CSFB (Circuit Switch Fall Back) Explanation & Optimization

I have received a lot of requests to explain CSFB as it is one of the more complicated LTE KPIs. So, here is a brief explanation on how CSFB works and what are various ways to optimize the KPI itself.

If a handset is camped on LTE and it does not support VoLTE, then it needs to perform a CSFB to initiate a voice call. CSFB (Circuit Switched Fall Back) is a mechanism that sends the user from 4G to 3G/2G (CS RAT) where it can successfully complete a CS Voice Call. If CSFB is not enabled in the network, the incoming voice calls will fail, however outgoing voice calls can still work depending on the mobile. However, let’s start from the basics to understand this in detail.

The figure explains an incoming CSFB call when the UE is in idle mode. If the UE is in idle mode, the core needs to send a paging message to inform the UE that a voice call is being made to the UE and it needs to perform a CSFB. Once, the UE reads the paging message, it will initiate access starting with RACH and followed by RRC setup. The RRC Setup Complete message will contain the Extended Service Request (ESR) and the eNB passes this message to the EPC in S1 Initial UE Message. ESR is the indication to the EPC that the UE has successfully received the page and it wants to undergo CSFB process.

Based on this, the MME will send Initial Context Setup Request to the eNB with CSFB indicator. It is at this point that the eNB will be informed that the CSFB process needs to be started. So, most of the CSFB related counters peg CSFB attempt at this point. This also means that if the paging fails or ESR is not decoded by the eNB, the CSFB KPI on the eNB will not show any degradation. The question is why the eNB does not peg any CSFB counters for paging or ESR? The answer is that the paging is broadcasted and the eNB does not know if that paging message is intended for the UE in its coverage area as the UE is in idle mode and it has no RRC connection to the eNB. The ESR is a NAS message and this a communication between UE and MME as the eNB does not read the NAS messages. So, the only message that informs the eNB about the CSFB process is the MME’s response to the ESR NAS message that has CSFB indicator included.

 

Once the eNB gets the Initial Context Setup Process, it may send a Security Command and RRC Reconfiguration to the UE to create a RAB and as the UE responds to these messages, the eNB sends a Initial Context Setup Response to the MME. After this, the process may vary depending on the type of CSFB implementation done in the network. Usually, there are they types of CSFB mechanisms that are used but there are other variations as well.

  1. Blind CSFB: This is the most common type of CSFB implementation where the eNB will send the UE blindly to 3G/2G. In this case, the eNB will send a RRC Release with the target RAT’s frequency and the UE will be redirected to that frequency. The UE will search for that carrier and will try to connect to it and send a paging response to initiate the MT CS Call (Mobile Terminating). The gain of this type of implementation is that it is very quick and since most of the networks have a better coverage of CS RATs (2G and 3G) so a blind redirection should not be an issue usually. The drawback is that if the CS RAT has bad coverage or poor quality in that area, the call might fail or take extensively long time.
  2. Measurement Based CSFB: This means that the eNB will send a RRC Reconfiguration with measurement control and the UE will measure the 3G or 2G cells and once it finds a cell that meets the required thresholds, it will send a Measurement Report (MR) to the eNB. The eNB will then send a RRC Release to the UE and the UE will try to initiate a CS call on that RAT. Over here, the CSFB can also be based on handover in which case, the eNB will initiate a handover to the target RAT’s cell and send the UE to that specific cell for which the MR has been generated by the UE. This type of implementation usually has more reliability but the delay is higher in call setup because the UE takes around 500 to 700 milliseconds to complete the measurement of the target CS RAT. Secondly, a handover-based implementation degrades the KPI as well since a small number of handovers fail in preparation phase. Usually, such an implementation is governed by a timer such that if the UE is unable to find 3G then if the timer expires, the eNB will blindly send the UE to 2G. This reduces the delay and improves the reliability of the call.
  3. RIM or Flash CSFB : This is a mechanism in which eNB fetches System Information (SIBs) from the target RAT and sends the SIBs to the UE in the RRC Release message. This reduces the time, the UE takes to connect to the target RAT as it will not have to read all the SIBs again. The drawback of this approach is that there can only be a limited number of SIBs that can be sent in a RRC Release message and the UE might not be able to find that cell in the target RAT so then it will still have to go through all the SIBs. However, this approach can work with both blind CSFB or measurement-based CSFB redirections and usually it brings gain to both the implementations.

Once the UE moves to the target RAT, it still needs to connect to initiate the call. There is a possibility that the call setup fails in this part and the call might fail. However, once again, this will not be pegged in the CSFB KPI on the eNB as the for the eNB, the CSFB is successful when it sends the RRC Release. The eNB has no way of knowing whether the call was successful on 2G/3G. Thus, when there is a complaint of a call failure, just looking at the CSFB KPI does not give the complete picture. It is a good idea to check the CS paging success rate and call setup success rate on the CS RATs as well.

If the CSFB KPI is bad then it can basically mean two things. Either there is an issue in the EPC to eNB phase or there is an issue in the eNB to UE Release phase.

