In NetSim, all gNBs broadcast MIB and SIB1 control packets at 80 ms and 160 ms (and continue broadcasting at every 80/160 ms intervals subsequently). 


The UE makes an initial decision on which will be its serving gNB.

  • The UE attaches itself initially to the gNB from which it receives the highest SNR in the SSB channel
  • If SNR from multiple gNBs are equal, the UE will attach to the gNB with the lowest ID
  • The gNB to which the UE is connected by the user in NetSim GUI at the network design stage is only for visual purposes. It plays no role in determining which gNB the UE will attach to.

After cell selection

  • RRC Connection establishment involves the following control packets:
    • RRC_SI: Selected gNB will send system information to UE
    • RRC_SETUP_REQUEST: Setup request will be sent by UE to selected gNB.
    • RRC_SETUP: Selected gNB sends the setup to UE which contains RRCTransactionIdentifier, RRCResponsetype, PDCP Properties: UEID and GNBID, DiscardDelayTimer , T_Reordering, Hdr Type, SN=0, dcBit.
    • RRC_SETUP_COMPLETE: UE sends back the setup complete containing RRCTransactionIdentifier, SelectedPLMNIdentity, AMFIdentifier, Gaumi Type, Hdr Type, SN, dcBit.
  • UE will send the UE_MEASUREMENT_REPORT to the connected gNB.  The measurement report is sent by each UE to its serving gNB and it contains SINR from all gNBs
  • The flow of these packets can be seen in NetSim packet trace or packet animation


After the UE gets attached to the gNB the gNB- 5G Core connection establishment takes place.

If the SNR from another gNB is offset greater than SNR from serving gNB, it would lead to handover. After the handover procedure is completed RRC Reconfiguration would happen between target gNB and UE. The UE will then send the UE MEASUREMENT REPORT to this gNB.


Detailed information regarding the cell selection and attach procedure is explained in the sections  Cell Selection and UE attach procedure and 5G Core connection management process in the NetSim 5G Manual


Useful link


1. Overview of NetSim 5G library: https://tetcos.com/5g.html

2. Detailed documentation: See the 5G manual in https://tetcos.com/netsim-documentation.html