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ICND1 break down - Pins And Transmissions


A normal ethernet cable allows only one way data transmissions while a crossover cable allows both way data transmission.
This is achieved by connecting the transmission pins (TX+ & TX-) of one end to the receiving pins(RX+ & RX-) of the other end.

In a normal RJ45 / CAT5 ethernet cable, the pins are marked as 1. TX+, 2. TX-, 3. RX+, 6. RX- at both ends. To create a crossover ethernet cable we just connect the TX pins to the corresponding RX pins. In effect, we swap wires at the pins 1-3 and 2-6.

In a normal ethernet cable,
We have the following pin configuration at both ends:

1: TX+
2: TX-
3: RX+
6: RX-

straight-through vs crossover
straight-through vs crossover

While in a crossover cable, we have:

1: TX+ <—-> 3: RX+
2: TX- <—-> 6: RX-
3: RX+ <—-> 1: TX+
6: RX- <—-> 2: TX-

We let the Pins 4,5,7,8 remain unaltered.


pinouts on one end are reversed from the other, as if the wire had been rolled over and you were viewing it from the other side.

So When should we use straight-through cable, cross-over cable and rollover cable? The following are the general guidelines.

Straight-trough Cable: Connecting Router Ethernet (AUI) Port to Switch Port. Also use it to connect a host to a hub or switch.
Notice that the AUI port accept a DB15 male connector instead of a RJ-45 connector, therefore RJ-45 to DB15 adaptors are often used with the straight-through cable.
RJ-45 normal ethernet cable

Crossover Cable: Use to Connecting two similar devices. Again when the ports on the devices don't accept RJ-45 connector, adaptors are often used to adapt RJ-45 to DB15, DB9, USB etc.
    RJ-45 crossover ethernet cable

  • Two Hosts

  • Two Switches
  • RJ-45 to AUI adaptor
  • Two Hubs

  • A Hub to a Switch
  • DB15 adaptor module
  • A Host to a Router.
  • DB9 to RJ-45 adaptor
  • Two router interfaces
DTE/DCE serial crossover cable

Rollover Cable: Connecting your Pc to a router’s console port, particularly your access server. The DB-9 connector connect to the laptop, RJ-45 connector connect to the sonsole port on the router.
rollover cable


1-rollover; 2-crossover; 3-straight-through; 4-straight-through

Modern switches come with MDIX ports but the speed and duplex setting must remain on auto

ICND1 and ICND2 break down

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