Thoughts on Interaction between Power Management and Path Selection July 2007 Thoughts on Interaction between Power Management and Path Selection Date: 2007-07-15 Authors: Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <stuart.kerry@philips.com> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <patcom@ieee.org>. Michelle Gong, et al.

July 2007 Problem Statement In a multi-hop mesh network, both power management and path selection are important functionalities As described in document 11-07/1996r1, power management is important for MPs, not only just for battery-powered devices but also for some main-powered devices Interaction between path selection and power management have not been defined in the current 11s draft An MP can go into doze state independent of the path selection process If left unaddressed, the topology of the mesh network may be changing all the time and the path selection scheme becomes ineffective To ensure reliable packet delivery, network stability, but still to reduce power consumption as much as possible, we need to jointly consider path selection and power management Should MPs in PS mode participate in path selection? Should an MP continue to operate in PS mode after it becomes a forwarding node? Whether a routing metric that considers battery status needs to be defined? (Not addressed by this presentation) Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

An MP can operate in one of two states: awake state or doze state July 2007 An MP can operate in one of two states: awake state or doze state In the awake state, an MP is fully powered and it can receive or transmit frames. In the doze state, an MP can not receive or transmit and consumes much less power An MP in PS mode goes in and out of the doze state periodically A PS MP wakes up to receive and/or to transmit during the ATIM window following the mesh DTIM beacon The PS MP may stay awake beyond the ATIM window until all the buffered frames have been transmitted or received Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

July 2007 There are two path selection modes in HWMP: on-demand path selection and proactive tree building On-demand path selection A source MP broadcasts a Path Request (PREQ) with the destination MP specified in the destination list Upon receiving the PREQ, intermediate MPs creates a path to the source or updates its current path and rebroadcasts the PREQ Depending on the configuration, the intermediate MP with a valid route back to the source or the destination MP can send a unicast Path Reply (PREP) back to the source On receiving PREP, intermediate MPs create a path to the destination and also forward the PREP back to the source Michelle Gong, et al.

Two mechanisms can be utilized to proactively build a mesh tree July 2007 Two mechanisms can be utilized to proactively build a mesh tree Proactive tree building mode Proactive PREQ mechanism: The root MP sends a proactive PREQ periodically The recipient MP may send back a proactive PREP to establish a bidirectional path Proactive RANN mechanism: The root MP periodically floods a Root Announcement (RANN) message into the network Upon receiving a RANN, each MP that has to create or refresh a path to the root MP sends a unicast PREQ to the root MP The root MP sends a PREP in response to each PREQ Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

There are several problems regarding power save and path selection July 2007 There are several problems regarding power save and path selection In the current 11s draft, no interaction between power save and path selection has been defined Any MP may go into power save mode at any time, which can cause arbitrary changes in mesh topology Frequent route ups and downs can lead to instability of the routed network The network may even become disconnected if one or more MPs at critical locations go into power save and do not participate in path selection Not allowing MPs to go into power save is a simple yet problematic solution Power save capability is critical to battery-powered devices Even for main-powered devices, it would be ideal if they can go into power save mode when they don’t forward or transmit data traffic Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

Should a PS MP participate in path selection? July 2007 Should a PS MP participate in path selection? Path Selection is different from data packet forwarding: The goal of path selection is to discover a valid route from a source to a destination Data packet forwarding starts only after a valid route has been set up between the source and the destination What if a PS MP participate in path selection? Neighbouring MPs buffer path selection related frames and only transmit them to the PS MP after the mesh DTIM beacon Depending on the synchronization scheme, the hop count and how many PS MPs are on the selected route, there might be a few extra seconds delay added to the path selection procedure If the route stays active, subsequent data packets do not suffer such an extra delay Michelle Gong, et al.

Should a PS MP participate in forwarding? July 2007 Should a PS MP participate in forwarding? Packet forwarding starts after a valid path has been set up between the source MP and the destination MP What if a PS MP participate in forwarding? Neighbouring MPs buffer data packets that need to go through the PS MP and only transmit them after the mesh DTIM beacon Depending on the synchronization scheme, the hop count and how many PS MP are on the route, data packets may suffer a few seconds of extra delay This is not acceptable for voice and certainly not ideal for other types of traffic Michelle Gong, et al.

