5G: How the user plane data flow between CU- UP and DU-UP ?

Question

In CU-DU architecture, how user plane data is going to flow ?

Answer 1

DL Data

DL Data between CU-UP and DU-UP flows via NRUP protocol layer. NRUP is used for flow control between CU-UP and DU-UP.
And it is used for DL data only. NRUP sits at both CU-UP and DU-UP.
You may refer to 3GPP Spec 38.425 for more information.

You can think of the call flow as:
CU: EGTP-Upper=>SDAP=>PDCP=>NRUP=>EGTP-Lower => DU
DU: EGTP => NRUP => RLC => MAC => L1

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UL data:

DU: L1 => MAC => RLC => EGTP => CU
CU: EGTP-Lower =>PDCP => SDAP=> EGTP-Upper => UPF