LTE ERAB Success Rate

The second major KPI for LTE is the LTE ERAB Success Rate which is also part of the accessibility. After the UE has completed the RRC Connection which has been explained in my previous article, LTE KPI Optimization: RRC Success Rate, it needs to get a Bearer assigned to it to initiate services. The bearer can be default (usually Data QCI9) or dedicated (VoLTE QCI1). During initial access, the default bearer is added and that constitutes the major portion of the total ERABs.

Firstly, lets understand the definition and points where the ERAB KPI is pegged. After the UE sends the RRC Setup Complete message to the eNB, the eNB sends a S1 Initial UE Message to the MME indicating the purpose of the UE (Attach, TAU, CSFB, Service Request etc) and its credentials. Once the MME receives this message and it decides that a bearer is required, it will send an Initial Context Setup Request to the eNB. This message is considered as the ERAB Attempt as it contains the bearers to be added along with their QCI values. The eNB receives this message and adds the DRB (Data Radio Bearer) based on the bearer profile in Initial Context Setup Request. But before the eNB can add bearers, it needs to activate the security for the connection. This is done by the Security Mode Command which carries the ciphering and integrity protection algorithms. After this the eNB sends a RRC Connection Reconfiguration message to the UE which adds a DRB and it includes the configuration for the DRB like bearer identity, PDCP & RLC configuration (AM/UM etc). SRB2 is also added at this point with this message. The UE receives these messages and reconfigures the connection. Then the UE responds with Security Mode complete and RRC Connection Reconfiguration Complete messages. As the eNB receives these messages, it sends an Initial Context Setup Response to MME and this message is considered as the ERAB Success.

LTE ERAB Success Rate

Common Failures In ERAB Setup Phase

Now let’s understand the common failures that usually cause a ERAB setup failure. Most of the times, the ERAB setup failures can be divided into two broad categories

Ø  Radio Induced ERAB Setup Failures

Ø  MME Induced ERAB Setup Failures

Let’s have an in-depth look at both of them and find ways to tackle them

Radio Induced ERAB Setup Failures

  • Radio Link Failure

Consider a UE that receives Security Mode Command but fails to maintain radio connection afterwards. This can happen in following two scenarios:

1. N310 consecutive out-of-sync events and T310 expiry

N310 indicates an interval of 200 consecutive PDCCH decoding failures. Simply put, if the UE fails to decode PDCCH for 200ms, it will be considered one N310. However, from here onwards, it is a sliding window with 10ms granularity. So, if the N310 value is 2 then it means that if the UE fails to decode PDCCH for 210 ms, it will have exceeded the configured N310 threshold. Once, N310 has been exceeded, the UE starts timer T310 and if the UE is unable to retain the connection (still unable to decode PDCCH) before T310 expires, the UE will initiate RRC ReEstablishment. Let’s understand with an example. Consider N310 of 11 and T310 of 500ms, then the UE will initiate RRC Connection ReEstablishment after 800 ms (N310 = (200 + (10*10)) = 300ms + T310 = 500ms).

LTE ERAB Success Rate

2. Maximum RLC retransmission count exceeded

Consider that the UE receives both the Security Mode Command and the RRC Connection Reconfiguration message. Now, it has to transmit the Security Mode Complete and RRC Connection Reconfiguration Complete message in Uplink. However, if the eNB fails to decode these responses, it will send a NACK to the UE or the eNB may not send anything if it completely fails to even receive these messages. The RLC layer in the UE is configured to resend the message if the message is not acknowledged. So, the RLC layer will keep resending until a valid acknowledgement is received. But the RLC cannot resend the same message indefinitely and it has a upper limit of retransmissions. Once that limit is reached, the RLC will not retransmit again and the UE will consider that the radio link is compromised. This will trigger a RRC ReEstablishment Request.

However, in both these cases, the RRC ReEstablishment Request will be rejected by the eNB since processing this request requires to have a valid UE context at the eNB. But since the UE did not respond to Security Mode Command, so the eNB does not consider the context to be active yet and rejects the RRC ReEstablishment Request. At the same instance, the eNB will send Initial Context Setup Fail to MME indicating an ERAB Setup Failure.

  • Optimization

Such issues can be reduced by increasing the N310 & T310 value. For instance, if the value of N310 is increased from 2 to 6 and T310 is increased from 500ms to 1000ms, then the UE will wait longer and there is more chance that N311 will be triggered. N311 is the In-Sync value and so it is the opposite of N310. T310 stops if N311 is triggered. If N311 is 1 then it means that UE needs 100ms of successful PDCCH decoding to stop T310. So, there is a higher probability of triggering N311 if the value of N310 and T310 is big.