Outline Background on power management July 2007 Outline Background on power management Overview of path selection modes in HWMP Current problems regarding lack of interaction between path selection and power management Two important questions that need to be answered Should an MP in PS mode participate in path selection? Should an MP in PS mode participate in forwarding? How to consider PS MPs in the path selection process? Michelle Gong, et al.

July 2007 Main-powered and battery-powered devices should be treated differently during path selection Conserving power is much more important for battery-powered devices than for main-powered devices In a multi-hop mesh network, forwarding others’ traffic consumes significantly more power In a mesh network, even if a MP doesn’t have its own traffic to transmit, it may have to forward traffic for other nodes Rules have to be defined to ensure network connectivity as well as to conserve power For instance, if a valid path can be found without using battery-powered MPs as forwarding nodes, such a path should be chosen Battery-powered MPs may be chosen as forwarding nodes when a route cannot be found with only main-powered MPs Michelle Gong, et al.

Two more bits are needed to categorize different MPs July 2007 Two more bits are needed to categorize different MPs Currently, there is only one bit, i.e. power save support bit, defined in mesh capability field for power management purpose This bit is used to indicate whether an MP supports power save or not At least two more bits are needed in the mesh capability field to classify devices into three categories 00 Full Availability: outlet powered 01 Moderate availability: battery powered but not in critical battery state 10 Limited availability: battery powered and in critical battery state 11 Reserved During the peer link set up, neighbouring MPs learn of each other’s power category An MP’s power category can be updated through beacons or through peer link updates Michelle Gong, et al.

July 2007 Based on this classification, path selection messages can be selectively transmitted to MPs Battery-powered PS MPs is treated differently from outlet-powered PS MPs during the path selection process A wakeup bit is defined in PREQ, PREP, and RANN The path selection messages are propagated to battery-powered PS MPs when the wakeup bit is set to 1 Battery-powered MPs may ignore the path selection messages when the wakeup bit is set to 0 Michelle Gong, et al.

The source node decides whether to set the wakeup bit July 2007 The source node decides whether to set the wakeup bit For instance, the source node doesn’t set this bit when it tries to discover a route to the destination for the first time If a route could not be found, the source node can set the wake up bit in PREQ to wake up the battery-powered MPs Upon receiving the PREQ with the wake up bit set, the batter-powered MPs should process and re-forward the PREQ Once being waken up, the batter-powered MPs shall stay in awake state until a timer expires In the proactive tree building mode, the root MP sends out PREQ and/or RANN messages periodically The root MP can choose to set the wakeup bit in all or some of the messages In a globally synchronized network, PS MPs may wake up periodically to receive PREQ or RANN sent from the root MP Michelle Gong, et al.

July 2007 An MP should not go into power save mode when it is forwarding traffic for other MPs Staying in PS mode while acting as a forwarding node may introduce excessive delay to packet delivery and may significantly reduce throughput Several conditions may trigger an MP to go into PS mode If there has been no data traffic for a period of time, after a timer expires, the MP goes into PS mode If there is no active route going through this MP, after a timer expires, the MP goes into PS mode If the MP is in critical battery state, it shall go into PS mode MPs in critical battery state should not be a forwarding node and should not be waken up during the path selection process unless it is the destination node Michelle Gong, et al.

July 2007 Conclusion It is important to define interaction between power management and path selection To avoid network partition and the network stability problem To avoid excessive packet delay when data packets are being forwarded by PS MPs To help battery-powered MPs save power PS MPs can participate in path selection Outlet-powered MPs can be notified first during the path selection process Battery-powered MPs participate in path selection if a valid route can not be found with only outlet-powered MPs MPs in PS mode acting as forwarding nodes may cause poor network performance Excessive delay in packet delivery Low throughput We may need to analyze how much more power we can save if the MP has to wake up frequently to receive and forward packets Main-powered and battery-powered MPs should be treated differently by path selection protocols Using the default radio-aware routing metric Define a new routing metric that differentiates main-powered and battery-powered MPs Michelle Gong, et al.

Straw Poll 1 Should MPs in PS mode participate in path selection? Yes July 2007 Straw Poll 1 Should MPs in PS mode participate in path selection? Yes No Michelle Gong, et al.

July 2007 Straw Poll 2 Should MPs that act as forwarding nodes operate in PS modes? Yes No Michelle Gong, et al.

July 2007 Straw Poll 3 Should main-powered MPs and battery-powered MPs be treated differently during the path selection process? Yes No Michelle Gong, et al.