A networking standard that supports data transfer rates up to 100 Mbps (100 megabits per second). 100BASE-T is based on the older Ethernet standard. Because it is 10 times faster than Ethernet, it is often referred to as Fast Ethernet. Officially, the 100BASE-T standard is IEEE 802.3u.
100BASE-T is any of several Fast Ethernet 100 Mbit/s (12.5 MByte/s excluding 4B/5B overhead) CSMA/CD standards for twisted pair cables, including: 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 meters (as with 10BASE-T and gigabit Ethernet). All are or were standards under IEEE 802.3 (approved 1995).
100BASE-T is any of several Fast Ethernet 100 Mbit/s (12.5 MByte/s excluding 4B/5B overhead) CSMA/CD standards for twisted pair cables, including: 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 meters (as with 10BASE-T and gigabit Ethernet). All are or were standards under IEEE 802.3 (approved 1995).
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Last Updated: Sep 2009
Last Updated: Sep 2009
What is Fast Ethernet?
In the early days of Fast Ethernet, much vendor advertising centered on claims by competing standards that "ours will work better with existing cables than theirs." In practice, it was quickly discovered that few existing networks actually met the assumed standards, because 10-megabit Ethernet was very tolerant of minor deviations from specified electrical characteristics and few installers ever bothered to make exact measurements of cable and connection quality: if Ethernet worked over a cable, it was deemed acceptable. Thus most networks had to be rewired for 100-megabit speed whether or not they had supposedly been CAT3 or CAT5 cable plants. The vast majority of common implementations or installations of 100BASE-T are done with 100BASE-TX.
With all 100BASE-T Ethernet (including 100BASE-TX), the raw bits of a packet to be transmitted -- a series of 0 and 1 bits at 100 Mbit/s -- are typically transferred 4 bits at a time clocked at 25 MHz to the Ethernet hardware.
This limits the theoretical maximum data bit rate to 100 Mbit/s. The data signaling rate actually observed on real networks is far less than the theoretical maximum, due to the necessary header and trailer (addressing and error-detection bits) on every packet, the occasional "lost packet" due to noise, and time waiting after each sent packet for other devices on the network to finish transmitting.
With all 100BASE-T Ethernet (including 100BASE-TX), the raw bits of a packet to be transmitted -- a series of 0 and 1 bits at 100 Mbit/s -- are typically transferred 4 bits at a time clocked at 25 MHz to the Ethernet hardware.
This limits the theoretical maximum data bit rate to 100 Mbit/s. The data signaling rate actually observed on real networks is far less than the theoretical maximum, due to the necessary header and trailer (addressing and error-detection bits) on every packet, the occasional "lost packet" due to noise, and time waiting after each sent packet for other devices on the network to finish transmitting.
