# DAPS Handover

## Summary


In mobile cellular systems, a UE in an active call might move from one cell to another. Via a procedure called handover the network attempts to maintain the call. Call is handed over from source cell to target cell. However, for some time period the connection is lost. DAPS Handover is an attempt to eliminate this handover interruption time. 

During Dual Active Protocol Stack (DAPS) Handover, the UE maintains two protocol stacks. One stack continues to receive and send data on the source cell while the other handles traffic in the target cell. Within the RAN, packets are forwarded from source to target. 

In 4G/LTE, Make-Before-Break (MBB) Handover was introduced in Release 14. In 4G/LTE and 5G, DAPS Handover was introduced in Release 16. MBB Handover can be viewed as a simpler form of DAPS Handover.

## Discussion

### Could you explain DAPS handover?

In 4G/LTE, a basic handover procedure incurs an interruption time of about 30-60ms.  In some cases, interruptions of hundreds of milliseconds have been reported. With the introduction of 5G and its URLLC use case, this is no longer acceptable. URLLC demands a latency of 1ms. 

With DAPS, UE uses resources of both source and target cells even after receiving the handover command. UE suspends Signalling Radio Bearers (SRBs) towards the source cell. On those Data Radio Bearers (DRBs) configured for DAPS, the UE continues to send and receive data on the source cell while connecting to the target cell. Within the network, source eNB/gNB forwards uplink/downlink packets to the target eNB/gNB.  

Once UE connects to the target cell, the latter commands the UE to release source cell resources. If handover failures, UE continues on the source cell and resumes suspended SRBs.  


### What's the difference between MBB handover and DAPS handover?

With DAPS handover, UE makes the connection with target cell before breaking connection with the source cell. Hence, it's common in literature to call this "make-before-break". However, the 4G/LTE specifications uses the term MBB handover in a specific way. Consider the following definitions: 

    + **MBB Handover**: Maintaining source eNB connection after reception of RRC message for handover before the initial uplink transmission to the target eNB during handover.
    + **DAPS Handover**: A handover procedure that maintains the source eNB connection after reception of RRC message for handover and until releasing the source cell after successful random access to the target eNB.In MBB, source eNB decides when to stop downlink transmission. Once uplink transmission is achieved on the target, UE can break its connection with the source. In DAPS, UE releases its source connection only after the target explicitly signals the UE to do so. 

MBB is applicable for a change of SeNB in Dual Connectivity (DC) scenarios. It can even be used with RACH-less handover. However, DAPS can't be used alongside DC or RACH-less handover. These must be released by the RAN before initiating DAPS.  


### How does DAPS impact the PDCP layer?

At the UE, PHY/MAC/RLC layers are duplicated, one for source cell and one for target cell. A single PDCP entity deals with both source and target cells. The figure shows the functional view of a 5G UE PDCP layer. In both transmit and receive entities, PDCP has two sets of security functions/keys and header compression protocols. 

When transmitting, PDCP does routing, that is, decides whether packet should be sent on source or target cell. PDCP Data PDUs are sent to the target cell. PDCP Control PDUs are sent to source or target as associated. 

When receiving, PDCP detects and discards duplicates since the same SDU may be received on both source and target cells. 

In 4G/LTE, the PDCP layer for DAPS is similar except for integrity protection. 4G/LTE PDCP applies integrity only for control plane but DAPS is for the user plane. 


### What are the technical details about DAPS?

UE, source/target NG-RAN and source/target AMF should support DAPS. DAPS is selectively applied to DRBs that require it. 

Inter-RAT DAPS handover is not supported. DAPS FR2 to FR2 is not supported in Release 17. 

For basic handover, UE resets the MAC entity and re-establishes the RLC entity. For DAPS, UE creates a new MAC entity. For each DAPS-enabled DRB, UE creates an RLC entity (with DTCH logical channel) and reconfigures the PDCP entity. If handover fails, UE reports the failure via the source cell without triggering RRC connection re-establishment. 

Source gNB continues downlink transmission (and forwarding to target gNB) till it receives *Handover Success* from target gNB. Source gNB then responds with *SN Status Transfer* to target gNB. At this point, source gNB stops delivering uplink QoS flows to UPF. Source gNB no longer allocates PDCP SNs for downlink packets. 

Even after switching to target gNB, UE continues to send UL layer 1 CSI feedback, HARQ feedback, layer 2 RLC feedback, ROHC feedback, HARQ data (re-)transmissions, and RLC data (re-)transmissions to the source gNB. 

## Milestones

Dec  
2015

Samsung files a patent that mentions **make-before-break handover** as a technique to reduce data interruption time. They describe this type of handover as "with continued PDUs delivered from the source eNB until the UE is ready to receive data from the target eNB." 

Jun  
2017

LTE Release 14 reaches functional freeze. The release introduces support for **Make-Before-Break Handover**. 

Jul  
2020

3GPP finalizes Release 16 specifications. The release introduces support for **DAPS Handover** in both 4G/LTE and 5G.

Feb  
2022

MediaTek and Anritsu demonstrate successful DAPS handover. The test system uses MediaTek M80 5G modem and Anritsu's MT8000A Radio Communication Test Station with Rapid Test Designer application software. 

Mar  
2022

In the specifications of 5G Self-Organizing Network (SON), Mobility Robustness Optimisation (MRO) is defined for DAPS. Measurements that need to be collected are defined. DAPS use case is also described.