Lambda to the Home and path dependencies

As the debate on Fiber-to-the-Home shifts to how to manage the transition from copper/coax to fiber, there is more attention for the future upgrade paths. First of all the attention will shift to the backhaul/middle mile networks which aggregate the traffic generated by a large number of users. As access speeds grow the capacity of these networks will have to grow. A well established upgrade path.

Even in the access networks there is already lot of development to increase the access speeds even more, up to the Gbps territory and beyond. The active Ethernet technology is quite stable but in GPON there is a proliferation of  new standards, not all of them compatible.

Digging deeper into these options it turns out that not all fiber access networks are the same. Topology matters and creates path dependencies.

As an example: if you have deployed a passive optical topology with deep splitters (close to the customers) with a high split ratio (> 32), you cannot upgrade from GPON  to 10GPON (4 times faster) without investing in the fiber plant or losing speed. You will have to reduce the split ratio which means laying new fibers to the splitter. 

The alternative is to switch to a different technology, wave division multiplex (WDM), sending multiple colors over one fiber . Applying WDM requires ripping out the splitters in the field and replacing them with a wavelenght filter that separates the colors to individual customers. The filter selects one wavelength per user,  making the fiber plant dedicated to WDM.

Path dependencies all over the place, unless you have a point-to-point topology.

Last night I had the pleasure to have a conversation with Johan Heneas, CTO of INS Communications (Norway). INS is a systemintegrator specializing in WDM technologies. His explanation of modern WDM technology led to an unexpected revelation: it will allow cost-effective "Lambda to the Home" in point-to-point topologies in dense urban area's.

The newest thing in WDM is a passive filter for many wavelengths (somewhere between 16 and 64). If you increase the number of wavelengths the filter has to be sharper and sharper. These passive filters are built from glass and are  sensitive to temperature changes.  Which becomes a problem if you increase the number of wavelengths and  place the filter outside in the field.  The solution so far was to add a heater and stabilize the temperature, requiring electrical power and something that can break in the field. Yuk....

The clever solution is to add another material to the filter which temperature sensitivity is exactly the opposite of the glass, compensating the changes. And it works. According to Johan their operational experience with these filters in the outside fiber plant (in the Norwegian weather) are excellent.

The second trick is to have automatic tuned lasers in each customers modem for the upstream signals. Each customer has its own separate wavelength for upstream bandwidth. It is possible to use the same wavelength as the downstream link but you run in to problems with reflections and the like when the speeds are increased.  

To prevent a logistic nightmare (you don't want to mix up the right combination of upstream and downstream wavelength per user) automatic tuning has been developed.  A broad spectrum lightsource is filtered at the head-end in (for example) 40 specific wavelengths. These wavelengths are adjacent to the sending lasers wavelengths so they automatically pass through the same filter to the right customer. That seed-light is a tuning trigger for the upstream laser which "clicks" to the right wavelength. Clever.

The technology is not yet standardized and still more expensive per user than GPON or active Ethernet (depending on the implementation factor 2-4). With larger volumes the gap will become smaller I guess.

This higher cost can be justified in rural areas where the distances to a POP or aggregation point are long (30 km or more). A standard shared topology (PON) runs into problems (not enough light) at larger distances, requiring a lower split ratio, more fiber and a higher average costs of OLT's ("head-end equipment") per customer.  WDM has a much larger optical power budget (15 db more), allowing for much longer distances.

The surprise came when we applied the technology to a situation like in Amsterdam, in a dense urban area with a point-to-point topology. What could this WDM possible bring in this environment? A bypass of the middle mile , removing the limitations.....

The filter is situated in a POP (with 10.000 fiber connections terminating in a POP). You patch a customer specific fiber to the filter and patch a backhaul fiber to the other side of the filter. The backhaul fiber can be 10-20-30 km of length before you terminate it in a WDM line card. So up to 40 individual customers get  their own separate lightpaths , bypassing the normal backhaul concentration links.

The WDM line card could be situated in an institution (science or research or government or media or high security environment) creating a dedicated ultra-high-speed optical Lambda-to-the-home.

If you live in Amsterdam the WDM line card could even be located near the AMS-IX, giving a dedicated link from the home to the biggest Exchange in the world. Mindboggling.

I can very well imagine that there is a market for this kind of service offerings.

And again an example what the advantages are of minimizing path dependencies in your topology......