Network Slicing for 5G with SDN/NFV: Concepts, Architectures, and Challenges

TL;DR: This article presents the network slicing concept, with a particular focus on its application to 5G systems, and analyzes a proposal from ETSI to incorporate the capabilities of SDN into the NFV architecture.

Abstract: The fifth generation of mobile communications is anticipated to open up innovation opportunities for new industries such as vertical markets. However, these verticals originate myriad use cases with diverging requirements that future 5G networks have to efficiently support. Network slicing may be a natural solution to simultaneously accommodate, over a common network infrastructure, the wide range of services that vertical- specific use cases will demand. In this article, we present the network slicing concept, with a particular focus on its application to 5G systems. We start by summarizing the key aspects that enable the realization of so-called network slices. Then we give a brief overview on the SDN architecture proposed by the ONF and show that it provides tools to support slicing. We argue that although such architecture paves the way for network slicing implementation, it lacks some essential capabilities that can be supplied by NFV. Hence, we analyze a proposal from ETSI to incorporate the capabilities of SDN into the NFV architecture. Additionally, we present an example scenario that combines SDN and NFV technologies to address the realization of network slices. Finally, we summarize the open research issues with the purpose of motivating new advances in this field.

Summary

1. Introduction

• 5G systems are nowadays being investigated to satisfy the consumer, service and business demands of 2020 and beyond.
• These logical networks are referred to as network slices.
• Such self-contained networks must be flexible enough to simultaneously accommodate diverse business-driven use cases from multiple players on a common network infrastructure .
• The authors provide a comprehensive study of the architectural frameworks of both SDN and NFV as key enablers to achieve the realization of network slices.
• Furthermore, the authors identify the main challenges arising from implementing network slicing for 5G systems.

2.1 Resources

• A network slice is composed of a collection of resources that, appropriately combined together, meet the service requirements of the use case that such slice supports.
• In network slicing, the authors consider two types of resources: ● Network Functions (NFs): functional blocks that provide specific network capabilities to support and realize the particular service(s) each use case demands.
• Generally implemented as software instances running on infrastructure resources, NFs can be physical (a combination of vendor-specific hardware and software, defining a traditional purpose-built physical appliance) and/or virtualized (network function software is decoupled from the hardware it runs on).
• Heterogeneous hardware and necessary software for hosting and connecting NFs, also known as ● Infrastructure Resources.
• They include computing hardware, storage capacity, networking resources (e.g. links and switching/routing devices enabling network connectivity) and physical assets for radio access.

2.2 Virtualization

• Virtualization is a key process for network slicing as it enables effective resource sharing among slices.
• Resource abstraction is the representation of a resource in terms of attributes that match predefined selection criteria while hiding or ignoring aspects that are irrelevant to such criteria, in an attempt to simplify the use and management of that resource in some useful way.
• The resources to be virtualized can be physical or already virtualized, supporting a recursive pattern with different abstraction layers.
• The authors consider a framework with three kinds of actors: ● Infrastructure Provider (InP): owns and manages a given physical network and its constituent resources.
• In such a case, this second tenant would provide more advanced network services to its own users.

2.3 Orchestration

• Orchestration is also a key process for network slicing.
• In a slicing environment, where the players involved are so diverse, an orchestrator is needed to coordinate seemingly disparate network processes for creating, managing and delivering services.
• A unified vision and scope of orchestration has not been agreed upon.
• According to the Open Network Foundation (ONF) [4], orchestration is defined as the continuing process of selecting resources to fulfill client service demands in an optimal manner.
• The term continuing means that available resources, service demands and optimization criteria may change in time.

2.4 Isolation

• The isolation must be understood in terms of: ● Performance: each slice is defined to meet particular service requirements, usually expressed in the form of KPIs.
• Attacks or faults occurring in one slice must not have an impact on other slices, also known as Security and privacy.
• To achieve isolation, a set of appropriate, consistent policies and mechanisms have to be defined at each virtualization level, following the ideas introduced in Section 2.3.
• The policies (what is to be done) contain lists of rules that describe how different manageable entities must be properly isolated, without delving into how this can be achieved.
• The mechanisms (how it is to be done) are the processes that are implemented to enforce the defined policies.

3. ONF Network Slicing Architecture

• The SDN architecture provided by the ONF comprises an intermediate control plane that dynamically configures and abstracts the underlying forwarding plane resources so as to deliver tailored services to clients located in the application plane (see SDN basic model in [4]).
• Thus, the SDN architecture is an appropriate tool for supporting the key principles of slicing.
• Client support contains all that is necessary to support client operations, including policies on what the client is allowed to see and do [4], and service-related information to map actions between the client and the controller.
• Represents all the information the controller needs to interact with a set of underlying resources, assembled in a Resource Group, through one of its southbound interfaces, also known as Server context.
• The process of transforming the set of Resource groups accessed through server contexts to those defined in separate client contexts is not straightforward, and it requires the SDN controller to perform virtualization and orchestration functions.

