Figure 9: Gmii Block - Broadcom NetXtreme/NetLink BCM5718 Series Programmer's Manual

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BCM5718 Programmer's Guide

Pulse Amplitude Modulated Symbol (PAM5) encoding is leveraged for Gigabit Ethernet wire transmissions.

PAM5 uses five encoding levels: –2, –1, 0, 1, and 2. Four symbols are transmitted in parallel on the four twisted-

wire pairs. The four symbols create a code group (an eight-bit octet). The process of creating the code-group is

called 4D-PAM5. Essentially, eight data bits are represented by four symbols. Table 40-1 in the IEEE 802.3ab

specification shows the data bit to symbol mapping. The code group representation is also referred to as a

quartet of quinary symbols {TA, TB, TC, TD}. The modulation rate on the wire is measured at 125 Mbaud. The

resultant bandwidth is calculated by multiplying 125 MHz by eight bits, for

1000 Mbps wire speed.

125-MHz Ref Clock

Physical Layer

125-MHz Ref Clock

January 29, 2016 • 5718-PG108-R

Figure 9: GMII Block

8-bit Data Path

MAC Sublayer

Decapsulation

Encapsulation

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