Nokia IP routing for the 400GE
Entering the 400GE era
Relentless demand for more capacity at a lower cost per bit is forcing network providers to constantly rethink and reoptimize their network designs. Besides delivering more capacity for consumer internet and ultra-high-definition (UHD) video streaming services, they must provide high availability and low latency for the mission-critical and massive machine-type communication services that the cloud and 5G will enable.
Rapid advances in silicon are fueling a new generation of compact, pluggable coherent 400G optics that open exciting new avenues for optimizing IP-optical network designs. This is the second blog in a series about 400G as the universal currency for IP-optical convergence.
Evolution of coherent optics
Until recently, technology advances in coherent optics have focused on improving transmission performance with increasingly sophisticated digital signal processing (DSP) algorithms. Enormous progress has been made in this area, with probabilistic constellation shaping (PCS), introduced by Nokia in 2018, taking capacity close to Shannon’s limit to enable network operators extract maximum spectral efficiency from their networks. So where to next?
Improving optical transmission performance remains important for long-haul and subsea coherent transport applications, where fiber is expensive and scarce. But the main innovation focus has shifted to improving the power, space and cost efficiency of short- and intermediate-reach coherent optics. The introduction of 5G radio and evolution to distributed service architectures with compute, storage and peering resources located in the edge cloud are concentrating the bulk of capacity demands on fiber access, metro transport and regional data center interconnect (DCI).
Figure 1 shows the progress made in developing high-density optics, comparing progress in short-reach client optics for routers, and for pluggable coherent optics. Traditionally, there has been a sizable difference in the port densities of short-reach (gray) router optics and coherent line optics. The introduction of 400ZR and 400ZR+ pluggable transceivers closes the gap and removes the density penalty of using coherent optics in the same router ports designed for short-reach client optics.
The compact QSFP56-DD form factor offers tremendous port density but its power dissipation is nominally limited to 14.5 watts, although transceiver designs are now pushing this up to 20 watts. This constrains the capabilities and reach of coherent pluggables compared to designs for larger form factors such as CFP2 and line cards in WDM systems with coherent optics.
Network operators need a range of 400G transceiver options to optimally address the different capacity, cost, topology and reach requirements in wide area networks. Table 1 lists the various options and their key characteristics.
400ZR was one of the first efforts to standardize an interoperable 400G coherent interface specification. Developed by the Optical Internetworking Forum (OIF) and released in March 2020, 400ZR is profile-optimized for high-density access and point-to-point DCI applications. It can deliver 400 Gb/s up to 40km over a single dark fiber span without external amplification and support up to 64-channel WDM in the C-band and up to 120 km with external amplification. Although 400ZR can be supported in various pluggable form factors, QSFP-DD is the most prevalent implementation choice.
In contrast, 400ZR+ is a related, non-standard interworking specification that targets higher optical performance with flexible 100G–400G line rates and longer reaches by leveraging multiple modulation types (16QAM, 8QAM and QPSK) and high-gain forward error correction (open FEC). In 400G mode and depending on fiber quality and aging, 400ZR+ can reach up to 600 km with external amplification, and even further using subrates. It can also traverse a limited amount of reconfigurable add-drop multiplexer (ROADM) nodes with external amplification on the add/drop interfaces, albeit with reduced reach.
400G multihaul digital coherent optics (DCOs) are a new category of modular transceivers that package coherent optics in a larger form factor (CFP2) that can be equipped in routers and/or WDM transponder systems. Multihaul DCOs also support 100–400 Gb/s line rates using QPSK, 8QAM and 16QAM modulation. They leverage higher launch power to achieve longer optical reaches up to 750 km and can pass multiple ROADM hops.
400G Anywhere transceivers are conventional optical transponders that are performance-optimized to maximize wavelength capacity and reach. They take the form of integrated line cards that reside within WDM optical transport systems. State-of-the-art optical transponders can deliver wavelengths of 100–800 Gb/s over thousands of kilometers by applying sophisticated DSP techniques and high-gain forward error correction. Optical transponders are typically deployed in combination with ROADMs for regional and long-haul networks where fiber connectivity is scarce and costly. They are usually monetized as managed wavelengths for multiple services.
