Load Balancing and Rebalancing and VoLTE Offloading

Feature Description

The sections below describe the load balancing and rebalancing functionality available on the MME. The MME also supports VoLTE Offloading.

Load Balancing

Load balancing on the MME permits UEs that are entering into an MME pool area to be directed to an appropriate MME in a more efficient manner, spreading the load across a number of MMEs.

Load Rebalancing

The MME load rebalancing functionality permits UEs that are registered on an MME (within an MME pool area) to be moved to another MME in the pool. The rebalancing is triggered using an exec command on the mme-service from which UEs should be offloaded.

When initiated, the MME begins to offload a cross-section of its subscribers with minimal impact on the network and users. The MME avoids offloading only low activity users, and it offloads the UEs gradually (configurable from 1-1000 minutes). The load rebalancing can off-load part of or all the subscribers.

The eNodeBs may have their load balancing parameters adjusted beforehand (e.g., the weight factor is set to zero if all subscribers are to be removed from the MME, which will route new entrants to the pool area into other MMEs).

VoLTE Offloading

Offloading of a certain percentage of users can be configured using the mme offload command. The MME sends S1 Release (with cause "load balancing TAU required" for offload) to the configured percentage of UEs attached to the MME. The MME does not distinguish between VoLTE and Non-VoLTE subscribers. Some subscribers with voice bearers are also offloaded as a result calls are dropped. This feature enhancement is targeted to preserve VoLTE voice bearers during MME offloading. A new CLI keyword is added to the mme offload command to preserve VoLTE subscribers (QCI = 1) from offloading until voice calls are terminated.


Note

This feature enhancement is license controlled. Contact your Cisco Account or Support representative for information on how to obtain a license.


Relationships to Other Features

MME load balancing can be used in conjunction with congestion control. For more information on congestion control, refer to the Congestion Control section in the Mobility Management Entity Overview chapter of the MME Administration Guide.

How it Works

Load Balancing

Load balancing is achieved by setting a weight factor for each MME so that the probability of the eNodeB selecting an MME is proportional to its weight factor. The weight factor is set by the operator according to the capacity of an MME node relative to other MME nodes. The relative-capacity mme-service level command is used to specify this relative weighting factor.

Once set, the Relative MME Capacity IE is included in the S1AP S1 SETUP RESPONSE message from MME to relay this weight factor. If the relative MME capacity is changed after the S1 interface is already initialized, then the MME CONFIGURATION UPDATE message is used to update this information to the eNodeB.

Load Rebalancing

Call Handling and Other Messaging Considerations

The MME uses the mme offload mme-service exec level command to enable the operator to offload UEs for a particular mme-service for load rebalancing among MMEs in a MME pool area. The command enables the operator to specify a percentage of UEs to offload, and the desired time duration in which to complete the offload.

The operator can also include the keyword option disable-implicit-detach . By default, if the UE context is not transferred to another MME within 5 minutes, the UE will be implicitly detached. This option disables this implicit detach timer.

To offload ECM-CONNECTED mode UEs, the MME initiates the S1 Release procedure with release cause "load balancing TAU required".

To offload UEs which perform TA Updates or Attaches initiated in ECM-IDLE mode, the MME completes that procedure and the procedure ends with the MME releasing S1 with release cause "load balancing TAU required".

To offload UEs in ECM-IDLE state without waiting for the UE to perform a TAU or perform Service request and become ECM CONNECTED, the MME first pages the UE to bring it to ECM-CONNECTED state.

New calls are processed normally (as per the new call policy configuration). The offloading process does not reject INIT UE messages for new subscribers. To prevent new calls from entering this MME, set the relative-capacity on this mme-service to 0.

When Init UE messages are received for an existing offloaded subscriber, the ue-offloading state is set as MARKED and the offload procedure continues until the UE is offloaded.

Once a UE is offloaded, messages such as EGTP events, Create bearer, Update bearer, Idle mode exit, and Paging trigger are be rejected. HSS initiated events also will be rejected for offloaded UEs.

