1. Introduction

Remote devices and wireless sensor networks have become a key part of our lives and the existing 4G network cannot meet the requirements of low latency and higher data rates. In recent years, the emergence of the Internet of things (IoT) has revolutionized almost every aspect of the communication field. Next-generation wireless networks make it possible for IoT devices to transform businesses by providing higher data rates and ultra-reliable low-latency communication (URLLC). Different technologies have already been developed for IoT networks, such as ZigBee, SigFox, and long range (LoRa) ^[1][2][1,2]. However, these technologies are not compatible with the current cellular network, and there is a gap to be filled in order to address this challenge. The architecture of previous generations of mobile networks does not meet the diversity, scalability, and performance requirements because they are not flexible. The fifth generation (5G) network was designed to overcome these challenges, and can support three sets of use cases with diverse requirements in terms of data rate, reliability, latency, and massive connectivity. Services in 5G are classified into three main categories, namely:

It will allow the enterprise community to customize the services according to their own needs [3]. The concept of dividing the network into logical networks is not new as it has been used in VPN and virtual local area networks (VLANs). In 4G network slicing, dedicated core (DECOR) permits the implementation of multiple core networks within a common infrastructure [4]. Due to the limitations of evolved packet core (EPC) architecture, it does not offer flexibility and it also considers that the radio access network (RAN) is the same for all cases [5]. Different organizations in the past have contributed to fulfilling the requirements of vertical applications. The 5G Public–Private Partnership (5G-PPP) defines the relevant network resources for management and diversity with a five layer architecture [6]:

• Enhance Mobile Broadband (eMBB).
• Massive Machine-Type Communications (mMTC).
• Ultra-Reliable Low-Latency Communications (URLLC).

Network slicing can provide answers to fulfill the above-mentioned specifications by partitioning or slicing the physical network into multiple logical networks according to their requirements [3]. Each logical network should have its attributes, which allows for the better utilization and allocation of the resources as compared to previous mobile networks. Logical networks provide flexibility and can be modified or operated dynamically to meet the requirements of the business segments according to service level agreement. All such logical sub-networks are treated as network slices, and a network slice consists of a set of network resources and functions. For instance, a logical sub-network can be customized to meet URLLC requirements. With network slicing, control and predictability can be achieved by reserving and isolating network resources and functions. From a security perspective, a logical sub-network can be connected to a virtual private network (VPN) by isolating it from the Internet. Due to the isolation of logical sub-networks, it will be easy to customize the customer needs according to the business model, as it was time-consuming over common infrastructure in the prior models. According to a GSMA report published in 2018 [3], NS will be the key technology of 5G opening the business opportunity worth USD 300 billion by 2025.

2. Network Slicing in 5G

Slicing of the network should be done in an end-to-end (E2E) manner in order to provide the dedicated services to specific applications over a common physical infrastructure. There are some technical challenges for the development of network slicing over 5G network that need to be solved. These challenges include efficient management of resources, mobility prediction, security, and end-to-end orchestration. In Section 5 and Section 6, Authorwe discuss how ML techniques can be utilized to manage the resources intelligently and predict the mobility. Many research projects, such as 5GEx ^[7][8], 5GNORMA ^[8][9], 5G-MEDIA ^[9][10], and 5G-XHaul ^[10][11], have provided the study of NS enabling technologies using SDN and NFV. There is a need for optimization and new architecture to cope with new challenges. The NGMN defines the concept of NS in three layers ^[11][12]:

• Service instance layer.

• Network slice instance layer.

• Resource layer.

2.1. Service Layer

The service layer consists of specific services that are supported by the network operator or third party. It is linked to management and network orchestration (MANO) via an interface that allows it to create the dedicated slices for the specific application. The 5G network slicing architecture is shown in Figure 1. The services layer shows the supported applications and each application is depicted as a service instance.

Figure 1. 5G network slicing architecture.

2.2. Network Instance Layer

In network function layer, a blueprint is used to initiate the network slice instance and it presents the network attributes which are essential for it. This can also be shared over multiple services instances as it may consist of more than one sub-instances ^[12][13]. A network slice instance can be logically separated from the rest of network slice instances. Sub-network instance consists of logical or physical resources and set of network functions, defined by sub-network blueprint ^[12][13]. A sub-network instance can split into more than one slices.

2.3. Resource or Infrastructure Layer

It comprises of actual physical and virtual resources of the RAN, networking nodes, cloud nodes, and associated links. Through virtualization, the flexibility of network resources can be achieved, which helps to create the core network slices. Spectrum resources can be shared and flexible management of these resources can be achieved by slicing the RAN. Scheduling technique can be useful to fulfill the task of radio resource sharing. It helps to dedicate the resources according to the application requirements.

In addition to the layers, NGMN also described the E2E management and orchestration concept for NS. For a specific service, it called upon the request of applicable network function, manages the physical and virtual resources, allocates the relevant functions, and monitors each slice via application programming interface (API). It orchestrates the virtual resources and maps them to physical infrastructure resources.