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The ideal union of PAM and Ethernet

Posted: 16 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Ethernet? pulse amplitude modulation? PAM? physical layer? media access control?

PAM offers several advantages:
???Simplicity of receiver and transmitter design
???Easy transmission over a pair of wires
???More bits transmitted in a symbol time
???High data rate links

The many PAMs used in Ethernet
The type of Ethernet determines which type of PAM is used. For example, 100BASE-T2 (running at 100Mb/s) Ethernet utilises a five-level PAM modulation over two wire pairs. The IEEE 802.3an standard defines the wire-level modulation for 10GBASE-T as a Tomlinson-Harashima Precoded (THP) version of pulse-amplitude modulation with 16 discrete levels (PAM16) encoded in a two-dimensional checkerboard pattern known as DSQ128. Several proposals are being considered for wire-level modulation in the newly proposed IEEE 802.3an standard for 10GBase-T. These include PAM with 12 discrete levels (PAM12), 10 levels (PAM10), or 8 levels (PAM8), with and without THP. PAM8 is currently in the draft phase with the IEEE 802.3 working group.

Figure 3: Ethernet and PAM encodings.

PAM3 signalling at any given instant involves one of three states or significant conditions, such as power level, phase position, pulse duration, or frequency. The Ethernet speeds that use PAM3 are 100BASE-T4, 100BASE-T1, and 1000BASE-T1.

100BASE-T4 was an early implementation of Fast Ethernet. It requires four twisted copper pairs, but those pairs were only required to be category 3 rather than the category 5 required by TX. One pair is reserved for transmit, one for receive, and the remaining two will switch direction as negotiated. A very unusual 8B6T code is used to convert 8 data bits into six base-3 digits (the signal shaping is possible as there are nearly three times as many 6-digit base-3 numbers as there are 8-digit base-2 numbers). The two resulting 3-digit base-3 symbols are sent in parallel over three pairs using 3-level pulse-amplitude modulation (PAM3)

Table 1: PAM3 Encodings.

PAM4 signalling is utilised in 400G PHY, which has the advantages of a half baud rate, additional voltage levels, and an improved signal-to-noise ratio (SNR) than existing non-return-to-zero (NRZ) schemes.

Figure 4: PAM4.

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