# How does "bits per second" relate to wavelength?

I'm learning about optical networking at a new job, and am confused by some introductory terminology. I'm pondering this statement:

There is typically one transponder for every client signal and SFP transceiver, however as the DWDM wavelength is at least 10Gigabits per second, smaller client signals, such as 1GbE can be joined up together so that they share the same 10G wavelength.

I'm a bit thrown off by that phrase "10G wavelength."

My understanding is that a wavelength corresponds to a frequency of light, e.g. blue, red, and human-visible colors.

But in regard to a wavelength of light, e.g. something in the C-band - say 1535 nm - traveling down a fiber optic cable...what is the relationship between that 1535 nm wavelength and 10G?
Does that mean, e.g., that if ASK (amplitude shift keying) was the modulation mode, then there could be 10G pulses (whatever combination of "1" or "0") of 1535 nm light per second sent down the fiber?

• "Wavelength" as in "signal" basically. Like "the 10G cable". Sep 4, 2020 at 20:07

such as 1GbE can be joined up together so that they share the same 10G wavelength

That is trying to say that you can combine multiple 1G links (at the data link layer) to a single 10G link so you'll just be using a single wavelength.

DWDM systems use multiple, tightly arranged "colors" = wavelengths of infrared light (spaced less than one nm apart) to transport multiple, independent links on a single pair of fiber (sometimes even a single strand only).

Actually, that source is wrong, there are 1G DWDM transceivers as well.

But in regard to a wavelength of light, e.g. something in the C-band - say 1535 nm - traveling down a fiber optic cable...what is the relationship between that 1535 nm wavelength and 10G?

Each physical link uses one wavelength in the WDM system. The links are entirely independent, so each color could carry a completely different protocol - you could mux Gigabit Ethernet, 10G Ethernet, Fibre Channel, SDH, ... all onto a single fiber pair. (Some protocols like 40+ Gbit/s Ethernet may use multiple wavelengths for a single link split into lanes using their own CWDM scheme.)

Does that mean, e.g., that if ASK (amplitude shift keying) was the modulation mode, then there could be 10G pulses (whatever combination of "1" or "0") of 1535 nm light per second sent down the fiber?

Most 10G Ethernet variants use 64b/66b line code (-R PCS code), so the actual symbol rate is 10.3125 GBd = 10.3125 billion pulses per second (each one either light or dark).

• Trying to grok your answer (this is entirely new to me) - thank you. What property of a wavelength of light ties it to a certain number of pulses per second? I.e. why not just have a 1 Terrabit wavelength, and combine 1000 x 1GbE ethernet client signals? Is it just like any other physical equipment and it comes down to the physical capability of generating N pulses of light at wavelength X per second? Sep 4, 2020 at 19:04
• There is no direct connection between wavelength (=carrier frequency) and bit or symbol rate. Of course, the frequency needs to be sufficiently higher than the symbol rate, but that can be taken for granted with IR light (1550 nm ~ 193 THz). Due to material, laser and sensor limitations we can't get beyond some 50 GBd over substantial distances currently - single-lane 100G Ethernet uses PAM-4 amplitude modulation to run at 50+ GBd. Also, the fastest Ethernet currently is 400G.
– Zac67
Sep 4, 2020 at 19:31
• Yes, generating those fast pulses is a technological challenge, even with externally modulated lasers - 1 TBd symbols would be just 1 ps (picosecond) in length. Light in vacuum travels just 300 μm (micron) during that period.
– Zac67
Sep 4, 2020 at 19:32