What is Downlink Data Delivery Status (DDDS) in 5G NR

Downlink Data Delivery Status (DDDS) is the F1⁃U message sent by DU during the inter-DU-intra-CU handover phase to inform the gNB⁃CU about the highest PDCP PDU Sequence Number (SN) successfully delivered in sequence to UE and the highest NR PDCP PDU SN transmitted to the lower layers of PDCP (i.e. RLC layer).

DDDS enables fast packet retransmission in case of intra-DU handover.

Note: Such a handover can have several reasons which are beyond the scope of this article.

Let us understand DDDS with an example. Let us assume a gNB with a CU connected to two DUs. The DDDS is used during the handover between DUs (i.e. intra-DU handover phase). If we see the timing diagram of the handover below, we can better understand the case (ref [1]).

Fig. 1: UE connectivity with gNB

Initially, a UE is connected to gNB-DU1. Now, for some reason, the UE experiences an outage. Consequently, the DU sends the “Radio Link Outage” notification to gNB-CU over the F1-U interface, which is part of the DDDS frame of the concerned data radio bearer.

It can be seen that at Steps 5 and 6 of Fig. 2 below, the gNB⁃CU sends a UE Context Modification Request message to the source gNB⁃DU, which includes a generated RRCReconfiguration message and indicates to stop the data…