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 Gigabit Ethernet Networking, by William Lane (Sams), starting at \$0.01

How Gigabit Ethernet Works
 Author: Gabriel Torres Type: Tutorials Last Updated: October 10, 2005 Page: 2 of 3 Select Page to Load Introduction Data Transmission Performance Issues
Data Transmission

On 10BaseT standard each bit that the computer wants to transmit is physically coded into a single transmitting bit, i.e., for a group of eight bits being transmitted, eight signals will be generated on the wire. Its 10 Mbps transfer speed means that its clock is of 10 MHz, but just because each clock cycle a single bit is transmitted. On other standards this is different.

100BaseT uses a coding scheme called 8B/10B, where each group of eight bits is coded into a 10-bit signal. So, differently from 10BaseT, each bit does not directly represents a signal on the wire. If you make the proper math, with a 100 Mbps data transfer rate, the clock rate of 100BaseT is of 125 MHz (10/8 x 100).

So, Cat 5 cables are certified to have a transmission speed of up to 125 MHz.

What Gigabit Ethernet does is to change the coding. Instead of making each bit to be coded into a single signal like 10BaseT or to code each 8-bit group into a 10-bit signal, it codes two bits per signal. So, a signal over a Gigabit Ethernet cable represents two bits, instead of a single bit. In order words, instead of just using two voltages on a signal representing merely ”0“ or ”1“, it uses four different voltages, representing ”00“, ”01“, ”10“ and ”11“.

Also, instead of using just four wires of the cable, Gigabit Ethernet uses all wires.

On top of this, all pairs are used in a bi-directional fashion. As we’ve seen above, both 10BaseT and 100BaseT uses different pairs for transmission and reception; on 1000BaseT, as Gigabit Ethernet cabling is also called, the same pairs are used for both data transmission and reception.

The beauty of Gigabit Ethernet is that it still uses the 100BaseT/Cat 5 clock rate of 125 MHz rate, but since more data is transmitted per time, the transfer rate is higher. The math is quite simple: 125 MHz x 2 bits per signal (i.e., per wire pair) x 4 signals per time = 1.000 Mbps.

This modulation technique is called 4D-PAM5 and it actually uses five voltages (the fifth voltage is used for its error-correction mechanism).

So it is a mistake to say that Gigabit Ethernet runs at 1.000 MHz. It doesn’t. It runs at 125 MHz just like Fast Ethernet (100BaseT), but it achieves a 1.000 Mbps because it transmits two bits per time and uses the four pairs of the cable.

In the table below you can check Gigabit Ethernet cabling pinout. ”BI“ stands for bi-directional, while DA, DB, DC and DD stand for ”Data A“, ”Data B“, ”Data C“ and ”Data D“, respectively.

 Pin Color Function 1 White with Green +BI_DA 2 Green -BI_DA 3 White with Orange +BI_DB 4 Blue +BI_DC 5 White with Blue -BI_DC 6 Orange -BI_DB 7 White with Brown +BI_DD 8 Brown -BI_DD

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