Software-defined networking (SDN) is generating big interest as a technology that makes networks programmable and less expensive to build and operate. This is achieved by separating the control and the data-forwarding functions of network equipment, and centralizing the control functions of multiple network elements. More generally, SDN is a framework for the automatic and dynamic management of multiple network elements. Major carriers that already have announced SDN initiatives include Telefonica, AT&T and DT.

So far, SDN control has referred to the control of network functions that reside above the physical layer such as packet forwarding and in the carrier’s data centers. But can there be a Software-Defined Access Network? Absolutely yes! The SDN framework also applies to access networks, where virtualizing access-network control and management functions result in enormous gains in service flexibility and reliability, particularly in multi-operator environments.

There are three models for a software-defined access network (SDAN), which can apply to any access technology. Unlike traditional SDN, the control and configuration of physical layer parameters also are an integral aspect of a SDAN.

The first model of a SDAN uses software to centrally control and manage lines connected to the access hardware. Control functions include the optimization of the physical layer configuration of the broadband connection, for example changing parameters such as data rates, coding schemes and power levels to ensure a reliable connection that meets all service requirements.

Management functions include network diagnostics and analytics that drive maintenance operations and marketing campaigns. Such functions can include line diagnostics used by technicians for isolating and correcting problems in the outside plant, and targeted recommendations for service upsell based on line qualification and past usage records.

Centralizing these functions enables the use of advanced analysis and resource allocation algorithms implemented in standard servers instead of relying on the more limited and inflexible functionality available in access hardware. Most importantly, this model allows for homogeneous, vendor-agnostic management of the access hardware, and does not require hardware changes since it relies on existing management interfaces.

The second model of an SDAN uses software to manage virtualized access networks. The return on investment in new access infrastructure (e.g. FTTN, FTTH) is improved when the infrastructure can be shared among multiple, competing service providers. The physical network is partitioned into virtual networks corresponding to the customers of each of the retail providers.

Software maps the physical access hardware (e.g. DSL access multiplexers or optical network terminals) to virtual hardware. The virtual hardware can be controlled and managed by the service providers (or virtual network operators), while providing for determinacy in the operation and performance of the underlying physical network. This model greatly improves the virtual network operator’s information about and control over the access network, almost as though the operator owns its own hardware. As opposed to the use of Bit-stream Access (Layer 2) unbundling, this model lets the virtual network operator design its own competitive service products and make real-time changes on the network.

The following figure shows an architecture that combines the above two models.

The access hardware interfaces with multi-tenant software that implements a Virtualization and Management layer. The tenants include the virtual network operators and the infrastructure provider. Each has access to functions for diagnostics, analytics and optimization, which are configured to meet their specific objectives. For example, a service qualification function for operator A can favor higher access speeds and accept a small amount of stability loss, while operator B may prefer to offer its customers the most reliable services, even if that means that access speeds are conservatively set.

This architecture allows individual operators to develop competitively differentiated services. Further, the shared management system improves coordination for trouble resolution: when an issue emerges, the system can clearly identify the source(s) of the problem.

The third model of an SDAN uses software to manage virtualized access network technologies that extend beyond the access segment to the home network and to the end-devices. Examples of such technologies include Wi-Fi network traffic management and speed-boost capabilities by using multiple fixed or wireless links. A virtual network operator may offload traffic from a customer’s mobile device to a Wi-Fi access point connected to a fixed access line within its control. Or it can deliver a boosted service to a customer by combining traffic streams from the Wi-Fi access point of the customer, and from the Wi-Fi access point of a second customer in the vicinity of the first customer. Virtual centralized control of such technologies requires coordination of multiple network elements that can include access hardware, home networks, end-devices and edge routers.