Apple Exposure Notification framework configuration

The Apple contact-tracing portal explains Apple's approach, and in particular its Apple Framework API.

Operation overview

Configuration

All the configuration is done through ENExposureConfiguration, and in particular:

• ENExposureConfiguration.attenuationLevelValues,
• ENExposureConfiguration.daysSinceLastExposureLevelValues,
• ENExposureConfiguration.durationLevelValues,
• ENExposureConfiguration.transmissionRiskLevelValues and
• ENExposureConfiguration.metadata.attenuationDurationThresholds

(bold below indicates these configuration parameters)

Results

Results are returned via ENExposureDetectionSummary and ENExposureInfo.

Diagrams

Summary of operation

totalRiskScore = transmissionRiskValue * durationRiskValue * daysSinceLastExposureRiskValue * attenuationRiskValue

Though the formula’s range can be up to 4096, the framework limits totalRiskScore to the upper limit of ENRiskScore, which has a maxium value of 255.

• Transmission Risk: Transmission risk is intended to reflect the status of infection in the affected user and its effect on risk of transmission. The value is based on the affected user’s symptoms, when symptoms first appeared, level of diagnosis verification, or other determination from the app or a health authority.
• Duration: Cumulative duration of the exposure. The framework measures this value.
• Days: Days since the exposure incident. The framework measures this value.
• Attenuation: The attenuation (transmission power - RSSI) can vary during an exposure event. Attenuation values >0 are weighted by the duration at each risk level and averaged for the overall duration. The framework measures and calculates this value.

Example

API details

• ENExposureConfiguration
  • ENExposureConfiguration.attenuationLevelValues
  • ENExposureConfiguration.daysSinceLastExposureLevelValues
  • ENExposureConfiguration.durationLevelValues
  • ENExposureConfiguration.transmissionRiskLevelValues
  • ENExposureConfiguration.metadata.attenuationDurationThresholds (default value is [50, 70])
• ENExposureInfo
  • ENExposureInfo.attenuationDurations An array of durations at certain radio signal attenuations (2 buckets, threshold 50dB)
  • ENExposureInfo.attenutationValue The attenuation (transmission power - RSSI) can vary during an exposure event. Attenuation values >0 are weighted by the duration at each risk level and averaged for the overall duration. (this sentence is very confusing, and Google is much less precise on this)
  • ENExposureInfo.metadata.attenuationDurations An array that contains the duration, in seconds, at certain attenuations, using an aggregated maximum exposures of 30 minutes.
• ENExposureDetectionSummary
  • ENExposureDetectionSummary.attenuationDurations (2 buckets, threshold 50dB)
  • ENExposureDetectionSummary.metadata.attenuationDurations (3 buckets, configurable via two parameters in metadata.attenuationDurationThresholds)

App-specific post-scoring

In addition to this, we need to understand what is done with ENExposureDetectionSummary and ENExposureInfo results.