MAC Improvements for Fairness and Power Saving in IEEE802.15.3

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: A proposal for MAC Improvements for fairness and power saving Date Submitted: 8 February, 2001 Source: Dr. William Shvodian Company: XtremeSpectrum Address: 8133 Leesburg Pike, Suite 700, Vienna, Virginia 22182 Voice: +1.703.749-0230 X364, FAX: +1.703.749.0249, E-Mail: bshvodian@xtremespectrum.com Re: A proposal for modifications to the 802.15.3 MAC Abstract: This proposal presents some modifications to the proposed 802.15.3 MAC that improve the ability to provide fair, guaranteed bandwidth allocations with bounded delay variation and support low power devices. Purpose: To provide an improvement to the current version of the 802.15.2 MAC 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. <William Shvodian>, <XtremeSpectrum>

2. Overview of Recommendations • Change SC-TDMA for fairness and improved QoS capability • Allow RTS/CTS to cover multiple packets, not a single packet • Divide Frames into Fixed Assignment slots and Demand Assignment slots • Add a polled response mode <William Shvodian>, <XtremeSpectrum>

3. Slot Cycle TDMA Issues • Slot allocation does not equal bandwidth allocation • Delay variation can fluctuate • Power management: All terminals must listen at all times in order to receive and to know when to transmit. Receivers can be shut down based on packet duration, but must listen to all asynchronous packet headers and idle mini-slots even if it only has isochronous traffic. • RTS/CTS required (I believe) even for isochronous slots <William Shvodian>, <XtremeSpectrum>

4. Issue: Slot Cycle Fairness • 1/n of the slots does not equal 1/n of the bandwidth • Suppose 2 terminals are allocated ½ the slots, of a 22 Mbps piconet • One is transmitting 53 byte ATM cells and the other is transmitting 1500 byte packets for data traffic. • The 1500 byte stream could take 95% of the bandwidth for the duration of a large file transfer, leaving the 53 byte stream with ~ 750 Kbps • This is an extreme case, but is meant to illustrate that “equal slots” and “equal bandwidth” are not equivalent when slots are variable length • 500 byte isochronous with 1500 byte asynchronous would cause a similar, though less drastic situation <William Shvodian>, <XtremeSpectrum>

5. Slot Cycle Fairness (cont.) Slot 1 Slot 2 53 1500 53 1500 53 1500 53 1500 <William Shvodian>, <XtremeSpectrum>

6. SC-TDMA suggestion #1 • Suggestion: Assign each terminal a maximum slot time that it can transmit for instead of allowing it to transmit one packet per slot. • Assignments are chosen so that each gets at least its minimum required bandwidth during the superframe • Benefits: • Terminals have guaranteed bandwidth and latency, and jitter is bounded <William Shvodian>, <XtremeSpectrum>

7. SC-TDMA suggestion #1(cont.) Slot 1 Slot 2 53 53 53 53 53 53 53 53 53 53 53 53 1500 53 53 53 53 53 53 53 53 53 53 53 53 1500 53 53 53 53 53 53 53 53 53 53 1500 minislot 53 53 53 53 53 53 53 53 53 53 53 53 1500 Time 1 Time 2 <William Shvodian>, <XtremeSpectrum>

8. SC-TDMA suggestion #1 Issues • This solves the fairness problem for SC-TDMA, but other issues remain: • Allocated asynchronous slot times may have to be at least the maximum packet size. Assigning a terminal an async slot of size less than it needs to transmit a packet can cause a backup in the terminal’s outbound queue and waste bandwidth. Packet size will often not be known a priori. Assigning only maximum sized slots for all terminals will not be efficient. Contention Access Period Contention Free Period Beacon Asynchronous slots Isochronous slots <William Shvodian>, <XtremeSpectrum>

9. SC-TDMA Suggestion #2 • In order to support the assignment of a Slot duration instead of a number of packets, the RTS/CTS should cover the duration for the entire transmission, not just for the first packet • The transmission duration can be up to it’s maximum allowed value for that terminal. • The maximum transmission duration value could be the same for all terminals for fairness, or it could be assigned separately for each terminal. <William Shvodian>, <XtremeSpectrum>

