Understanding 800G Optical Performance
As optical networking vendors announce 5th generation coherent DSP chips, supporting higher capacity wavelengths, some confusion has developed over the actual performance of 800G wavelengths in real world WDM operating networks, as opposed to highly controlled and optimized field trials or lab experiments. Over standard SMF-28 fiber, carriers should expect 100 – 220 Km optical reach in most WDM applications. While that’s significantly less than some of the industry grabbing 800G headlines, the differences are easily explained through a short review of the factors affecting optical reach performance.
The new 5th generation coherent DSPs represent significant technical achievements and push wavelength capacities close to the Shannon theoretical limits. However, the real benefit may be less about headline grabbing 800G wavelengths, and more about enabling 400-600G wavelengths over regional to subsea distances.
Capacity vs Reach Tradeoff
In coherent optics, modulation is used to encode digital ones and zeros into symbols that are transmitted at the baud rate, or symbol rate. Higher order modulations, such as 32QAM or 64QAM, encode more bits per symbol, but with a tradeoff of shorter optical reach compared to lower complexity modulations, such as QPSK or 8QAM.
Higher order modulations utilize more constellation points, which are spaced more closely together, resulting in higher optical to signal noise ratios (OSNR) requirements at the receiver. The higher OSNR requirements, for the 32QAM and 64QAM modulations, result in shorter optical reaches for these wavelengths.
Due to the capacity versus optical reach trade-off in coherent optics, high capacity 600G – 800G wavelengths have been limited to shorter reach applications, where lower order modulations (200G – 400G wavelengths) can be used on long haul (LH), ultra-long haul (ULH) and subsea wavelengths.
One of the key benefits of 5th Gen DSPs is inclusion of higher +90 Gbaud rates. The higher baud rates enable 400G – 600G wavelengths to use lower order modulation formats (8QAM or PCS shaped 16QAM) suitable for LH/ULH/Subsea applications. Unfortunately, 800G wavelengths, still require the use of 32QAM and 64QAM modulations, or equivalent probabilistic shaped 64QAM versions, limiting their optical reach.
800G: Real World Performance
Modulation format isn’t the only factor that limits higher-capacity wavelengths in real-world networks. Optical fiber type, fiber attenuation, and span distances (losses) also play primary roles in defining optical reach.
Vendors strive to differentiate their products and performance, but the reality is that most modern 4th and 5th generation coherent DSPs combined with leading optical component technologies (modulators, drivers, lasers), result in in similar capacity vs reach curves across the industry. In addition, how far an optical pulse propagates within a fiber is driven by the physics of light for a specific fiber type, route, and span losses. For 800G wavelengths, network operators can typically expect 100 – 200 Km distances over standard SMF-28 (0.23 dB/km) networks, under real world operating conditions.
But that doesn’t mean vendors don’t like to push the performance envelope if given an opportunity. There are a couple of strategies that can be used to boost performance on field trials and demos.
First, on real world networks transponders typically incorporate 2 to +3 dB of spare margin to accommodate such factors as component aging (10 – 15 year), temperature variation, component variance, polarization dependent loss (PDL), polarization mode dispersion (PMD), and nonlinearity impairments. The spare margins are a necessity to ensure proper operation over the life of the WDM network.
Field trials and demos are usually conducted without the 2 to +3 dB spare margin typically allocated in real networks, resulting in performance increases of 40 – 50%. Similarly, adjacent channels near the test wavelength can be removed (guard bands) to reduce or eliminate nonlinearity penalties, further improving performance.
Fiber type, attenuation, and span loss play an enormous role in defining maximum optical reach. Transoceanic subsea networks are capable of very long optical distances primarily due to their ultra-low-loss (ULL) fiber , low noise amplifiers (repeaters), and relatively consistent, short span distances. Boosting 800G performance on terrestrial networks is possible using similar low loss (LL) or ULL fiber types (< 0.18 dB/km), as opposed to standard single-mode fiber (e.g., SMF-28), which has losses of 0.22 – 0.25 dB/km. The small difference in the fiber attenuation specifications may may not seem great, it has a very large impact on the optical reach performance. Unfortunately, the specialized LL/ULL fibers that boost performance are not nearly as common in terrestrial networks as standard single-mode fiber (SMF-28 at 0.23 dB/Km).
Using these field demo strategies can result in significantly longer 800G optical reach than what is achievable in a real world production networks using standard SMF-28 fiber. As a result, operators should some caution when evaluating field trial or demo results to ensure they have accurate performance estimates over their own networks and fiber types, along with normal margin allocations.
400ZR Impact on 800G Wavelengths
The introduction of 400ZR optics is having an impact on high capacity network planning for metro edge and DCI applications. The new 400ZR standard defines an interoperable, pluggable, 400G coherent module for distances up to 120 Km, that incorporates the DSP inside the module. Integration of the coherent DSP into optical modules has long been an industry objective, but only recently become viable with availability low power 7nm CMOS technology. The small size, low cost, lower power and pluggable form factor, make the 400ZR an obvious solution for short reach high-capacity transport applications.
The introduction of 400ZR optics may reduce the demand for 800G optics, since both interfaces are targeting same short-reach applications. As shown in previous section, these high capacity (800G) wavelengths result in somewhat limited optical reach over standard SMF-28 fiber. This new 400ZR optical modules are projected to have wide adaptation for high-capacity short reach applications due to their 1) lower costs, 2) lower power, 3) smaller size, 4) pluggable form, and 5) interoperability, and will directly compete with 800G interfaces on these same deployments
Taking Light to the Limit
At any given optical reach, there is an upper limit on how much information can be transmitted over a communications channel. This theoretical limit was defined in Claude Shannon’s renowned paper “A Mathematical Theory of Communications” written in 1949 while a researcher at Nokia Bell Labs. While vendors like to differentiate their products and performance, modern coherent optical interfaces all operate under the same “physics of light” using very similar underlying core technologies.
The benefit of 5th generation DSPs, isn’t the 800G wavelengths that grab industry attention, but rather support of higher baud rates (+90 Gbaud), which enable 400G – 600G wavelengths over regional, LH, ULH, and subsea networks. For short reach (< 120 Km) applications, 400ZR / ZR+ optics are expected to dominate the application space due to their small size, low power, low costs, and interoperability. There will be some applications for 800G wavelengths, over specialty ULL fiber types, or on some short reach applications not utilizing 400ZR optics, but 400ZR optics will compete in this same short reach, high-capacity application area. For carriers, it’s important understand actual, realistic network performance that can be expected when using high capacity wavelengths, as opposed to optimized demo experiments.
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