Detach events are processed as usual.


Important

Emergency attached UEs in Connected or Idle mode are not considered for offloading.

VoLTE Offloading

The mme offload command is enhanced with the keyword preserve-volte-subscribers , this keyword enables preservation of subscribers with voice bearers (QCI=1) from offloading until the voice bearers are deactivated. In any MME service both VoLTE and Non-VoLTE subscribers are present. The offload command now has options to configure the percentage of total subscribers to be offloaded and to preserve VoLTE subscribers from offloading until voice calls are terminated. With this feature enhancement if VoLTE preservation is not enabled, all subscribers are marked for offloading. But when the keyword preserve-volte-subscribers is enabled, Non-VoLTE subscribers are first marked for offloading based on configured offload-percentage. If the configured offload-percentage is greater than the available Non-VoLTE subscribers, VoLTE subscribers are also marked for offloading but the VoLTE UEs will be offloaded only when voice bearers are deactivated.

Configuring Load Balancing and Rebalancing

Configuring Load Balancing

Set the relative capacity of an MME service to enable load balancing across a group of MME services within an MME pool.

Use the following example to set the relative capacity of this MME service. The higher the value, the more likely the corresponding MME is to be selected.

config 
   context context_name 
      mme-service  mme_svc  -noconfirm 
         relative-capacity rel_cap_value 
         exit 

Notes:

  • relative-capacity rel_cap_value -- This command specifies a weight factor such that the probability of the eNodeB selecting this MME is proportional to this value in relation to other MMEs in a pool. rel_cap_value define the relative capacity by entering an integer from 0 to 255. The default relative capacity for an MME service is 255.
  • The weight factor of the MME is sent from the MME to the eNodeB via S1-AP messages using the Relative MME Capacity S1AP IE in the S1AP S1 Setup Response. If the relative MME capacity is changed after the S1 interface is already initialized, then the MME Configuration Update message is used to update this information to the eNodeB.

Verifying Load Balancing

Enter the show mme-service all causes the MME to generate a display similar to the following to indicate the configured relative capacity:

show mme-service all 
Relative Capacity:      50 

Performing Load Rebalancing (UE Offloading)

Start Offloading

The following example command rebalances (offloads) 30 percent of all UEs from the specified MME service (to other MME services in the MME pool) over the course of 10 minutes.

mme offload mme-service mme_svc time-duration 10 offload-percentage 30 -noconfirm 

This command can also be entered with the disable-implicit-detach option. By default, if the UE context is not transferred to another MME within 5 minutes, the UE will be implicitly detached. This option disables this implicit detach timer.

mme offload mme-service mme_svc time-duration 10 offload-percentage 30 disable-implicit-detach -noconfirm 

Stop Offloading

To stop the offloading process, issue the command with the stop keyword option.

mme offload mme-service mme_svc stop -noconfirm  

Verifying Load Rebalancing (UE Offloading)

The following command shows the offload configuration as well as the status of the rebalancing.

show mme-service name svc_name offload statistics   
show mme-service name mme1 offload statistics 
Current Offload Status: In Progress 
Implicit Detach Status: Enabled 
Time Duration Requested: 600 secs 
Percentage of Subscribers Requested: 30 
Total Number of Subscribers: 0 
Total Number of Subscribers to be Offloaded: 0 
Total Number of Subscribers Offloaded: 0 
Total Number of Subscribers Received Context Transfer: 0 
Remaining Time:  0 secs 
Where the Current Offload Status field will report one of the following:
  • Not Started No UEs marked for offloading and no UEs currently being offloaded.
  • In Progress MME is currently offloading marked UEs.
  • Completed Offload procedure is completed or has been terminated by operator using stop keyword.

