Idle-mode Signaling Reduction

Idle-mode Signaling Reduction (ISR) allows a UE to be registered on (and roam between) E-UTRAN and UTRAN/GERAN networks while reducing the frequency of TAU and RAU procedures and overall signaling.

Feature Description

Idle mode Signaling Reduction (ISR) allows the UE to be registered in UTRAN/GERAN at the same time it is registered in E-UTRAN. ISR requires functionality in both the UE and the network (i.e. in the SGSN, MME, S-GW and HSS) to activate ISR for a UE. The network can decide for ISR activation individually for each UE.

ISR allows the UE to roam between LTE & 2G/3G while reducing the frequency of TAU and RAU procedures caused by UEs reselecting between E-UTRAN and GERAN/UTRAN, when operated together. It not only reduces the signaling between UE and network, but also reduces the signaling between E-UTRAN & UTRAN/GERAN.

When ISR is activated, the UE is registered with both the MME and S4 SGSN. Both the S4 SGSN and the MME have a control connection with the S-GW. The MME and S4 SGSN are both registered at the HSS. The UE stores MM parameters from S4 SGSN (e.g. P-TMSI and RA) and from MME (e.g. GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common for E-UTRAN and GERAN/UTRAN accesses. In an idle state the UE can reselect between E-UTRAN and GERAN/UTRAN (within the registered RA and TAs) without any need to perform TAU or RAU procedures with the network. SGSN and MME store each other's address when ISR is activated.

How it Works

ISR Activation

ISR does not entail any changes to the initial attach procedure at the MME or S4 SGSN. ISR is only activated when the UE is registered with both the MME and S4 SGSN. This happens for the first time when the UE has a previous state at either the MME or S4 SGSN and relocates to the other node. This is achieved via TAU/RAU procedures or via inter-RAT procedures. Both the S4 SGSN and the MME then have a control connection with the Serving GW. The MME and S4 SGSN are both registered at the HSS.

The UE stores Mobility Management (MM) parameters from the SGSN (P-TMSI and RA) and from MME (GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common for E-UTRAN and GERAN/UTRAN accesses. In the idle state, the UE can reselect between E-UTRAN and GERAN/UTRAN (within the registered RA and TAs) without any need to perform TAU or RAU procedures with the network. The SGSN and MME store each other's address when ISR is activated.

Figure 1. ISR Activation During MME to SGSN Relocation


Notes:
  • S3 Fwd relocation request/context response would indicate ISR support at MME via indication flag (ISRSI).
  • If the SGSN also supports ISR, it activates and indicates so using ISRAI flag to the S-GW in an S4 modify bearer request message.
  • The SGSN uses Context Ack/Fwd Relocation Complete response to indicate to MME that ISR has been activated. This ensures that the MME does not delete UE context.
  • The MME also expects the HSS to not send a Cancel-Location-request to the MME.
Figure 2. ISR Activation During SGSN to MME Relocation


Notes:
  • S3 Fwd relocation request/context response indicates ISR support at SGSN via indication flag (ISRSI).
  • If the MME also supports ISR, it activates and indicates so using ISRAI flag to the S-GW in a S11 Modify Bearer Request message.
  • The MME uses the Context Ack/Fwd Relocation Complete notification to indicate to the SGSN that ISR has been activated. This ensures that the SGSN does not delete the UE context.
  • The MME sends a t3423 timer and sends the appropriate EPS Update result IE to UE in a TAU accept.

ISR Deactivation

The UE and the network run independent periodic update timers for GERAN/UTRAN and for E-UTRAN. When the MME or SGSN do not receive periodic updates, the MME and SGSN may decide independently for implicit detach, which removes session management (bearer) contexts from the CN node performing the implicit detach and it also removes the related control connection from the S-GW. Implicit detach by one CN node (either SGSN or MME) deactivates ISR in the network. It is deactivated in the UE when the UE cannot perform periodic updates in time. When ISR is activated and a periodic updating timer expires, the UE starts a Deactivate ISR timer. When this timer expires and the UE was not able to perform the required update procedure, the UE deactivates ISR.

All special situations that cause context in the UE, MME and SGSN to become asynchronous are handled by ISR deactivation. The normal RAU/TAU procedures synchronize contexts in MME and SGSN and activate ISR again when wanted by the network.

ISR Behavior with Circuit Switched Fallback

ISR capability impacts some MME messaging when Circuit Switched Fallback (CSFB) is also implemented.
  • When receiving a Paging Request from the MSC/VLR, the MME must initiate paging in both the E-UTRAN and the UTRAN/GERAN domains (as a UE in idle mode may be in either cell coverage).
  • When the MSC/VLR initiates a Non-EPS Alert Procedure, the MME must inform the peer SGSN of the request. If there is signaling activity in the UTRAN/GERAN domain, the SGSN can inform the MME (via the S3 interface) to allow the MME to indicate activity to the MSC/VLR.
  • IMSI-detach is allowed from the SGSN.

