reed that the first aggregation network does not need to employ MPLS switching (although the packets may carry MPLS labels), SO that it will most probably be 6.6 Transport Network based on either plain Ethernet with ancillary mechanisms or on CE. For the latter case, S-tagged Q-in-Q VLANs will be used to control the forwarding, resulting in a stack of the type X/GTP/UDP/IP/S-tag/C-tag/Ethernet. However, if an MPLS label stack is inserted to identify tunnels, this instead will be X/GTP/UDP/IP/MPLS/Ethernet. In some cases the transport provider may wish to preserve the user's Ethernet, in which case an Ethernet pseu- dowire may be built, resulting in X/GTP/UDP/IP/Ethernet/PW15/MPLS/Ethernet ( (where the first Ethernet header is the user's and the second is the wholesaler's), or more generally X/GTP/UDP/IP/Ethernet/PW/MPLS/S-tag/C-tag/Ethernet. For the case where CE is not used, required carrier grade features are often provided by IP mechanisms, for example, IS-IS for forwarding behavior, bidirectional forwarding detec- tion (BFD) for OAM, IP-FRR for resilience, etc. This requires some tunneling mechanism, most commonly GRE (RFC 2890), although an MPLS label stack may be employed (RFC 4023). When using GRE the stack will be IP/GTP/UDP/IP/Ethernet/GRE/IP/Ethernet, or IP/GTP/UDP/IP/Ethernet/MPLS/IP/Ethernet where the first IP is the UE's, the second the BS, and the third the CSG. More complex cases may be found, for example using Virtual Extensible LAN (VXLAN) (RFC 7348) or other modern tunneling mechanisms. In fiber-rich environments, and when higher data rates are required (especially for fronthaul), the lower layers may be circuit-switched instead of, or in addition to, packet switched. In such cases OTN (ITU-T C G.709) and dense wavelength division multiplexing (DWDM) (ITU-T G.694.1) technologies will be employed. The ITU-T is presently studying the application of OTN to 5G transport (ITU-T G Suppl. 67). Similarly, point-to-point microwave may be used instead of fiber as a constant bit rate transport medium; alth