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Understanding modern wiring requires a brief history. With each new category, the cable's performance characteristics increase dramatically.

Category 1 wiring, though sometimes disputed as a viable part of the category hierarchy, supported only voice communications, such as POTS.

Category 2 wiring passed local-talk data at up to 4 Mbps.

Category 3, one of the most widely deployed UTP products, was adopted in 1991 at the advent of the wildly popular 10-Mbps Ethernet network. Cat 3 also supported 4-Mbps token-ring environments.

The Category 4 standard was instituted in 1993 with 16-Mbps token-ring networks in mind and supported data speeds to 20 Mbps.

Then in 1994, Category 5 wiring specifications were adopted to support Fast Ethernet networks, with speeds up to 100 Mbps. As impressive as Category 5 wiring was in comparison with the previous wiring standards, it has been superseded by the 1998 Category 5E (Enhanced Category 5) wiring standard.

Cat 5E supports the same 100 Mbps but has much higher performance specifications and was enacted to support 1000Base-T (Gigabit Ethernet, or 1-Gbps) networks along with applications like 155-Mbps ATM. In addition to its greater throughput, Category 5E is usually tested to a bandwidth of 155 MHz, despite its 100-MHz specified bandwidth. Testing is more stringent with Cat 5E than it was with Cat 5 and includes additional measurements, several of which help to better quantify the UTP cable's noise characteristics.