Xn interfaces between base stations without the exorbitant cost of direct fiber interconnect requires a non-trivial network architecture. 6.6 Transport Network The second set of justifications consists of business constraints. Provision of transport services requires transport resources (fiber plant, microwave links, optical muxes, Ethernet switches, etc.) and transport expertise (operations, administration, maintenance, perfor- mance measurement, efficient APS, etc.), neither of which is at the heart of the mobile operator's business. It thus makes sense for the mobile operator to farm out the transport segment to a wholesale provider (or to an internal, but separate, transport division - see below) with the needed resources and expertise. In many cases the mobile operator provides its own transport, but generally through a separate "transport" division or business unit. Quite often this transport network is used for multiple services - residential, business, and mobile. Although a mobile operator may decide to build out its own transport network, this case occurs most often when the mobile operator is the successor of an incumbent telephony provider, or the result of a merger of ser- vice providers with different specialties, or part of a diverse telecommunications company. Even in such cases the mobile operator may contract a wholesale provider to augment its footprint areas where it needs to provide mobile coverage but has no transport network. A wholesale provider is typically a network operator with extensive fiber and switch- ing resources with which it delivers a variety of services, of which mobile backhauling is one. Mobile backhauling mandates certain specific requirements, such as synchronization, but is otherwise a relatively straightforward use of the wholesaler's network and expertise. A recent trend is for wholesale providers to additionally supply power and shelving for servers, in order to host (and perhaps even provide) virtualized services including virtual RAN functions (see Section 6.2). Bas