Referring now to FIG. 1, first and second devices 10 and 12 include physical layers 14-1 and 14-2 that are connected by cable 18 that includes four pairs of twisted pair wires (A, B, C and D). The physical layers 14 usually include digital signal processors (DSPs) and autonegotiation controllers (both not shown). The DSP of the first device receives and decodes signals from the second device. The DSP of the first device codes and transmits signals to the second device. The four pairs of twisted pair wires are typically labeled A (1,2), B (3, 6), C (4,5), and D (7,8). In 10BASE-T and 100BASE-TX mode, only pairs A (1,2) and B (3,6) are required to autonegotiate and to establish a link. In 1000BASE-T mode, however, two pairs of twisted pair wires are required to autonegotiate and four pairs are required to establish a link.
In 10BASE-T, 100BASE-TX, and 1000BASE-T modes, the physical layer performs autonegotiation before a link is established. During autonegotiation, the devices 10 and 12 negotiate the operating speed of the link as well as other functional capabilities of the devices. A device can advertise operating speeds that are less than or equal to the maximum operating speed of the device.
Compliant cable for Ethernet over twisted pair cables requires four pairs of twisted pair wires to be available even if they are not used. Referring now to FIG. 2, in some situations (for example due to cost or malfunctions), cable 20 provides only two operable pairs of twisted pair wires and is therefore non-compliant. The 1000BASE-T mode cannot establish a link over the non-compliant cable 20. However, 1000BASE-T autonegotiation on the non-complaint cable can be completed successfully. When the two devices attempt to establish the 1000BASE-T link, the link fails because only two pairs of twisted pair wires are available. The devices cycle through successful autonegotiation and unsuccessful link modes and are never able to exchange data.
To establish the 1000BASE-T link over compliant cable, the following steps are usually performed. In a first step, autonegotiation determines that both devices have 1000BASE-T capability. One device is designated the master device and the other device is designated the slave device. In a second step, after autonegotiation completes, the master device begins transmitting on all four pairs. Embedded in the transmit signal is an indication that the master device has not locked onto the transmit signal of the slave device. The slave device recovers the data on all four pairs of twisted pair wires. During the recovery time, the slave device does not transmit any data.
In a third step, after the slave locks onto the transmit signal of the master device and before starting echo cancellation, the slave device begins transmitting on all four pairs of twisted pair wires. Embedded in the transmit signal of the slave device is an indication that the slave device has not locked onto the transmit signal of the master device. In other words, the slave device has not completed echo cancellation. In a fourth step, both the master and slave devices transmit on all four pairs of twisted pair wires. The master and the slave devices perform echo cancellation and recover the data from each other. Once echo cancellation is complete and both devices lock onto the transmit signals, both devices transmit an indication that they are locked and ready to transmit and receive data. Then, the link is brought up. The DSP of each device must be locked onto the signals and the coding on all four pairs of twisted pair wires must be correct before the indication is sent.
If the second to fourth steps do not complete within a predetermined amount of time, the master and/or slave device times out and stops transmitting. Control returns to the first step. The time limit is usually governed by a maxwait timer in the autonegotiation controller.
To recover the incoming signals, the DSPs in the master and slave devices perform the following steps: When a transmit signal is detected on the twisted pair wire, the DSP starts tracking the incoming transmit signal. The DSP equalizes the transmit signal and locates an ideal sampling point to capture the transmit signal. The sampling process requires some time to complete before the DSP locks onto the transmit signal. During the sampling and locking process, the error rate decreases as the DSP converges to the ideal sampling point and locks onto the transmit signal.
Once the signal is locked, a physical coding sublayer (PCS) circuit of the DSP determines whether the incoming signal was encoded correctly to determine the validity of the incoming signal. It is possible for the DSP to lock onto a signal that was incorrectly encoded. An example in 1000BASE-T encoding is a signal that is all 0's. Since 0 is a valid signal level, the DSP will lock onto the 0. Since there are no transitions in the signal, any sampling point is the ideal sampling point. Since 1000BASE-T coding does not allow a signal to remain at 0 indefinitely, the coding of the received signal must be checked to determine the validity of the recovered signal.