Network Slicing in 5G: Survey and Challenges

TL;DR: The state of the art in 5G network slicing is reviewed and a framework for bringing together and discussing existing work in a holistic manner is presented, to evaluate the maturity of current proposals and identify a number of open research questions.

Abstract: 5G is envisioned to be a multi-service network supporting a wide range of verticals with a diverse set of performance and service requirements. Slicing a single physical network into multiple isolated logical networks has emerged as a key to realizing this vision. This article is meant to act as a survey, the first to the authors� knowledge, on this topic of prime interest. We begin by reviewing the state of the art in 5G network slicing and present a framework for bringing together and discussing existing work in a holistic manner. Using this framework, we evaluate the maturity of current proposals and identify a number of open research questions.

Summary

I. INTRODUCTION

• Mobile devices have become an essential part of their daily lives and as such the mobile network infrastructure that connects them has become critical.
• It is conceivable that the eventual 5G system would be a convergence of two complementary views that are currently driving the research and industrial activity on 5G.
• The authors take this view in this article as it is intertwined with the 5G mobile network architecture, although the evolutionary view also has architectural implications.
• This article is, to the authors' knowledge, the first survey on this important topic.
• The authors then present a generic 5G architectural framework made up of infrastructure, network function, and service layers as well as the cross-cutting aspect of service management and orchestration (MANO).

A. Use Cases and Requirements

• The ITU and 5G-PPP have identified three broad use case families: enhanced mobile broadband, massive machine type communications and critical communications.
• Within those, it is possible to define several specific use cases [1] ranging from general broadband access with global coverage to specialized networks for sensors or extreme mobility.
• Alternative architectural proposals for 5G have emerged recently to accommodate use cases with diverse requirements; the authors outline two such proposals next.

B. Architecture

• NGMN's architectural vision [1] advocates a flexible softwarized network approach.
• This views network slicing as a necessary means for allowing the coexistence of different verticals over the same physical infrastructure.
• The overall NGMN architecture is split into three layers: infrastructure resource, business enablement and business application.
• Realizing a service in this proposal follows a topdown approach via a network slice blueprint that describes the structure, configuration and work-flows for instantiating and controlling the network slice instance for the service during its life cycle.
• 5G-PPP's architectural vision [3] offers a more elaborate examination of the roles and relationships between different parts of the 5G network.

A. Infrastructure Layer

• The infrastructure layer broadly refers to the physical network infrastructure spanning both the RAN and the CN.
• It also includes deployment, control and management of the infrastructure; the allocation of resources (computing, storage, network, radio) to slices; and the way that these resources are revealed to and can be managed by the higher layers.

Existing work:

• The related work focuses on two main subjects; the composition of the network infrastructure and its virtualization.
• This not only means virtualization and full isolation of the underlying resources (processing, storage, network and radio) among slices but also the capability to support different types of control operations over the resources in a virtualized manner based on the service requirements.
• This characteristic of providing a virtualized end-to-end environment, that can be potentially opened up and fully controlled by third parties, is one of the key features that separates network slicing from the already existing network sharing solutions [5] [4] .
• The dedicated spectrum approach is easier to implement, especially with dedicated radio hardware per slice, since isolation of radio resources is guaranteed through the static fragmentation of the spectrum but it can result in inefficient use of radio resources.

C. Service Layer & MANO

• This observation regarding the operation of slicing in the context of 5G naturally leads to two new high-level concepts: (1) a service layer that is directly linked to the business model behind the creation of a network slice; and (2) network slice orchestration for the hypervision of a slice's life-cycle.
• In the first case, the slice orchestrator will be assigned the more complex task of identifying the appropriate functions and technologies that will guarantee the fulfillment of the requirements described in the slice's manifest, while in the second case things are more simplified since the required building blocks of the slice are already identified in its description.
• Another very important issue is how to map and stitch together the components that are available to the various layers of the architecture in order to compose an end-to-end slice.
• Two types of mapping have been considered: (1) the functional/SLA mapping of the service requirements to network functions and infrastructure types; and (2) the mapping of network functions and infrastructure type to vendor implementations [6] [7] .

IV. CHALLENGES

• From the last section, it is apparent that 5G network slicing has already received fair amount of attention from the research community and industry.
• At the same time, there are several aspects key to end-to-end network slicing that are not well understood as captured by the illustration in Fig. 3 .
• The authors now elaborate on several significant outstanding challenges that need to be addressed to fully realize the vision of network slicing based multi-service softwarized 5G mobile network architecture.

