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18
(C) Herbert Haas
2005/03/11
1000BaseT
Defined by 802.3ab task force
UTP
Uses all 4 line pairs simultaneously for duplex
transmission! (echo cancellation)
5 level PAM coding
• 4 levels encode 2 bits + extra level used for Forward
Error Correction (FEC)
Signal rate: 4 x 125 Mbaud = 4 x 250Mbit/s data
rate
• Cat. 5 links, max 100 m; all 4pairs, cable must
conform to the requirements of ANSI/TIA/EIA-568-A
Only 1 CSMA/CD repeater allowed in a
collision domain
It is very difficult to transmit Gigabit speeds over unshielded twisted pair cables.
Only a mix of multiple transmission techniques ensure that this high data rate can
be transmitted over a UTP Cat5 cable. For example all 4 pairs are used together
for both directions. Echo cancellation ensures that the sending signal does not
confuse the received signal. 5 level PAM is used for encoding instead of 8B10B
because of its much lower symbol rate. Now we have only 125 Mbaud x 4
instead of 1250 Mbaud.
The interface design is very complicated and therefore relatively expensive.
Using Cat 6 or Cat 7 cables allow 500 Mbaud x 2 pairs, that is 2 pairs are
designated for TX and the other 2 pairs are used for RX. This dramatically
reduces the price but requires better cables, which are not really expensive but
slightly thicker. Legacy cable ducts might be too small in diameter.