Access to real-time communication has become very critical to consumers and businesses today in the wake of the widespread adoption of smartphones and the demand for immediacy in customer service and information exchange.
The new set of extreme real-time communications applications (such as neuro-haptic control, telesurgery, advanced robotics, cognitive control system, massive IoT, and augmented reality) pose a new and diverse set of stringent requirements with respect to the capacity, bandwidth, latency, and reliability. To support such applications and use cases with vastly heterogeneous needs, multiple logical networks with speciﬁc functionality running on top of a unified physical infrastructure are among the founding principles of next-generation networks as mandated by IMT2020. This creates a new paradigm in cellular networks where a particular service provider focuses on specific applications/services, i.e., an mMTC provider, or a provider of long-distance telesurgery, sitting on top of an infrastructure provider like current day operators like AT&T, Jio, Vodafone, etc.
This principle lays the foundation for split and slicing, which spans from a set of devices to applications going through an access network, transport, edge, and core. 3GPP has standardized RAN Slicing as part of Release 15: a higher-layer split was specified with a well-defined interface (F1) between two logical units: The Centralized Unit (CU) and the Distributed Unit (DU) and well recognized by vendors and operators.
In RAN virtualization, the DU is connected to the radio via a packet interface based on enhanced Common Public Radio Interface (eCPRI) or Radio over Ethernet (RoE IEEE1914.3). There are multiple ways to divide functions between the DU and the radio; these are referred to as lower-layer split (LLS) options. The main intention of the horizontal split is to enable gains from centralization. However, it also allows Network Functions (NFs) to be placed in CU and DU according to a performance criterion such as latency and to adapt the placement to the characteristics of the x-haul (back-haul, mid-haul, front-haul) transport network between CU, DUs, and RUs.
RAN Slicing offers a way of realizing a versatile radio access network through customized virtual subnetworks satisfying heterogeneity of the requirements- high reliability, low latency, high data rates, and low energy consumption. It is emerging as a key enabler to customize and manage virtualized base stations while sharing the radio resources among them, with the objective of accommodating operators, service providers, OTT providers, and meeting their requirements.
Several comprehensive surveys have studied different slicing architectures and their applications, including vehicular networks, information-centric networks, isolated wireless network slicing, etc. In summary, all these surveys converge on the following key design principles for modern RAN realization:
A control plane/user plane split to support the software-defined networking principle and a radio protocol stack layer-based split to allow flexible placement of processing functions between a central and distributed unit
The slice state is maintained at the edge of the transport network. ‘Slice State’ refers to a state related to the RAN functions (RU, DU, or CU) connected to the transport network (E.g., the IP/MAC addresses of RAN functions). The transport network is unaware of this information and is only concerned about forwarding packets between transport tunnels. This assumption allows to reconfigure RAN functions without affecting the transport network flexibly
Transport Tunnels are pre-provisioned but are potentially dynamic
Ethernet encapsulation is adopted as the common data plane abstraction for RAN realization
SDN makes the network “smarter” by being able to analyze itself and integrate real-time information about networking activity with the applications
Slices may use different technologies and RAN functional splits; the slicing overlays the functional splits for heterogeneous applications and services. Slice-specific isolation of radio resources to fulfill slice-specific Service Level Agreements (SLAs) under a Quality of Service (QoS) framework can be achieved. This can be done by resource scheduling on MAC and efficiently managing the allocation of time-frequency resources or on higher layers.
As shown in Figure 3, there are different approaches to achieve a realistic RAN slicing and split solution. These approaches are driven by the various trade-offs such as spectrum, network infrastructure, applications/service to be supported, technologies, available CAPEX, and planned OPEX, which can be presented to an infrastructure provider/operator. There are multiple architecture initiatives available such as ORAN, Open-RAN, and Standardization efforts under ITU-T & 3GPP to address slicing and splitting.
RAN Slicing and splitting is a new paradigm focusing on creating highly efficient and performance-agnostic next-generation cellular networks. These networks leverage SDN and NFV- technologies well established in the data networking space. RAN Slicing and Splitting would radically transform the Cellular RAN network frameworks. They make room to allow conventional and large structures to be divided into customizable modular components that can be coded and programmed to offer just the right connectivity level. Each element within the system would be running on the framework of its choice. The Network Slicing concept would facilitate the development of networks, making them more flexible and versatile.
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