doc.: ieee 802.11-01/144 submission march 2001 mathilde benveniste, at&t labs - researchslide 1...
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March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 1
doc.: IEEE 802.11-01/144
Submission
An E-DCF Proposal Using TCMA
Mathilde BenvenisteAT&T Labs, Research
Relevant submissions: IEEE 802.11-00/375 (.ppt and .doc); -00/456; -00/457; -01/002; -01/004; -01/019; -01/117
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 2
doc.: IEEE 802.11-01/144
Submission
EDCF proposed features
Basic Contention Resolution: BackoffA backoff counter drawn from a random distribution [0, CWsize]
Class Differentiation
By Arbitration Time, UAT The waiting time to start countdown after a transmission (DIFS for legacy)
By Retrial Persistence Factor, CWPFactor The coefficient multiplying the contention window size on retransmission
By Transmit Lifetime Limit, TLT The maximum time allowed to transmit a packet
[When legacy stations are present, for priorities above those assigned to legacy]
By Contention Window size, CWSize
Adaptation to TrafficAP updates CWSize as neededEnable finer CWSize adjustment upon retrial
Elimination of stale packetsPackets can be discarded based on age limit that depends on class
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 3
doc.: IEEE 802.11-01/144
Submission
EDCF Parameter Set element
CPM urgency class index for “contention” priority
TxOp Limit limit on transmission duration (μsec)
UCIi contains parameters defining Urgency Class i
Element ID
Length
CPM UCI0 UCI1 UCI2 UCI3TxOp Limit
Octets: 1 1 1 242
ASCi CWPFactori CWSizei TLTi
1 1 2 2Octets:
ASCi arbitration slot count
CWPFactori contention window persistence factor (1/16)
CWSizei contention window size
TLTi transmit lifetime (TU)
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 4
doc.: IEEE 802.11-01/144
Submission
Priority to Urgency Class mapping
There are 4 urgency classes, 0, …, 3 [another number can be specified, if desired]
IEEE 802.11E allows ten values: the integers between and including 0 and 7 as well as the values allowed by IEEE 802.11.
IEEE 802.11 allows two values: “Contention” or “ContentionFree”.
CPM, the urgency class index for priority “Contention”, is specified in the
EDCF Parameter Set Element.
Priority Value Urgency Class
1 0
2 0
0 (Default) 1
3 1
4 2
5 2
6 3
7 3
Contention CPM
ContentionFree Not mapped since f rames withthis priority are not
transmitted during thecontention period.
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 5
doc.: IEEE 802.11-01/144
Submission
ExampleThe backoff value m at T0 is equal to 1 for all nodes; m(A)= m(B)=m(C)=1
Transmit urgency ranking: (C, B, A); hence, UAT(A) >UAT(B)>UAT(C)
Differentiation by the Arbitration Time
TCMA Protocol Backbone
• Each urgency class is assigned a different urgency arbitration time (UAT) whose length decreases with increasing urgency.
• Arbitration Time = interval that the channel must be sensed idle by a node before decreasing its backoff counter.
For stations with classification i= 0,1,...
UATi = aSIFSTime + aASCi x aSlotTime
aASCi = arbitration slot count for class iTime
Node C
Node B
Node A transmission
time slot
UAT
End of lasttransmission
T0
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 6
doc.: IEEE 802.11-01/144
Submission
Backward Compatibility
In the presence of legacy stations, there can be urgency classes above legacy.UATs are selected as follows:
For classes with urgency below legacy aASCi >=2 (UAT>=DIFS ), and X=0
For classes with urgency above legacy aASCi = 1 (UAT=PIFS), and X=1
Since all residual backoff values are 1 or greater, transmission waiting time >= PIFS+1 x aSlotTime = DIFS
no collisions with PCF or HCF
Multiple classes with urgency above legacy are obtained with different CWSize i values
Backoff Time = (Random() + X) aSlotTime
where X = 0 for all STAs and ESTAs with ASCi>1
X = 1 for ESTAs with ASCi =1
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 7
doc.: IEEE 802.11-01/144
Submission
Slow Adaptation to Traffic (SAT)
Using CWSizei in the EDCF element, the AP may adjust the contention window in response to traffic conditions.
Upon joining a BSS or IBSS, or whenever they detect any change in the advertised values of CWSizei, ESTAs set their CWSizei to the value in the EDCF Element.
The new window is used when a new packet arrives or upon retrial of a failed transmission.
Retrial persistence factorThe new CW used for retrial of a failed transmission, is obtained by multiplication
with the persistence factor aCWPFactori .
new CWi = ((current CWi + 1) x (aCWPFactori /16) - 1
SAT, which chooses a window appropriate for current traffic, obviates the need for large retry adjustments.
High urgency classes benefit from smaller aCWPFactori values.
One can still use the aCWPFactori value effective now, 2 x 16.
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 8
doc.: IEEE 802.11-01/144
Submission
Removal of Stale Packets
Time-sensitive packets are obsolete if delayed excessivelyDiscarding excessively aged packets relieves congestion without impact on QoSThe TLTi (Transmit Lifetime) limit, which is the maximum number of time units (TUs)
allowed to transmit an MSDU, is differentiated by urgency class i. The timer is started when the MSDU enters the MAC.TLTi = 0 indicates no restriction
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 9
doc.: IEEE 802.11-01/144
Submission
Competing Traffic Streams at a Node
Packets generated by stations with multiple traffic types will not be disadvantaged Multiple backoff timers shall be maintained for each class at a node, each adhering
to backoff principles consistent with that class
In case of a tie, the higher urgency packet is selected. The packet not selected
starts a new backoff counter.
