Driven by a new generation of consumer and business services and applications, the demand for bandwidth continues to pose significant challenges for operators of transport networks.
While this bandwidth growth is ubiquitous across all networks, metro networks are projected to be particularly affected. According to a recent industry report, bandwidth in the metro is expected to grow by more than 80% in the next three years. This type of growth in services and applications will significantly strain network operators' capital budgets and will challenge their operational organizations to keep up with the demand.
Facilitating cost-effective and efficient network growth requires taking a fresh approach to transport networks. Recent innovations in "optical express" capabilities for transport networks promise to help network operators reduce costs, simplify operations, and extend service flexibility to a wider variety of network types.
DWDM and optical express
Introduced in the 1990s, DWDM has proven a cost-effective approach for managing simple point-to-point bandwidth growth in transport networks. DWDM increases fiber efficiency by enabling multiple optical signals to be carried across a single fiber, with each signal transmitted on a different wavelength, or color of light. The advent of tunable and pluggable transceivers and advancements in passive DWDM filter technology (mux/demux) have created the technological foundation for efficiently expanding network capacity in point-to-point applications without the substantial capital and operational expense required when lighting additional fibers.
In addition, because DWDM signals operate at 1550 nm (or within the C-Band), network operators can take advantage of cost-effective and readily available optical amplification to provide connectivity across greater distances without the need for costly electrical regeneration.
However, while point-to-point DWDM can provide some relief for bandwidth growth, it requires that all services be terminated at every location. This type of approach is only effective at reducing fiber costs and will not significantly reduce equipment costs or the operational complexity associated with significant network growth. As bandwidth demands continue to increase, it becomes extremely cost-prohibitive for network operators to continue to terminate services at every intermediate location.
Optical express capabilities can solve this problem. With optical express, individual wavelengths can passed through intermediate locations exclusively in the optical domain. The economic benefits of optical express are significant, as optically expressing a service through an intermediate node is orders of magnitude less expensive than terminating that signal electrically (Figure 1).
|Figure 1. The economic benefits of optical express – 20X less the cost of electrical termination.|
The economic benefits of optical express become even greater the more locations and the more traffic there are in a network. Figure 2 illustrates a five-node hub-and-spoke network where the total network traffic demand (the sum of all traffic from the hub to each of the end nodes) grows from 1x10G shared among all four destinations to a dedicated 10G to each of the four destinations.
|Figure 2. A typical five-node hub-and-spoke network configuration.|
The typical economic crossover point between networks with optical express and those without is roughly 2x10G of total traffic. As illustrated in Figure 3, this could be a case in which one location requires a dedicated 10G and the other three locations share the remaining 10G, or each pair of two nodes shares 10G. In either case, optical express capabilities will substantially reduce the total number of interfaces required by the network and subsequently provide a more cost-effective architecture.
|Figure 3. Possible optical express configurations for 2x10G in a five-node network.|
ROADMs and optical express
While DWDM can provide a mechanism for optical express, providing this capability with fixed-filter DWDM platforms is a manually intensive affair. With fixed-filter systems, technicians must manually install fiber jumpers at intermediate locations, as well as manually and frequently balance wavelength power levels throughout the network. This high level of manual effort will significantly hamper bandwidth growth and significantly increase operational expenses.
Reconfigurable optical add/drop multiplexers (ROADMs) overcome these operational challenges. ROADMs enable remote provisioning of optical express connections and provide automated, per-channel power balancing across the network (Figure 4). As a result, when turning up a new service, manual effort is required only at the endpoints of the service (to physically install the new interface modules), while zero manual effort is required at the intermediate locations. ROADMs provide all of the economic benefits of optical express while eliminating the operational difficulties of fixed-filter hardware.
|Figure 4. Optical express facilitated by ROADM technology.|
ROADMs provide numerous other network benefits, including facilitating troubleshooting and reducing network complexity. However, the three primary and most significant benefits ROADM-based networks provide are:
- accelerated service-activation times
- reduced regeneration costs
- improved network reliability.
By eliminating manual intervention and dramatically simplifying network engineering, it is easy to see how ROADMs can dramatically accelerate service activation times – often by as much as 80% (Figure 5). Additionally, because ROADMs can balance signal power levels more accurately than manual balancing, they provide better overall optical performance in the network. This improved optical performance will result in a reduction in the number of times a signal will require costly electrical regeneration before reaching its destination. Elimination of these expensive electrical regeneration points can save as much as 30% in equipment costs.
|Figure 5. Key benefits of ROADMs.|
Finally, based on network studies, a full 75% of service interruptions in a typical network are the result of human error. Because ROADMs significantly lessen manual and physical interactions when activating services, they have proven to reduce service interruptions by as much as 63%.
With the rapid rise in bandwidth requirements, the benefits provided by optical express capabilities in ROADMs are becoming more essential in a greater number and wider variety of networks. As a result, the industry is starting to witness the emergence of new optical express innovations and system designs that promise to reduce overall costs, as well as first costs for deployment.
These innovations include multi-degree ROADM technology with all of the necessary components packaged into a high-density, single-slot module (e.g., eight-degree ROADM cards). These highly flexible colorless and directionless ROADMs provide an ideal option for dense metro, regional, and mesh-based applications, as well as select long-haul applications.
The power of automated edge OADM technology
Despite the benefits and advances such as multi-degree capabilities, ROADM deployment faces a significant challenge – the systems' cost, due largely to the wavelength-selectable switch (WSS) technology that is the key enabler of traditional ROADMs. This has resulted in ROADMs being deployed primarily in very high-density core metro and/or regional networks.
The advent of automated, non-WSS, edge-optimized ROADM capabilities provide an innovative approach to extending optical express capabilities to more network types, as well as extending deployment closer to the network edge.
These highly integrated single-slot blades include per-channel power balancing, variable gain amplification, and operationally efficient optical express capabilities without the expensive WSS component. They cost-effectively provide network operators the ability to achieve the full benefits of optical express in edge applications.
The adaptability factor
There is one overarching fact about all network requirements – they are not static. It can be difficult for network operators to predict when and where significant traffic growth will occur. Optimal networking equipment therefore will provide not only the flexibility to support multiple configurations in a single device, but also the ability to adapt from any initial configuration to any other configuration. e.g., grey light, passive DWDM, automated edge OADM, and multi-degree ROADM configurations. The best of these platforms will enable such migration as seamlessly as possible by simply adding components as necessary, without the need to swap out existing equipment and without complex recabling or reconfiguring of existing systems.
Optical express expansion
As network operators struggle to design future-proof transport networks capable of efficiently and cost-effectively supporting traffic growth without reducing service profitability or unreasonably burdening their operational organizations, optical express capabilities will become more and more vital. With significant advancements in ROADM technology, coupled with the innovations such as automated edge OADM, cost-effective and operationally friendly optical express capabilities are now accessible to a wider variety of networks and can be extended closer to the network edge.
Rob Shore is director of product marketing at Coriant. He has more than 21 years of telecommunications experience. Prior to his current role in marketing, Rob worked in Coriant's technical sales organization managing the Solution Sales Management team for the Europe, Middle East, and Africa (EMEA) regions. He has also held positions in the North America sales organization, market management, segment marketing, Tier 3 technical support, product development and solution validation at Coriant.