Standard Ethernet Cable types

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A standard Ethernet cabling type is Category 5 Unshielded Twisted-Pair commonly known as CAT 5 UTP. The connector on the end of a typical Cat5 UTP cable is an RJ-45 connector. This type of connector has a tab on the bottom that snaps into place when the connector is correctly placed into the device. (You can usually hear the "snap" sound, unless you're in a very loud wiring closet !)

The cable will contain separate wires inside, and the endpoints of these wires are referred to as pins. While you now know that bits are sent over these wires, it's important to know that the same set of pins is always used to transmit, and a separate set of pins is always used to receive.

Pins1 and 2 Transmit
Pins 3 and 6 Receive

Crosstalk is caused by the electromagnetic interference mentioned a moment ago. Basically, a signal "crosses over" from one pair of cables to another, causing the signals to become unusable.

NEXT (near - end crosstalk) is a condition generally cause by crossed or crushed pairs of wires. The conductors inside the wires don't even have to be exposed - but if the conductors are too close, the signal traveling on one wire can actually interfere with the signal on another wire. The "near-end" is a relative term, referring to the end of the cable being tested (as opposed to far-end crosstalk, or FEXT)

In a typical RJ-45 connection, the crosstalk is actually at its highest level as data enters the cable.

You may occasionally see the tern PSNEXT. This is short for "Power Sum Near End Cross Talk", and refers to the calculation carried out when a NEXT test is run. When the NEXT results for each pair of wires is added, the result is the PSNEXT value.

In the following exhibit, we've got three separate physical connections:
 -  Cable 1 :- Straight-through cable
 -  Cable 2 :- Crossover calble
 -  Cable 3:- Rollover Cable

 -  A laptop connect to a switch (Cable 3)
Here we need a rollover cable. All eight wires in the cable will "roll over" to another pin at the remote end, with the wire on Pin1 and one end rolling over to Pin 8 at the other end, the wire on Pin 2 a one end rolling over to Pin 7 at the remote end, and so forth.

You may also need an adapter for your rollover cable, since one end of the cable is a DB-9 connector, and few if any of today's laptops have such a port. You probably do have USB ports on your laptop, and you can get an adapter that allows you to connect a rollover cable to your laptop's USB port from just about any cable dealer.

A for the connection to the switch, you need to make sure you connect the RJ-45 connector on the other end of the rollover cable tot the Colsole port of the switch, I'll drive this point home at lease one more time elsewhere in the course....

 -  Two switched connected to each other (Cable 2) :-  we may have occasion to connect two similar devices directly with Ethernet, which can cause a problem since both devices will use the same pair of wires for transmitting data.
It's very common to connect two switches to allow them to send data over that connection, called a trunk. You'll learn all about the particulars of trunking in you CCNA studies, but the first thing we have to do is make sure we have the correct cable!

Here is need a crossover cable. The wire connected to Pin 1 on one side will no longer be connected to Pin 1 on the other, as it was in a straight-through cable. Four wires will "cross over" in a crossover cable :
Local Cable End                       Remote Cable End
Pin 1                                                   Pin 3
Pin 2                                                   Pin 6
Pin 3                                                   Pin 1
Pin 6                                                   Pin 2

 -  A PC connected to a switch (Cable 1) :- here we need a straightthrough cable. A straightthrough cable is used to connect a PC to a switch or hub. In a straight-through cable, the wire connected to Pin1 on one side is connected to Pin 1 on the other, the wire connected to Pin 2 on one side is connected to Pin2 on the other, and so forth.

So physically if we see the connector diagrams, if the colors of the cables are exactly the same its a straigtthrough cable, if 4 colors are changed its a crossover cable, and if all the colors change its a roll over cable.
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