Auto-negotiation

Autonegotiation is an extension of the link test methods used by 10Base-T and 10Base-FL to verify the integrity of the link between devices. Autonegotiation advertises a device's abilities by encoding a 16-bit data packet, called a link code word (LCW), within a burst of 17 to 33 link pulses, called a fast link pulse (FLP) burst. FLP bursts have an approximate duration of 2 �s and are transmitted in 16.8 �s intervals (the same interval as for the normal link pulses used by 10Base-T and 10Base-FL).

However, this does not hold true for the Half Duplex/Full Duplex (HD/FD) selection. If a non-autonegotiating device running FD is connected to a negotiating device, the negotiating device runs at HD, and the link does not operate properly.

Note: A Gigabit Ethernet device negotiates at HD or FD, speed is always one Gigabit.

Note: M5520-TX (P/N M5520-100TX) boards manufactured with a Quality Phy do not auto-negotiate with Xircom brand adapter cards. If you have this problem, disable auto-negotiation on the affected ports, and set the port speed and duplex state manually.

Note: You may experience difficulties with auto-negotiation between some releases of the 10/100Base-TX Module (M5510-100TX, M5520-100TX, M5510R-100TX, M5512R-100TX) and adapter cards using physical interfaces manufactured by National Semiconductor. The symptom is loss of connectivity. If you do. do one of the following:

• Disable auto-negotiation
• Use a patch cable that is longer that 5 meters

Note: The factory default for the National Phy Mode is Enable.

The LCW contains two fields (the selector field and the technology ability field), which together serve to identify a device's capabilities.

It may seem that because the FLP and the normal link pulse use the same interval at the same frequency, older devices may not be compatible with auto-negotiation. This is, however, not the case. For example, a 10Base-T device that does not have auto-negotiation capabilities sees FLP bursts simply as a link test signal. A 10Base-T device will respond to the FLP burst with its usual normal link pulse signal. At the other end of the link, a 10/100-capable device will recognize normal link pulse and choose 10Mbps mode operation.

Auto-negotiation attempts to find the greatest common denominator for the two devices on the link in the following order of preference:

1. 100Base-TX full-duplex
2. 100Base-T4
3. 100Base-TX
4. 10Base-T full-duplex
5. 10Base-T half-duplex

Note: T4: 100 Mbps with 8B/6T coding scheme

Once the greatest common denominator of settings is determined, each device equipped with auto-negotiation will configure itself automatically. In certain cases where automatic configurations are not desired, auto-negotiation provides a way for these settings to be overridden manually.

Note: Auto-negotiation should be disabled only on 50-series modules that have remote fault detection enabled. Do not disable auto-negotiation on 80-Series gig links.

Table�44 lists the gigabit modules that do not support auto negotiation:

Note: If a Gigabit module that does not support the autonegotiation is connected to a device that does, disable autonegotiation to ensure proper operation.