4. NFV Reference Architectural framework

• It lacks capabilities that are vital to efficiently manage the lifecycle of network slices and its constituent resources.
• The authors commence providing a brief overview of the NFV architectural framework, and later describe the integration of the two SDN controllers .
• ● Management and Orchestration (MANO): performs all the virtualization-specific management, coordination and automation tasks in the NFV architecture.
• Each controller provides a different level of abstraction.
• While the IC provides an underlay to support the deployment and connectivity of VNFs, the TC provides an overlay comprising tenant VNFs that, properly composed, define the network service(s) such tenant independently manages on its slice(s).

5. Network Slicing use case with SDN-NFV Integration

• The authors describe an SDN-enabled NFV deployment example that illustrates the network slicing concept, with several slices running on a common NFVI .
• Each slice consists of VNFs that are appropriately composed and chained to support and build up the network service(s) the slice (and thus the tenant) delivers to its users.
• The example considers that the tenants access NFVI resources from three InPs.
• The TC, deployed as a VNF, relies on the capabilities provided by virtual switches/routers (in the form of VNFs as well) to enable the VNF composition, forwarding pertinent instructions to such virtual switches/routers via its southbound interfaces.
• To access, reserve and request such resources, the tenant’s RO interacts with the VIM(s) /WIM(s) by means of interfaces that those functional blocks expose and that tenant’s RO consumes.

6. Challenges and Research directions

• When network slices are deployed over a common underlying substrate, the fulfillment of performance isolation requirement is not an easy task.
• Thus, it is required to design adequate resource management mechanisms that enable resource sharing among slices when necessary without violating their required performance levels.
• This automation enables both the RO and slice-specific functional blocks to be authorized to perform the corresponding management and configuration actions in a timely manner.
• The innovative partnerships between several players, each providing services at different positions of the value chain, and the integration of new tenants such as verticals, OTT service providers, and high-value enterprises, empowers promising business models.

Figures

Figure 2. InPs and tenants as virtualization actors. This scenario shows the recursion principle, where these actors happen in a vertical multi-layered pattern.

Figure 4. Complex client-server relationships enabled by the recursion in the SDN control plane, adapted from [7].

Figure 1. 5G network slices running on a common underlying multi-vendor and multi-access network. Each slice is independently managed and addresses a particular use case.

Figure 5. Integrating SDN controllers into the reference NFV architectural framework at the two levels required to achieve slicing.

Figure 3. ONF SDN Network Slicing architecture [5].

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Network Slicing for 5G with SDN/NFV: Concepts,
Architectures and Challenges
J. Ordonez-Lucena
1
, P. Ameigeiras
1,2
, D. Lopez
3
, J.J. Ramos-Munoz
1,2
, J. Lorca
4
, J. Folgueira
5
1
Department of Signal Theory, Telematics, and Communications, University of Granada.
2
Research Centre for Information and Communications Technologies, University of Granada.
3
Technology Exploration & Standards, Telefónica I+D – Global CTO.
4
Radio Access Networks–Innovation, Telefónica I+D – Global CTO.
5
Transport & IP Networks, Telefónica I+D – Global CTO.
Abstract— The fifth generation of mobile communications is anticipated to open up innovation
opportunities for new industries such as vertical markets. However, these verticals originate
myriad use cases with diverging requirements that future 5G networks have to efficiently support.
Network slicing may be a natural solution to simultaneously accommodate over a common
network infrastructure the wide range of services that vertical-specific use cases will demand. In
this article, we present the network slicing concept, with a particular focus on its application to 5G
systems. We start by summarizing the key aspects that enable the realization of so-called network
slices. Then, we give a brief overview on the SDN architecture proposed by the ONF and show
that it provides tools to support slicing. We argue that although such architecture paves the way
for network slicing implementation, it lacks some essential capabilities that can be supplied by
NFV. Hence, we analyze a proposal from the ETSI to incorporate the capabilities of SDN into the
NFV architecture. Additionally, we present an example scenario that combines SDN and NFV
technologies to address the realization of network slices. Finally, we summarize the open research
issues with the purpose of motivating new advances in this field.
Keywords – 5G, Network Slicing, SDN, NFV
1. Introduction
5G systems are nowadays being investigated to satisfy the consumer, service and business
demands of 2020 and beyond. One of the key drivers of 5G systems is the need to support a variety
of vertical industries such as manufacturing, automotive, healthcare, energy, and media &
entertainment [1]. Such verticals originate very different use cases, which impose a much wider

