The first category cable ever created was labeled Category 3, and was originally manufactured for use in telephone wiring. Later it was used as an Ethernet cable carrying data up to 10 Mbit/s. This was followed by a Category 4 cable that carried 16 Mbit/s. These were then replaced by Category 5 cables which were then upgraded to the Category 5 enhanced aka Cat5e. These carried up to 100 MHz and made all previous category cables basically obsolete. The Cat 5e is still relevant today and is still widely used in many installations.
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Cat 6 cables then hit the market amid some controversy. A lot of installers started picking Cat6 cables over Cat5e due to the way they were marketed instead of for their specifications. Cat6 cables are more expensive than Cat5e, and can be overkill for today's home network speeds and what installers are using them for. They have been advertised as being a much better solution than Cat 5e cables, but that is not necessarily the case. Most networks are still gigabit or less and Cat5e can handle gigabit speeds without a problem. The biggest misconception is that they are rated at a much higher megahertz than other cables, up to 250 MHz.
This is not an accurate description though when it comes to the cable's actual ability to pass a signal. Another misconception is that the Cat6 will carry a signal farther than other cables. This is not true; Cat 6 cables cannot carry a signal farther than the Cat 5e although the signal itself will be stronger. There is also Cat 6a on the market, which stands for "augmented Cat 6." This cable is defined as having frequencies up to 500 MHz. Although this enhanced version of the cable will have a higher performance level, that means that it is physically bigger also. Cat 6a cables are the biggest currently on the market, which means they are heavier and need more room to bend.
Category 7 cables do exist, but they are not rated or regulated for use in the United States. There has been some debate on why Cat 7 cables were not sanctioned by the TIA for use in the U.S. The reason being that Cat 7 cables have a 600 MHz bandwidth, and Cat 6a already has a 500 MHz bandwidth. There would not be a significant enough upgrade to make a whole new cable classification. In Europe the new cable type caught on in popularity with the belief that the added bandwidth would give installers a "safety margin." Cat 7 cables are sometimes referred to as "Ultrafast Ethernet." Most of the Cat7 cables you see on the market here in the U.S. don't follow the European standard, but come with a specification that was made up by an offshore factory to sell more cables.