Transcript 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.

<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: [email protected]
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
minislot
53
53 53
53
53
53
53
53
53
53
53
1500
53
1500
Time 1
Submission
Time 2
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:
Beacon
– 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
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
Beacon
Slot Cycle TDMA Suggestion #3 (cont.)
Contention
Access 1
Period
Contention Free Period
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
Submission
Slide 11
<William Shvodian>, <XtremeSpectrum>
<February 2001>
doc.: IEEE 802.15-01/120r0
Slot Cycle TDMA Suggestion #3 (cont.)
Beacon
Superframe
Contention
Access 1 2
Period
Contention Free Period
4
5 1 2 3 4 1
Frame
2 1 2
4
5
5 1 2 3 4 1
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 SCTDMA 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>