As data consumption grows and networks evolve, selecting the right open line system (OLS) for IP over DWDM (IPoDWDM) is more important than ever. Among the available options, reconfigurable optical add-drop multiplexer (ROADM)-based architectures stand out for their flexibility and ability to reduce power and cost by avoiding unnecessary regenerations.

The point-to-point (P2P) OLS solutions discussed in the previous blog are ideal for simple IPoDWDM topologies and low-capacity scenarios. They offer straightforward planning and minimal operational overhead. On the other hand, more complex topologies – such as longer rings, chains or hybrid mesh-ring networks – benefit greatly from the flexibility and optical bypass capabilities that ROADM architectures provide. 

ROADMs are devices that direct individual wavelengths through or into a network node. This enables more dynamic routing, simplifies scaling and avoids unnecessary optical-electrical conversions, helping to reduce cost and power use. Choosing between a point-to-point or ROADM-based architecture means balancing operational simplicity with potential savings in capex and energy – a decision shaped by network design, traffic patterns and operational preferences.

ROADM networks provide additional benefits besides providing optical interconnections in between routers. ROADMs enable bridging optical connectivity across metro, regional and core network segments and support transparent wavelength services in wholesale environments. With flexgrid capabilities, they accommodate current and future spectral demands, including 800Gbit/s, 1600Gbit/s and beyond. They also improve topological agility, making it easy to add spurs or ring extensions without disrupting service. Spectrum services – such as alien wavelengths, partial-to-full allocations or dynamic spectrum sharing – can be provisioned flexibly, with integrated monitoring providing clear demarcation and visibility.

Network control and service assurance

While simple P2P links may require only minimal management, more dynamic ROADM-based networks benefit from connection-oriented service management and robust FCAPS (fault, configuration, accounting, performance and security) capabilities. These functions – including provisioning, wavelength assignment and fault resolution – are typically handled by an optical network controller, such as our Mosaic Network Controller.

ROADM options for IPoDWDM

Depending on network goals, operators can tailor their ROADM architecture for flexibility and uniformity or reduced cost.

1. Colorless ROADM: Simple and flexible

Many operators prefer colorless ROADM add/drop ports and uniform node configurations across the complete network. In this context, ‘colorless’ means that software can assign any wavelength to any port, eliminating the need for extensive preplanning. This also enables remote provisioning – simply patch a new transceiver in the IP router to a free ROADM port and configure the device accordingly from remote.

There are multiple ways to implement a colorless architecture. One common approach is the route-and-select (RS) model, which offers additional control and protection against misconfiguration. RS ROADMs use two wavelength-selective switches (WSSs): the first routes each incoming wavelength to the pass-through path or local drop, while the second selects outgoing signals from either the local add or the through direction. This ensures only configured wavelengths are inserted into the network, helping to block misconfigurations at the add side.

Adtran’s FSP 3000 12-degree RS ROADM card enables highly compact ROADM nodes for IPoDWDM use cases. Combined with a twin EDFA amplifier card in a small “pizza-box” shelf (as shown in the figure below), this setup supports a full nodal degree serving a fiber pair for one specific direction. Operators typically deploy one shelf per direction for redundancy. Routers often connect in direct attach mode, linking directly to a WSS port without needing a separate multiplexer. This ensures simple setup and strong optical performance. For example, in a node with fibers connecting from two different directions, a so called two-degree node, one port of the 12-port WSS serves the opposite line direction, leaving 11 ports free for local add/drop traffic. A 1x8 splitter can be added if more local ports are required. The equipment shown in the picture below is the same for all networks nodes – whether it’s a spoke site, hub site, interconnection point or a link to additional spurs or subtending rings. All these nodes can be realized with the same uniform device per direction.

2. ROADM with quasi-colorless ports: Cost-effective option 

Colorless ROADMs offer maximum flexibility and full software control, but at a price. For more static environments, a ROADM with wavelength specific ports can provide a simpler, more affordable alternative. While traditional fixed wavelength ROADMs offer only one specific wavelength per port, Adtran’s design enables a more versatile approach.

A good example is Adtran’s FSP 3000 two-degree ROADM for intermediate nodes in chains or rings. It combines a low-cost two-degree ROADM with an eight-port wideband add/drop filter. The filter provides access to eight 425GHz-wide subbands rather than just individual wavelengths. This offers two key benefits: first, routers can use any wavelength within those ranges, maintaining the benefits of remote wavelength configuration within those limits; second, the design is future-ready – supporting pluggables speeds up to 1.6T. 

For cost efficient terminal sites at the end of these ROADM chains or rings, filters and amplifiers can be combined to increase add-drop capacity. Where connectivity to additional network domains is required, the same filter can be used in a ROADM to terminate wavelengths locally as needed.  

In today’s fast-moving optical landscape, selecting the right OLS for IPoDWDM is about more than just capacity. Choosing between P2P OLS and ROADM-based systems means balancing operational simplicity with savings in capex and power consumption – a decision that depends not only on topology and traffic requirements but also on organization and operational preferences. By selecting the architecture that best fits your network – whether it’s ultimate flexibility or cost-optimized performance – you can build smarter, more resilient infrastructure. We’re here to help you design the most efficient ROADM-based OLS for your network.