Service Specific Access Control (SSAC)

Under disaster or emergency events, the flood of simultaneous voice call requests is usually the main cause for network congestion. Hence, a limiting mechanism is required to differentiate between bandwidth consuming real-time services and bandwidth-efficient data service.

In contrast, since LTE only has a PS domain, DSAC (Domain Specific Access Control) is not applicable. Nevertheless, the same requirements for priority calls and means for providing contact in case of disaster still apply to LTE. Therefore, means of access control depending on the service is required within the PS domain. For this reason, Release 9 introduces a function called Service Specific Access Control (SSAC) allowing IMS voice and video phone calls to be controlled separately from other traffic.

SSAC shall provide mechanisms to minimize service availability degradation (i.e. radio resource shortage) due to the mass simultaneous mobile originating session requests and maximize the availability of the wireless access resources for non-barred services.

 

How SSAC takes place in a UE:

  1. User initiates a call
  2. UE gets the SSAC related parameters from NW/Lower layers
  3. UE  checks for the disaster situation
  4. UE calculates the SSAC  timers and factors
  5. UE makes call on the basis of above timer/factor values

 

Implementation in a UE:

UE checks for emergency session and network type.

For emergency [Reference : TS 124 173 V10.0.0 (2011-05) Section J 2.1.1 ]or non LTE NW type, the SSAC is other than LTE then skip the rest of steps below and continue with session establishment as SSAC feature is only applicable for LTE (EUTRAN) only UE. It is not used for UTRAN and GERAN network type.

UE should get the following information :

  • BarringFactorForMMTEL-Voice: barring rate for MMTEL voice
  • BarringTimeForMMTEL-Voice: barring timer for MMTEL voice
  • BarringFactorForMMTEL-Video: barring rate for MMTEL video
  • BarringTimeForMMTEL-Video: barring timer for MMTEL video

 

After having the values to perform the following barring action should be taken

[Reference : TS 124 173 V10.0.0 (2011-05) Section J 2.1.1 ]:

If video is offered in the multimedia telephony communication session and back off timer for video is already running then reject the multimedia telephony communication session establishment and skip the rest of steps below.

If the timer is not running, draw a random number that is uniformly distributed in the range  0 <= number < 1 and if it is lower than the BarringFactorForMMTEL-Video, then skip the rest of steps below and continue with session establishment.

Otherwise draw a new random number that is uniformly distributed in the range 0 <= number < 1 and start the back off timer with the duration calculated by the given formula and reject the multimedia telephony communication session establishment –

Ty = (0,7 + 0,6*rand2) * BarringTimeForMMTEL-Voice

References:

  • 3GPP TS 22.011
  • 3GPP TS 36.331
  • 3GPP TS 24.173
  • 3GPP TS 27.007

Author: Arindam Ghosh and Bidyut Maji