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MIB Attributes for 40 MHz Scanning in 2.4 GHz

MIB Attributes for 40 MHz Scanning in 2.4 GHz. Authors:. Date: 2007-09-14. Abstract. This presentation investigates the power consumption associated with scanning for 40MHz operation in 2.4GHz. The analysis is targeted at a handheld VoIP device

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MIB Attributes for 40 MHz Scanning in 2.4 GHz

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  1. MIB Attributes for 40 MHz Scanning in 2.4 GHz Authors: Date: 2007-09-14 Eldad Perahia, Intel Corporation

  2. Abstract • This presentation investigates the power consumption associated with scanning for 40MHz operation in 2.4GHz. The analysis is targeted at a handheld VoIP device • Talk time and standby time is first calculated without scanning for 40 MHz operation, as a baseline • Talk time and standby time is then calculated with scanning for 40 MHz to quantify the battery life “cost” to a handheld device • Results are used to select values for MIB attributes Eldad Perahia, Intel Corporation

  3. Assertions/Assumptions • A handheld VoIP device includes active components beyond the 802.11n radio: codec, LCD, host, memory • Talk time analysis based on G.711-20ms • A handheld VoIP device must periodically receive beacons to be able to receive incoming calls • A handheld VoIP device must periodically scan for roaming to maintain connectivity in order to receive calls • Analysis assumes a single STA in a BSS • This assumption results in a worst case power consumption degradation due to additional scanning Eldad Perahia, Intel Corporation

  4. Determine amount of time the radio spends in one of four states: Tx (1.2 Watts) Rx (0.6 Watts) Idle or waiting to receive (0.4 Watts) Power down or deep sleep (0.01 Watts) The rest of the phone has two states: During call (0.4 Watts) Standby (0.01 Watts) Performance will be based on talk time and standby time Conversion from power to time is based on battery life of 4 Watt-hours Power Consumption Calculation Eldad Perahia, Intel Corporation

  5. Assumptions Single STA in BSS U-APSD Period of codec: 20ms Voice packet length: 242 bytes With ideal scenario of single STA and U-APSD: STA wakes from sleep and transmits uplink VoIP packet to AP without contention or collision STA receives ACK from AP STA receives downlink VoIP packet from AP again without contention or collision STA transmits ACK to AP Remainder of time is spent in sleep mode, which is very low power In non-ideal conditions, STA would spend much more time awake contending for the channel or waiting to receive packet from AP Call Parameters Eldad Perahia, Intel Corporation

  6. In order for the handheld device to receive a call in a timely fashion (within a few seconds), it must awake from sleep to receive beacons The longest interval that is considered is 1 sec (beacon interval = 100 msec; DTIM interval = 10) Further assumptions OFDM 6 Mbps Packet length = 100 bytes Beacon Eldad Perahia, Intel Corporation

  7. In order for a handheld device to maintain connectivity to receive a call in a timely fashion when moving between BSSs, it must scan for neighboring APs Scan interval is for a single channel for example if 3 channels were to be scanned in 1 sec, the scan interval would be set to 333 msec The maximum scan interval considered is 1000 msec Assumption Active scan Probe request and probe response same length as beacon Scanning for Roaming Eldad Perahia, Intel Corporation

  8. Call Time • Combine the amount of time spent in each state during different conditions • Tx: call, roaming scan • Rx: call, roaming scan, beacon • Idle/waiting to receive: call, roaming scan, beacon • The remaining time is assumed to be spent in sleep state (very optimistic) Max scanning Min data rate Min scanning Max data rate Eldad Perahia, Intel Corporation

  9. Standby Time • Combine the amount of time spent in each state during different conditions • Tx: roaming scan • Rx: roaming scan, beacon • Idle/waiting to receive: roaming scan, beacon • The remaining time is assumed to be spent in sleep state (very optimistic) Max scanning Min data rate Min scanning Max data rate Eldad Perahia, Intel Corporation

  10. Power consumption computation based on active scan A range of dot11BSSWidthTriggerScanInterval values examined, 10 sec to 5 min Scanning for 40 MHz Mode Note: “number of channels” is set to 6, since it is assumed that the two other channels are scanned more frequently for roaming scanning Eldad Perahia, Intel Corporation

  11. Two choices associate in 20 MHz and no scanning associate in 40 MHz and scan (40 MHz w/ no scanning shown for reference) In order to examine the worst case reduction due to 40 MHz scanning, we compare to the best case (DTIM = 10; roaming scan interval = 1 sec) dot11BSSWidthTriggerScanInterval 10 sec results in 1% reduction in call time and 19% reduction in standby time 60 sec results in 0.2% reduction in call time and 4% reduction in standby time 300 sec results in 0.04% reduction in call time and 0.8% reduction in standby time MIB values: 10 sec: min 60 sec: default 300 sec: max Call Time & Standby Time w/ Scanning for 40 MHz Eldad Perahia, Intel Corporation

  12. With a default value of dot11OBSSScanActiveDwell set to 10 msec, the scan fits within a 20 msec voice codec period With a minimum value of dot11OBSSScanActiveDwell set to 5 msec, the scan fits within a 10msec voice codec period The percentage of time spent scanning is based on dot11OBSSScanActiveTotalPerChannel, the number of channels, and dot11BSSWidthTriggerScanInterval With dot11OBSSScanActiveTotalPerChannel minimum and default value equal to 20 msec, the percentage of time spent scanning is small for the range of dot11BSSWidthTriggerScanInterval Active Scan Dwell & Total Per Channel Eldad Perahia, Intel Corporation

  13. With passive scanning, we must ensure that the probability of the scan interval overlapping with a beacon is high Each curve represents a different value for dot11OBSSScanPassiveDwell The horizontal axis represents a range of dot11OBSSScanPassiveTotalPerChannel values To allow for passive scanning with VoIP traffic without losing a packet during the scan default value of dot11OBSSScanPassiveDwell set to 10 msec min value of dot11OBSSScanPassiveDwell set to 5 msec With dot11OBSSScanPassiveDwell set to 5 msec, a dot11OBSSScanPassiveTotalPerChannel minimum value of 200 msec results in 87% probability a dot11OBSSScanPassiveTotalPerChannel default value of 250 msec results in 92% probability Passive Scan Dwell & Total Per Channel Eldad Perahia, Intel Corporation

  14. dot11OBSSScanActivityThreshold is a percentage of time spent transmitting and receiving packets For an infinite length VoIP call, the time spent ranges from 1% to 4% For a 15 minute call every 60 minutes, the time spent ranges from 0.2% to 1% Since the percentages are so low with individual voice calls, dot11OBSSScanActivityThreshold needs to be very small otherwise a 40 MHz BSS could contain many active VoIP clients that do not scan, causing interference dot11OBSSScanActivityThreshold = 0 for min, default, max; in other words, delete exemption Any device that wants to avoid scanning for 40 MHz simply associates as 20 MHz (Supported Channel Width Set field of the HT Capabilities element set to 0) Activity Threshold Eldad Perahia, Intel Corporation

  15. Summary of MIB Variables Eldad Perahia, Intel Corporation

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