Tunable Optical Transceivers – When To Use?

Already for some time, tunable optical transceivers for use in DWDM systems, such as XFP and SFP+, have been available for the optical networking industry. Current evolution of tunable optical transceivers typically supports full ITU-T C-band, allowing dynamic tuning of transmitter wavelength with 50GHz steps. However, even longer time conventional or fixed wavelength DWDM transceivers are in the market. We feel that some of our customers are puzzled about choosing between fixed and tunable optical transceivers. I will try to shed some light on this by expressing my opinion.

What did I believe when I first heard about tunable optical transceivers? Well, I was thinking that it is revolutionary and that tunable transceivers will be part of future Metro Ethernet and Optical Transport Networks. I was imagining that in the ecosystem of Software Defined Networks, network intelligence will make decisions about required bandwidths in different directions, will dynamically add or drop some wavelengths, and that an essential part of that will become tunable optical transceivers. What is reality now, after some years with tunable optics in the market?

At first, Metro Ethernet. Nowadays, Metro Ethernet mainly consists of IP nodes, such as switches and routers, interconnected by several 10G interfaces. The main use case of DWDM in metro networks is the economy of black fiber. Usually, operators implement DWDM by using colored transceivers directly in 10G ports and connecting to the line through Passive optical multiplexers. These IP nodes focus on L2/L3 packet processing, but their optical network functionality is quite limited. From currently popular IP platforms, only a few actually support tuning of wavelengths by CLI commands.

Then there is Optical Transport Networks (OTN), which focuses on carrying payload and multiplexing, switching, and supervising networks in the optical Layer 1 domain. The focus of current systems is packaging of payload in the most effective transport containers, and for Layer 1 management, OTN uses packages of Wavelength Selective Switches (WSS), also called ROADM (reconfigurable optical add-drop multiplexer), which allow switching wavelengths in multi-direction nodes from ring to ring, branch to branch. WSS/ROADM technologies allow rapid implementations of backup hot standby transport optical routes. But what about tunable optical transceivers in OTN..? Still, the mainstream is conventional fixed-wavelength DWDM transceivers.

How many optical ports from the total actually implemented are with tunable optical transceivers? Hard to say about this research's precision, but we found such a graph online, saying that from all optical ports shipped, only about 0.5% are tunable, and we tend to believe it:

So, what is the current use case for tunable optical transceivers? When to consider the implementation of a DWDM network based on tunable transceivers? Take into account that tunable transceivers are much more expensive compared to conventional fixed wavelength DWDM transceivers. I see two main applications:

Spare Part – when you're running a big DWDM network with a high number of nodes, and let’s say use up to 80 (50GHz spacing) different wavelengths, spare part management quickly could become a nightmare. You need to have a couple of transceivers of each wavelength and possibly in different locations, so that your field technicians can access network nodes quickly enough. In this case, a few tunable modules in place of a few hundred fixed ones is a very good and cost-efficient idea. If your platform supports in-port tuning, it’s excellent – if not, you will additionally need a special programming board to allow the tuning module to operate at the necessary wavelength. (As us – we can deliver compatible tunable optical XFP and SFP+ modules and corresponding boards).

Really big transport network with 400G or 1T applications on the horizon. Yes, with already now available coherent optical technologies, such as Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK), it is possible to fit 100G transmission bandwidth within a DWDM 50 GHz channel. So if 100G is too little for you, then future 400G and 1T transmission formats are expected to be bulky and not fit within 50GHz spacing. These future new data rate formats require that the channel plan/spacing is flexible, that your OTN system can adapt to new rates, and can re-arrange channel spacing to find a place for new rates in it.

Thank you for reading so far – this is just an opinion, if you have to add something, always welcome sales@edgeoptic.com.