ess Framing Downlink Fiber (optional) Alignment 10GbE Fiber or PoE Ethernet Cascade Fiber Downlink eCPRI De- eCPRI De- eCPRI eCPRI Compress IQ Broadcast Time/Offset Framing Compress Framing (optional) Alignment FIGURE 2-44 FHGW/AU U-Plane functionality. FHGW, fronthaul gateway. Chapter 2 System components, requirements, and interfaces 95 FHGW/AU eCPRI/7-2x IEEE1588/PtP to RU 10GbE (optional) to RU 10GbE to RU 10GbE FHGW/AU to RU 10GbE Transcode to RU 10GbE to RU 2x10/25 GbE 10GbE to DU, to RU eCPRI/7-2x 10GbE cascaded IEEE1588/PtP to RU 10GbE FHGW/AU FIGURE 2-45 FHGW/AU baseline connectivity requirements. FHGW, fronthaul gateway. Cell site router/gateway The cell site router provides access from the CU and DU to the mid- and backhaul aggrega- tion network to the CU and the core network. In its simplest form, when it provides connec- tivity between a (known) CU and (known) DU instantiation, little more than basic connectivity between the two units over an IP interface needs to be provided. In such case, the cell site router functionality can be absorbed in the DU unit. High-capacity 5G cell sited may need more router capacity to bifurcate a single backhaul IP connection to multiple lower capacity interfaces to subunits. Other use cases for cell site routers include separation of traffic between backhaul network types (say, enterprise and core networks). Cell site router functionality includes the following: Time and frequency synchronization-these functionalities are often included in the cell site router given that this is the first entry point for a connection carrying IEEE1588 traffic from the mid-haul link. Security-when required for the operator network, the cell site router can provide IPSec for securing the link. Traffic switching and routing toward other cell sites that are chained or otherwise connected to the main site. QoS-wireline traffic prioritization and shaping. We discuss this in more detail in the chapter 7. Cell site routers can either be logical (software) functions implemented on the CU and DU