range of requirements than existing services do nowadays. Today’s networks, with their “one-size-
fits-all” architectural approach, are unable to address the diverging performance requirements that
verticals impose in terms of latency, scalability, availability and reliability. To efficiently
accommodate vertical-specific use cases along with increased demands for existing services over
the same network infrastructure, it is accepted that 5G systems will require architectural
enhancements with respect to current deployments.
Network softwarization, an emerging trend which seeks to transform the networks using software-
based solutions, can be a potential enabler for accomplishing this. Through technologies like
Software-Defined Networking (SDN) and Network Function Virtualization (NFV), network
softwarization can provide the programmability, flexibility, and modularity that is required to
create multiple logical (virtual) networks, each tailored for a given use case, on top of a common
network. These logical networks are referred to as network slices. The concept of separated virtual
networks deployed over a single network is indeed not new (e.g. VPN), although there are
specificities that make network slices a novel concept. We define network slices as end-to-end
(E2E) logical networks running on a common underlying (physical or virtual) network, mutually
isolated, with independent control and management, and which can be created on demand. Such
self-contained networks must be flexible enough to simultaneously accommodate diverse
business-driven use cases from multiple players on a common network infrastructure (see Figure
1).
Figure 1. 5G network slices running on a common underlying multi-vendor and multi-access
network. Each slice is independently managed and addresses a particular use case.
WiMAX
access
2G/3G/4G/5G
access
Satellite
access
Access node
Transport/Aggregation node
Core node
xDSL/Cable
access
Edge
Cloud
Core
Cloud

In this paper, we provide a comprehensive study of the architectural frameworks of both SDN and
NFV as key enablers to achieve the realization of network slices. Although these two approaches
are not yet commonplace in current networking practice, especially in public wide area networks
(WANs), their integration offers promising possibilities to adequately meet the slicing
requirements. Indeed, many 5G research and demonstration projects (such as 5GNORMA, 5GEx,
5GinFIRE, or 5G!Pagoda) are addressing the realization of 5G slicing through the combination of
SDN and NFV. Thus, we present a deployment example that illustrates how NFV functional
blocks, SDN controllers, and their interactions can fully realize the network slicing concept.
Furthermore, we identify the main challenges arising from implementing network slicing for 5G
systems.
The remainder of this paper is organized as follows: Section 2 provides a background on key
concepts for network slicing. Sections 3 and 4 describe the SDN architecture from the ONF and
the NFV architecture from the ETSI, respectively. Section 5 shows a network slicing use case with
NFV and SDN integration, and Section 6 provides the main challenges and future research
directions.
2. Background on key concepts for Network Slicing
In this section, we provide a background on key aspects that are necessary to realize the network
slicing concept.
2.1 Resources
In its general sense, a resource is a manageable unit, defined by a set of attributes or capabilities
that can be used to deliver a service. A network slice is composed of a collection of resources that,
appropriately combined together, meet the service requirements of the use case that such slice
supports. In network slicing, we consider two types of resources:
● Network Functions (NFs): functional blocks that provide specific network capabilities to
support and realize the particular service(s) each use case demands. Generally implemented
as software instances running on infrastructure resources, NFs can be physical (a
combination of vendor-specific hardware and software, defining a traditional purpose-built
physical appliance) and/or virtualized (network function software is decoupled from the
hardware it runs on).
● Infrastructure Resources: heterogeneous hardware and necessary software for hosting and
connecting NFs. They include computing hardware, storage capacity, networking

resources (e.g. links and switching/routing devices enabling network connectivity) and
physical assets for radio access. Suitable for being used in network slicing, the
aforementioned resources and their attributes have to be abstracted and logically
partitioned leveraging virtualization mechanisms, defining virtual resources that can be
used in the same way as physical ones.
2.2 Virtualization
Virtualization is a key process for network slicing as it enables effective resource sharing among
slices. Virtualization is the abstraction of resources using appropriate techniques. Resource
abstraction is the representation of a resource in terms of attributes that match predefined selection
criteria while hiding or ignoring aspects that are irrelevant to such criteria, in an attempt to simplify
the use and management of that resource in some useful way. The resources to be virtualized can
be physical or already virtualized, supporting a recursive pattern with different abstraction layers.
Just as server virtualization [2] makes virtual machines (VMs) independent of the underlying
physical hardware, network virtualization [3] enables the creation of multiple isolated virtual
networks that are completely decoupled from the underlying physical network, and can safely run
on top of it.
The introduction of virtualization to the networking field enables new business models, with novel
actors and distinct business roles. We consider a framework with three kinds of actors:
● Infrastructure Provider (InP): owns and manages a given physical network and its
constituent resources. Such resources, in form of WANs and/or data centers (DCs), are
virtualized and then offered through programming interfaces to a single or multiple tenants.
● Tenant: leases virtual resources from one or more InPs in the form of a virtual network,
where the tenant can realize, manage and provide network services to its users. A network
service is a composition of NFs, and it is defined in terms of the individual NFs and the
mechanism used to connect them.
● End user: consumes (part of) the services supplied by the tenant, without providing them
to other business actors.