Service Specific Access Control (SSAC) in 3GPP Release 9

In an emergency situation, like Earthquake or Tsunami, degradation of quality of service may be experienced. 

Degradation in service availability and performance can be accepted in such situations, but mechanisms are desirable to minimize such degradation and maximize the efficiency of the remaining resources.

When Domain Specific Access Control (DSAC) mechanism was introduced for UMTS, the original motivation was to enable PS service continuation during congestion in CS Nodes in the case of major disaster like an Earthquake or a Tsunami.

In fact, the use case of DSAC in real UMTS deployment situation has been to apply access control separately on different types of services, such as voice and other packet-switched services.

For example, people’s psychological behavior is to make a voice call in emergency situations and it is not likely to change. Hence, a mechanism will be needed to separately restrict voice calls and other services.

As EPS is a PS-Domain only system, DSAC access control does not apply.

The SSAC Technical Report (see Reference) identifies specific features useful when the network is subjected to decreased capacity and functionality. 

Considering the characteristics of voice and non-voice calls in EPS, requirements of the SSAC could be to restrict the voice calls and non-voice calls separately.

For a normal paid service there are QoS requirements. 

The provider can choose to shut down the service if the requirements cannot be met. In an emergency situation the most important thing is to keep communication channels uninterrupted, therefore the provider should preferably allow for a best effort (degradation of) service in preference to shutting the service down. During an emergency situation there should be a possibility for the service provider also to grant services, give extended credit to subscribers with accounts running empty. Under some circumstances (e.g. the terrorist attack in London on the 7 of July in 2005), overload access control may be invoked giving access only to authorities or a predefined set of users. It is up to national authorities to define and implement such schemes.

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

3GPP TR 22.986 - Study on Service Specific Access Control

http://blog.3g4g.co.uk/2009/05/service-specific-access-control-ssac-in.html

http://www.aptsoftware.com/blog_deatil.aspx?Id=POST150429103329&title=Service%20Specific%20Access%20Control%20(SSAC)