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Measurement Pilot Frame

Measurement Pilot Frame. Steve Emeott, Walter Johnson, Floyd Simpson, Stephen Wang, Tim Wilson Motorola. Channel Acquisition and Signal Quality Measurements. Mobile stations will employ several functions that require measurements of AP signal quality as input Discovery of 802.11 service areas

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Measurement Pilot Frame

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  1. Measurement Pilot Frame Steve Emeott, Walter Johnson, Floyd Simpson, Stephen Wang, Tim Wilson Motorola Stephen Wang, et. al.

  2. Channel Acquisition and Signal Quality Measurements • Mobile stations will employ several functions that require measurements of AP signal quality as input • Discovery of 802.11 service areas • Identification and ranking BSS transition (handover) candidates • Initial transmit rate selection • Flexible techniques for measuring signal quality are required to accommodate an increasing range of applications • Mobile stations roaming into an 802.11 coverage area must be able to rapidly detect a transmittable channel, preferably passively and in compliance with DFS regulations (5 GHz band), among the available channels prior to active probe and/or association • Handheld stations camped on an 802.11 system must be able to take measurements while still offering long standby times • Complexity of mechanisms used to take measurements must be suitable for wide range of wireless networks Stephen Wang, et. al.

  3. A voice station associated with a foreign network scans for WLAN coverage Challenge: The station (possibly with a call in progress) has to locate the transmitting channel by conforming to DFS regulations as quick as possible. AP1 AP2 Use Case 1 – Serving Channel Acquisition • When a station is approaching a WLAN from a foreign network (e.g. WAN), it has to quickly locate the transmitting channel among 11 (2.4GHz band) or 19 (5 GHz band) possible channels. • Because of DFS regulations for the 5GHz band, a station must first passive scan up to 19 channels before it can detect the active channel and transmit a probe request.  • Fast detection of a transmittable channel leads to shorter channel acquisition time and lower power consumption.  Stephen Wang, et. al.

  4. A voice station roams into 802.11a coverage   Challenge: The station (possibly with a call in progress) has to scan for neighbor beacons.  AP1 AP2 Use Case 2 – Neighbor AP Scanning • Neighbor report may not always include the optional TSF Offset field (e.g. due to accuracy requirements). • Information carried in Neighbor report may be stale, making it less useful for accurate prediction of neighbor AP TBTT. • Without precise information about neighbor TBTT, the station must passively scan neighbor beacons or transmit probe request (provided DFS requirement is met in the 5 GHz band). • This scanning time is a function of the beacon interval, unless something new is done. Stephen Wang, et. al.

  5. A voice station with a call in progress roams within a WLAN wireless network   Challenge: For a typical 100ms beacon interval, beacon frames may be too sparse to provide real-time link quality measurements. AP1 AP2 Use Case 3 – Handover Trigger • Stations must make timely handover decision based on real-time link quality measurements. • Measurement accuracy is compromised if too much time elapses between consecutive measurements • Ill advised handover decisions may result if a STA is unable to update its signal quality estimate in a timely fashion (resulting in call dragging or handover flurries) Stephen Wang, et. al.

  6. Use Case 4 – Fast Neighbor Report • In most cases, neighbor report is generated based on information gathered via beacon reports • Measuring STAs must suspend current tasks in order to actively probe or passive sniff up to a full beacon interval on each neighbor channel. This is costly to the measuring STA in terms of resource and battery life. • Active probe may not always be possible in the 5GHz band. • The stretched beacon reporting time results in longer neighbor report turnaround time, affecting STA’s ability in making timely transition decisions. Stephen Wang, et. al.

  7. Proposal: Measurement Frame • Definition: A measurement frame is a new management frame (for example, using frame type 6) that contains only the fields needed by a station to take a link margin measurement plus those required by a station to formulate a valid probe request frame • Necessary Fields include: • Timestamp (8 octets) • Measurement Frame Interval (2 octets) • Beacon Interval (2 octets) • Capability info (2 octets) • RSN Capabilities (2 octets) • DS Parameter set (3 octets) • Country info (3 octets) • Max regulatory power (1 octet) • AP Max Power (1 octet) • AP Power Used (1 octet) • AP Noise Floor (1 octet) • Total Frame Body Length = 26 Octets Stephen Wang, et. al.

  8. Measurement Pilot Bandwidth Consumption • Total bandwidth consumption is less than 1% in an 11a network • Each measurement pilot MMPDU is 54-octet long (24-octet MAC Header + 26-octet Frame Body + 4-octet FCS) • Total length of PPDU is 624 bits (144-bit PLCP Preamble + 48-bit PLCP Header + 432-bit MMPDU) • 100 Measurement Pilot frames transmitted per second (10ms interval) • AP transmits at a fixed rate of 6 Mbps Stephen Wang, et. al.

  9. Bandwidth Consumption Breakeven Point– Measurement Pilot vs. Active Probe – Assumptions: Probe Request (min size): 72 octets Probe Response (min size): 106 octets Measurement Pilot: 54 octets Measurement Pilot Interval: 10ms STA Probe Interval: 250ms Time Span: 10 secs Stephen Wang, et. al.

  10. Impact of Sniffing Time on Battery Life WLAN Recv Power: 900 mW GSM Standby Power: 14 mW Battery Energy: 2400 mW-Hr Sniff Interval: 30 sec No. of Channels: 19 Stephen Wang, et. al.

  11. Beacon versus Measurement Pilot Stephen Wang, et. al.

  12. Measurement Pilot Generation • Measurement Pilot shall be optional and may be configured by MIB variable. • A measurement pilot generation function shall transmit the Measurement Pilot frame when the value of the TSF Timer (in us) modulo the Measurement Pilot interval equals 0. • The Measurement Pilot frames shall be addressed to the broadcast destination address. • The Measurement Pilot frames shall not be buffered for power save reasons. • Beacon transmission takes precedence over Measurement Pilot transmission. Stephen Wang, et. al.

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