Feb. 2002 doc.: IEEE 802.15-02/076r2 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: Modified CTRB Parameters Date Submitted:
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Transcript Feb. 2002 doc.: IEEE 802.15-02/076r2 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: Modified CTRB Parameters Date Submitted:
Feb. 2002
doc.: IEEE 802.15-02/076r2
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: IEEE802.15.3: Modified CTRB Parameters
Date Submitted: 6 February, 2002
Source: Dr. William Shvodian
Company: XtremeSpectrum, Inc.
Address: 8133 Leesburg Pike, Suite 700, Vienna, VA 22182
Voice: 703-269-3047 , FAX: 703-269-3092 , E-Mail: [email protected]
Re: [ ]
Abstract: This points out problems with the current CTRB parameters and proposes a solution that will
help to simplify the 802.15.3 MAC.
Purpose: To provide information for comment resolution of LB12, Comment Number 1333, 1491 and
792
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
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Overview
• Problems with Current CTRB
parameters
• Proposed Modified CTRB
• Proposed 1 s resolution for CTR, CTA
and piconet synchronization timing
Submission
Slide 2
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Problems with Current CTRB
Parameters
• The Allocation Period currently does not have an
integer relationship to the Superframe Duration. The
Allocation Period can be any allowed time value
So:
• Either the PNC has to adjust the superframe duration
based on the application needs – which will create
interoperability problems
• Or, the GTS position changes on a superframe by
superframe basis. Consequently, pseudostatic GTS
slots are not possible in this scenario. It was shown
in submission 01/436 that pseudo static GTS slots
are needed for isochronous streams to avoid
transmission stoppage due to corrupted Beacons.
Submission
Slide 3
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Problems with Current CTRB
Parameters (continued)
• Maximum Allocation Delay attempts to put a
tight bound on delay variation, but it can only
control the delay variation of the GTS slot, not
the actual transmitted frames (details to
follow)
• Allocation for GTS slots with an allocation
period greater than a superframe is not
possible given the CTRB in Raju’s proposed
text change for EPS.
Submission
Slide 4
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Adjusting the Superframe Duration to fit
the most QoS demanding application
• This is asking for interoperability problems.
– What if two simultaneous applications are running
that require different superframe durations? (One
wants a slot every 33 ms +/- 1 us, the other wants
one every 32 ms +/- 1 us.) These applications can
not be both satisfied simultaneously. “Hey, my
camcorder works great except when my kids are
watching a DVD in the next room.”
• Recommendation: Don’t adjust the SF
duration to match application requirements
Submission
Slide 5
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Sources of Delay Variation - Clock Error
• The application clock and the PNC clock are close,
but not perfectly synchronized.
• The clocks will drift relative to each other, causing
delay variation at the receiver.
Submission
Slide 6
G
T
S
Beacon
G
T
S
Beacon
G
T
S
Frame arrives too
late and has to wait
an entire superframe
Beacon
Beacon
Frame
arrives just
in time for
GTS slot
G
T
S
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Sources of Delay Variation – Channel throughput
must be greater than application throughput
• Because of clock uncertainty, the allocated slot time must occur
slightly more often than source arrivals in order to ensure the
queue doesn’t grow to infinity
• Also, if ACKs are used, extra throughput is required for potential
retransmissions.
• Hence, the queue will periodically be empty during an assigned
slot and the slot will go unused resulting in delay variation
Submission
G
T
S
Slide 7
G
T
S
Beacon
Frame arrives too late and
has to wait an entire
superframe
Beacon
G
T
S
Beacon
Beacon
Frame arrives just in time
for GTS slot
G
T
S
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Frame delay variation versus GTS slot
Maximum Allocation Delay
• Summary of delay variation sources:
– Any retransmissions at all will cause the frame delay
variation to exceed the Maximum Allocation Delay for
the GTS slot assignments.
– Retransmissions require that the GTS throughput is
actually higher than the application throughput to
handle retransmissions.
– The applications will not be synchronized with the PNC
clock – and they should not be. This will cause
slippage between the packet arrivals and the slots.
• Recommendation: Eliminate Maximum
Allocation Delay from the CTRB
Submission
Slide 8
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Bounding the superframe duration
bounds the delay variation
• Limiting the superframe duration limits
the delay variation to:
– 1 X Superframe duration for static GTS,
– 2 x Superframe duration for dynamic GTS
Submission
Slide 9
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Delay Variation Smoothing
• In order to compensate for MAC delay
variation, the receiver will use a
smoothing buffer at the convergence
layer so that packets are passed
smoothly to the application.
Submission
Slide 10
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Change to GTS Minimum and Desired
GTS Time
• Minimum and Desired GTS Times don’t tell
PNC the units of time to be allocated. If the
PNC allocates more than the minimum but
less than the desired, it needs to know what
increment of time to allocate
• Replace Minimum and Desired GTS time with
GTS time Unit, Minimum GTS time units and
Desired GTS time units
Submission
Slide 11
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Channel Time Requests and GTS slots
for extremely low latency applications
• Voice is probably the most latency sensitive
application – as low as 10 ms for toll quality
voice
• Depending on the superframe duration,
Multiple GTS slots per superframe could be
required to meet the target latency
• If multiple slots per SF are required, a DEV
needs to be able to request multiple slots per
SF, where the PNC attempts to spread them
out as smoothly as possible
Submission
Slide 12
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Channel Time Requests and GTS slots
for Sub rate slot allocations
• On the other hand, some applications
may require slots less often than once
per superframe.
• A DEV needs to be able to ask for a slot
every n superframes
Submission
Slide 13
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Solution: CTR Type/Interval
• The value of the CTR Interval and CTR
Type fields determine how often the
GTS should occur.
– If CTR type is one, the GTS slots should
occur every CTR Interval superframes.
– If CTR type is zero, there should be CTR
Interval slots per superframe.
Submission
Slide 14
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
CTRB Comparison
Baseline CTRB (from Raju’s doc)
octets: 1
1
2
2
2
2
Target
DEVID
Stream
Index
Allocation
Period
Minimum
GTS
Time
Desired GTS
Time
Maximum
Allocation
Delay
Proposed CTRB
bits: 8
8
1
15
16
8
8
Target
DEVID
Stream
Index
CTR
Interval
Type
CTR
Interval
CTR
Time Unit
Minimum
CTR Time
Units
Desired
CTR Time
Units
Submission
Slide 15
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Limiting SF Duration (Comments 792, 1491)
• Baseline slot resolution is 8 s. This is 800
bits at 100 Mbps
• Currently superframe duration is limited to
100 ms.
• Changing the resolution to 1 s will limits the
superframe duration to 65.535 ms which will
reduce the potential delay variation, allow
more efficient slot assignments and more
efficiently use the 16 bits of resolution
Submission
Slide 16
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Recommendation Summary
• PNC chooses SF duration
• Adopt new CTRB parameters
– DEV chooses CTR Time unit
– DEV requests Minimum and Desired number of time units
– DEV requests CTR Interval (slots per superframe or
superframes per slot) instead of Allocation Period
– Maximum Allocation delay deleted
• DEV requests enough time for retransmissions
• Jitter Buffer used to smooth out the delay variation
• Change CTR and CTA, and piconet synchronization
time parameters to 1 s resolution instead of 8 s
Submission
Slide 17
Dr. William Shvodian
Feb. 2002
doc.: IEEE 802.15-02/076r2
Conclusions
• Delay variation is limited by superframe
size and is minimized by a smoothing
buffer at the receiver
• New CTRB parameters greatly simplify
the PNC’s slot request/allocation
processing
Submission
Slide 18
Dr. William Shvodian