair 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.
[2] It is backwards compatible with the normal link pulses (NLP) used by 10BASE-T.[3] The protocol was significantly extended in the Gigabit Ethernet standard, and is mandatory for 1000BASE-T gigabit Ethernet over twisted pair.
Because this introduced a new speed option for the same wires, it included a means for connected network adapters to negotiate the best possible shared mode of operation.
The autonegotiation protocol included in IEEE 802.3 clause 28 was developed from a patented technology by National Semiconductor known as NWay.
The company gave a letter of assurance for anyone to use their system for a one time license fee.
Simple network testing utilities such as ping may report a valid connection.
However, network performance will be significantly impacted by transmission aborts and subsequent Ethernet frame drops that result from a duplex mismatch.
Currently, most network equipment manufacturers recommend using autonegotiation on all access ports and enable it as a factory default setting.
Other than speed and duplex mode, autonegotiation is used to communicate the master-slave parameters for gigabit Ethernet.
These link integrity test (LIT) pulses are sent by Ethernet devices when they are not sending or receiving any frames.
However, if the half-duplex device receives data while it is sending, it senses a collision and aborts transmission and then attempts to resend the frame.
The full-duplex device will report frame check sequence (FCS) errors on the aborted transmissions.
Depending on timing, the half-duplex device may sense a late collision, which it will interpret as a hard error rather than a normal consequence of CSMA/CD and may not attempt to resend the frame.
The full-duplex device does not detect any collision and assumes the frame was received without error.
It uses differential Manchester encoding (DME) pages to negotiate capabilities in a half-duplex manner.
