I have a question regarding the 1000Base-LX or SX standard. The question is mainly on the electrical characteristics of the output and input requirements of the 1000Base-X.

The standard is based on the IEEE 802.3z. After trying to read through the standard document, I couldn't figure out what is actually the output and input requirement of the 1000Base-LX/SX interface. In other words, what is the standard output to an optical transmitter and what is the standard input requirement from an optical receiver that conforms to 802.3z.

802.3z documents

I'm fairly new to Ethernet and I have to admit, I dont quite understand all the wordings especially when I'm simply looking for the input/output standards. Thanks for any help in advance.


The reason I'm asking is because I'm developing a low-power receiver with differential output of VCM=0.2V and Vswing=0.2V (single ended), with SFP interface to interface it to a typical RJ45-SFP media converter (gigabit ethernet).

I need to know if my receiver output is compatible with the typically-used 1000BASE-X-compliant media converters out there. If it is not compatible, maybe I would need to use a limiting amp in my receiver to boost the signal, but I'd prefer to keep the limiting amp out to preserve my low-power spec.

  • \$\begingroup\$ When you are talking about input/output requirements, it's about voltage and timings ? \$\endgroup\$
    – zeqL
    Feb 25, 2014 at 13:20
  • \$\begingroup\$ It's actually more on the voltage levels. To add to zeqL's answer, I'm looking specifically at 1000BASE-SX and 1000BASE-LX. But i could not find the definitions for the output and input voltage levels for these two. \$\endgroup\$
    – user37749
    Feb 26, 2014 at 1:21

1 Answer 1


I think you missed an important figure of the Ethernet Standard about the 1000Base-X part:

enter image description here

This figure explains many things:

  • A PHY is composed of three parts: PCS (Physical Coding Sublayer), then PMA (Physical Medium Attachment) and finally the PMD (Physical Medium Dependent) part.
  • In case of 1000Base-X, this term "Base-X" is used to cover to three type of "physical" medium: 1000Base-LX, 1000Base-SX and 1000Base-CX, but as these medium share a common PCS and PMA, and their specifics PMD are quite similar, a common name is used: 1000Base-X.

As a reminder:

  • 1000Base-LX and SX: long wavelength (LX) and short wavelength (SX) laser, so optical fiber
  • 1000Base-CX: short-haul copper, usually a coax cable.

So you need to figure out which 1000Base-X medium you are using and just have to look to the appropriate section in the standard (802.3 Standard part 3)

  • Section 38 for 1000Base-LX and SX
  • Section 39 for 1000Base-CX
  • \$\begingroup\$ Hi zeqL, let me correct myself. I'm referring to 1000BASE-LX and 1000BASE-SX (for fiber application). In the document, I could not find the voltage level standard for these two, especially on the output (to the transmitter) and input requirement (from the receiver). \$\endgroup\$
    – user37749
    Feb 26, 2014 at 1:23
  • \$\begingroup\$ Or is it that the voltage levels are not defined by the 1000BASE-LX/SX. Instead, these voltage levels are defined by the optical interface that is used, such as SFP? Because, there are media converters that uses SFP, and there are others that uses different types of optical transceiver. \$\endgroup\$
    – user37749
    Feb 26, 2014 at 1:26
  • \$\begingroup\$ Yes voltages are defined by the transceiver. So for SFP you need to look for SFP electrical characteristics. \$\endgroup\$
    – zeqL
    Feb 26, 2014 at 12:31

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