TWDM-PON unbundling for infrastructure sharing
TWDM-PON unbundling – the ability to separate traffic by wavelength – is one of the unique properties of TWDM-PON (Time and Wavelength Division Multiplexed Passive Optical Network). Unbundling encourages service providers to consider sharing network infrastructure, which accelerates fiber deployments and creates new revenue streams from wholesale services.
Unbundling is now seen as a compelling driver for the adoption of TWDM-PON, along with the superior bandwidth and the potential for service convergence that promise increased revenues and operational efficiency.
TWDM-PON offers 4 wavelength pairs (1 upstream wavelength and 1 downstream wavelength per pair) and each pair can be configured as either 10/10, 10/2.5, or 2.5/2.5 Gbps. Separate pairs can be used for different services (residential, business, backhaul) or allocated to different service providers. In the latter case, each service provider has the flexibility to choose the bandwidth on their wavelength pair and modify it independently of other service providers on the same TWDM-PON infrastructure.
Unbundling on TWDM-PON can be introduced gradually, wavelength by wavelength, as needed. TWDM-PON can coexist on the same infrastructure with GPON technology and fixed wavelength TWDM-PON (Universal TWDM) as they each use different wavelengths (see Figure 1). This makes it possible to launch unbundling using fixed wavelength TWDM-PON, which has a lower initial investment cost, and upgrade to full TWDM-PON with tunable optics in the future.
Fixed and tunable TWDM-PON both use the same hardware so no major changes are required in the network. There is no impact on the existing users; only the optics in the access node need to be changed. This flexibility makes TWDM-PON much better for unbundling than other PON technologies. Unbundling results in faster network monetization and encourages service providers to cooperate to deploy fiber infrastructure more quickly as costs and business risk can be shared.
This flexibility makes TWDM-PON much better for unbundling than other PON technologies. Unbundling results in faster network monetization and encourages service providers to cooperate to deploy fiber infrastructure more quickly as costs and business risk can be shared.
Figure 1: The wavelength plan ensures the co-existence of different technologies on the same PON
3-layer network model
In considering how to approach unbundling with TWDM-PON, it is helpful to look at the network as consisting of 3 layers.
- The bottom layer consists of passive infrastructure such as ducts, poles, dark fiber and splitters. These elements are not subject to frequent technology or market changes and can be used for decades.
- The middle layer consists of active elements such as access nodes and end-user modems. These elements are subject to moderate change due to innovations in technology.
- The upper layer consists of the retail services provided to consumers and businesses. They are subject to significant changes due to constant shifts in the market and customer preferences.
Figure 2: 3-layer network model
TWDM-PON can be unbundled in 3 ways, as shown in Figure 2.
- Passive sharing (unbundling at the bottom layer)
One organization (e.g. a municipality or wholesale operator) builds the passive infrastructure and multiple service providers are allocated a dedicated wavelength for their own active equipment. The service provider operates the access node and provides retail services. This scenario also interests alternative operators looking to co-invest in a common passive fiber network and share the risk and cost.
- Active sharing (unbundling at the middle layer)
The organization that builds the passive layer also provides and maintains the middle layer (active equipment). Multiple service providers offer retail services on top. Each service provider has a dedicated wavelength, but does not own or operate the access node.
- Full separation (unbundling at each layer)
One organization, such as a municipality, deploys the passive network. Another organization, such as a wholesaler, provides the active equipment. And multiple service providers each have a dedicated wavelength to provide retail services on top.
Each of these models can be applied to both greenfield (new fiber networks) and brownfield deployments (upgrading existing fiber networks).
TWDM-PON unbundling in brownfield deployments
Where an operator already has a fiber network the benefit of introducing TWDM-PON unbundling is to increase revenues through wavelength wholesale. This is simple as TWDM-PON and GPON can coexist on the same fiber plant, using the same passive plant and access node. This is an easy way for a new service provider to enter a market and offer services to end-users.
To add TWDM-PON to a GPON network, new network elements are needed. A wavelength multiplexer (WMUX) combines the different TWDM-PON wavelengths coming from different ports on a line card onto the same fiber. A coexisting element (CEx) combines GPON and TWDM-PON signals onto the same fiber. Then a network management tool is used to allocate and manage TWDM-PON wavelengths for all service providers.
1. Active sharing
In an active infrastructure sharing scenario, the wholesale operator owns the passive and active layers of the network and allocates wavelengths to different service providers. All the end-user traffic for a given service provider is aggregated and delivered to the service provider either in a local central office or deeper in the network in a regional aggregation center.
Growth can be managed gradually. If in the 1st year there is only 1 service provider then only 1 wavelength needs to be introduced. The network operator can keep 1 or more wavelengths for itself or even keep 1 as a spare for future enhancements or operational use.
Active sharing in a brownfield deployment has low complexity as all network elements and activities are owned and managed by the wholesale provider.