  • EPC to eNB Phase: I call the portion where the MME tells the eNB that a CSFB is required as the “EPC to eNB Phase”. In idle mode, this phase describes the portion from Initial Context Setup Request to Initial Context Setup Response while in connected mode, this covers the UE Context Modification Request to UE Context Modification Response. Usually, the failures in this phase happen in the connected mode and they are mostly related to conflict with other procedures. For instance, if the UE is performing a handover and MME sends a UE Context Modification Request to the eNB for CSFB, the eNB might reject it saying that the UE is already undergoing handover procedure. Such issues can be mitigated by reducing handovers – if the number of handovers is huge or some vendors offer features to prioritize CSFB over other procedures. However, such failures do not cause a call failure as the MME will resend the CSFB request to the target eNB. If both the connected mode and idle mode phases show failures then it usually indicates a CSFB license issue or the feature is not activated.
  • eNB to UE Release Phase: Once the eNB has responded to the MME with Initial Context Setup Response or UE Context Modification Response, then the eNB will start the procedure over the air interface. In case of blind CSFB, the eNB just sends a RRC Release to the UE. If the RRC Release is not sent, then usually it is related to configuration of UTRAN frequencies or neighbors depending on the vendor. If the CSFB is measurement based and it is taking too long then verify if the UE is sending the Measurement Report in time (within 700 milliseconds). If it is taking longer than that then that normally indicates that the target thresholds (B1/B2) are too difficult. For instance, if the target RAT is 3G and the B1 threshold is set to -8 dB EcNo, the UE might find it difficult to find a cell that meets this threshold and will take a longer time to send the Measurement Report. So reducing the threshold to -12 or -14 will help in resolving this issue.

These are some of the basics on CSFB and I will write another article on how to reduce the CSFB call setup time and also about issues that cause a E2E CSFB call setup failure like TAC definition, Inter-MSC issues and paging failures.

In case of any queries or feedback, please drop a comment below and I would love to respond and help.

The following two tabs change content below.

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 delivered and led a number of projects in different regions across the globe including Pakistan, Bahrain, UAE, Qatar, Nigeria, Turkey and Oman. 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.

41 thoughts on “CSFB (Circuit Switch Fall Back) Explanation & Optimization”

  1. 1. In Blind CSFB , UE will be redirected ONLY to any of the 3G Neighbor Cells defined in a particular eNodeB, right?
    In case of multiple 3G carriers , one carrier can be prioritized for blind redirection?
    And similarly with Blind Flash CSFB , it will fetch SIBs of only those Cells which are defined as Neighbors in a particular eNodeb?

    2. Handover Conflicts can be resolved by a Parameter in eRAN v12 (Huawei) although as per my knowledge it is only limited to Handover Preparation Phase and does not work in Handover Execution Phase , is it correct? Secondly what other procedures can it conflict with?

      1. i actually meant that for blind redirection it will be any one of the cell which are defined as NBRs (assuming only one carrier of 3G) in that particular enodeb
        Correct?

      2. Hi Ali, I am facing CSFB Call setup time issue. we are getting CST above 7 sec in a cluster. What are the possible measures to reduce it. Further after some analysis we find that we are facing large call alerting time on DCCH. If you can recommend some measures it would be helpful.

  2. Hi Mr khalid , i facing Browsing issue in the LTE Network (huawei and new lunched) , what i mean you can browsing some web sites while you can not browsing others , for example youtue.com ok , but you can not access gmail.com or google.com and etc..

    1. Well, there can be different reasons for access issue to only certain websites.

      1. Have you checked same websites on another device using the same network? If the result is no different, you might need to check with your provider as they might have them filtered on the Firewall.
      2. Have you tried accessing those websites without having the DNS to resolve them? If that is okay, try sending a basic request to any site by connecting to port 80. It is a good test because it ensures that you are able to connect to the web server.
      3. It would also be helpful to run a trace test for problematic sites as the traceroute would reveal which exact device is responsible for dropping packets.
      4. Lastly, changing the default MTU setting on your gateway device could also help as I have experienced once before.

  3. Hi , i have a small doubt, why we are Multipling RRC & Erab for calculating CSSR ? Why can’t we sum rrc & erab. Any specific reason for Multiplying.

  4. Hi Ali, I came across your site and you have excellent articles about LTE. You express complex functionality in very simple way…

    I have a request, if you can also cover hardware part from RF design point of view –

    1- Power dimensioning, Ref power calculation for various bandwidths, how PA power is split when you increase/decrease bandwidth/Txports
    2. Hardware Configuration, How to configure various bands through single, multiple antennas, 2T2R/4T4R configurations, when do we need couplers, combiners, hardware limitations in terms or power/ports etc.

  5. Hi, here in explanation it is mentioned that, “if the paging fails or ESR is not decoded by the eNB, the CSFB KPI on the eNB will not show any degradation”. ESR in RRC Setup complete msg is earlier process before MME sent initial context setup request with CSFB indicator. And because of CSFB indicator only eNB get to know about CSFB initiation.

    So how come eNB can decode ESR. It means for incoming call when UE is in idle mode, CSFB KPI will not show any degradation every time.

