Connectix says that there are a number of choices of cabling system, and all have their pros and cons

Category 5 was first published as a standard in 1995 and appeared in:  ANSI/TIA/EIA-568-A, EN 50173, ISO 11801. In the American standard (TIA), the term ‘Category’ refers to the individual product performance (cables and connectors etc.) and the overall channel performance. For example Category 5 defines a range of electrical performances up to 100 MHz.

In the European standard, EN 50173 and the international ISO standard, ‘Category’ only refers to a product performance. The overall link and channel performance is rated by ‘Class’. So for example a Cat5 cable terminated with Cat5 connecting hardware would give a Class D channel and link performance.

When gigabit Ethernet was introduced in 1998 (1000BASE-T), it was discovered that the original Cat5 specification wasn’t good enough to guarantee error-free performance. The original Category 5 standard was only suitable for LAN transmission standards up to 155 Mb/s, including all the various forms of fast Ethernet up to 100 Mb/s.

To ensure reliable operation of gigabit Ethernet, extra technical requirements were added to the original Cat5 specification, such as Return Loss, Delay, Delay Skew and Power Sum Crosstalk measurements. The improved range of parameters became known as enhanced Category 5 or Cat5E and, although it still had a 100MHz channel, it had a tighter electrical specification. To differentiate it from the ‘old’ Cat5, the expression ‘Cat5E’ was introduced.

Standards were updated in 1999/2000 to take into account the new Cat5 requirements. However, in September 2002, the standards were replaced by ANSI/TIA/EIA-568-B, EN 50173 2nd Edition, ISO 11801 2nd Edition.

The old Cat5 standards are now obsolete and nobody (in theory) makes these products anymore. This means that the term ‘Cat5E’ will gradually fall out of use because today’s standard products all meet the latest Category 5 /Class D specifications and are, by default, Cat5E.

 In the summer of 2002, Category 6/Class E was introduced, and is defined in the following standards: ANSI/TIA/EIA-568-B2-1 EN 50173 2nd Edition, ISO 11801 2nd Edition.  Cat6 requires the same electrical parameters as Cat5E but with a required performance up to 250 MHz. The benefits of its higher bandwidth are that data rates of up to 10 Gb/s can be offered to LANs, and superior video transmission performance is also possible. (The lower twist rate of Cat5E means less differential delay between pairs which can improve performance for RGB transmissions.)  

However, even faster systems are on the horizon. Designed to go hand in hand with the new ten gigabit Ethernet standard 10GBASE-T, is Category 6A which can offer speeds of 500 MHz. This makes it an ideal data centre technology - data centre standards, TIA 942 and EN 50173-5, require that Cat 6 is the minimum performance for data centres and computer rooms.

In conclusion, if users are happy with the LAN technology of today then Cat5E, with its 100 MHz bandwidth and max speed of 1000 MB/s, is adequate for their needs. But we all know that LAN technology changes significantly on about a two year cycle, so if users want a system that will see them into the foreseeable future, encompass the next generation of LAN technology, all without recabling their building, then Category 6 would be the technology for them.”