In a nutshell, the "categories" refer to the bandwidth or capacity of the cabling system. The categories apply only to "unshielded twisted pair" (UTP) cable systems -- i.e. wires or cables that look like "telephone" cables. UTP is the type of cable that is installed almost universally these days for voice and data communications.
The transmission performance of the cabling system is one factor that limits the rate at which data can be transmitted over a network. Standards specify different levels of performance and assign a numerical "category" to identify each level of performance. Higher bandwidth cable systems (such as Category 5e or 6) can carry more information per second than lower capacity systems (such as Category 3). If we think of cabling systems as being analogous to a plumbing system, a Category 6 plumbing system is constructed with bigger pipes than a Category 3 system. It will therefore carry a larger volume of water per unit of time.
Categories are defined in American standard EIA/TIA 568-B. Category 1 and 2 cables do not exist anymore. Category 3 is intended to carry signals up to a frequency of 16 megahertz (MHz), so it has a bandwidth of 16 MHz. Category 3 is only recognized by EIA/TIA-568-B for use in the backbone. Enhanced Category 5 (Category 5e) is intended for signals up to 100 MHz, and Category 6 is intended for signals up to 250 MHz, so they have bandwidths of 100 and 250 MHz respectively.
Category 4 cable was intended for signals up to 20 MHz and is no longer recognized by EIA/TIA-568-B. Category 5 cable was intended to be used at up to 100 MHz (the same as Category 5e), but Category 5 was required to meet fewer parameters than are now specified for Category 5e and Category 6 cables. These new parameters were found to be important to permit cables to carry signals at higher data rates.
The standard for augmented Category 6 (Category 6a) cabling was approved in 2008. This cabling system will support signals of up to 500 MHz and data rates of up to 10 Gb/s. Aside from cost, a significant disadvantage of Category 6a cable is that it is much larger in diamater than lower-performing cables. Hence, it will fill up an available pathway much more quickly.
Within the frequency range specified for Category 3 (i.e. frequencies up to 16 MHz), the transmission performance of Category 5e (or higher) is better to support the greater capabilities of systems designed to run on them. However, a LAN designed to operate on a Category 3 cabling system, for example, will not work any faster on a Category 6 or 6a cabling system.
While the cabling standards specify the bandwidth of the installed cabling, the actual rate at which data can be transmitted over a cabling system depends on the design of the electronic equipment attached to it. As noted above, the bandwidth of the cabling system is specified in MHz. However, the data rate is specified in bits per second – today, that is usually millions (megabits per second – Mb/s) or billions (billions of bits per second – Gb/s).
As users demand interoperability between electronic equipment provided by different manufacturers, the method of operation of this electronic equipment is specified by other standards. For example, traditional 10 megabits per second (10 Mb/s) Ethernet intended to run over UTP is standardized as "10BaseT" and can be transported on Category 3 cabling within the distances specified by EIA/TIA-568-B.
The related document, Cabling Capacities, provides further information on the maximum data standardized rates that may be carried on various categories of cabling. It will also assist in deciding if an existing cabling system may be re-used or if it should be replaced.
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