NSA Call Flow (5G Non-Stand Alone ) and Log Analysis NSA UE

What is NSA and NSA call flow?

NSA stands for Non-stand-alone, which means there is an LTE cell required to connect a 5G cell. And NSA call flow is the set of message flows between UE and Network. lest discuss all these in detail.

Table of Contents Or Jump To The Page Content from Here

In NR different types of deployment options are introduced like 2A, 3A, etc.

ENDC is a technology that enables the introduction of 5G services in a predominantly 4G network.

An approach that allows the UE to connect simultaneously to LTE Master Node eNB (MN-eNB) and 5G-NR Secondary Node gNB (SN-gNB).

Also Read:- 5G Architecture and what is the difference in 4G and 5G architecture?

Firstly, UE registers to LTE the network and then adds NR via RRC Connection Reconfiguration process.

Based on Telecom Italia deployment, eNB is used for both signaling(C-plane) and data exchange(U-Plane) but gNB interface is used only for data exchange.

NSA CALL FLOW basic Setup is Mentioned Below:

Also Read:- UE power on procedure in 5G: Initial Access or Cell Selection

Log Analysis NSA call Flow:-

Upper layer Indication

If the 4G network supports EN-DC, the first indication that it gives to the UE is through SIB-2, containing 5G-NR PLMNs via the PLMN-InfoList-r15. To check this message, open ‘RRC NAS OTA” window in 4G logs and look for the first System Information Block message. Toggle down and observe the below-mentioned IE.

Attach Request Message in DC

UE in Attach Request sends an indication to network that it supports DC-NR. Message can be checked in QXDM, under “RRC NAS OTA” window during 4G attach procedure.

Restrict DC-NR

If the network restricts NR request of the UE, it sends an Information element “Restrict DCNR” supported in Attach Accept message.

UE Capability Inquiry

In QXDM, the capability messages will be seen in “RRC NAS OTA” window during 4G attach procedure.

Information Elements in UE Capability Information Message

Supported UL/DL bandwidths by 5G-NR bands:

Check the second UE capability information sent by UE for 5G-nr and look for supported bandwidth information element for a specific band for both DL and UL as below

Supported ULDL bandwidths by 5G-NR bands

MIMO Support for NR bands:

Modulation Type:

This information element describes the modulation type supported for both Uplink/Downlink. In below example, for DL 265QAM is supported and for UL 64QAM is supported

Subcarrier Spacing:

EN-DC support:

EUTRA–NR Band Combination:

The snapshot shows an example of EN-DC band combination that the given UE supports i.e. 7A-20A-NR78A

RRC Connection Reconfiguration Message

RRC Connection Reconfiguration message is used by the network to manage the layer-wise configuration of the connection between UE and the Network. With respect to NR, some important Information elements sent in this message are:

Measurement Report Configuration
Adding/Releasing NR

Configuration of NR Cell Measurement Reporting: During EUTRA attach procedure, eNB sends an RRC Connection Reconfiguration message that includes measurement objects for 5G NR frequencies. This

procedure is important for NR Addition (not during “Blind Addition”). Information elements that completes this procedure are:

a) Adding a “measObjectID”: A new measObjectID is added for r-15 frequencies. We can see this IE under measconfig in RRC Connection Reconfiguration message. This message contains NR cell carrier frequency, – band information, subcarrier spacing etc supported by the network.

measconfig in RRC Connection Reconfiguration message

b) Adding a “ReportConfigID”: A new “ReportConfigID” is added to configure measurement reporting for NR cell. We can see this IE under “ReportConfigToAddModList” in RRC Connection Reconfiguration message. This message contains- event type, threshold Power etc.

c) Adding a “MeasID”: A new “MeasID” is added to link the “ReportConfigID” and ”MeasObjectID ”

Based on “ReportConfigID” and “MeasObjectID” we have set in previous slides, we see in this snapshot that “measID 4” is linking them. All the reporting parameters that we gave in “reportConfigID 4” will be applied to the r-15 frequency i.e. the cell mentioned in “MeasObjectID 2”

Measurement Reporting for NR cell

After configuration for measurement reporting is sent by the network to the UE in RRC Connection Reconfiguration message, the UE , based on the event type, starts reporting measurements.

The measurements usually consist of Physical parameters of NR cell such as:

Physical Cell ID
RSRP/RSRQ values.

Based on these measurement reports, the NR is added to the LTE cell.

“NR Addition” logging at the NR side

NR Addition process begins with LTE eNB sending “gNB Addition request ” to NR gNB. The message carries the RRC and Radio Bearer configuration as seen in below snapshot. gNB replies with an acknowledgement to this request. In QXDM, we can only see the request part of the message(i.e RRC and Radio Bearer configuration )in NR logs as this communication is between two networks.

To check these PDU’s, open the logs using NR log mask and open the “RRC NAS OTA window”. You will see RRC_Reconfig, Radio_Bearer_Config message and MIB.

NR Cell Addition at LTE

After receiving the acknowledgment from gNB, the 4G eNodeB sends an RRC Connection Reconfiguration message to the UE for 5G addition. This message can be seen just after measurement report for 5G NR is sent by the UE (unless it’s a blind Addition).
The message carries the RRC and Radio Bearer configuration along with information used by UE to RACH on NR cell.
Under nonCriticalExtension , “nrconfig-r15” setup message , we can see the HEX PDU’s for “SecondaryCellGroupConfig-r15” and “nr-RadioBearerConfig-2-r15”. These PDU’s are basically the RRC and Radio Bearer configuration message we saw in NR logs.

UE response to NR Cell addition request:

RRC Connection Reconfiguration Complete: This message is sent by the UE when the procedure is a success i.e. the NR cell is added.

RACH on NR: After RRC Connection Reconfiguration message is sent to UE, it reads the MIB of NR cell, sends a RACH to the NR and request for UL GRANT. The message will be seen in “RACH window”. Success in the message indicates that UE has successfully attached to the NR cell. If the RACH procedure FAILS, a FAILURE string in place of SUCCESS is seen and the UE retries RACH Attempt.

SCG Failure: If the UE fails to attach on NR cell an “scgfailureinformation” is sent by the UE to EUTRAN with failure type: Rlc-MaxNumRetx .

5G-NR Cell Release:

5G NR cell release message is passed through RRC Connection Reconfiguration message sent by LTE Cell.

In QXDM,Check “RRC NAS OTA” window in LTE logs and check RRC Connection Reconfiguration message for NR Cell Release.

NSA call flow 5G-NR Cell Release: — 5G-NR Cell Release:

Identification of IE and Radio Messages in QXDM logs

DL-UL Throughput on NR: To check the DL and UL throughput, open QXDM with NR mask and open “Tput” window. Note: In logs, the white logging is for DL and yellow one is for UL.

DL Split Bearer:

Filterer View of NR L2 DL window of NR logs, find “DL Config” followed by NR5G L2 DL Config log which confirms whether the current configuration is SCG split bearer or SCG bearer, RLC path: LTE AND NR indicates split bearer.

UL Split Bearer:

Filterer View #9 NR L2 UL window match “UL config”, NR5G L2 UL Config log can confirm which path is being taken while in Uplink for transmission. In below case, the network is configured for “NR only”.