Breaking through network capacity limits

With the increasing demand for high-speed data, internet and video services, carriers encounter unending network upgrades to support higher and higher transport capacities. Many carriers are reporting 30 – 50 percent annual bandwidth growth over their core networks. Capacity requirements will only continue to increase due to the shift to cloud based applications, 5G wireless networks, work from home initiatives, and wider deployment of the Internet of Things (IoT).

Many carriers operate regional and long-haul (LH) networks over leased fiber pairs, commonly referred to as “dark fiber”, instead of physically owning their own fiber cables and routes. Unfortunately, once these existing dark fiber networks are filled, operators are forced to lease additional fiber pairs to expand their network capacity. Leasing fibers over regional or long-haul distances, ranging from 800 – 3,000 km, can be prohibitively expensive, in addition to months long delays needed to install and turn-up new overlay WDM networks.

Nokia’s C+L WDM systems provide the ideal solution for increasing network capacity, without the expense or time delays incurred with additional dark fiber leases.

Reaching C-Band limits

Until recently, the optical networking industry kept pace with ever-increasing bandwidth requirements by utilizing higher speed wavelengths. Capacity increased by simply deploying faster and faster wavelengths, initially from 10G to 100G wavelengths, and more recently from 100G to 200G to +400G wavelengths. In addition, spectral efficiency improved by reducing channel sizes to better match wavelength modulation and baud rates. These two options of increasing wavelength capacity and reducing channel spacing are interrelated, but both are constrained by the overall Shannon Limit. At any given optical reach, there is an upper limit on how much information can be transmitted over a fiber route. This theoretical limit was defined in a famous paper titled “A Mathematical Theory of Communications” written by Claude Shannon in 1949 while a researcher at Nokia Bell Labs, simply referred to as the “Shannon Limit”.

Modern coherent DSPs offer multiple baud rates, multiple modulation formats, and strong Forward Error Corrections (FEC) to provide optimized wavelength capacity at any reach – from metro to subsea distances. These advancements in DSPs and coherent optics have resulted in tremendous capacity improvements over the last 10 years, while reducing optical networking cost per gigabit. Unfortunately, with modern transponders operating so close to Shannon limit, future capacity and spectral efficiency improvements will be fairly limited.

How does a carrier efficiently and cost effectively expand their network capacity? By adding L-band capability to their existing networks, carriers can double their overall network capacity. Doubling capacity over existing fibers, by utilizing combined C+L WDM systems, is especially critical for carriers operating over leased fiber, since the C+L systems eliminate the need for leasing additional dark fibers.

 Figure 1) Network Capacity Options

Flexible upgrade options

A key aspect of Nokia’s C+L WDM lines systems is the efficiency and cost effectiveness of network expansions. In other words, how easy is it to add L-band capacity, to existing, deployed C-band networks. Carriers have different operational and budget requirements for network expansions, so supporting modular, flexible upgrade options ensures wide industry adoption – as opposed to “one size fits all” constraints found on some C+L WDM platforms.

Many carriers choose to deploy their core networks with standalone C-band WDM systems. Once the C-band channels are filled, L-band equipment is added to existing ROADM and ILA sites. Adding the L-band equipment as needed, reduces the network’s “initial deployment cost” – it’s the ultimate “pay as you go” process, as shown in Figure 2a.

Another option is to deploy combined C+L nodes as part of the initial network deployment, but only at intermediate line amplifier (ILA) site. Since long haul ILA nodes tend to be located in remote locations, installing C+L ILA nodes as part of the initial network build, eliminates the time and operations cost to revisit those remote sites during L-band upgrades. When additional network capacity is needed, only the ROADM sites need to be upgraded with additional L-band equipment, as shown in Figure 2b. Since ROADM nodes are typically located in staffed central offices and data centers, these upgrades can be performed quickly and efficiently.

Figure 2) Flexible Upgrade Options

Nokia – industry leader in C+L systems

Nokia has been the industry leader in C+L WDM networks, since the introduction of its first generation (Gen 1) 1830 PSS C+L system in early 2017, gaining wide industry adoption and global deployments.

Figure 3) Global Industry Adoption – Nokia C+L

In 2020, Nokia released its second-generation C+L WDM (Gen 2) platform, integrating higher performance and higher node capacities, while reducing physical node sizes. Based on Nokia’s iRDM32 C-band and iRDM32 L-band integrated ROADM cards, the latest Nokia Gen 2 C+L systems provide industry leading performance, integration, and flexibility. Available supporting both CDC-F (colorless, directionless, contentionless, Flexgrid) and C-F (colorless, Flexgrid) architectures, the 1830 C+L Gen 2 systems are ideal for C-Band, L-Band, or combined C+L band applications.

Figure 4) Nokia 1830 PSS C+L Gen 2

The additional optical layer flexibility provided by CDC-F ROADMs enables powerful optical layer restoration, improves network capacity and utilization, while reducing overall networking costs. Network operations that once required costly, time-consuming, manual changes can now be fully automated. Within a CDC-F ROADM, it is the add/drop block, using either multicast switch (MCS) or MxN WSS technology, that enables any transponder to be connected to any incoming port (colorless), any port to be connected to any ROADM line degree (directionless), while avoiding wavelength interference (contentionless), all without requiring any manual or operator intervention.

Both MCS and MxN WSS technologies perform the same CDC-F ROADM add/drop function. Multicast switch (MCS) based architectures offer slightly lower ROADM node costs than MxN WSS options, while MxN WSS-based systems support very large add/drop port counts (> 600 transponder ports) and are better suited for use with new, low power coherent pluggables. Fortunately, Nokia’s 1830 PSS systems support both MCS and MxN WSS units on their C-band and C+L WDM platforms, so carriers can choose the best option for their individual network applications. For more in-depth information on relative benefits of MCS and MxN WSS add/drop technologies, please read the “Revolutionizing optical networks with key new technologies” blog.

Benefits of C+L networks

Over the past ten years, increasing wavelength capacities have resulted in a tremendous increase in overall network capacity, while dramatically reducing the cost per gigabit for optical networks. However, as wavelength and fiber capacities approach Shannon limits, future bandwidth and spectral efficiency improvements will be much more limited.

Combined C+L ROADM networks enable carriers to double their network capacity, without the need for additional, costly dark fiber leases. Modular and flexible C+L WDM systems ensure carriers can upgrade their networks in the most efficient and cost-effective manner. Nokia has been the industry leader with thousands of C+L WDM nodes globally deployed. In 2020, Nokia introduced the second generation 1830 PSS C+L (Gen 2) platforms based on new high performance, high capacity iRDM32 cards. The new Gen 2 WDM systems support both CDC-F and C-F ROADM architectures, as well as both multicast switch (MCS) and MxN WSS add/drop technologies, so carriers can choose the best fit for their networks.

Nokia – Taking networks beyond the limit.