Similarly, if the RLC retransmission count threshold is increased from 8 to 16, then the RLC will retransmit 16 times instead of 8 times which will increase the probability that the eNB might be able to decode the message and prevent RLF.

  • No Response From UE

In this case, the UE receives the Security Mode Command and the RRC Connection Reconfiguration messages in downlink but does not respond to these messages in uplink. This can result in the Inactivity Timer expiry and the eNB will send a UE Context Release Request to the MME during ERAB setup phase which will cause the ERAB setup failure. Let’s see why this scenario happens in live networks. Once a UE receives a downlink message which needs a response, it will need an uplink allocation to send a response. In order to get an uplink allocation, the UE requests the eNB by using a Scheduling Request Indicator or SRI. The UE sends a SRI based on the SRI Configuration shared with it in the RRC Connection Setup Message. The SRI Configuration tells the UE about the periodicity of the SRI and it determines the subframe where the UE will send the SRI. So, the eNB will look for that UE’s SRI in that subframe only and based on that, the eNB allocates an uplink resource to the UE by instructing the UE on the PDCCH. Now, the vendors have moved to adaptive SRI intervals which can result in a new SRI configuration in the RRC Connection Reconfiguration message. There are UEs that do not support this change of SRI configuration and they keep using the old SRI configuration. So, once they have received the Security Mode Command and the RRC Connection Reconfiguration messages in downlink and they want to respond in uplink, they will have to send a SRI first. The UE will be sending SRI according to the old SRI Configuration shared in RRC Connection Setup message while the eNB will be looking for the UE’s SRI in the subframe defined in SRI Configuration of RRC Connection Reconfiguration message. This will result in a scenario where the eNB will consider that there is no response from UE and once the inactivity timer is expired, the ERAB setup will fail.

This can also happen if the UE is in poor coverage or if the PUCCH has high interference. The UE will keep sending SRIs in the correct location on PUCCH but the eNB might not be able to read them resulting in a similar scenario as explained above.

  • Optimization

If such a scenario is observed consistently, it will be a good idea to shift from an adaptive SRI period to a fixed SRI period. This will avoid reconfiguring the SRI periodicity and will prevent this issue.

Also, using PUCCH enhancements like IRC on PUCCH can help reduce the probability of such issues.

  • RLC Mode Issue

This is rarely seen in networks when a UM mode (Unacknowledged Mode of RLC) QCI is used for UEs which do not support UM mode. A common example is the QCI7 which is a Non-GBR QCI defined for live streaming or voice services and it usually works in UM Mode. But there are many UEs which do not support UM mode and the eNB simply fails to add a bearer with UM mode for them. This issue can be seen from the counters as it will show that ERAB failures on Radio interface are happening only on QCI7 or any other QCI which is set to UM Mode.

  • Optimization

Simply changing the RLC mode for the QCI from UM to AM should solve this issue.

  • Security Mode Failure

Another issue that is a bit rare is the Security Mode Failure issue. In this case, the UE receives the Security Mode Command from the eNB but responds with a Security Mode Failure message. Consequently, eNB sends Initial Context Setup Failure to the MME resulting in ERAB setup failure. This happens if the security configuration on the eNB is not supported by the UE or sometimes it can happen if the UE cannot handle both the Security Mode Command and the RRC Connection Reconfiguration together. In most of the cases, this turns out to be the terminal issue.

MME Induced ERAB Setup Failures

Let’s have a look at the MME induced ERAB failures. This may come as a surprise but most of the MME induced ERAB setup failures in commercial networks are actually caused by the radio interface and not the MME. I know it is hard to understand but those of you who have delved themselves in RRC and S1 traces will understand it more clearly once I explain this issue.

As explained in the section above, when the UE experiences a RLF after receiving the Security Mode Command, it can try RRC ReEstablishment which actually tells the eNB that there was a RLF on the UE’s side. Consider a UE experiencing a RLF before it receives the Security Mode Command. The UE can only send a RRC ReEstablishment after security is activated but if the UE experiences a RLF before the Security Mode Command has been received, it cannot send a RRC ReEstablishment Request.

