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User Guide
54
All the different test results are provided for each Media Dependent Interface (MDI). An
MDI corresponds with a single Ethernet pair transceiver. There are four of them in
1000BASE-T (MDI-0, MDI-1, MDI-2 and MDI-3) and two in 100BASE-TX / 10BASE-T
(MDI-0 and MDI-1). Using the MDIs to supply information about wiring is the most
logical choice: Pin assignment in a connector may change but MDIs are always the
same and, at logical level, communication between MDIs is very simple: MDI-0 is
connected to MDI-0, MDI-1 is connected with MDI-1, etc.
Old Ethernet stations had a fixed pin-out for MDI-0, MDI-1, MDI-2 and MDI-3. This fixed
assignment for stations corresponds to the MDI status. These older stations were
connected to Ethernet hubs and switches through a straight cable and to a second
station through a crossover cable. For this reason, MDI pair assignment in hubs and
switches was required to be complementary to the wiring of stations. This second pair
assignment corresponds to the MDIX status:
In old devices with static MDI pair assignment, interface description could be based
either on MDI-0 to MDI-3 or pair 1 to pair 8 but with most of the current Ethernet devices
the situation is different. Modern Ethernet stations and switches can switch their ports
between MDI and MDIX status before link establishment to make sure that an MDI port
Table 3.1: Pair assignment for MDI and MDIX port modes
MDI-0 MDI-1 MDI-2 MDI-3
MDI 1-2 3-6 4-5 7-8
MDIX 3-6 1-2 7-8 4-5
Figure 3.1: T-568A and T-568B wiring standards. Pair numbering (1, 2, 3, 4) is a way to
identify pairs in the cable and is not to be confused with MDI numbering (MDI-0, MDI-1,
MDI-2, MDI-3).
12345678
T-568B Pin/Pair assignment
1
3
2
4
12345678
T-568A Pin/Pair assignment
1
2
3
4
(a) (b)
Pair Number
Pair 1: Blue
Pair 2: Orange
Pair 3: Green
Pair 4: Brown
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