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 transmission for UE.

The source gNB⁃DU could respond with a Downlink Data Delivery Status (DDDS) frame via F1⁃U to inform the gNB⁃CU about the highest PDCP PDU SN successfully delivered in sequence to UE and the highest NR PDCP PDU sequence number transmitted to the lower layers, which is the key step for the fast retransmission of lost PDCP PDUs (shown in the dashed rectangle in Fig. 2). Based on this information, the gNB ⁃CU is able to identify the unsuccessfully transmitted PDCP PDUs in the source gNB⁃DU side. The unsuccessfully delivered PDCP PDUs will be sent from the gNB-CU to the target DU for retransmission

Fig. 2: Intra-DU handover

Edit: Section 5.4.2 of TS 38.425 contains more comprehensive detail for DDDS which I did not refer to earlier.

Reference

[1] H. He et al. “Mechanism of Fast Data Retransmission in CU⁃DU Split Architecture of 5G NR”, DOI: 10.19729/j.cnki.1673􀆼5188.2018.03.007

[2] TS 38.401, “5G NGRAN Architecture Description” R16 V 16.8.0