A. RAN Virtualization

• As already discussed in Section III-A2, the main challenges for infrastructure virtualization lie in the RAN.
• Solutions that pre-allocate distinct spectrum chunks to virtual base station instances are straightforward to realize and provide radio resource isolation but have the downside of inefficient use of radio resources.
• This can potentially be addressed by adapting SD-RAN controllers like [13] .
• This presents an additional outstanding challenge as it is unclear whether multiple RATs can be multiplexed over the same possibly specialized hardware or each needs its own dedicated hardware; the answer to this question might depend on the set of RATs under consideration.
• RaaS paradigm requires going beyond radio resource and physical infrastructure sharing [4] [9] to have the capability to create virtual RAN instances on-the-fly with tailored set of virtualized control functions (e.g., scheduling, mobility management) to suit individual slice/service requirements while at the same time ensuring isolation between different slices (virtual RAN instances).

B. Service Composition with Fine-Grained Network Functions

• Coarse-grained functions are easy to compose as fewer number of interfaces need to be defined to chain them together but this comes at the cost of reduced flexibility for the slices to be adaptable and meet their service requirements.
• Fine-grained network functions do not have this limitation and are more desirable.
• However the authors lack a scalable and interoperable means for service composition with fine-grained functions that could be implemented by different vendors.
• The straightforward approach of defining new standardized interfaces for each new function is not scalable as the functions increase in number and the granularity becomes finer.

C. End-to-End Slice Orchestration and Management

• The problem of describing services has already been identified in the literature (Sec III-C1) but without satisfactory resolution.
• A good approach to address this void is to develop domain-specific description languages that allow the expression of service characteristics, KPIs and network element capabilities and requirements in a comprehensive manner while retaining a simple and intuitive syntax (e.g., in the philosophy of [14] ).
• This concerns holistic orchestration of different slices so that each meet their service/SLA requirements while at the same time efficiently utilizing underlying resources.
• While underlying principles from these other contexts can be leveraged, mechanisms targeting 5G network slicing should be suitably adapted and extended considering additional types of resources.
• Specifically, not just the resources found in cloud environments (memory, storage, network), but also radio resources need to be included, considering their correlation and how adjusting one resource type could have a direct effect on the efficiency of some other and therefore on the overall service quality.

V. SUMMARY

• To this end, the authors present a common framework for bringing together and discussing existing work in a holistic and concise manner.
• This framework essentially groups existing slicing proposals according to the architectural layer they target; namely the infrastructure, network function, and service layers along with the MANO entity.
• With respect to this framework, the authors evaluate the maturity of current proposals and identify remaining gaps.
• While several aspects of network slicing at the infrastructure and network functions layers are quickly maturing, issues like virtualization in the RAN are unresolved.
• Also, approaches for realizing, orchestrating, and managing slices are still in their infancy with many open research questions.

Figures

TABLE I APPROACHES FOR ADDRESSING DIFFERENT ASPECTS OF NETWORK SLICING AND THEIR (DIS)ADVANTAGES

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Network slicing in 5g: survey and challenges" ?

This article is meant to act as a survey, the first to the authors ’ knowledge, on this topic of prime interest. The authors begin by reviewing the state of the art on 5G network slicing and they present a framework for bringing together and discussing existing work in a holistic manner. Using this framework, the authors evaluate the maturity of current proposals and identify a number of open research questions. 

Q2. What can be done to provide isolation guarantees for the underlying infrastructure?

technologies like Kernel-based Virtual Machines (KVM) and Linux Containers (LXC) can provide isolation guarantees in terms of processing, storage and network resources at the OS or process level. 

Q3. What is the way to address this void?

A good approach to address this void is to develop domain-specific description languages that allow the expression of service characteristics, KPIs and network element capabilities and requirements in a comprehensive manner while retaining a simple and intuitive syntax (e.g., in the philosophy of [14]). 

Q4. What is the key feature that separates network slicing from the already existing network sharing?

This characteristic of providing a virtualized end-to-end environment, that can be potentially opened up and fully controlled by third parties, is one of the key features that separates network slicing from the already existing network sharing solutions [5] [4] 

Q5. What is the first type of mapping?

The first type of mapping refers to the way that MANO chooses appropriate high-level network elements that are required to create a slice for a given service in order to meet its functional requirements and SLA. 

Q6. What are the main issues in the RAN?

While several aspects of network slicing at the infrastructure and network functions layers are quickly maturing, issues like virtualization in the RAN are unresolved. 

Q7. What is the purpose of the service/slice instance?

The service/slice instance created based on the blueprint may be composed of several sub-network instances, each in turn comprising of set of network functions and resources to meet the requirements stipulated by the service in question. 

Q8. What is the main reason why a large number of architectural proposals for network slicing?

This is why a large number of architectural proposals [4] [7] [6] for network slicing expect the deployment of generic software-defined base stations composed of centralized baseband processing units and remote radio heads as the logical next step. 

Q9. What does virtualization mean for network slicing?

This not only means virtualization and full isolation of the underlying resources (processing, storage, network and radio) among slices but also the capability to support different types of control operations over the resources in a virtualized manner based on the service requirements. 

Q10. What is the need for a further mapping of these elements to concrete vendors?

Once the type of functions and infrastructural elements required for the slice have been identified, there is a need for a further mapping of these elements to concrete vendor implementations.