# On the Rollout of Network Slicing in Carrier Networks: A Technology Radar
## 7 TN Domain
### Transport Technologies  
The system architecture represented in Figure 2 shows the entities on the path for an E2E slice, assuming a 3-tier deployment for the gNB: RU, DU and CU-UP. As seen, the user plane from the device to the application includes the following TN segments: fronthaul (O-RAN fronthaul interface), midhaul (3 GPP F1-U interface), backhaul (3 GPP N3 interface) and DN (3 GPP N6 interface). The F1-U and N3 interfaces use GTP-U to transport packets (IPv4, IPv6, Ethernet or unstructured) in the PDU session established between the UE and the DN. The fronthaul interface carries the radio frames in the form of In-Phase (I) and Quadrature (Q) samples, using eCPRI [64] encapsulation over Ethernet or UDP over IP. The mission of TN slicing is to implement slicing-featured capabilities across all these connectivity segments, ensuring that the SLA requirements are met not only at the 3 GPP network function layer, but also at the transport underlay. Examples of these capabilities include guaranteed QoS (e.g., bandwidth reservation, upper latency bounds, controller delay variation), isolation, protection and reliability (e.g., disjoint routes). As outlined in Section 2.3 and shown in Figure 8, there exist multiple networking solutions that can be used for the provision of these capabilities, ranging from soft slicing to hard slicing, with some trade-off solutions in between (i.e., hybrid slicing). Next, we present the journey we foresee towards the realization of an end-to-end TN slicing, based on a step-wise integration of different transport technologies into carrier networks. Today's scenarios are based on 5G NSA, where the slice data path only includes two segments: backhaul segment (between the physical gNB and S/P-GW) and DN segment (between S/P-GW and application). Typical implementations for these scenarios bet on the use of **traditional L2/L3 overlay** solutions, mainly VPN techniques. Layer 2 VPN techniques are, for example, Virtual Private LAN Service (VPLS), which sets up a Ethernet-based communication over MPLS tunnels, or Virtual Leased Line (VLL), which emulates a pipeline between two given endpoints. Layer 3 VPNs can be realized, for example, via Virtual Private Routed Networks (VPRN), featuring MPLS-based VPNs. Two main reasons explain the preference for these technologies. On the one hand, they do not have an impact in the underlay. Taking into account the brownfield design of today's transport networks, it is natural for operators to reuse as much as possible existing overlay technologies, especially if they are carrier-grade and widely available along the operator footprint. On the other hand, they are cost-efficient, with a performance that is enough to satisfy the transport requirements of NSA slices. In the short term, early B2C slicing offering will require evolving transport networks with different solutions such as the listed below. * **DiffServ Code Point (DSCP)**. The integration of 5 GC will lead to a change in the backhaul segment, now based in the N3 interface. Additionally, the DU-CU disaggregation will produce the F1 interface for the mid-haul segment. With this scenario, the setup is as follows: a GTP tunnel encapsulating user packets with slicing information captured in the [PLMN ID, S-NSSAI, 5 QI] triplet transverses the backhaul and midhaul segments. Since the IP underlay across these segments is not able to interpret these 3 GPP signalling identifiers, it is not possible for the border router (see Figure 7A) to apply the constraints represented by this tuple. In this regard, the UPF and RAN shall perform transport level packet marking in downlink and uplink, respectively, by setting the DSCP in the outer IP header [65]. This information is used by the corresponding border routers in the IP underlay to differentiate traffic from different slices [66; 67]. * **Segment Routing (SR)**, and its use across backhaul and midhaul segments.