Relentless traffic growth leads to a ROADM rethink

Technology briefing: ROADMs

Lumentum has developed an optical switch to enable reconfigurable optical add-drop multiplexers (ROADMs) to cope with the traffic growth expected over the next decade. 

The company’s MxN wavelength-selective switch (WSS) will replace the existing multicast switch used in colourless, directionless and contentionless ROADMs. The Lumentum TrueFlex 8x24 twin switch will enable networking nodes of 400-terabit capacity.

“This second-generation switch is what will take us into the 100 gigabaud and super-channel era of network scalability,” says Brandon Collings, CTO of Lumentum.

ROADMs

ROADMs sit at the mesh nodes in an optical network. Their function is to pass lightpaths destined for other nodes in the network - referred to as optical bypass - and enable the adding and dropping of wavelengths at the node. Such add/drops may be rerouted traffic or provisioned new services. 

As network traffic continues to grow, so do the degrees of a ROADM and the ports of its sub-systems. The degree of a ROADM is defined to the number of connections or fibre pairs it can support. In the diagram, a ROADM of degree three is shown.

A multicast switch-based 3-degree CDC ROADM. Source Lumentum.

It is rare to encounter more than five or six fibre routes leaving any given mesh node in a network, says Lumentum. “But in those fibre routes there is typically a large number of fibres - 64 or 128,” says Collings. “Operators deploy a conduit of fibre between cities.”

When the C-band fills up, an operator will light another fibre pair, taking up another of the ROADM’s degrees. ROADMs built today have 16 degrees. And since a fibre’s C-band can occupy some 30 terabits of data, this is how 400-terabit mesh nodes will be achieved.

“That is a pretty big node but that is the end [of life] capacity,” says Collings. “I don’t think you will find a 400-terabit node today but we build our networks so that they get there, five to eight years from when they are deployed.”

This raises another issue: the length of time it takes for any generational change of a ROADM design to take hold in the network.

“When a new approach comes along, it takes a couple of years for everyone to figure out how they will use it,” says Collings. Then, once a decision is made, it takes another two years to deploy followed by five to eight years before the ROADM node is filled.  

“Nothing happens quickly in this business,” says Collings. “But the upside, from a business point of view, is that as things are designed in, they have a long deployment cycle.”

Lumentum illustrates the point with its own products. 

The company is seeing growing demand for its dual TrueFlex WSS deployed in route-and-select ROADM architectures. “But we are still seeing growth on the older broadcast-and-select architectures underpinned by singe 1x9 WSSes,” says James Goodchild, director, product line management for wavelength management products at Lumentum.

CDC ROADMs

A colourless, directionless and contentionless (CDC) ROADM uses a twin multicast switch for the wavelength add and drop functions. The input fibre to each degree’s WSS is connected to the output path WSS of each of the ROADM’s other degrees. The input WSS also connects to the drop multicast switch (see diagram above).

Using a WSS on the input path means that only wavelengths of interest are routed to the WSS’ output ports. Hence the ROADM’s reference as a route-and-select architecture.

Using a 1xN splitter array instead of a WSS for the input path results in a broadcast-and-select ROADM. Here, the input fibre’s wavelengths are broadcast to all the N output ports. The high optical loss associated with the splitters is the main reason why CDC ROADM designs have transitioned to the WSS-based route-and-select architecture. 

This second-generation switch is what will take us into the 100 gigabaud and super-channel era of network scalability

However, there is still an optical loss issue to be contended with, introduced by the add or drop multicast switch. Accordingly, along with the twin multicast switch are two arrays of erbium-doped fibre amplifiers (EDFAs). One EDFA array is on the drop ports to the MxN multicast switch and the second amplifier array boosts the outputs of the add-path multicast switch before their transmission into the network.

The MxN multicast switch comprises 1xN splitter arrays, N being the number of add-drop ports, and Mx1 selection switches where M is the number of directions the ROADM supports. A typical multicast switch is 8x16: eight being the ROADM’s number of directions and 16 the drop-port count.  

Each of the N splitter arrays sends the signals on a drop port to all the Mx1 selection switches where each one pulls off the channel to be dropped. Having a selection switch at each of the multicast switch’s N drop ports is what enables contentionless operation, the avoidance of a collision when the same wavelength is droppedat a node from different degree directions.

MxN switch

Lumentum’s decision to develop the MxN switch to replace the multicast switch follows its study to understand how optical transmission networks will evolve with continual traffic growth.

One development is the adoption of higher-baud-rate, higher-capacity coherent transmissions that require wider channel widths. A 400-gigabit wavelength requires a 75GHz channel compared to the standard 50GHz fixed grid used for 100- and 200-gigabit transmissions. Future transmission speeds of 800 gigabits will use two such channels or 150GHz of spectrum, while a 1 terabit signal is expected to occupy 300GHz of fibre spectrum. “This is how we anticipate coherent transmission evolving,” says Collings.    

Moving to wider channels also benefits the ROADM’s cost. If operators continued to use 50GHz channels, the channel count would grow exponentially with the growth in traffic. In contrast, adopting wider channels means the add-drop port count grows only linearly with traffic. “Using wider channels, the advantage is you don't have to support 600 ports of add-drop in your ROADM networks,” says Collings.

But wider channels means greater amplification demands on the EDFA arrays, an issue that will only worsen over time.

Multicast switch-based designs don’t support the wider channels we know are coming

Losing the amp   

Because the power spectral density is constant, the power in a channel increases proportionally with its width. For example, a 75GHz channel has 2dB more power compared to a 50GHz channel spacing, a 150GHz channel 5dB more while a 300GHz channel has an extra 8dB.

The EDFA array is engineered to handle the worst case power requirement that occurs when all 16 optical transceivers into the multicast switch go to the same ROADM degree. Here the EDFA must be able to boost all 16 channels.

For a multicast switch with 16 ports, 22dBm amplification is needed for a 150GHz channel which requires going from an uncooled pump design to a cooled pump one. Equally, 25dBm amplification is needed for 300GHz channels. And as the number of degrees grows, so do the demands on the amplification until no practical amplifier design is possible (see diagram).  

The EDFA requirements to compensate for the optical loss of the multicast switch. The complexity of the EDFA design grows with the multicast switch's port count until it becomes insupportable. Source: Lumentum.

“This is not an issue today because we use very modest-sized channels and we engineer our systems to accommodate them,” says Collings. “But if you look forward, you realise they [multicast switch-based designs] don’t support the wider channels we know are coming.”

Using a WSS-based MxN switch solves this issue because, as with the input port WSS of a route-and-select architecture, the switch has a lower optical loss - under 8dB - compared to the 17dB of the splitter-based multicast switch. 

The sub-8dB loss is below the threshold where amplification is needed: the optical signal is sufficiently strong at the drop port to be received, as are the added signals for transmission into the network. The resulting removal of the EDFAs simplifies greatly the complexity, size and cost of the CDC ROADM.  

“The MxN is a WSS - it’s a router - so it sends all of the light in the direction it is supposed to go,” says Collings. “You can push through the MxN switch channels of any width and of any power because there is no amplifier that needs to be there and be designed appropriately." 

The resulting second-generation CDC ROADM design is shown below.

Source: Lumentum

Lumentum's Goodchild says the 8x24 twin implementation of the MxN switch will be available in the first quarter of 2019. 

“Certain systems vendors already have access to samples,” says Goodchild.  

Further reading 

2D WSSes, click here

ROADMs and their evolving amplification needs, click here