Categories of twisted pair cabling systems

The standards categories:

Category	Maximum data rate	Usual application
CAT 1 (de facto name, never a standard)	Up to 1 Mbps (1 MHz)	analog voice (POTS) Basic Rate Interface in ISDN Doorbell wiring
CAT 2 (de facto name, never a standard)	4 Mbps	Mainly used in the IBM cabling system for Token Ring networks
CAT 3	16 Mbps	Voice (analog most popular implementation) 10BASE-T Ethernet.
CAT 4	20 Mbps	Used in 16 Mbps Token Ring. Otherwise not used much. Was only a standard briefly and never widely installed.
CAT 5	100 MHz	100 Mbps TPDDI 155 Mbps ATM. No longer supported; replaced by 5E. 10/100BASE-T 4/16MBps Token Ring Analog Voice
CAT 5E	100 MHz	100 Mbps TPDDI 155 Mbps ATM Gigabit Ethernet. Offers better near-end crosstalk than CAT 5
CAT 6	Up to 250 MHz	Minimum cabling for data centers in TIA-942. Quickly replacing category 5e.
CAT 6E	Up to 500 MHz (field-tested to 500 MHz)	Support for 10 Gigabit Ethernet (10GBASE-T.) May be either shielded (STP, ScTP, S/FTP) or unshielded (UTP). This standard published in Feb. 2008. Minimum for Data Centers in ISO data center standard.
CAT 7 (ISO Class F)	600 MHz 1.2 GHz in pairs with Siemon connector	Full-motion video Teleradiology Government and manufacturing environments Fully Shielded (S/FTP) system using non-RJ45 connectors but backwards compatible with hybrid cords. Until February 2008, the only standard (published in 2002) to support 10GBASE-T for a full 100m.

Cabling systems are categorized in terms of the data rates that they can sustain effectively. The specifications describe the cable material as well as the types of connectors and junction blocks to be used in order to conform to a category. For ISO/IEC standards, the category refers to the cable and class refers to the connector. ISO are international standards and can be followed anywhere in the world. A standard is different than a code in that standards are voluntary and provide a guideline so that all manufacturers have a minimum set of parameters to work with. A code, on the other hand, is law and is enforced through an AHJ (Authority having Jurisdiction). 

The two most widely-installed categories are CAT 3 (voice) and CAT 5e, traditionally, however with new installations, category 5e for voice and one high speed to support 10GBASE-T for long term usage are the most common. According to a BSRIA study, 78% of all new installations are shielded. While the two cables may look identical, CAT 3 is tested to a lower set of specifications and can cause transmission errors if pushed to faster speeds. CAT 3 cabling is near-end crosstalk-certified for only a 16 MHz signal, while CAT 5 cable must pass a 100 MHz test. CAT 5E replaced CAT 5 in both standards groups.

The CAT 6 specification, published in 2002 (the same time as ISO Category 7/class F), improves on CAT 5E in terms of near-end crosstalk, balance, and bandwidth. According to IEEE, 70% of new installs in 2004 were CAT 6.

Category 7/Class F (ISO/IEC), published in 2002, is a fully shielded system. The shielding provides superior noise immunity and performance. Until February 2008, it was the only published standard to support 10GBASE-T to a full 100m.

CAT 7A/Class FA and Category 6A/Class EA specifications were published in February 2008.

Read more about Categories of twisted pair cabling systems: - IT Knowledge Exchange offers expert advice on cabling. See answers or ask your own questions. - Siemon offers a free e-book, '10 Gigabit Network Cabling Basics.' (Registration required.) - NetworkStuff provides a brief description of CAT 7. - Siemon offers a white paper, "Facts and Fallacies of Testing Next Generation Cabling."

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