Network virtualization in the data center and the wide-area network became a mainstream trend in 2014. Discussions around how the network would adapt to support the highly virtualized compute and storage infrastructure in data centers has given rise to network solution vendors – old and new – who are working on approaches to deliver what they see as the optimal network virtualization offering. This trend has likewise reached the mobile network infrastructure industry, which has started to create value from virtualization technology. While virtualizing functions of the evolved packet core has already reached an advanced state, the virtualization of the radio access network is still in its infancy.
The transition toward a virtualized RAN will come in phases. An initial step has been made with the introduction of the centralized RAN, i.e. partitioning base stations into a baseband unit and a remote radio head component. Even though both network functions are implemented with vendor-specific, dedicated hardware, baseband processing can now be physically separated from radio frequency processing, allowing both components to be installed at different locations. Pooling BBUs at base station hotels and installing compact RRHs at macro and small cell site locations generates significant benefits. They include more effective interference coordination, reduced operational complexity and simplified site acquisition among others.
A direct consequence of migrating to a centralized RAN architecture is the need to have access to optical fiber infrastructure for interconnecting RRHs and BBUs – a fixed network application commonly known as fronthaul. Fiber connectivity is required to meet the stringent requirements for high capacity in the order of gigabits per second and latency in the order of tens of microseconds. Furthermore, the common public radio interface protocol utilized by the centralized RAN architecture has service performance requirements that do not allow mapping the native data signal into common transport protocols such as Ethernet, MPLS or OTN. Mobile network operators have therefore turned to acquiring dark fiber and passive wavelength services from their landline service providers.
Leasing dark fiber services is not like buying most other telecom services because it is more like a physical asset than a service. Outages frequently caused by fiber breaks are typically much longer than for lit services. Commonly established service level agreements for dark fiber service do not foresee fault detection by the dark fiber provider, reflecting the passive nature of the service. Those SLAs are not adequate for fronthaul applications. Increased availability and proactive service restoration are desired performance characteristics, expressed by many mobile network operators. New dark fiber monitoring solutions designed for high scalability and maximum cost-efficiency relax this situation and enable fiber network operators to proactively monitor their passive, fiber-based infrastructure and services. The new solutions prevent interference with services and applications provisioned on top of the passive infrastructure by spectral separation, therefore maintaining full transparency and providing complete control to shape every aspect of the network to the client organization’s need.
The subsequent step of evolving into a fully virtualized cloud RAN architecture comprises the virtualization of the BBUs pooled at base station hotels and driving the RRHs. Running baseband processing at virtual machines provisioned on commercial servers allows load balancing and switching network capacity and resources on demand from cell sites in light traffic areas to regions experiencing heavy mobile traffic. C-RAN promises to trade the additional cost of fiber-based fronthaul for a reduction in equipment cost, energy consumption and network operation expenses. It also helps to speed up the deployment of new sites and allows faster activation of additional capabilities in the RAN. The LTE-Advanced toolkit is full of features that enable operators to squeeze more out of their spectrum by implementing intelligent algorithms for spectrum coordination and interference control. A server-based C-RAN implementation will simplify the introduction of future LTE-A functionality by adding software appliances.
Moving to a virtualized C-RAN architecture also paves the way for mobile edge computing, a new industry initiative launched by the corresponding Industry Specification Group of ETSI. MEC provides IT and cloud-computing capabilities within the RAN in close proximity to mobile subscribers. For application developers and content providers, the edge of the RAN offers a service environment with ultra-low latency and high bandwidth as well as direct access to a real-time radio network for offering context-related services. The “Internet of Things” will also welcome these capabilities, promising better performance and more efficient service implementation. MEC closely follows the principles of cloud computing and network function virtualization and can be seen as a cloud service performing specific tasks that could not be achieved with traditional network infrastructure while running at the same virtual machines and servers that are used for virtual baseband processing.
Virtualized C-RAN and MEC are the two main industry trends that will help to realize the full potential of the LTE-A RAN. The implementation of both concepts will trigger exciting new developments that will enable a different class of services, applications and mobile user experience. Mobile base stations will be transformed into intelligent service hubs that are capable of delivering highly personalized services and machine-to-machine communication directly from the very edge of the network while providing the best possible performance in mobile networks.
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