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Sunday, July 26, 2009

1000base-T Gigabit Ethernet

1000base-T is the copper based version of the gigabit Ethernet standard defined by 802.3ab which, since it is over 6 months old, is available free of charge from the enlightened IEEE. Great work. In passing, if you want to see sophistry raised to an art form read the EIA's justification for charging for their specifications. (Note: The original EIA statement is unfortunately no longer avilable on-line. This is a great loss to both the development of the English language in general, and comedy writing in particular.) The following notes apply to the 1000base-T spec:

  1. The standard defines auto-negotiation of speed between 10, 100 and 1000 Mbit/s so the speed will fall to the maximum supported by both ends - ensuring inter-working with existing installations.

  2. The cable specification base-line is ANSI/TIA/EIA-568-A-1995 (which you have to pay for). This means that if you know your cat5 cable was manufactured to this standard (there was a lower spec 1991 version of this specification) then it will support Gigabit Ethernet. Cat5 cable manufactured to the old specification may work or it may not - you need to run some tests. Cat5e and cat6 being higher spec cables will clearly support Gigabit Ethernet.

  3. Maximum runs are the standard 100m (~300ft).

  4. Gigabit Ethernet uses all 4 pairs (8 conductors). The transmission scheme is radically different (PAM-5 a 5 level amplitude modulation scheme) and each conductor is used for send and receive.

  5. Crossed Gigabit Ethernet cables must cross all 4 pairs.

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