Figure 3: Active sharing: TWDM-PON unbundling x
2. Passive sharing
Where an operator owns a fiber network, other service providers can be given use of the passive layer but deploy their own access network equipment in the active layer.
The benefit for the network owner is again to increase revenues through wholesaling but with lower capital and operating costs because service providers will supply and manage their own access nodes. The advantage for a service provider is to have more flexibility to manage and evolve the access layer according to their business priorities.
New service providers just need to connect a fiber from their access node into a WMUX.
Passive sharing in a brownfield deployment has medium complexity as most activities are managed by one operator (the wholesale provider).
Figure 4: Passive sharing: TWDM-PON unbundling
3. Full separation
In existing networks, the network owner usually takes care of the bottom and middle layers, so full sharing is not likely to happen.
TWDM unbundling in greenfield deployments
Networks can be designed from the start to allow an open access model. This is typically the case for municipal projects, utility projects, or joint ventures with co-investment in a common network.
A new network can initially be based either on GPON or TWDM-PON but should preferably be ready for TWDM-PON unbundling, i.e., with access nodes, WMUX and CEx elements deployed as required. TWDM-PON brings far greater flexibility for service providers to choose their own quality of service and evolution path.
Unbundling encourages investment in fiber networks as risk and costs can be shared. The advanced properties of TWDM-PON will attract more service providers, resulting in better network monetization and more choice for end-users.
4. Active sharing
In an active sharing scenario in a greenfield deployment, 1 organization builds the passive and active network layers.
When a service provider requires access to the network, a new wavelength can be assigned. The aggregated traffic for each service provider is delivered either locally or into a central location.
This type of unbundling could be interesting for governments, municipalities, utilities, or operators acting as network providers or wholesalers.
Complexity is again low as everything is managed by the wholesale provider.
5. Passive sharing
A wholesale provider deploys the passive network then invites service providers to deploy their own access equipment and provide services.
This scenario requires WMUX and CEx equipment to be available from day 1. The responsibility for operating the passive network can either be taken by the wholesale provider or there can be a commercial agreement between service providers that 1 of them takes the lead.
This scenario has medium complexity as most activities are managed by the wholesale provider.
6. Full separation
Full separation is much more likely to happen in greenfield deployments. However, we can consider that it is just a special case of active sharing. A network provider builds the passive network while a single wholesaler provides the active layer and engages with service providers.
Managing wavelengths in unbundling scenarios
A key question when considering unbundling using TWDM-PON is governance, i.e., how wavelengths should be managed and by whom.
In TWDM-PON unbundling, each service provider is assigned a dedicated wavelength. To switch between service providers, an end-user needs to be transferred to a different wavelength. With fixed-wavelength TWDM-PON, a truck roll and new end-user equipment are required. An end-user modem must be shipped or else the optics must be changed in the optical network unit (ONU) to match the wavelength of the new service provider.
On the other hand, unbundling with fixed wavelengths on GPON or Universal TWDM is easy to oversee because there is no need to introduce a mechanism to prevent non-authorized changes of an end-user’s wavelength.
With tunable TWDM-PON, an operator can change an end-user’s wavelength directly from the central office, which eliminates the need for truck-rolls and the need to swap the end-user equipment (assuming interoperability between the new service provider’s access node and the incumbent service provider’s ONU).
In the active sharing and full separation models, this is not an issue because 1 organization (the wholesale provider) controls all the active elements. Therefore, the assignment of wavelengths does not require complex multi-vendor inter-node synchronization mechanisms.
However, in a passive sharing model, where each service provider has its own access node, communication on a higher level is needed (between the OSS/BSS of all service providers and inter-node communication) to ensure synchronization between all access nodes.
A standard protocol for this communication is being discussed within the Broadband Forum which would prevent unauthorized wavelength switching and ensure transparency for both end-users and service providers. The standards-based applications are expected to be ready in 2018-2019 timeframe.
|1. Active||2. Passive||3. Full||4. Active||5. Passive||6. Full|
|Passive layer management||Wholesaler||Wholesaler||Unlikely||Wholesaler||Wholesaler||3rd party network provider|
|Active layer management||Wholesaler||Service Provider||Wholesaler||Service Provider||Wholesaler|
|WMUX and CEx management||Wholesaler||Wholesaler||Wholesaler||Wholesaler or Service Provider||Wholesaler|
Figure 5: Managing infrastructure and wavelengths in TWDM-PON unbundling scenarios
The promise of unbundling
TWDM-PON technology promises network operators new revenue streams as it provides more capacity and flexibility for offering wholesale services compared to other technologies. It’s also of great interest to regulators, governments, and local municipalities as the potential unbundling and infrastructure sharing scenarios will ensure competition and choice for end-users.
Our authors look forward to your questions and comments.