Optimizing Superframe Intervals with Enhanced Scheduling Beacons

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Modifying Superframe Intervals and Set up the Scheduling Beacons ] Date Submitted: [24 June, 2008] Source: [Ning Gu, L. Zhang, H. Liu, Z.Zhao ] Company [Vinno Technologies Inc., Inst. of Mirco-system and Info Tech] Address [Room 402 Building D, Pioneering Park, No. 2 Shangdi Xinxi Rd, Peking, China] Voice:[+86 10 8278 2373], FAX: [+86 10 8278 2373 - 830], E-Mail:[guning@vinnotech.com, zhangliang@vinnotech.com] Re: [IEEE 802.15.4e group] Abstract: [An improved scheduling beacon structure is proposed toeffectively decrease the packets collision possibility ] Purpose: [To promote discussion in IEEE802.15.4e Study Group] Notice: This document has been prepared to assist the IEEE P802.15. 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 acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

2. Objectives An improved scheduling beacon structure is proposed toallocated time slots in distributed fashion.

3. Existing Problems • Original definition of scheduling beacons in 15.4 is not specific and adaptive. • Original SF(superframe) structure leads to low throughput because at least half of the beacon interval in one network may be used for sleep mode as BO>SO. (Beacon Order. Superframe Order) • Too many nodes working in shared time slots (SF overlap) might cause following problems: • Some nodes might overhear packets that destined to other nodes. • Heavy loaded network (too many nodes) might easily result in a “channel access failure” when performing CSMA/CA procedure.

4. The Definition of Scheduling Beacon • Scheduling beacon is a method to configure superframe features of beaconed coordinator, which include BO and beacon relative time Tx, etc. • Time slots resources for scheduling beacon can be reused as long as they do not interfere with each other. • Superframe overlapping can be prevented by choosing appropriate beacon start time.

5. Modification for Scheduling Beacons Suggested modification and improvements: • 15.4 regulates that “BO and SO shall be equal for all SF on a PAN”, which might be replaced with “BO shall be equal for all SF on a PAN”. • The structure of scheduling beacons contain beacon slot, P-slot, C-slot, MS and inactive period. • B-slot: beacon slot, for beacon transmission and reception, SF can be synchronized during this slot • P-slot: for sending data to parent node, each node is allocated an unique P-slot by its parent node • C-slots: for parent node to receive data from all the child nodes, • M-slot: management slot, for link management between nodes, its working mode is indicated in beacon frame. • Inactive period: during a SF, all the other slots beside the above slots • Might consider inserting scheduling beacon information in the payload of beacon frame, which includes the start time Tx and the network information.

6. Scheduling Beacon Explanation • Suggest to apply the TDMA algorithm in scheduling beacon structure. • BaseSlotLength = 3*2SO backoff, recommended value of SO is 0 or 1, BS (beacon slot), each P-slot, C-slot and M-slot occupies one BaseSlotLength. • BI (Beacon interval) = 3*2BO backoff, recommended value of BO is between 7 and 10, which would result in between 1.5% and 0.1% duty cycle. • The number of C-slots equals to the maximum number of child nodes that the parent node could have. • Nodes shall enter into inactive period during its unallocated C-slot(s). • All the transmission are finished in the allocated time slot, nodes shall enter into inactive period after transmission if the slot is not over. • The working mode of MS is broadcasted through beacon, those nodes which have nothing to do with current MS shall enter into inactive period. • Except root node, all node may have two MS, one for communicating with its parent node while the other one for communicating with its child nodes. • Before transmitting its own beacon, i.e. set up its own sub-network, nodes must scan the time slots to find the available slots.

7. Scheduling Beacon Structure As represented in the right figure, if node 2 and node 3do not interfere with each other, they can reuse the same time slots. Note: node 2 and node 3 do not have any child node, so all of their C-slots are unallocated

8. Modified Beacon Frame Structure

9. Scheduling Beacon Field Explanation • Tx Field: symbols difference between parent’s beacon transmission time and device’s beacon transmission time. • Child Number Field: the maximum child nodes the device could have. • Slot Allocation Field: responsible for assigning slots to all of the device’s child node. • Slot Number sub-field: the number of slots allocated to device’s child nodes. • Short Address sub-field: the address sequence of child nodes that occupy the time slots. Each device has one time slot • M-Slot Mode Filed: specify the working mode for the current management slot. • Join: set to true if M-Slot is used for node join • Broadcast Flag: set to true if device has broadcasting data for all the child nodes

10. Start a Scheduling Beacon • A potential scheduling beaconed device might take the following steps to start scheduling beacons: • The device shall first perform ED and active scan to find potential parent node and locate its B-Slot. • Choose candidate parent and perform association in parent’s M-Slot if JOIN bit in M-Slot mode field is true. • Obtain the beacon information of neighbor beaconed routers by scanning. • Based on scan result obtained in step3, the following steps might be taken by the device:

11. Start a Scheduling Beacon (continued) • if no other beacons can be scanned, the device may transmit its own scheduling beacons during inactive period. Note: the transmitting start time, i.e., Tx, is decided by “StartTime” in start request primitive and shall be chosen a value as small as possible.

12. Start a Scheduling Beacon (continued) • if other beaconed routers are scanned, the device may obtain these routers’ SF information, and start its own scheduling SF. The device inserts scheduling Beacon to the SF by deciding an appropriate Tx time to make sure there is no SF overlapping.

13. Start a Scheduling Beacon (continued) • if this device want to be a beaconed router and it can not find an appropriate Tx time to avoid the overlapping with other beacons from any network depth, it shall perform channel hopping and repeat the above steps.

14. Start a Scheduling Beacon (continued) • if none of the above is available, this device shall work as an end device.

15. The Benefits from Improved Scheduling Beacons • Network scale is enlarged because more routers can be arranged to work in original inactive period. • Network throughput is increased since inactive period is used for communication. • Packets confliction possibility is decreased and more power can be saved.

16. Thank you!