## 3 Services Layer: From Telephony to Virtual Reality
### Discussion on the Needs from User's Perspective  
According to the ETSI F5G Group [2], the types of users can be classified into three main categories, namely residential users, business and vertical industries. Each type of user enjoys a different gamut of services, for which the requirements vary. The network/service provider, in order to meet the service requirements, has to ensure sufficient values of key indicators, such as QoS and QoE, over the whole service life cycle. On one hand, QoE has a subjective nature, and apart from the network performance, it depends on the user's background, expectations, emotional state, etc. On the other hand, QoS directly depends on the number of network and physical layer performance metrics such as bit error rate (BER), latency, jitter, etc.  
For residential users, the most important requirement for real time services is low latency, and for non-real time services, is the sufficient data rate. In business services, smart offices and cloud services are the key elements for the F5G development, requiring sufficient bandwidth, security and high availability. For vertical industries, services such as smart cities require high connectivity, availability, security, operational and energy efficiency. Overall, there is a list of various metrics that have to be ensured by the network provider in order to maintain the QoS for this wide range of services in the F5G era, which highlights the indispensable transformation of the whole network ecosystem in the coming years in order to achieve it.  
Figure 2: Representative services over the five fixed network generations.  
The F5G services, based on their requirements, can be classified into three categories, as illustrated in Figure 3, namely enhanced fixed broadband (eFBB), full fiber connection (FFC) and guaranteed reliable experience (GRE). The eFFB category includes services that require fixed broadband connections with data rates greater than 1 Gb/s per user and per transmission direction. Representative services of this family are ultra-high definition (UHD) video streaming, cloud office and virtual/augmented reality. Next, the FFC category demands full fiber coverage and massive/dense fiber connections. Smart infrastructure (cities, campuses, homes and manufacturing), enterprise cloudification and the interconnection with the 5G network can be incorporated into this family. Further, the GRE model mainly includes services which aim to maximize the user's QoE, such as online e-learning, online gaming, virtual/augmented reality, real time social networking and e-health. The main metric of interest here is the low latency in order to ensure seamless service operation along with great QoE.  
An important challenge for the network providers on the road towards F5G is to sufficiently support services that are located at the intersection of two or three families. For example, data center interconnection requires, concurrently, full fiber connectivity, low latency and a high data rate. Further, UHD live streaming and online virtual and augmented reality require both low latency and sufficient data rates. In our opinion, in F5G as well as en route towards F6G, a significant number of services with multiple key requirements (low latency, great connectivity and high data rate) that have to be met concurrently, will emerge, and a considerable amount or research has to be carried out in the near future in order to define specific roadmaps and guidelines on how to efficiently support them.  
Figure 3: Representative services for the three F5G service families.