NRZ

What NRZ Actually Is

Non-return-to-zero represents a one as one amplitude and a zero as another, and never returns to a neutral level between symbols. One symbol, one bit. It is the simplest line code that works, and it carried optical Ethernet from the first gigabit link through to 100GBASE-SR4.

Because there are only two levels, the whole amplitude range is one eye opening. The receiver has a single decision threshold to place and a lot of room either side of it, which is why a 10G NRZ link is specified to run error-free with no error correction at all.

The Symbol Rate Is the Whole Problem

One bit per symbol means the symbol rate equals the bit rate. A 25G NRZ lane is 25 gigabaud; a 50G NRZ lane would be 50 gigabaud, and a 100G one would be 100 gigabaud. The Nyquist frequency is half the symbol rate, so that last case asks a printed circuit board, a connector and a package to pass energy cleanly at 50 GHz.

Copper loss rises with the square root of frequency, and connector reflections and crosstalk get worse faster than that. Somewhere around 28 gigabaud the channel stops being a design problem and becomes a physics problem. The industry did not abandon NRZ because PAM4 was elegant; it abandoned NRZ because the board ran out of bandwidth.

What You Gave Up by Leaving It

Splitting the same amplitude range into four levels costs roughly 9.5 dB of signal-to-noise ratio in theory, and more in practice once transmitter nonlinearity is counted. On NRZ that margin was spent on cheap components, long multimode reaches and relaxed manufacturing tolerances.

It also changed what a working link looks like. NRZ links either ran clean or were broken. PAM4 links run with a constant background of raw errors that forward error correction repairs, so the operational habit of treating any error count as a fault has to be unlearned — see bit error rate for what the two counters actually mean.

Where You Still Meet It

Everything at 25G per lane and below is NRZ: 10GBASE-SR and LR, 25GBASE-SR and LR, 40GBASE-SR4 as four 10G lanes, and 100GBASE-SR4, PSM4, CWDM4 and LR4 as four 25G lanes. That is still an enormous installed base, and it is the reason 100G optics remain cheap.

It matters at breakout boundaries. A 100G NRZ port breaking out to four 25G lanes and a 100G PAM4 port carrying a single 100G lane are electrically unrelated even though the datasheet says 100G, and mixing them is a common cause of a port that never links. The standard lookup will tell you which generation a given PMD name belongs to, and the breakout configurator which splits are legal.

Related Terms

PAM4 is what replaced it, SerDes is where the choice is made, FEC is the price PAM4 pays, and TDECQ is the compliance test that replaced the NRZ eye mask.