the timing signal between each NIC card in the system. Figure 10-37 illustrates the typical deployment scenario using a virtualized DU. Once the DU server gets the accurate time and phase signal from a connected PRTC, the NIC card can be configured as a T-GM to distribute the clock to other connected servers in daisy chain configuration (or other NIC cards in the same server). PRTC/T-GM PRTC/T-GM ###### ...... vDU with PCle timing NIC PCle timing NIC PCle timing NIC timing NIC ###### Timing sync through external connection Backup timing Backup timing from network Primary timing from network from network to legacy BBU Midhaul network Midhaul network a) Cell site with single DU server b) Cell site with multiple DU servers and legacy BBU Figure 10-37 vDU Deployment at Cell Site Location The (multiple) interfaces on the time sync NIC are used to connect to mid-haul network elements, front-haul routers, and RU devices. If any legacy BBU or eNB is also deployed at the site, it is pos- sible to use timing from the DU or take timing from another network source such as a router/switch in the mid-haul or back-haul network. Because the CRAN architecture using the vDU is heavily dependent on the time sync NIC card in- side the server, the reliability of the solution is reduced using this approach-especially when the single NIC is used as a source of PTP and daisy chained to other vDUs in the site. Similarly, this approach affects the timing error and scalability of the timing source at the site. The daisy chaining adds hops between the vDU and the associated RUs, adding to the time error. And since many of these NIC cards have a very limited number of interface ports, using a daisy chain takes two ports from each card (they typically only have three or four ports). This limitation also makes it very hard to scale the timing network without introducing excessive hops and NIC cards. For this reason, a network-based front-haul timing distribution is highly recommended when more than one vDU server is deployed at the site. 10.5.