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Old 03-12-2009, 04:53 PM   #1 (permalink)
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Default Ethernet

Ok, the doctor needs a consult.

Does anyone, anyone at all know why the Ethernet specification allows for two sending and receiving lines in CAT5 cable?

I have been looking forever and no one seems to know, or at least talk about it.


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Old 03-12-2009, 09:45 PM   #2 (permalink)
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Default Re: Ethernet

Dr. House,

I found this at Wikipedia:

1000BASE-T details
In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation and a 5-level pulse amplitude modulation (PAM-5) technique. The symbol rate is identical to that of 100BASE-TX (125 Mbaud) and the noise immunity of the 5-level signaling is also identical to that of the 3-level signaling in 100BASE-TX, since 1000BASE-T uses 4-dimensional Trellis Coded Modulation (TCM) to achieve a 6 dB coding gain across the 4 pairs.

The data is transmitted over four copper pairs, eight bits at a time. First, eight bits of data are expanded into four 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; this is similar to what is done in 100BASE-T2, but uses different parameters. The 3-bit symbols are then mapped to voltage levels which vary continuously during transmission.
source: Gigabit Ethernet - Wikipedia, the free encyclopedia

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Old 03-12-2009, 11:12 PM   #3 (permalink)
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Default Re: Ethernet

That sums it up, Gigabit Ethernet is the ONLY one what uses all four pairs in a cat5e or cat6 cable.
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