Inside Singapore’s OpenNet

If you’re a network techie like me, you will probably be very interested to know how Singapore’s OpenNet fibre-to-the-home network works. What is the technology that runs the fibre-to-the-home network? How is it related to the fibre optic networks that are already common place in enterprise and campus environments? There isn’t much engineering information you can easily find, at least not from IDA and OpenNet themselves. However, with help from Google search, I’ve managed to find a variety of information that should interest network engineering people.

Let’s get one important terminology straightened out first. This new network that we’re talking about is what the Info-communications Development Authority (IDA) calls our Next Generation National Broadband Network, often abbreviated to Next Gen NBN, and sometimes further to NGNBN. This NGNBN is our new Fibre-to-the-Home network project, and it is part of IDA’s IN2015 masterplan.

NGNBN is a name made up by IDA. People from other parts of the world will know what ADSL and Cable means, because they are standard industry terms. NGNBN is a Singapore thing.

My apologies if this is starting to sound like an IDA presentation with all the fuzzy high-level talk mostly uninteresting to techies… but it is important to understand (if you don’t already know) some overall structure so that you can appreciate how all the bits fall into place later.

Our NGNBN is made up of several layers:

1. The passive network layer, comprising the dark fibre that reaches into the customer premises (e.g. homes).
2. The active network layer, comprising the actual network switching.
3. The retail services layer, where service providers come in to sell customers network access.

Layers of Network

Network people like layering. The physical network medium, as you already know, comprises optic fibre. The optic fibre that reaches your home is single-mode fibre. Two cores of single-mode fibre is terminated in a Termination Point (TP) in your home. The TP is a small box (125mm x 80mm x 20mm) with two SC connectors. (I know, to network engineering people, this sounds oversized, but the reason is that the TP box includes a reel to take up some excess length of fibre cable.)

All the dark fibre cables from customer premises are aggregated into a Central Office (i.e. Telephone Exchange). There are 9 such locations:

• Tuas
• Jurong West
• Bukit Panjang
• Ayer Rajah
• Ang Mo Kio
• Orchard
• East (it’s at French Road)
• Bedok
• Tampines

Active network switches are only deployed in the Central Offices. As you know, single-mode fibre can carry signals far enough. But, are you wondering, how is it possible to have so many fibre cores from customers individually terminate into a Central Office?

The answer is in the optical technology used – GPON (Gigabit Passive Optical Network) [see Wikipedia]. Unlike Gigabit Ethernet on fibre (e.g. 1000BASE-SX or 1000BASE-LX), GPON transmits and receives on the same fibre core. Furthermore, a single core of fibre from the Central Office can be split up to 128-way toward the customer-end. In order words, a single fibre core at the Central Office is split 128-way toward the customer-end, ultimately terminating in 128 termination points (or 64 customer TP boxes, since each TP in the NGNBN design has two fibre termination points). The splitting can happen much nearer towards the customer end, such as in the MDF room within the customer building, and is accomplished with passive optical splitters.

At the Central Office end, the fibre core connects into an Optical Line Terminal (OLT). The customer-end will have a patch cord from a port on the TP to an Optical Network Terminal (ONT) or Optical Network Unit (ONU). The OLT at the Central Office is where the active network infrastructure begins. The optical distribution from Central Office to customer is entirely passive.

The OLT and ONT (or ONU, but I’ll just omit mentioning ONU for the rest of this post) terminate their respective ends of the passive optical network. The OLT-ONT can provision a variety of native services, including IP-over-Ethernet. The OLT interfaces upstream with service providers’ network services, and presents those native services to the customer at the ONT.

OLT to ONT Connection

As mentioned earlier, a single OLT port connects to multiple ONTs. Downstream traffic is transmitted on one wavelength, and upstream traffic is transmitted on a different wavelength. This means that downstream traffic from the OLT is seen by all ONTs, and all ONTs share the same upstream channel to transmit traffic.

Each ONT processes only the traffic is addressed to it, even though it can see all the traffic on that shared optical medium. Encryption is used to prevent an ONT from reading traffic addressed to some other ONT. Upstream traffic from the ONTs are time-division multiplexed on the shared optical medium.

The GPON standard (ITU-T G.984) permits several bit rates, so I’m not sure what bit rate our NGNBN has adopted. However, the industry has converged on 2.488 Gbps downstream and 1.244 Gbps downstream. This bandwidth has to be shared by all the ONTs. However, the OLT controls the optical medium and allocates time slots to ONTs. Each ONT can be provisioned with different downstream and upstream bandwidths, enabling service providers to sell different tiers of services to customers.

GPON also permits Dynamic Bandwidth Allocation (DBA) where the OLT can grant additional time slots to ONTs depending on its bandwidth demand.

Layers of Separation

Remember how networks are naturally layered, and how the NGNBN design is also layered? Well, this layering is a key foundation to enabling competition. Here’s how the NGNBN layering works:

• Wholesale Wirelines (Layer 1 Open Access): This passive optical network starting from the TP at the customer end, and the passive dark optical fibre leading up to the Central Office. The operator at this layer is the Passive Infrastructure Company, or NetCo. OpenNet is the only NetCo at this time. It’s possible (although not in the NGNBN plan) for there to be several NetCos, either operating exclusively in different geographic regions, or competitively in the same locality.
• Wholesale Bandwidth Services (Layer 2 and Layer 2 Open Access): This is the active network infrastructure, starting with the OLT in the Central Office. The ONT in the customer premise is part of this layer too. The operator at this layer is Active Infrastructure Company, or OpCo. Nucleus Connect is the first OpCo awarded by IDA. SingTel became the second OpCo in September 2010.
• Retail Services: This is the layer that provides services that customers actually buy, including Internet access services, Voice-over-IP telephone services, IP television, etc. Commercial and industry customers may even have access to a new breed of services such as CCTV networks, telemedicine, etc. The operator at this level is known as the Retail Service Provider (RSP). Right now, there are several RSPs. You probably already know who they all are… SingTel, StarHub, M1 and SuperInternet.

The way the NGNBN has been engineered thus enables a lot of flexibility and room for competition. You can buy services from different RSPs. Although there are multiple OpCos, you as a customer probably don’t get to choose OpCos directly since it will depend on which OpCo the RSP connects to. (But it is technically possible for RSP to connect to multiple OpCos, and offer customers the choice of OpCos.)

So in principle, right now, since there are two ports on the TP box, you could connect two ONTs from two different OpCo, and from the ONTs, buy multiple services from a variety of RSPs. This probably sounds rather unnecessary right now, because all the RSPs just basically offer Internet access services (some value added with telephone and/or television services), but there could well be a whole new breed of services in future.

Just so you know, the initial NetCo OpenNet is owned by SingTel (and several other partners), while the initial OpCo Nucleus Connect is owned by StarHub. So you’ve got to wonder if there is really fair competition? Well, the answer is that OpenNet and Nucleus Connect are contractually obligated to provide fair non-discriminatory services to upstream providers.