Whangarei is the first town off the blocks in the ultra-fast broadband space race, with Crown Fibre selecting Northpower for negotiations back in September, then the ceremonious connection of a school a few days ago — a completely meaningless event as there is still no solid definition around the UFB that differentiates this from any other fibre connection in the country. But Whangarei is a nice place, so I thought I’d have some fun and look at it as a short case study.
Deployment area
Crown Fibre has released a fact sheet which includes the following map of the deployment area. The deployment will cover the urban area of Whangarei that they claim consists of a “population of nearly 52,000, including more than 2,000 business premises, and over 300 medical and other healthcare services, with over 20 schools.” It’s also important to know how many homes there are, so for that we turn to the 2006 census and find that there are 28,149 occupied dwellings in Whangarei District. The district actually includes a large rural area not currently covered by Northpower’s indicated UFB deployment, but we can refine this using the meshblock data to find there are 16,851 occupied dwellings in the indicated deployment area.
But what about the rest of Northland? There are plenty of smaller centres with reasonable populations, for example Dargaville with around 4,500 residents, or Kaitaia and Kerikeri with around 5,000 each. I think we should have a road map for these centres. This is where Telecom should have an advantage over other proposals, if only they hadn’t spent so much time trying to derail the project and had instead embraced it from the start — but I want to talk about that in a future post, so for now let’s move onto the network design.
Network basics
The network will use GPON to deliver wholesale Ethernet services for use by the majority of customers, but the physical network will also support wholesale dark fibre for use by service providers, enterprise customers, and big institutions. The optical distribution network (ODN) has a dedicated fibre between every premise and the local convergence point, also known as a fibre distribution hub (FDH). These are small passive cabinets, and there is usually one for every 200 to 400 premises. We locate the splitters in the FDHs because this gives us the optimal flexibility, future proofing, and manageability. Feeders run from the FDH back to the central office where the OLT is housed. The term central office typically creates visions of a large exchange building, but this is not the case in our network. We only have a small amount of equipment and small number of fibre terminations to house, so a large exchange building is quite simply overkill and would be a massive unwarranted cost. A roadside cabinet or portable telecommunications hut will be sufficient in most cases. The OLT is then connected back to the aggregation switch where handovers are made to service providers.
You may be wondering about how I arrived at the ranges shown on that drawing. The range between the aggregation switch (BSA) and OLT can be whatever is supported by the installed optics, and then much longer in cases where DWDM is used to provide transport. I use <70 km as a rule of thumb for discussing concepts. The range between the OLT and the ONT is a factor of the split ratio, the GPON type, and number of connectors and splices. The typical example I use to get my <10 km rule of thumb is described below.
To start with we must know our power budget, which is the difference between the launch power and the recieve sensitivity as described in ITU-T G.984.2. For class B optics we have a 26 dB budget and for class C optics we have a 31 dB budget. This budget gets consumed as follows.
- 3 dB reserve for cable breaks and repairs.
- 0.5 dB for each pair of mated connectors. We have a connector at the port on the OLT and another at the port on the ONT. Ideally in between these everything is fusion spliced to reduce faults and maintenance, but I’ll allow for another connector at the NAP in case pre-connectorised drops are being used. 3 × 0.5 = 1.5 dB
- 0.05 dB for each fusion splice. Let’s allow for 10 fusion splices between the OLT and farthest ONT. 10 × 0.05 = 0.5 dB
- 3.5 dB for a every 1:2 split. Higher ratio splitters are just trees of binary splits, so the formula is log2(n) where n is the split ratio. Let’s allow for a typical 1:32 split ratio. log2(32) * 3.5 = 17.5 dB
At this point we’ve accumulated 22.5 dB, leaving us with around 4 dB for class B and 8 dB for class C. The downstream operates at 1490 nm and the upstream at 1310 nm wavelengths. In G.652 fibre these have a typical attenuation of around 0.2 dB per km and 0.3 dB per km respectively. Of course we need bidirectional communication to make things work so we’re bound by the higher 0.3 dB figure. From this we can conclude a range of around 10 km for class B and around 25 km for class C.
I’ve used a 1:32 split ratio above, but the Crown Fibre fact sheet for Whangarei tells us that they’ll be using a 1:24 split ratio. This doesn’t entirely make sense as splitters work as a binary tree. The fact sheet says, “GPON is split 1:24 enabling up to 24 customers to receive 100Mbps downstream and 50Mbps upstream,” from which I’m inferring that they’ve simply said 1:24 because 2.4 Gbps × 1:24 = 100 Mbps, and 100 Mbps is some magic number for Crown Fibre. Of course this also doesn’t make sense as some customers will elect to take lower speed services than others, and GPON’s dynamic bandwidth allocation (DBA) means that you oversubscribe the bandwidth, but I won’t digress down that path now.
One benefit of a smaller split ratio is less attenuation, which means we get a longer range. Remember the splitters are a binary tree, so in the case of a 1:24 split, we could have a range to 16 of the premises of up to 10 km as described above, but for 8 of the premises we gain an extra 3.5 dB because we drop one split, giving us an extra 10 km range to those 8 premises. Conceivably we could use such a splitter scheme for example as follows, allowing us to reach some more distant premises.
But there isn’t any difference between that and the following. The truth is the splitter scheme can vary to suit the specific situation.
This is where I’ll leave part 1 for today. In part 2 we’ll explore some conceptual topologies for Whangarei.