Showing posts with label NGAP. Show all posts
Showing posts with label NGAP. Show all posts

Wednesday, 7 October 2020

Understanding the Dual Active Protocol Stack (DAPS) Handover in 5G


In this video I explain the principles and signaling procedures related to the DAPS handover.

The DAPS handover is a new feature for URLLC services defined by 3GPP in Rel. 16.

Friday, 17 July 2020

A Look into 5G Virtual/Open RAN - Part 7: Change of gNB-CU-UP without Handover

This will be the last part of my series about Virtual/Open RAN signaling procedures. In this final post (although not the last one on this blog) I would like to present a very unique procedure that emerges from the facts of virtualization and automation of the RAN. And again I would like to present the big picture overview of the scenario that is called "Change of gNB-CU UP" (without handover). The full message flow (ladder diagram) can be found in 3GPP 38.401, chapter 8.9.5.

In the same chapter one can read that the trigger point for starting a change of the gNB-CU UP is quite vague. 3GPP writes: "e.g. a measurement report". However, which particular measurement event should trigger such a procedure? Even when looking into the Rel. 16 versions of 3GPP 38.331 (NR RRC) it becomes evident that all measurement events that are not dealing with NR sidelink or V2X connectivity are triggered by changing reference signal strength or rising interference. 

However, in case of a gNB-CU UP change without handover the UE does not move to a different cell. This makes me think - correct me if I am wrong - the true trigger points for this procedures come form a different entity, e.g. from the AI-driven policies and algorithms of the RAN Intelligence Controller (RIC) that is a fundamental element of the Open RAN architecture.


So what is necessary from a signaling perspective to change the gNB-CU UP during an ongoing connection?

There are new transport network resources aka GTP/IP-Tunnels required to steer the user plane traffic to and through the RAN. A new F1-U tunnel is necessary as well a a new NG-U tunnel, because also the user plane traffic between RAN and the UPF in the 5G core network must be exchange using a new route.

When it is clear which new UP transport tunnels need to be established (and which old ones need to be deleted) it is really simple to understand the overall scenario.

A F1AP UE Context Modification procedure is performed to switch the F1-U tunnel. NGAP Path Switch procedure is performed to switch the NG-U tunnel. And an E1AP Bearer Context Modification procedure is the prerequisite, because it delivers the new UL GTP-TEID for the F1-U tunnel as well as the new DL GTP-TEID for the NG-U tunnel.

Unfortunately the authors of 3GPP 38.401 are not very precise when mentioning protocol procedures defined in other specs. Thus, they speak about "bearer modification" when looking at F1AP and "Path Update" for NGAP.

It is not a big deal, but something you just need to know if you want to analyze real-world message flows of this scenario.

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Tuesday, 30 June 2020

A Look into 5G Virtual/Open RAN - Part 6: Inter-gNB CU Handover involving Xn

In previous blog posts I have discussed intra-gNB-DU handover and inter-gNB-DU handover scenarios.Now it is time to look at inter-gNB-CU handover that uses the Xn interface.

At the RRC protocol layer there will be the measurement setups and measurement reports as in the intra-gNB handover cases. And F1AP UE Context Setup and Release Procedures are identical with the ones discussed for inter-gNB-DU handover. Only the cause values are expected to be different, e.g. "successful handover".

Thus, I do not want to  focus here on la adder diagram call flow (that is by the way very well described in 3GPP 38.401, chapter 8.9.4), but invite you to have a look at a "big picture" that you see below.

(click image to enlarge)

What characterizes the inter-gNB handover is the transfer of the UE RRC/NGAP context form the source gNB-CU to the target gNB-CU. When the Xn interface is available to connect two neighbor gNBs this context transfer is executed using the XnAP Handover Preparation procedure. The Initiating Message of this procedure transfers the UE context parameters to the target gNB-CU. Then embedded in the Successful Outcome message the handover command is sent in return to the source gNB-CU that forwards it to the UE. In addition a temporary user plane transport tunnel for the purpose of data forwarding is established and later on released on the Xn user plane interface.

Once the UE performed the handover on the radio interface all the transport tunnels for the payload transmission need to be switched from the old gNB to the new one. This includes the tunnel to the UPF that is managed by the NGAP. Thus, the target gNB-CU starts the NGAP Path Switch procedure. 

In the target gNB environment it is necessary to establish a new F1AP UE context, new E1AP Bearer Context and new F1-U payload transport tunnel. All this happens BEFORE the Handover Command is sent to the source gNB/UE. And once there is an indication that the handover is completed all the radio and transport resources controlled by the source gNB will be released.

So the figure above looks complicated, but actually the underlying logic of context/data forwarding, radio resource allocation and transport tunnel switching is quite simple.

Special note: In case there is no Xn interface available the UE context/handover information can be transmitted using NGAP Handover Preparation procedure on the source side of the connection and NGAP Handover Resource Allocation procedure on the target side of the connection.

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Tuesday, 28 April 2020

Comparing S1AP and NGAP UE Context Release


As an addition to my blog post about the 5G RAN Release procedure I would like to have an in-depth view at the details of NGAP UE Context Release Complete message.

Indeed, the S1AP (known from E-UTRAN) and the NGAP are very similar protocols and when reading the 3GPP specs it is obvious that many message names are identical and the procedures fulfill the same purpose when looking at call scenarios.

