Power save for wireless mesh

1. Power save for wireless mesh Mathilde Benveniste benveniste@ieee.org Mathilde Benveniste

2. Introduction • A wireless mesh may consist of a mix of power-saving and non-power-saving MPs • It is assumed that all mesh points support power saving for peer MPs • The goal of a PS protocol is to • minimize time a PS MP must be awake • reduce the power overhead associated with power save (e.g. TIM transmission, and PS Poll and trigger frame transmissions) • Power save protocol that requires no beacon TIMs, PS Polls or trigger frames Mathilde Benveniste

3. Description of proposed PS protocol • Every power-saving (PS) MP must be awake to receive buffered frames from its peers at a known time • The time interval an MP must be awake, called a “receive window” (RW), is defined in terms of • an offset Rwoffset • a duration RWLength, and • a period RWP • Each MP advertises its RW(s) in its beacon • An MP with buffered unicast and multicast frames for a PS MP may transmit them during an RW of the PS MP • A PS MP (one that has the PM bit set in its frames) wakes up at the start of the RW and may go to sleep if no frames are received during the RW; otherwise, it must stay awake until t receives a frame with EOSP=1 • The frames that can be sent by a buffering MP to a PS MP in a RW cannot take longer than MaxSPLength Mathilde Benveniste

4. Initialization and adaptation of RWs • When a PS MP joins a mesh, it selects an RW, which is advertised in its beacon* • The RW choice reflects the expected traffic streams for the PS MP • A RW can be adapted over time at the request of the buffering MP in order to avoid overlap with the RWs of other PS MPs, alleviate queue buildup at the buffering MP, or increase the efficiency of power save • A PS MP may modify its RW by announcing the new RW on its beacon • A PS MP may increase the RW period interval and reduce RW duration when it shifts from an active call to stand by • Adaptation of the RWs may be directed from the outside (e.g. by a central controller) • Each periodic RW has an associated ID • The ID is used for modification and/or deletion of the RW • A PS MP may have multiple RWs • A PS MP may receive traffic from multiple buffering MPs, each possibly using different RWs • Different traffic streams may lead to different RWs at a PS MPs, with possibly different durations and period intervals *Selection of the initial RW and way to adapt to traffic is not specified; however the RW selection rules determine the effectiveness of power save Mathilde Benveniste

5. MP1 MP2 MP3 RWs of a PS MP • All MPs in this example are PS MPs with bi-directional traffic • The Receive Windows (RWs) repeat at equal intervals, the RW Period Interval • A DTIM interval contains several RW Period Intervals • The RW specification may change over time depending on the activity of the MPs MP1 MP2 MP3 RW Period Length RW offset Receive Transmit Beacon Mathilde Benveniste

6. Octets: 1 1 1 1 1 2 1 Element ID Length RW ID RW Duration RW Period Length RW Offset Max RW Length Receive Window Setup • The RW Setup information element is used to publicize the time when a PS MP is awake to receive frames and by a buffering MP to request a PS peer MP to set up a periodic sequence of RWs • This information element is transmitted in the beacon of the PS MP • It is also found in individually addressed PS action frames sent by a buffering MP to the PS MP to modify the parameters of an existing RW or to request a new RW • The RW period interval is a fraction of the DTIM interval, which is obtained from the DTIM interval by division by an integer • Each RW is identified by a RW ID • Modification of an RW is signaled by using the RW ID with the new parameters • A 0 value in the RW Duration field signals deletion of an established RW Mathilde Benveniste

7. Octets: 1 1 1 1 6 Element ID Length RW ID Reply Code Alternate RW Receive Window Response • The RW Setup Response element is used to reply to an RW Setup Request. • The alternate RW is the description of suggested receive period parameters to be used in a new RW Setup Request • The reply code explains whether the request is accepted and if an alternate is supplied Mathilde Benveniste

8. Reply Code Meaning 0 Accept 1 Reject 2 Reject: suggested window supplied Other Reserved Reason Code • The RW Setup Response element is used to reply to an RW Setup Request. • The alternate RW is the description of suggested service period parameters to be used in a RW Request • The reply code explains whether the request is accepted and why Mathilde Benveniste

9. MP1 MP2 MP3 Examples of RWs for a linear mesh MP1 All three MPs are power saving • Initial RWs are set upon association with mesh • MPs adapt their RWs based on when traffic is received and buffered • MP1 and MP3 set the RWs of MP2 by requesting an RW to occur when traffic is buffered • MP2 sets the RW of MP1 and MP3 • The buffering MPs awaken to transmit during the RWs of their peer PS MPs • Each MP must awaken twice • MPs may adapt their RWs for contiguous awake intervals • MP1 may request MP2 to delay its RW • MP2 requests MP3 to delay its RW before it agrees to MP1’s request • Transmission MP1-MP3 is delayed and followed immediately by transmission from MP3-MP1 • All MPs awaken once per period 1 MP2 MP3 MP1 2 MP2 MP3 MP1 MP2 3 MP3 Receive Window Receive Window Transmit Window Transmit Window Mathilde Benveniste

10. MP1 MP2 MP3 Examples of RWs for a linear mesh MP1 MP 1 is not power saving • Initial RWs are set upon association with mesh • MPs adapt their RWs based on when traffic is received and buffered • MP2 sets the RW of MP3 • MP3 sets the RW of MP2 by requesting an RW to occur when traffic is buffered • The buffering MPs is awake to transmit during the RWs of their peer PS MPs • Each PS MP must awaken twice • MPs may adapt their RWs for contiguous awake intervals • MP2 requests MP3 to delay its RW • MP2 adapts the RW used to receive from MP1 by publicizing in its beacon • Transmission MP1-MP3 is delayed and followed immediately by transmission from MP3-MP1 • All PS MPs awaken once per period v 1 MP2 MP3 MP1 v 2 MP2 MP3 MP1 v MP2 3 MP3 Receive Window Transmit Window Mathilde Benveniste

11. Advantages of proposed PS protocol • Buffering MPs need not broadcast TIMs on their beacons • PS MPs need not awaken to listen to the beacons for the TIMs • RWs need not start or include a beacon • PS MPs need not send PS Polls or trigger frames to retrieve buffered frames Mathilde Benveniste