nalities as well, and more fully warrant its name. The most common function is aggregation of all traffic types originating in the cell, including 4G, 5G sub 6 GHz (FR1), 5G mmWaves (FR2), small cell transit traffic, Wi-Fi hot spot backhauling, etc. The CSG is responsible for homogenizing traffic across cellular generations and wireless technologies, minimizing transport expenses (including energy efficiency and multiplexing/duplexing in order to minimize OOF fiber expenses), constructing a single frequency and time-of-day reference clock, providing a cell-wide heartbeat to ensure connectivity to the cell and all its components, and initiat- ing failover self-healing procedures as needed. It may additionally incorporate a micro-data center platform for edge computation. 6.6.5.1 xHaul Network Topology We concluded in the previous subsection that direct connection of base stations to the mobile core is infeasible, and that a non-trivial xHaul network, with various transport-specific functionalities, is required. The classical backhaul network topology comprises a "core edge" switch or router con- necting via an "aggregation network" to CSGs. The connectivity topology of this network can be star, tree, "hub and spoke," or a ring or collection of rings (ring-subring, ladders, etc.), as depicted in Figure 6.6.5. In geographically large jurisdictions, where cell count and distances overly burden such a topology, the connectivity between the core edge and the cell sites might be divided into "first aggregation" (or pre-aggregation) networks and second aggregation network (sometimes called "access" and "aggregation" networks, respectively). The first aggregation networks have hub and spoke or ring topology, while the second aggre- gation network comprises rings or some form of partial mesh. This basic model supports various variants. For example, Xn interfaces between cell sites are supported by hair-pinning at the core edge (or second aggregation switch) rather than continuing on to the core. Fronthaul requires an exten