Editing Fast Ethernet (section)

=== 100BASE-T4 ===
<!-- This section is linked from [[Ethernet over twisted pair]] -->
'''100BASE-T4''' was an early implementation of Fast Ethernet. It required four twisted copper pairs of [[Category 3 cable|voice grade twisted pair]], a lower-performing cable compared to [[Category 5 cable]] used by 100BASE-TX. Maximum distance was limited to 100 meters. One pair was reserved for transmit and one for receive, and the remaining two switched direction. The fact that three pairs were used to transmit in each direction made 100BASE-T4 inherently half-duplex. Using three cable pairs allowed it to reach {{nowrap|100 Mbit/s}} while running at lower carrier frequencies, which allowed it to run on older cabling that many companies had recently installed for 10BASE-T networks.

A very unusual [[8B6T]] code was used to convert 8 data bits into 6 base-3 digits (the signal shaping is possible as there are nearly three times as many 6-digit base-3 numbers as there are 8-digit base-2 numbers). The two resulting 3-digit base-3 symbols were sent in parallel over three pairs using 3-level [[pulse-amplitude modulation]] (PAM-3).

100BASE-T4 was not widely adopted but some of the technology developed for it is used in [[1000BASE-T]].<ref name="TDG_ETH_2nd" /> Very few hubs were released with 100BASE-T4 support. Some examples include the [[3com]] 3C250-T4 Superstack II HUB 100, [[IBM]] 8225 Fast Ethernet Stackable Hub<ref>{{cite web |url=https://www.ibm.com/common/ssi/cgi-bin/ssialias?appname=skmwww&htmlfid=897%2FENUS196-117&infotype=AN&mhq=IBM%20Network%20Station%208361%20Series%20100&mhsrc=ibmsearch_a&subtype=CA |title=IBM 8225 Fast Ethernet Stackable Hub Hardware Announcement |website=[[IBM]] |date=May 28, 1996}}</ref> and [[Intel]] LinkBuilder FMS 100 T4.<ref>{{cite web|url=https://techlibrary.hpe.com/docs/products/eos/3Com-End-of-Sale%20dates.pdf|title=3Com Product End of Sale dates|website=[[Hewlett Packard Enterprise]]}}</ref><ref>{{cite web | url=https://manualzz.com/doc/3244934/intel-express-100base-t4-user-s-manual | title=Intel Express 100BASE-T4 User's Manual | website=Manualzz}}</ref> The same applies to [[network interface controller]]s. Bridging 100BASE-T4 with 100BASE-TX required additional network equipment.