5G Call Flows & Signaling

This article “5G Call Flows and Signaling – How a 5G UE performs Initial Access” explains 5G call flows in a very simple, yet an effective manner. One of the most complex area for beginners is to understand 5G Call Flows and Signaling procedures. 3GPP defines them but they are not easy to understand.

So, in this video, I have explained the 5G Call Flows in a simplified manner and I have also included the contents of some of the important messages to show what actually goes on between UE/Mobile and the 5G network during the 5G Initial Access or Network Entry procedure.

Moreover, I have highlighted some of the important 5G KPIs in the end that will help the audience to correlate the 5G Call Flows and Signaling procedure with the 5G KPI itself. This is also one of a popular 5G Interview topic as well and many questions are asked from this Signaling Procedure.

5G Call Flows and Signaling – How a 5G UE performs Initial Access

Note: To have sound understanding of 5G call flows and signaling, it is better to have a bit of background that you can achieve in my old article/video – 5G Architecture – A summary of 5G Network Deployment Options

A LTE eNB looks for 5G capable mobiles or handsets and as soon as it finds a 5G capable mobile in its coverage, it tries to add the 5G leg for it. However, for optimum 5G user experience, it is important to ensure that only mobiles in 5G coverage are allowed to connect to 5G. For this purpose, there is a concept of measurement thresholds and only if the mobile finds the 5G coverage to be above the threshold, only then it is allowed to attach to 5G leg. This threshold is called a 5G NR B1 Threshold.

Most of the current 5G implementations are Non-Stand Alone (NSA). So, for 5G network access, the mobile needs to connect to LTE first. The LTE site should be configured with the 5G node relations (frequency and cell relations). The LTE site checks mobile’s capability and if the mobile is 5G capable, it sends a B1 Measurement Control to the mobile This B1 measurement control message is basically a request to the mobile to measure the 5G signal in order to verify whether the mobile has 5G coverage or not. If the mobile is in 5G coverage, it will send a measurement report to the eNB. Based on this measurement report, the eNB will add the 5G node for the mobile using X2 signalling (SgNB Addition procedure). Once, the 5G NR leg is added, the mobile connects to the 5G NR leg by using RACH procedure. This is type of a handshake to get uplink synchronization for the mobile. The mobile sends a RACH preamble and based on that the gNB sends RAR which carries the TA (Timing Advance) value. The mobile corrects its timing and sends MSG-3 which indicates the success of 5G access. As there is no RRC in 5G NSA, so this MSG-3 is not a RRC Setup Request (unlike LTE) but it is actually just a PUSCH transmission with CRNTI. This PUSCH allocation is provided by the gNB to the UE in the MSG-2. After this, the gNB will send a MSG-4 which will again be just a PDSCH transmission and once the PDSCH will be successfully decoded by the UE, the MSG-4 will be considered successful and the gNB will start transmitting data packets to the UE. This is where the data flow starts in both downlink and uplink directions.

In case you have any queries or want to connect with Ali Khalid, please feel free to drop a comment or follow him on LinkedIn. Also, if this has been helpful, then please subscribe to our Youtube channel – Our Technology Planet for more exciting stuff and videos.

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Ali Khalid

5G NR | VoLTE | LTE-A | Massive MIMO | NB-IoT | NDO Network Specialist at Ericsson, Australia
Ali Khalid is a Senior LTE/VoLTE RNPO, NB-IoT and 5G Solution Architect who has successfully led and delivered a number of projects in different regions across the globe including Pakistan, Bahrain, UAE, Qatar, Oman, KSA, Nigeria, Turkey, Poland and Japan. He is currently working in Strategic Competence Unit (SCU), a highly experienced global team at Ericsson, Australia. In case of any questions or feedback, please feel free to drop a comment below or connect with him on LinkedIn.

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  4. Thanks Ali for the simplifying the explanation.
    My question is this, since the sgNB addition is completed before RACH process in 5G-NSA, what happens if the RACH process fails?
    1. Will the UE loose 5G connection?
    2. Is data transmission to the UE started after sgNB addition success or RACH success?
    3. Which KPIs will be impacted if there’s a RACH failure? only RACH SSR or sgNB drop inclusive?
    4. If there is sgNB failure after RACH failure, will it be counted as a service drop?

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