Level indicates whether the voltage level of the link negotiation signal was normal or low. Communication might not be reliable if the level is low. If the link level is low, the warning icon appears next to the cable icon on the HOME screen.

The OneTouch AT does not specifically measure link pulse voltages. It sets trigger points around a midpoint noise threshold and detect if the link pulse exceeds these thresholds. The values are specific to our circuit and not absolute voltages. The threshold for high link pulses are roughly 26% above and 25% below midpoint scaled to the OT AT circuit.

The link level is measured prior to linking at .525 Volts positive. If at least 5 pulses were found across all pairs, link is considered adequate. If less than 2 pulses were detected on the normal Rx pair (3, 6), link is measured at .42 Volts negative and checked again. 5 pulses passes, usually there are hundreds or even thousands. These values are somewhat empirical and arbitrary.

In general, if the link shows low the OneTouch AT is likely not to link at all. This is useful because it will offer a diagnosis for why link didn’t happen. The pulse counts on the pairs are also used to determine if there is an MDIX or crossover. Errors can happen there especially on “green” switches that reduce link pulses to conserve power. The errors will exhibit as 1, 2 or 3, 6 being the Rx pair on an MDIX port. It won’t show low link in those cases as link level is still fine.

The notion of voltages and ground are somewhat arbitrary due the design of the measurement circuit and the floating nature of Ethernet signals. These values have an empirical component determined through testing and measuring with numerous switch and hub ports.