Cable Types Explained

Category 5e
Cat 5e cable is an enhanced version of Cat 5 that adds specifications for far end crosstalk. It was formally defined in 2001 as the TIA/EIA-568-B standard, which no longer recognizes the original Cat 5 specification. Although 1000BASE-T was designed for use with Cat 5 cable, the tighter specifications associated with Cat 5e cable and connectors make it an excellent choice for use with 1000BASE-T. Despite the stricter performance specifications, Cat 5e cable does not enable longer cable distances for Ethernet networks: cables are still limited to a maximum of 100 m (328 ft) in length (normal practice is to limit fixed ("horizontal") cables to 90 m to allow for up to 5 m of patch cable at each end, this comes to a total of the previous mentioned 100 m maximum). Cat 5e cable performance characteristics and test methods are defined in TIA/EIA-568-B.2-2001. 

Category 6
Category 6 cable, commonly referred to as Cat-6, is a cable standard for Gigabit Ethernet and other network protocols that is backward compatible with the Category 5/5e and Category 3 cable standards. Compared with Cat-5 and Cat-5e, Cat-6 features more stringent specifications for crosstalk and system noise. The cable standard provides performance of up to 250 MHz and is suitable for 10BASE-T, 100BASE-TX (Fast Ethernet), 1000BASE-T / 1000BASE-TX (Gigabit Ethernet) and 10GBASE-T (10-Gigabit Ethernet). Category 6 cable has a reduced maximum length when used for 10GBASE-T; Category 6a cable, or Augmented Category 6, is characterized to 500MHz and has improved alien crosstalk characteristics, allowing 10GBASE-T to be run for the same distance as previous protocols. Category 6 cable can be identified by the printing on the side of the cable sheath.
Category 6 cable contains four twisted copper wire pairs, just like earlier copper cable standards. Although Cat-6 is sometimes made with 23 gauge wire, this is not a requirement; the ANSI/TIA-568-B.2-1 specification states the cable may be made with 22 to 24 AWG wire, so long as the cable meets the specified testing standards. When used as a patch cable, Cat-6 is normally terminated in 8P8C modular connectors, often incorrectly referred to as "RJ-45" electrical connectors. Cat-6 connectors are made to higher standards that help reduce noise caused by crosstalk and system interference. Attenuation, NEXT (Near End Crosstalk), and PSNEXT (Power Sum NEXT) are all significantly lower when compared to Cat-5/5e. Some Cat-6 cables are too large and may be difficult to attach to 8P8C connectors without a special modular piece and are technically not standard compliant. If components of the various cable standards are intermixed, the performance of the signal path will be limited to that of the lowest category.
The cable is terminated in either the T568A scheme or the T568B scheme. So long as both ends of a cable are terminated using the same scheme, it doesn't matter which scheme is used; they are both straight through (pin 1 to 1, pin 2 to 2, etc) and the pairing is the same. Mixing T568A-terminated patch cords with T568B-terminated horizontal cables (or the reverse) does not produce problems in a facility. The T568B Scheme is by far the most widely used method of terminating patch cables. Crossover is used for hub to hub, computer to computer, wherever two-way communication is necessary. All gigabit Ethernet equipment, and most new 10/100Mb equipment, supports automatic crossover, meaning that either a straight-through or crossover cable may be used for any connection. However, older equipment requires the use of a straight-through cable to connect a switch to a client device, and a crossover cable to connect a switch to a switch or a client to a client.

Category 6a
The latest standard from the TIA for enhanced performance standards for twisted pair cable systems was defined in February 2008 in ANSI/TIA/EIA-568-B.2-10. Category 6a (or Augmented Category 6) is defined at frequencies up to 500 MHz—twice that of Cat 6.Improved specifications, particularly in the area of Alien Cross-talk (AXT) as compared to Cat6 UTP which exhibited high alien noise in high frequencies.
The global cabling standard ISO/IEC 11801 will soon be extended by the addition of amendment 2. This amendment defines new specifications for Cat. 6A components and Class EA permanent links. These new global Cat. 6A / Class EA specifications require a new generation of connecting hardware offering far superior performance compared to the existing products which are based on the American TIA standard.
The most important point is a performance difference between ISO/IEC and EIA/TIA component specifications for the NEXT transmission parameter. At a frequency of 500 MHz an ISO/IEC Cat. 6A connector performs 3 dB better than a Cat. 6A connector conforming with the EIA/TIA specification. 3 dB equals 100 % increase of Near End Crosstalk noise reduction when measured in absolute magnitudes. The maximum allowed length of a Cat-6 cable is 100 meters (330 ft) when used for 10/100/1000baseT. This consists of 90 meters (300 ft) of solid "horizontal" cabling between the patch panel and the wall jack, plus 10 meters (33 ft) of stranded patch cable between each jack and the attached device. Since stranded cable has higher attenuation than solid cable, exceeding 10 meters of patch cabling will reduce the permissible length of horizontal cable. When used for 10GbaseT, Cat-6 cable's maximum length is 55 meters (180 ft) in a favorable alien crosstalk environment, but only 37 meters (120 ft) in a hostile alien crosstalk environment such as when many cables are bundled together. 10GbaseT runs of up to 100 meters (330 ft) are permissible using Cat-6a.

Installation Caveats
Category 6 and 6a cable must be properly installed and terminated to meet specifications. Incorrect installation practices include kinking the cable or bending it with too tight a radius. Incorrect termination practices include untwisting the wire pairs or stripping the outer jacket back too far. Shielded Category 6a cable must have the foil grounded at one end to achieve specified alien crosstalk performance. Unshielded Category 6a cable does not have this limitation, but has a larger diameter. To ensure that an installation will meet the requirements for the network protocol it will be used for, a new installation is usually certified using a so-called cable certifier, validator or qualification tester.

Category 7
Category 7 cable (Cat 7), (ISO/IEC 11801:2002 category 7/class F), is a cable standard for Ethernet and other interconnect technologies that can be made to be backwards compatible with traditional Cat 5 and Cat 6 Ethernet cable. Cat 7 features even more strict specifications for crosstalk and system noise than Cat 6. To achieve this, shielding has been added for individual wire pairs and the cable as a whole. Category 7 is not recognized in EIA/TIA standards but it is used and marketed in industry.
The Cat 7 cable standard has been created to allow 10 Gigabit Ethernet over 100 m of copper cabling (also, 10-Gbit/s Ethernet now is typically run on Cat 6a). The cable contains four twisted copper wire pairs, just like the earlier standards. Cat 7 can be terminated either with 8P8C compatible GG45 electrical connectors which incorporate the 8P8C standard or with TERA connectors. When combined with GG45 or TERA connectors, Cat 7 cable is rated for transmission frequencies of up to 600 MHz. 

Category 7a
Category 7a (or Augmented Category 7) is defined at frequencies up to 1000 MHz, suitable for multiple applications in a single cable (just like all other categories) including CATV (862 MHz) . Simulation results have shown that 40 Gigabit Ethernet is possible at 50 meters and 100 Gigabit Ethernet is possible at 15 meters.[1] Mohsen Kavehrad and researchers at The Pennsylvania State University believe that either 32 nm or 22 nm circuits will allow for 100 Gigabit Ethernet at 100 meters.
However, similar studies in the past have shown that Cat5e could support 10G, so these should be read with caution. Furthermore, the IEEE is currently not looking into 40G or 100G for Cat7a. It may in the future, but there is absolutely no guarantee that such applications will ever exist.
Cat7a is currently a draft in ISO standards for channel and permanent link. Component performance is yet to be looked into. TIA/EIA currently is not working on any Cat7a standard.