Friday, February 8, 2013

Crossover Cables vs. Straight Through Cables


LAN Cabling
Ethernet patch cables can be wired in three different ways; the two main ways are called straight through and crossover. The third type is called rolled and has only specialized applications.
Straight through cables are used to patch between different types of equipment; for example, PCs to a hub.
Crossover cables are generally used to patch between similar types of equipment; a PC to another PC for example.
Some modern hubs don't care if you use crossover cables or straight through cables, they work out what you're using and configure themselves accordingly.

Inside the UTP patch cable there are 8 physical wires although the network only uses 4 of them (the other 4 are simply wasted). The 8 wires are arranged in what's known as pairs and one pair is used to send information whilst the other pair is used to receive information.
Color Codes
The standards say that Ethernet connectors should be cabled with specific colors on specific pins. There are two standard layouts - if a cable has the same layout on both ends it's a straight through cable. If a cable has one layout on one end and the other layout on the other end then it's a crossover cable. Whilst not universal, the color codes shown below are generally used on professional cables.

The straight through and crossover cables look like the diagram below:-




If a cable has 568A color wiring on both ends then it's a straight through cable.


If a cable has 568B color wiring on both ends then it's also a straight through cable.


If a cable has 568A color wiring on one end and 568B color coded wiring on the other end, then it's a crossover cable.

While the colors are standardized and usually followed, that's not the important part. What's more important is that one "pair" (wires that are twisted together inside the cable sheath) is used for the transmit side and another pair for the receive side. If pairs aren't used then it's likely your cable will not work. Pairs are identified by the colors. The orange wire and the orange with white stripe (or sometimes white with orange stripe) wire are a pair. The brown wire and the brown with white stripe wire are a pair. Etc.

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