10. Slot Cycle TDMA Suggestion #3 • Assign real-time “isochronous” slots at the beginning of each frame, then use the remaining bandwidth for non-real-time traffic using Slot Cycle TDMA (or CSMA/CA) • No explicit assignments are made for asynchronous traffic. However, the coordinator may use assigned slots for low power devices • A superframe can be divided into frames, where each frame consists of first all of the isochronous slots followed by all of the asynchronous slots. • A beacon occurs once per superframe • Isochronous users get guaranteed bandwidth with bounded latency and latency variation <William Shvodian>, <XtremeSpectrum>

11. Slot Cycle TDMA Suggestion #3 (cont.) Contention Access Period Contention Free Period Beacon 1 2 4 5 1 2 3 4 1 2 4 Fixed Assignment slots Demand Assignment slots Mini slots • First station in the Demand Assignment portion of the CFP may rotate through for fairness, specified in Beacon • Unassigned bandwidth shared among users with asynchronous traffic • Assigned and unassigned “slots” are bounded time periods rather than single packets <William Shvodian>, <XtremeSpectrum>

12. Slot Cycle TDMA Suggestion #3 (cont.) Superframe Contention Access Period Contention Free Period Beacon 1 2 4 5 1 2 3 4 1 2 1 2 4 5 5 1 2 3 4 1 Frame Frame Fixed Assignment slots Demand Assignment slots Mini slots • CFP can consist of multiple frames • This limits packet delay variation which reduces buffer needs • What happens at the end of a frame if not enough time to transmit a packet, but more time than a mini-slot? Maybe a special RTS to denote that the packet will follow the FA slots <William Shvodian>, <XtremeSpectrum>

13. Advantages of suggestion #3 • Terminals can be assigned any fraction of a superframe for real-time traffic. • Fixed Assignment slots are fixed time for the duration of a superframe • Supports low power mode • Reduces delay variation • Fixed Assignment (isochronous) slots do not require RTS/CTS • Coordinator doesn’t need to know async packet sizes or async throughput a priori, yet all remaining bandwidth is used efficiently and fairly • Only terminals with asynchronous streams need to listen to the asynchronous slots. • Coordinator only needs to do scheduling of Assigned Slots. However, Demand Assignment slot durations can be allocated for each terminal individually or all terminals can use the same time duration like token ring • Simplifies the assignment calculations and assignments as compared to SC-TDMA with mixed isochronous and asynchronous slots • If coordinator is not capable of scheduling Fixed Assignment slots, all terminals can use SC-TDMA for bandwidth sharing <William Shvodian>, <XtremeSpectrum>

14. Suggestion #4 • Add a polled response mode to support very simple MACs in devices used for infrequent batch file transfers like cameras • Special Frame Type for poll messages • The device participates in limited communication with the coordinator and target device, but only transmits during the CP or when it receives a poll command. This enables a device with a stripped down MAC to transfer files efficiently. • A separate period of the frame/superframe may be reserved for polled traffic. <William Shvodian>, <XtremeSpectrum>

15. Power saving modes and features • Sleep mode • Assigned slots only mode • Fixed Assignment slots can be assigned for isochronous streams for QoS or asynchronous traffic flows to support power saving • Header abort mode: Terminals can shut down for the duration of a packet if the destination MAC address is not theirs • Polled Only MAC <William Shvodian>, <XtremeSpectrum>

16. Summary • Suggestion #1: Assign terminals a maximum time duration for transmission instead of allowing single packet per slot • Suggestion #2: RTS/CTS covers the duration of all of the packets to be transmitted during a terminal’s transmission interval • Suggestion #3: Separate the frames into Fixed Assignment slots and Demand Assignment slots. Use SC-TDMA (or CSMA/CA) only in the DA slots • Suggestion #4: Polled only mode MAC <William Shvodian>, <XtremeSpectrum>