Now, consider that the UE experiences RLF after RRC Setup Complete message and before Security Mode Command, this UE will go to idle and retry a new RRC connection by sending another RRC Connection Request. Let’s say that the UE sends a RRC Connection Request to another eNB (eNB2) and that eNB2 will start processing it. The eNB2 does not know that the eNB1 already has a ERAB setup process going on for this UE. The eNB2 will send a S1 Initial UE Message to MME for this UE and the MME will see that it already has another ERAB setup process going on with eNB1. So, for MME to initiate the new ERAB setup process by sending Initial Context Setup Request to eNB2, it needs to first stop the process on eNB1, as it cannot have separate context of same UE on two different eNBs. As a result, the MME will send a UE Context Release Command to eNB1 asking to abort the ERAB setup process. The eNB1 is trying to find the UE over the air interface and once it receives the Context Release Command from MME, it will consider that the MME aborted the ERAB setup and will peg it as a MME induced ERAB setup failure. eNB1 will send an Initial Context Setup Failure to MME and the ERAB setup on eNB1 will be pegged under MME induced failure. However, this issue was actually caused due to radio issue but the eNB1 was not able to find that out.

LTE ERAB Success Rate

This issue can also happen if the UE sends the second RRC Connection Request to the same eNB or even to the same cell. At RRC level, the eNB does not check TMSI value and the UE is referenced by its CRNTI. So, if the same UE sends another RRC Connection Request to the same eNB, it will allocate a new CRNTI and will consider it a new connection. But when the eNB will send S1 Initial UE Message to MME, the MME will check the TMSI and will send UE Context Release Command to the previous session resulting in ERAB setup failure on the first process.

Another scenario that can cause a MME induced ERAB Setup failure is the Initial Context Setup Timer on the MME. If that timer is set to small value and eNB is waiting for the UE to respond to Security Mode Command, the MME will send UE Context Release Command due to timeout. This will also result in a MME induced ERAB Setup Failure.

  • Optimization

There is no real optimization on the first scenario as it is purely a coverage issue and coverage enhancement by physical or soft changes can be done to mitigate it. The second scenario can be minimized by increasing the Initial Context Setup Timer on the MME.

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.

85 thoughts on “KPI Optimization: LTE ERAB Success Rate”

  1. Once again excellent sharing !
    Can you share more detail on Abnormal release.MME in H**** Vendor?
    How to minimize it

          1. Thanks you to increased my knowledge on KPI ERAB failures caused and it remedy in excellent way.

      1. Dear Ali,
        Can you pls share some RCA for all the major LTE KPI.As i m new to R.F world so if you can share root cause analysis for major lte kpi,s its very helpfull to me

    1. This timer is on the MME as the MME sends the Initial Context Setup Request and the eNB needs to respond to it. So, the timer value can be checked with MME guys. However, if you put a trace on a top site, you can parse the ERAB Setup Failures and look into the MME induced ERAB failures. Now if you calculate the time delta between Initial Context Setup Request and UE Context Release Command, there will be a value that will be higher than others. The timer induced setup failures will have a the same time value but other type of failures will be random. This should give you an idea of the timer settings on the MME.

  2. Salam Ali,

    Thanks for this great & simple topic about ERAB.

    Would you please clarify the L.E-RAB.FailEst.RNL & L.E-RAB.FailEst.RNL.other counters under Huawei KPI’s,

    and which of the above reasons will be counted for the L.E-RAB.FailEst.RNL.other counter?

  3. Excellent Content , Can you please share the optimisation plan for ‘E-RAB Drops due to UE lost’ cases …..

  4. Hi Khalid,
    Regarding RLC Mode issue you had suggested to go for AM instead of UM. It is costly as more resource is consumed for AM mode than UM. The UE mode configuration is driven by eNB RRC . So for sure the UE ERAB failure is not due to Mode support issue at UE side.

    1. It is costly, that is why vendors have introduced enhancements on the RLC layer. If these enhancements are activated, the eNB changes the RLC mode to AM for the UEs that do not support a UM mode for their bearer. Coming to the second point – the QCI is sent by the core and if the core sends a QCI to the eNB that requires a UM mode, the eNB will reject it since the the UE does not support UM mode and consequently, the ERAB setup will fail.

  5. In general at S1AP the ERAB setup can fail if the corresponding QCI is not supported at eNB as eNB is developed to support only certain QCI only or due to QoS IE parameter encode issue or due to ERAB ID clash issue, or Even as part of the RRM RBC the setup can fail due to restriction in QCI support (resource crunch), but I strongly feel the mode failure is not an issue. The QCI is decided based on service flow between the AF and the UE at application interface level and eNB decides the Mode.

    RRC level – The bearer setup happens using RRC Reconfig and the UE uses the config send by eNB RRC. The UE RRC uses the eNB RRC config to set up it’s corresponding layer. The UE RRC can report config failure only due to ASN violation, Mandatory field missing or it can even initiate Re-establishment. There is no way for UE to report mode not supported.

