Extreme Networks ExtremeWare Version 7.8, Troubleshooting Manual

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100 Advanced System Diagnostics and Troubleshooting Guide
Additional Diagnostics Tools
length. For example, a 2% error in the default value of the speed of wave propagation results in a
two-meter error for a 100-meter cable.
Cable Pair Information. Twisted pair conductors in the RJ-45 Ethernet cable are connected to pins of
the PHY in the following pairings: 1-2, 3-6, 4-5, and 7-8. In a straight-through (MDI) cable, these
conductor pairings are associated with channel designators (A, B, C, and D), which refer to the roles
that the MDI pairs might assume under the IEEE definition.
Table 9 lists the logical channel assignments for straight-through (MDI) and crossover (MDI-X) cable
types as defined by IEEE 802.3 clause 40.8.
Cable Fault Information (no Gigabit link established). When the Gigabit link cannot be established,
the CDM tests use time domain reflectometry (TDR) to diagnose cable faults. The results of the
diagnostic test are then interpreted to determine the type of cable fault (described below) and—when
applicable—the approximate distance to the cable fault.
• Short —If the positive and negative conductors of the same cable pair make an electrical contact, the
cable diagnostic reports that condition as a short on that pair. The possible causes are:
— The cable pair conductors might be shorted if they are both touching metal in a wiring closet
patch panel.
— Within the cable, the insulation on cable pair conductors might be worn from constant flexing
and fail, allowing the conductors to make electrical contact.
• Open
—If one or both of the conductors in a cable pair are severed, the cable diagnostic reports that
condition as an open on that cable pair. The possible causes are:
— The conductors of this cable pair might have been left unconnected at the wiring closet patch
panel.
— The conductors of this cable pair might have been severed within the cable.
— The far end of the cable is not connected.
• Impedance mismatch (impedance error) —When the effective impedance on a cable pair is not 100
Ohms, the cable diagnostic reports that condition as an impedance mismatch. The possible causes
are:
— Different quality cables—for example, a 100-Ohm segment and a 120-Ohm segment—are joined
through a cable extender.
—
A cable of unsatisfactory quality is used.
Table 9: IEEE 802.3 Cable Mapping
Contact MDI MDI-X
1 BI_DA+ BI_DB+
2 BI_DA– BI_DB–
3 BI_DB+ BI_DA+
4 BI_DC+ BI_DD+
5 BI_DC– BI_DD–
6 BI_DB– BI_DA–
7 BI_DD+ BI_DC+
8 BI_DD– BI_DC–