Nortel Networks 207344-B manual Figure B-2. MDI-X to MDI-X External Crossover RJ-45 Example

Cables and Connectors

The switch module receives data on pins 1 and 2 and transmits its data on pins

3 and 6. The host module ports implement this MDI standard crossover internally and are called MDI-X (the X for crossover), as shown in Figure B-1. All EtherSpeedII switch module 10BASE-T ports are MDI-X standard.

Two simple rules govern all usual RJ-45 connections from each switch module 10BASE-T/100BASE-TX port to the 10BASE-T/100BASE-TX port on another device:

•A straight-through cable (MDI to MDI-X) is customarily used for connection to a station NIC, to a transceiver, or to a cable plant patch panel.

•A crossover cable or adapter (MDI-X to MDI-X; as shown in Figure B-2) is customarily used for connection to another switch or hub port.

1

Receive

2

3

Transmit

6

MDI-X port

Twisted

pairs

1

Receive

2

3

Transmit

6

MDI-X port

9730EA

Figure B-2. MDI-X to MDI-X External Crossover (RJ-45 Example)

Making the crossover externally with a crossover cable or adapter that exchanges the TD and RD wire pairs (as shown in Figure B-2) is possible but has the following disadvantages:

•A crossover cable is not standard. You may need to special order one or make your own.

•Crossover cables and standard cables that resemble one another in a network can cause confusion.

Caution: Make any crossover cable you use look obviously different from normal straight-through cables and put a tag on each end; for example, use a distinctive color like yellow and put a tag labeled “X-over” on each end.

207344-B B-3