A traditional central office started with the need to house the fiber terminating electronics that connect the new market to the outside world. There also is the need to house and light the electronics facing the customers – although in some network design configurations some of the customer facing electronics can be housed in remote huts in neighborhoods.
A traditional central office needs room for a lot of other equipment. First is significant space for batteries to provide short-term backup in case of power outages. For safety reasons the batteries are often placed in a separate room. Central offices also need space for the power plant used to make the conversion from AC power to DC power. Central offices also usually need significant air conditioning and need room to house the cooling units. If the fiber network terminating to a central office is large enough there is also the requirement for some kind of fiber management system needed to separate the individual fibers in a neat and sensible way. Finally, if the above needs meant building a large enough space, many ISPs also built space to provide working and office space for technicians.
Lately I’ve seen several fiber deployments that don’t require the large traditional central office space. This is largely due to the evolution of the electronics used for serving customers in a FTTP network. For example, the OLTs (optical line terminations) electronics has been significantly compressed in size and density and a shelf of equipment can now perform the same functions that would have required much of a full rack a decade ago. As that equipment has reduced in size, the power requirements have also dropped, reducing the size of the power plant and the batteries.
I’ve seen several markets where a large cabinet provides enough room to replace what would have required a full central office a decade ago. These are not small towns, and two of the deployments are for towns with populations over 20,000.
As the footprint for the ‘central office’ has decreased there’s been a corresponding drop in costs. There are several supply houses that will now pre-install everything needed into the smaller cabinet / hut and deliver the whole unit complete and ready to go after connecting to power and splicing to fiber.
What I find interesting is that I still see some new markets built in the more traditional way. In that same market of 20,000 people it’s possible to still use a configuration that constructs several huts around the city to house the OLT electronics. For purposes of this blog I’ll refer to that as a distributed configuration.
There are pros and cons to both configurations. The biggest benefit of having one core hut or cabinet is lower cost. That means one pre-fab building instead of having to build huts or cabinets at several sites.
The distributed design also has advantages. A redundant fiber ring can be established with a network consisting of at least three huts, meaning that fewer parts of the market will lose service due to a fiber cut near to the core hub. But the distributed network also means more electronics in the network since there is now the need for electronics to light the fiber ring.
The other advantage of a distributed network is that there are fewer fibers terminating to each hut compared to having all customer fibers terminating to a single hut. The distributed network likely also has smaller fibers in the distribution network since fiber can be sized for a neighborhood rather than for the whole market. That might mean less splicing required during the initial construction.
Anybody building a new fiber network needs to consider these two options. If the market is large enough then the distributed network becomes mandatory. However, many engineers seem to be stuck with the idea that they need multiple huts and a fiber ring even for smaller towns. That means paying a premium price to achieve more safety against customer outages. However, since raising the money to build a fiber network is often the number one business consideration, the ability to save electronics costs can be compelling. It would not be unusual to see the single-hub configuration save half a million dollars or more. There is no configuration that is the right choice for all situations. Just be sure if you’re building FTTP in a new market that you consider the options.