However, the difference is visible in the details as one can see when looking at the figure below.

While the S1AP UE Context Release Complete message does not contain any additional information we find in the NGAP UE Context Release Complete the identity of the last serving 5G cell, represented by the NR-CGI, the last visited Tracking Area Identity (TAI) and a list with the IDs of the PDU sessions (E-RABs) that have been terminated when the UE context was released.

This additional information in very valuable for network troubleshooting, since in LTE (S1AP) only the ID (ECGI) of the initial serving cell or a new serving cell ID at inter-node handover was signaled. And if you wanted to know how many E-RABs have been terminated with a S1AP UE Context Release procedure it was necessary to look back into the full sequence of call-related S1AP messages starting with the messages for Initial Context Setup.

All in all, with 5G NGAP trace analysis and the life of RAN engineers becomes easier. Thank you, 3GPP! 

Comparision of S1AP and NGAP UE Context Release Complete Messages

Friday, 24 April 2020

A Look into 5G Virtual/Open RAN - Part 3: Connection Release and Suspend

The 3rd post of this series introduces the details of connection release in the 5G RAN.

Indeed, we find most of the release causes known from E-UTRAN in the 5G specs and it is clear that all protocols that have been involved in the connection setup need to be perform a release procedure at the end of the connection.

However, again the split into different virtual functions brings the demand for some addition messages.

This is illustrated in figure 1 for the a release due to "user inactivity", which means: the gNB-CU UP detected that for a define time (typical settings for the user inactivity timer are expected to be between 10 and 20 seconds) no downlink payload packets have been arrived from the UPF to be transmitted.

So the gNB-CU UP sends an E1AP Bearer Context Inactivity Notification message to the gNB-CU CP that triggers the release procedures on NGAP, F1AP, RRC and E1AP. The RRC Releases message is transported over the F1 interface to the gNB-DU where is forwarded across the radio interface to the UE.


Figure 1: Connection Release due to "user inacativity"
An alternative to the connection release is the RRC Suspend procedure shown in figure 2. Here the UE is ordered to switch to the RRC Inactive state, which allows a very quick resume of the RRC connection when necessary.

Figure 2: RRC Connection Suspend

In case of suspending the RRC connection the RRC Release message contains a set of suspend configuration parameters. The probably most important one is the I-RNTI, the (RRC) Inactive Radio Network Temporary Identity.

If the RRC connection is suspended, F1AP and E1AP Contexts are released, but the NGAP UE Context remains active. Just NGAP RRC Inactivity Transition Report is sent to the AMF.

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Friday, 21 February 2020

EPS Fallback in 5G Standalone Deployments

It can be expected that later this year some mobile network operators will launch their initial 5G standalone (5G SA) deployments.

Nevertheless there will remain areas with temporary or permanently weak 5G NR coverage. One possible reason might be that even when 5G and LTE antennas are co-located, which means: mounted at the same remote radio head, the footprint of the 5G NR cell is significantly smaller when it uses a higher frequency band than LTE - see figure 1.

Figure 1: Smaller footprint of co-located 5G NR cell with higher frequency
Especially UEs making Voice over New Radio (VoNR) calls from the 5G cell edge have a high risk of experiencing bad call quality, in worst case a call drop. To prevent this the UE is forced  during the voice call setup towards 5G core network (5GC) to switch to a LTE/EPS connection where the radio conditions are better for the voice service.

The same procedure for which the term "EPS Fallback" was coined by 3GPP also applies when the UE is served by a 5G cell that is not configured/not optimized for VoNR calls or when the UE does not have all needed VoNR capabilities.

Figure 2: Two options for EPS fallback

When looking at the RAN there are two options for executing the EPS Fallback as shown in figure 2.

In option A the 5G radio connection is released after the initial call attempt is successfully finished and with the 5G RRC Release the UE is ordered to reselect to a 4G cell where a new radio connection is started for the VoLTE call. In this case the UE context is transferred from the AMF to the MME over the N26 interface. 3GPP seems to use also the term "RAT fallback" for this option.

Option B is to perform a 5G-4G inter-RAT handover. Here the session management and user plane tunnels in the core network are handed over from SMF/UPF to MME/S-GW in addition. This is realized with the GTPv2 Forward Relocation procedure on N26 interface.

All in all the EPS fallback is expected to cause an additional call setup delay of approximately 2 seconds.

For the inter-RAT handover case it is easy to detect from signaling information that an EPS fallback was triggered. In the source-eNodeB-to-target-eNodeB-transparent-container sent by the gNB to the eNB a boolean "IMS voice EPS fallback from 5G" indicator will be found that is set to "true". This container is named according to the receiving entity and will be carried by the NGAP Handover Preparation, GTPv2 Forward Relocation Request and the S1AP Handover Request messages.

If a redirection for Voice EPS Fallback is possible or not is indicated in the NGAP Initial Context Setup Request, Handover Request (during 5G intra-system handover) and Path Switch Request Acknowledge (after Xn handover) messages, all sent by the AMF to the gNB.

Further the NGAP protocol provides the cause value "IMS voice EPS fallback or RAT fallback triggered" in the PDU Session Resource Modify Response message indicating that a requested VoNR session cannot be established.