    Do you say even the QCI 1, QCI 2 all can be configured to AM mode instead of UM. If so then we end up more jitter and more delay in voice packet which is not agreed for these service.

    1. For the Mode issue, I have seen it in three networks. It was verified from the UE’s capability that it does not support UM mode and then from the CHR as well that it is rejected due to the mode support. So, I have no doubt on the issue itself.
      Coming to the second thing about QCI1 – ofcourse it needs UM. The idea is not that we need to change mode to AM but if QCI-7 is being used then enabling this enhancement resolves the ERAB failures due to this issue. It is not applicable to QCI1 as that is only used in the network for VoLTE and UEs supporting VoLTE also support UM.

      1. You are right about this case, I also encounter the same issue on my network specially with roaming users that mostly used QCI7 for some services and normally the eNb cfg will reject them. Going in CHR analyze for this type of error will tell the same exact reason, and also the bit 7 in the FGI will show 0. So phones that don’t support Volte and are LTE mostly have QCI 7 configured for UM only mode. We cannot change the phone cfg but a work around is map QCI7 with AM and you are set to go. And yes MME can solve this issue easily :).
        Nice explanation keep up the good work.

  6. “However, in both these cases, the RRC ReEstablishment Request will be rejected by the eNB since processing this request requires to have a valid UE context at the eNB. But since the UE did not respond to Security Mode Command, so the eNB does not consider the context to be active yet and rejects the RRC ReEstablishment Request. At the same instance, the eNB will send Initial Context Setup Fail to MME indicating an ERAB Setup Failure.”

    “As explained in the section above, when the UE experiences a RLF after receiving the Security Mode Command, it can try RRC ReEstablishment which actually tells the eNB that there was a RLF on the UE’s side. Consider a UE experiencing a RLF before it receives the Security Mode Command. The UE can only send a RRC ReEstablishment after security is activated but if the UE experiences a RLF before the Security Mode Command has been received, it cannot send a RRC ReEstablishment Request.”

    In first paragraph you said that UE will send RRC Re-establishment request but it will be rejected and in 2nd paragraph you said that it is unable to send RRC Re-establishment request

    Can you clear this confusion

    1. The difference is the stage at which the RLF occurs. If the RLC occurs before the UE gets the SMC, then there will be no RRC ReEstablishment but if the RLF occurs after the UE gets the SMC, then the UE will send a RRC ReEstablishment.

  7. Dear Khalid,

    I am facing one issue in our network, few sites in LTE initial erab setup success is poor,the problem is that failure is just in one cell and other two cells is 100%, what could be the possible reason for this.

    Regards
    Abhijeet

  8. HI Khalid,

    Have u seen any cell that is having high Erab failures bur RRC and S1 is ok
    What could be the reason for this?

    Thanks

  9. Dear Bro,

    Excellent sharing & very clean and in details about the KPI & love the detail explanation regarding MME induced fails because of Radio issues

  10. So, if the N310 value is 2 then it means that if the UE fails to decode PDCCH for 210 ms, it will have exceeded the configured N310 threshold. Once, N310 has been exceeded, the UE starts timer T310 and if the UE is unable to retain the connection (still unable to decode PDCCH) before T310 expires, the UE will initiate RRC ReEstablishment. Let’s understand with an example. Consider N310 of 10 and T310 of 500ms, then the UE will initiate RRC Connection ReEstablishment after 800 ms (N310 = (200 + (10*10)) = 300ms + T310 = 500ms).

    In above example if we took N310 as 10 than as per calculation it should be like N310=200+(9*10)=290ms
    My calculation is right?

  11. Hi All,

    I am looking for the E-RAB Successful Attach time (Secs or Mins) KPI for Huawei LTE. What will be the Counter for this in Huawei PRS or U2000 Performance Query

    Thanks

  12. Dear Khalid,

    Thanks a lot for this useful knowledge sharing. I also face one issue in our network about ERAB EST degradation on one or 2 cells of different sites for a duration of time. I asked our customer to do trace and for some cases they didn’t confirm that issue is related to UE behavior. N310 and T310 are set on their max value in our network and i have increased upper limit of UL re-transmissions on site. Yet, issue is not recovered and as previously site was working with same settings (even lower limit of re-transmissions) and KPI was perfect I doubt issue can be due to this. Do you have any idea about this issue and what should I do as an optimizer?

    1. Without a trace analysis, it is difficult to root-cause the issue. You will need to check whether it was a radio drop during ERAB or a MME induced ERAB failure which can also be due to radio issue. Use the call flows explained in the article as a guide for trace analysis.