    Please correct me if i am wrong.

    1. eNB sends the ESR to the MME – it is more like a forwarding of a message. So, the MME is the entity that reads the ESR and then sends the CSFB indication to the eNB. Now, in case of incoming call for idle mode, once the UE gets the page, it sends the ESR and the eNB will forward the ESR to the MME. MME will send Init Context Setup with CSFB indication and the eNB will peg CSFB preparation at this point.

  6. Can You Please explain the Scenario/ Architecture of E-UTRAN sharing between Two Operator & What are the Possible causes of CSFB failure and low Throughput in E-UTRAN Shared network ( Two Operators ).

  7. Dear,
    In RIM Message, ENB will send to UE the SI of the declared neighbors.
    i would like to know that ENB send the SI of each neighbor is the same order like i declared in adjacencies table ? or enb can do an ordering based on the measurement report got from UE ?
    Thank You.

    1. This is vendor specific implementation. Usually, vendors tend to send the SI of the neighbor that was reported in MR by the UE and then the remaining cells are chosen based on priorities defined (for instance co-located cells)

  8. Salam Ali Khalid,

    Thank you for this excellent topic , according to your experience what should be the best strategies for CSFB

    Blind or with mesurément ?

    redirection or handover ?

    BR

  9. Hi Mr. Ali,

    Quick question, does Location-Requesting-Udate have A number information?, is this a most or due to the blind CSFB it’s not needed?

    Thanks for the awesome article.

  10. In our network we are facing CSFB issue in one cell.During busy hour lots of Ue-Context modification with CSFB attempt & all the Ue-Context Modification attempt is successful. But no Ue-Context Release after successful Ue-Context modification. As this Ue-Context modification & Ue-Context Release part is included in our CSFB formula .So CSFB formula is showing almost 0

    1. When the UE context modification is successful but there is no UE context release, the most common issue is that the UTRA configuration is not there. For instance, if the freq for UTRA is not defined or there is no UTRA NBR cell defined (especially Huawei system) then there will be no UE context release.

  11. Hi Ali,
    You didn’t mention R9 redirection or SIB skipping method of CSFB where target frequency is indicated like R8 redirection. However, UE then reads mandatory SIBs 1, 3, 5 and 7 only, skipping all other SIBs and undergo normal call setup procedure (also referred to as Differed Measurement Control Reporting or DMCR).

    Secondly you mentioned that HO based CSFB will cause higher delay as compared to the blind redirection. I think below will be the CSFB types sorted from lowest to highest time/delay:

    1- R9 SI Tunneling or RIM
    2- PS Handover (non blind) (blind handover will be fastest)
    3- R9 Redirection with SIB Skipping (DMCR)
    4- R8 Redirection

  12. Hello Ali, I do not have a GSM/CSFB back ground but the article highlights complex terms in a very easy way. I have a question and not sure this thread is still active or not. But trying to turn OFF UMTS and trying to find the best way to reduce the “Hurt” to customers. yes we still have some using 3G in poor coverage areas. With LTE more pervasive now, what are some of the ericsson Opto Parms I can trial/look at to test to manage this endeavor?

    My initial thoughts are for the LTE phone not to be able to read the PHICH or anything 3G. extend LTE Idle Mode Parms.. like QrxLevMin parms.. to try and extend (although weak) cell-edge users.
    advice is greatly appreciated.

    regards
    Mike C

    1. You have the correct idea. In case you have no 3G and you want to retain all the users on LTE, this can be done with the basics like you mentioned (cell selection and reselection parameters). Physicals can also be looked into to provide max coverage atleast on the lower band. You may use a RS boost on lower band to increase LTE’s coverage.

  13. In NSN in case of CSFB, what are the specific counters using for CSSR (Call setup success rate) & CST (Call setup time) 4G cell wise?

  14. Hi Ali Khalid,

    Having one issue,, CSFB is successful but FLash CSFB is not happening at enodeB..
    Please suggest…

  15. in CSFB (without measurement ) it goes blindly to higher frequency priority , how ue find the cell then? for example we have 2-3 cells with same frequency , is it based on SIB data ?
    thanks,

    1. Yeah, the UE will blindly look for cells on that frequency and the cell that meets the selection criteria will be used. If the UE has System information for that cell, it will not have to read the SIBs and call setup will be faster.

  16. Hi sir ,

    Our issue when the mobile is in connected mode ( in LTE ) and after csfb the data connection of 3G is disapearing during 5s .

  17. Hi Khalid,

    I have a problem with my CSFB function, i have 3g overlaping 4g coverage, i have problem with a specific zone that the users report missed call without notification or alert of incoming calls. Please Help

    1. Usually this happens in LAC/MSC borders. But it will be better to root-cause with an E2E trace plus DT logs or if you have CEM, you can use that. Break down the issue in parts to see where the call fails – pre ESR (paging), at ESR to RRC Rel, on UMTS paging response or bearer establishment.

Leave a Reply to Ali Khalid Cancel reply

Your email address will not be published. Required fields are marked *