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Timing Accuracy For eCPRI Fronthaul And Open RAN

Considerations in working with new network interfaces.

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Open RAN is an eCPRI-based open fronthaul interface between the radio unit (RU) and distributed unit (DU) [1]. This white paper discusses timing accuracy for eCPRI fronthaul transport networks. The eCPRI interface is supported by packet switched fronthaul transport networks. When the fronthaul network is used to synchronize an eCPRI node, it must provide timing at the user network interface according to the relevant 3GPP timing accuracy category. Time error budgets for 4 timing accuracy categories are given by the eCPRI Transport Network Requirements Specification [2]. Time synchronization for eCPRI nodes can be accomplished using the precision time protocol (PTP) per IEEE 1588 [3]. The ITU-T maintains a set of recommendations including G.8271.1 [4] and G.8273.2 [5] that cover PTP based synchronization.

The ITU-T recommendations define the characteristics of telecom boundary clocks (T-BC) and telecom time slave clocks (T-TSC) that are used for PTP based synchronization; and they describe methods to estimate time error accumulation in networks. The time error characteristics of the clocks within DUs, RUs, and switches and routers have a direct bearing on how many T-BC hops can be supported by the time error budget of a fronthaul network. The greater the number of T-BC hops the lower the number of primary reference time clocks (PRTC) that will be required and the lower the deployment cost. Further, the greater the number of allowed T-BC hops the greater the flexibility in the deployment.

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