    1. Check if you have zero RRC and S1 attempts as well. If you have attempts at RRC and S1 (lots of attempts) and no ERAB attempts then verify if it is a TAC problem (if the cells have different TACs). If you have no RRC attempts as well then better check it for sleeping cell.

  13. Thanks Khalid Excellent explanation. !!!

    I have a case here for QCI-1 , After E-RAB Setup Request from MME, eNB Send RRC reconfiguration to UE but after 10 ms, eNB sending E-RAB Setup Response Message to MME with Protocol ID “ErabFailedToSetupListBearersUres (29) “and Failure Cause ” Failure-In-Radio-Interface-Procedure (26)”.
    I am unable to find reason that why eNB did not wait for RRC Reconfiguration Complete message before sending E-RAB Setup Message Response.

    RRC reconfiguration time is 8 Sec.
    We are using variable SRI in our N/W, with UM.

    I am unable to upload snapshot here, So writing timings of messages.
    E-RAB Setup Request (MME-eNB)— RRC Reconfiguration (eNB-UE)–> (3 ms)

    RRC Reconfiguration (eNB-UE) — E-RAB Setup Response (eNB-MME)–> (2 ms)

    E-RAB Setup Response (eNB-MME)– UE Context Release Command (eNB-MME)- (0.3 ms).

    Based on Trace This decision is totally taken by eNB here.

    Suggest if you have any Idea over it.

    Thanks

    1. No I have not seen this issue so far. But since, the eNB is responding after a fixed time to MME without waiting for the RRC Recfg from the UE so yeah you are right the issue is at the eNB. Can you check if there is any license or resource limitation? Do share your findings!

  14. Hi Khalid,
    Can ypu pls share LTE MIMO in detail & throughput optimization through MIMO,that would be very useful as MIMO is not covered anywhere in detail.

    Thnx

  15. Great explanation! Please shed some light on below topics when you can:

    1. VoLTE Optimization
    2. Carrier Aggregation troubleshooting
    3. Load Balancing+Inter-Layer working Strategy

  16. Hi Khalid,

    Please help me to find out the rca for initial erab setup failure at all new on aired sites in LTE while rrc and s1 is ok in Ericsson L17Q2.00 VERSION

  17. Dear Mr.Ali,
    Could you share me a knowledge about LTE call setup success rate failure due to E-RAB.FailEst.MME .What is the main reason for this issue and what shall I check?

    1. Consider a UE experiencing a RLF before it receives the Security Mode Command. The UE can only send a RRC ReEstablishment after security is activated but if the UE experiences a RLF before the Security Mode Command has been received, it cannot send a RRC ReEstablishment Request. If UE experiences RLF after RRC Setup Complete message and before Security Mode Command, this UE will go to idle and retry a new RRC connection by sending another RRC Connection Request. This will initiate a new S1 Initial UE message from eNb to MME for the same UE which will make the MME send a UE Context release to the first ERAB attempt session resulting in ERAB Setup Failure Due to MME.

  18. Aoa,

    Thank for sharing detailed information

    I have a query that ERAB SR is degraded with the counter “ERAB failure due to RRC reestablishment request” and ” RRC reconfiguration failure”. I have checked there is no rise in RLC retransmission, no specific QCI release increase, no security mode failure, no UE context release by eNB due to inactivity. I didn’t find any counter for T310 timer expiry as I am working on ZTE
    What next should I check?

    1. If the counters are showing that the failure is due to RRC Reestablishment during ERAB Setup then it is typically a radio issue. Some vendors have enhanced switches that enabled RRC Reestablishment during ERAB setup – check if you have that enhancement as that will resolve most of the issue.

  19. Thanks

    I found only one switch “interEnbReestabSwch” and it is ON. If RRC reestablishment switch is disabled, no rrc reestablishment attempt would have been pegged.

    1. No, this one should be the switch that enables inter-eNB Reestablishments and context fetching. The ERAB setup is not related to Inter-eNB as it is specific to the serving cell. The RRC Reestablishment failure during ERAB setup happens because most of the vendors do not support reestablishment in this phase (i.e. pre-context setup). So, if you do not have another switch that allows this then you will have to work on why reestablishment is happening during ERAB setup phase and that usually comes down to radio quality or you might be able to improve your KPI by increasing the rlf timers.

  20. Thanks a lot brother

    Statistically RF conditions seem ok, CQI and RTWP is fine no degradation in any other KPI. Meanwhile I have asked for trace to NOC team.

    You mean T310? if yes so it is set at 2000ms which is Max value. I can increase N310.

    1. Usually, E-RAB failures due to MME are caused by uplink issues. So, check the uplink related parameters or if the cells are overshooting that might also cause this issue.

Leave a Reply

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