Standards Compliance

The ISR capability complies with the following standards for 3GPP LTE/EPS wireless networks:
  • 3GPP TS 23401-970
  • 3GPP TS 29274-940
  • 3GPP TS 23272-990
  • 3GPP TS 24301-950

Configuring ISR

This feature requires that a valid license key be installed. Contact your Cisco Account or Support representative for information on how to obtain a license.

Use the following example to enable the ISR feature on the specified MME service 

config 
   context <context_name> 
      mme-service <mme_svc_name> -noconfirm 
         isr-capability 
         exit

Verifying ISR Configuration

Use either of the following commands to display information to verify if ISR is enabled. 

show mme-service all  
show mme-service name <mme_svc_name>

The output of this command displays the entire configuration for the MME service specified. 

show mme-service name mmesvc1  
ISR Capability   : Enabled

Monitoring and Troubleshooting ISR

ISR Bulk Statistics

The following MME Schema bulk statistics have been introduced for the Idle-mode Signaling Reduction feature:

  • isr-activated

The following eGTP-C Schema bulk statistics have been introduced for the Idle-mode Signaling Reduction feature:

  • mobility-sent-cspagingind
  • mobility-recv-cspagingind
  • mobility-sent-alertmmenotf
  • mobility-sent-retransalertmmenotf
  • mobility-recv-alertmmenotf
  • mobility-recv-retransalertmmenotf
  • mobility-sent-alertmmeack
  • mobility-sent-retransalertmmeack
  • mobility-recv-alertmmeack
  • mobility-recv-retransalertmmeack
  • mobility-sent-alertmmeackaccept
  • mobility-sent-alertmmeackdenied
  • mobility-recv-alertmmeackaccept
  • mobility-recv-alertmmeackdenied
  • mobility-sent-ueactivitynotf
  • mobility-sent-ueactivitynotf
  • mobility-sent-retransueactivitynotf
  • mobility-recv-ueactivitynotf
  • mobility-recv-retransueactivitynotf
  • mobility-sent-ueactivityack
  • mobility-sent-retransueactivityack
  • mobility-recv-ueactivityack
  • mobility-recv-retransueactivityack
  • mobility-sent-ueactivityackaccept
  • mobility-sent-ueactivityackdenied
  • mobility-recv-ueactivityackaccept
  • mobility-recv-ueactivityackdenied
  • mobility-sent-detachnotf
  • mobility-sent-retransdetachnotf
  • mobility-recv-detachnotf
  • mobility-recv-retransdetachnotf
  • mobility-sent-detachack
  • mobility-recv-detachack
  • mobility-sent-detachackaccept
  • mobility-sent-detachackdenied
  • mobility-recv-detachackaccept
  • mobility-recv-detachackdenied

ISR Show Command(s) and/or Outputs

This section provides information regarding show commands and/or their outputs in support of ISR.

Only those counters which relate to ISR are shown. 

show mme-service statistics
Table 1. ISR Deactivation Statistics
Field Description

ISR Deactivation Statistics

S3 path failure

The total number of Idle mode Signaling Reduction (ISR) deactivations due to failure in the S3 interface.

SGSN local detach

The total number of Idle mode Signaling Reduction (ISR) deactivations due to SGSN detach notification.

SGW relocation

The total number of Idle mode Signaling Reduction (ISR) deactivations due to S-GW relocation of the session to an MME/SGSN which does not support ISR.

CN Node relocation

The total number of Idle mode Signaling Reduction (ISR) deactivations due to CN Node relocation of the session to an MME/SGSN which does not support ISR.

Implicit detach

The total number of Idle mode Signaling Reduction (ISR) deactivations due to an idle timeout (implicit detach) initiated by either the MME or Peer SGSN.

Other detach procedures

The total number of Idle mode Signaling Reduction (ISR) deactivations due to an idle timeout (implicit detach) initiated by either the MME or Peer SGSN.

Other reasons

The total number of Idle mode Signaling Reduction (ISR) deactivations due to a reason not otherwise classified by one of the other ISR Deactivation Statistics categories. 

show mme-service session full
Table 2. ISR Session Information
Field Description

ISR Status

Displays if the session is using Idle mode Signaling Reduction (ISR). Possible configurations are Activated or Deactivated.

Peer SGSN

Displays the IP address of the SGSN which has a context for this UE in support of Idle mode Signaling Reduction (ISR). A Peer SGSN address is only shown when ISR is activated for this session. 

show mme-service session summary
Table 3. ISR Session Summary
Field Description

Total ISR-activated sessions

The current total number of MME sessions which are activated for ISR. 

show egtpc sessions

Typically this command shows only one EGTP session (S11) per UE. When an ISR-activated UE is present, this command displays 2 EGTP sessions per UE.