These counters are reset each time an offload procedure is initiated, or when the following command is entered:

clear mme-service statistics offload 

Configuring VoLTE Offloading

The following configuration command is used to configure preservation of VoLTE subscribers from offloading during active calls (QCI=1); the offload command is enhanced with the key wordpreserve-volte-subscribers :

mme offload mme-service mme_svc_name { time-duration minutes offload-percentage percent [ disable-implicit-detach | preserve-volte-subscribers  ] ] | stop } [- noconfirm ] 

By default, the subscribers with voice bearer with QCI = 1 will not be preserved during MME offloading. Configuring the keyword preserve-volte-subscribers enables preservation of subscribers with voice bearer.

The following example command re-balances(offloads) 30 percent of Non-VoLTE subscribers from the specified mme-service (to other mme-services in the MME pool) over the course of 30 minutes with VoLTE preservation.

mme offload mme-service mmesvc  time-duration 30 offload-percentage 30 preserve-volte-subscribers 

Verifying VoLTE Offloading

The following show command display is used to verify if VoLTE preservation is enabled and the number of VoLTE subscribers preserved during offloading:

show mme-service name svc_name offload statistics 
 Current Offload Status : Completed 
 Implicit Detach Status : Disabled 
 Preserve VoLTE subscribers Status : Enabled  
 Time Duration Requested : 60 secs 
 Percentage of Subscribers Requested : 1 
 Total Number of Subscribers : 0 
 Total Number of Subscribers Marked for Offloading: 1 
 Total Number of Subscribers Offloaded : 0 
 Total Cumulative Number of Subscribers Offloaded: 2 
 Total Number of VoLTE Subscribers Preserved : 0  
 Total Cumulative Number of VoLTE Subscribers Preserved:7 
 Total Number of Subscribers Received Context Transfer: 0 
 Remaining Time : 0 secs 

Monitoring and Troubleshooting

The following sections describe commands available to monitor and troubleshoot this feature on the MME.

Show Command(s) and/or Outputs

This section provides information regarding show commands and their outputs in support of load rebalancing (UE offload).

The following show command displays current statistics for the Load Rebalancing feature.

show mme-service name mme_svc offload statistics 
Table 1. show mme-service name <mme_svc_name> offload statistics
Field Description

Current Offload Status

Current offload status of the specified mme-service. Possible values are Not Started, In Progress and Completed.

Implicit Detach Status

The Implicit Detach Status specified in the mme offload command. When enabled, if the UE context is not transferred to another MME within 5 minutes then it will be implicitly detached.

Preserve VoLTE subscribers Status

Is displayed as “Enabled” when the keyword preserve-volte-subscribers is configured in the mme offload command. The status is displayed as “Disabled”, when VoLTE preservation is not configured. By default VoLTE preservation is disabled.

Time Duration Requested

The time-duration value specified in the mme offload command (in seconds). This is the maximum allowed time for the offload procedure to complete.

Percentage of Subscribers Requested

The offload-percentage specified in the mme offload command (specified as a percentage of all UEs on this mme-service).

Total Number of Subscribers

The total number of UEs on the specified mme-service.

Total Number of Subscribers Marked for Offloading

Displays the total number of subscribers marked for offloading during the current MME offload.

Total Number of Subscribers to be Offloaded

Total number of UEs on the specified mme-service selected for offloading.

Total Number of Subscribers Offloaded

The total number of UEs which have been successfully offloaded from this mme-service (UE offloading State/Event = Done).

Total Cumulative Number of Subscribers Offloaded

Displays the cumulative count of subscribers offloaded.

Total Number of VoLTE Subscribers Preserved

Displays the number of preserved VoLTE subscribers during and after MME offload.

Total Cumulative Number of VoLTE Subscribers Preserved

Displays the total numbers of subscribers preserved before starting the offload timer when the mme offload command is executed.

Total Number of Subscribers Received Context Transfer

Total number of UEs which has been successfully context transferred to another MME.

Remaining Time

The number of seconds remaining to complete the offload procedure.

The following command also provides information relating to load balancing:

show mme-service session full all 

Only the output field which relates to load rebalancing is shown.

Table 2. show mme-service session full all
Field Description

UE Offloading

Displays the UE offload state. Possible values are None, Marked, In-Progress and Done.