d, especially if it is essential that at least one (or some) air interface service from the site continues its operation specifically if another technology experiences an outage. 9.1.5 Mobile Backhaul Tiers A transport network may be categorized into separate network tiers: access, distribu- tion, and core, which all have distinctive characteristics. Figure 9.1 shows three tiers of mobile backhaul, partitioning the e2e network into tiers with distinct characteristics, based on tiered network architecture, see e.g. [2]. In the access tier, individual base stations (macro- or small cell gNBs), distributed (gNB-DUs) units, as well as remote radio units (RUs) are connected to the network via first mile¹ access links. The access tier may consist of just a single link, a few links, and also possibly of some site switches/routers functioning as small access hubs. Access links may be - and often are - supplied by wireless transmission, otherwise fiber is the first option, wherever available. The access tier is very important, as a major part of the transport network cost origi- nates from here. Access links are needed in volumes, and even if the access link is shared 1 The term "last mile" is used as well. The terms "first mile" and "last mile" both refer to the link connecting the access device (base station in the 5G case) to the network. 9.1 5G Transport Network - High-speed backbone - IP, MPLS, Fiber - Scalable IP networking services-based architecture - Edge device (PE) for IP MPLS VPNs Distribution - Resiliency, Security, QoS - Distribution/collection of traffic flows to/from the access Access - High amount of multi-Gbps/10 Gbps access links, IP/IPsec connectivity from hosts - Possibly few link hops, access IP hub services - Automated mass operations - Simple, robust functions, resiliency where needed - Fiber, WDM, Wireless Figure 9.1 Mobile backhaul network tiers. for multiple base stations on the same site, there are still a large number of sites in any practical network that need to have connectivity to the netw