TV, VPN business services, as well as for emerging critical IoT services (all of them belong to the specialized services). So, all services are being transferred over IP-based networks, but network neutrality refers only to IAS and not to every IP-based network and service. For example, NGN VoIP as a PSTN/ISDN replacement is not part of the public Internet and network neu- trality does not refer to it. OTT voice services such as Skype and Viber are provided via IAS, over the public Internet, based on network neutrality principles. 8.7 Challenges Regarding QoS and Network Neutrality Horizontal dimension Transit ISP Broadband ISP ISP - Internet service provider Figure 8.5 Three dimensions of network neutrality. In general, one may distinguish three main dimensions regarding the relation between QoS differentiation and network neutrality [16], which are typically accompanied with separate policies (Figure 8.5): Vertical dimension. This dimension refers to connections to an ISP for transfer of Internet traffic to/from end-users and from/to service providers' data centers. Hence there are two sub-dimensions vertically: Users' vertical subdimension. This sub-dimension refers to the client's side (con- sidering the client-server Internet communication), that is the uplink/downlink relation between the ISP and its customers who use the IAS provided by that ISP. Service providers' vertical subdimension. This refers to the server's side (of the Inter- net client-server communication), i.e. this dimension refers to the relation between the ISP and service/application or content providers. Horizontal dimension. This refers to interconnections between the ISPs as the same level entities considering the network value chain. The horizontal dimension also COV- ers relations between different types of ISPs, such as broadband ISPs (which provide broadband access) and commercial ISPs (which provide transit of Internet traffic). Typically, their relations are based on peering or transit agreements. Diagonal dimension. This refers to