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					     <February 2001>                                                        doc.: IEEE 802.15-01/120r0

      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.

     Submission                                    Slide 1              <William Shvodian>, <XtremeSpectrum>
<February 2001>                        doc.: IEEE 802.15-01/120r0



             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

Submission               Slide 2   <William Shvodian>, <XtremeSpectrum>
<February 2001>                             doc.: IEEE 802.15-01/120r0



                  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


Submission                  Slide 3     <William Shvodian>, <XtremeSpectrum>
<February 2001>                                     doc.: IEEE 802.15-01/120r0


                  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



Submission                        Slide 4       <William Shvodian>, <XtremeSpectrum>
<February 2001>                                doc.: IEEE 802.15-01/120r0


                  Slot Cycle Fairness (cont.)

  Slot 1                          Slot 2



  53                             1500



  53                             1500



  53                             1500


  53                             1500




Submission                    Slide 5      <William Shvodian>, <XtremeSpectrum>
<February 2001>                                     doc.: IEEE 802.15-01/120r0

                  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




Submission                        Slide 6       <William Shvodian>, <XtremeSpectrum>
<February 2001>                                                                   doc.: IEEE 802.15-01/120r0


               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




Submission                                                  Slide 7           <William Shvodian>, <XtremeSpectrum>
  <February 2001>                                          doc.: IEEE 802.15-01/120r0



                 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.
   Beacon




            Contention               Contention Free Period
             Access
              Period

                                                       Asynchronous slots
                                                       Isochronous slots
  Submission                           Slide 8         <William Shvodian>, <XtremeSpectrum>
<February 2001>                          doc.: IEEE 802.15-01/120r0



                  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.
Submission                 Slide 9   <William Shvodian>, <XtremeSpectrum>
<February 2001>                                       doc.: IEEE 802.15-01/120r0



             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



Submission                        Slide 10        <William Shvodian>, <XtremeSpectrum>
 <February 2001>                                               doc.: IEEE 802.15-01/120r0


     Slot Cycle TDMA Suggestion #3 (cont.)
Beacon




         Contention               Contention Free Period
          Access 1    2   4   5   1     2        3   4     1             2           4
           Period

                                                     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
 Submission                           Slide 11           <William Shvodian>, <XtremeSpectrum>
 <February 2001>                                                  doc.: IEEE 802.15-01/120r0


     Slot Cycle TDMA Suggestion #3 (cont.)
                               Superframe
Beacon




         Contention                Contention Free Period
          Access 1 2   4   5 1 2 3 4 1          2 1 2     4       5     5 1 2 3 4 1
           Period


                           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
 Submission                          Slide 12                 <William Shvodian>, <XtremeSpectrum>
    <February 2001>                                           doc.: IEEE 802.15-01/120r0


                  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


    Submission                           Slide 13         <William Shvodian>, <XtremeSpectrum>
<February 2001>                            doc.: IEEE 802.15-01/120r0



                   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.


Submission                 Slide 14    <William Shvodian>, <XtremeSpectrum>
<February 2001>                            doc.: IEEE 802.15-01/120r0



             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

Submission                  Slide 15   <William Shvodian>, <XtremeSpectrum>
<February 2001>                        doc.: IEEE 802.15-01/120r0



                    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
Submission              Slide 16   <William Shvodian>, <XtremeSpectrum>

				
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