Packet Stream to Node A
Packet Stream to Node B
Access Buffer
Packet Stream to Node C
Contention for access
CHANNELTRANSMISSIONS
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 10
doc.: IEEE 802.11-01/144
Submission
Simulation Study Overview
Purpose: Compare performance of E-DCF to DCF
Two traffic patterns were considered: – non-bursty (Poisson arrivals) and– mixed (fixed and ON/OFF)
Features included in simulations• Differentiation by Urgency Arbitration Time (UAT)• … by contention window (CWSize)• … by retrial persistence factor (CWPFactor)
Features not included in simulations• ‘Slow’ adaptation to traffic (SAT)• Differentiation by Transmit Lifetime limit (SAT)
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 11
doc.: IEEE 802.11-01/144
Submission
Scenario I: Network Configuration
• Traffic• 30 stations generate 30 bi-
directional streams;• stream load split 1-to-2
between two directions
• WLAN Parameters• DS, 11 Mbps channel• buffer size=2.0 Mbits; no
fragmentation• RTS/CTS suppressed; max retry
limit=7
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 12
doc.: IEEE 802.11-01/144
Submission
Scenario I: Traffic Poisson arrivals
Packet Size 1504 Bytes Load Load LoadStream Priority Start Traffic IA BPS BPS BPS
Class Time Multiplier Time Seg1 Seg1 Seg11 Top 0 1 0.06016 200000 200000 2000002 Top 0 2 0.03008 400000 400000 4000003 Top 0 1 0.06016 200000 200000 2000004 Top 0 2 0.03008 400000 400000 4000005 Top 0 1 0.06016 200000 200000 2000006 Top 0 2 0.03008 400000 400000 4000007 Medium 0 1 0.06016 200000 200000 2000008 Medium 0 2 0.03008 400000 400000 4000009 Medium 0 1 0.06016 200000 200000 20000010 Medium 0 2 0.03008 400000 400000 40000011 Medium 30 1 0.06016 200000 20000012 Medium 30 2 0.03008 400000 40000013 Low 30 1 0.06016 200000 20000014 Low 30 2 0.03008 400000 40000015 Low 30 1 0.06016 200000 20000016 Low 30 2 0.03008 400000 40000017 Low 30 1 0.06016 200000 20000018 Low 30 2 0.03008 400000 40000019 Low 30 1 0.06016 200000 20000020 Low 30 2 0.03008 400000 40000021 Top 60 1 0.06016 20000022 Top 60 2 0.03008 40000023 Top 60 1 0.06016 20000024 Top 60 2 0.03008 40000025 Medium 60 1 0.06016 20000026 Medium 60 2 0.03008 40000027 Medium 60 1 0.06016 20000028 Medium 60 2 0.03008 40000029 Low 60 1 0.06016 20000030 Low 60 2 0.03008 400000
3000000 6000000 9000000
Star
t at T
=0
Star
t at T
=30
Star
t at T
=60
6 Mbps
9 Mbps
3 Mbps
•Frames size -- 1504 bytes•Includes 192 microseconds of PHY overhead•Independent packet arrivals •Exponential inter-arrival times
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 13
doc.: IEEE 802.11-01/144
Submission
Scenario I: Class Description
ClassI ndex
UAT I nitial BackoffRange
PersistenceFactor*
DCF-- All SI FS+2*TimeSlot [0, 31] 2TCMA-- 0 SI FS +TimeSlot [1, 15] 1.5-- 1 SI FS+2*TimeSlot [0,31] 2-- 2 SI FS+3*TimeSlot [0,31] 2
*Factor CWPFactor multiplies CWSize before retrial
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 14
doc.: IEEE 802.11-01/144
Submission
DC
FTC
MA
Throughput(bits/sec)
Dropped Packets(bits/sec)
Delay(sec)
Scenario I: Simulation Results -- TCMA vs DCF
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 15
doc.: IEEE 802.11-01/144
Submission
Scenario I: Simulation Results --TCMA CWPFactors
CW
PFa
ctor0
=2
CW
PFa
ctor0
=1.
5
Throughput(bits/sec)
Class 0 Delay(sec)
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 16
doc.: IEEE 802.11-01/144
Submission
Scenario II: Traffic -- 1 Low Priority and 17 Voice calls
Call Duration0 20 40 60 80 100 120 140
1
3
5
7
9
11
13
15
17
Cal
l
Time (sec)TrafficVoice - 285 Kbps per callFixed arrivals Frame size* - 356 bytes
Low Priority - 3,318 KbpsFrame size* - 1,728 bytesFixed arrivals 12 ms ON/88 ms OFF
WLAN ParametersDS, 11 Mbps channelbuffer size=2.024 Mbits; no fragmentationRTS/CTS suppressed; max retry limit=7
* Includes 192 microsec PHY overhead
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 17
doc.: IEEE 802.11-01/144
Submission
Scenario II: Offered LoadL
oa
d (
bit
s/s
ec)
Total
Voice
Video
Call Volume vs Time - No PHY Overhead
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (sec)
Lo
ad
(b
its
/se
c)
Total
Voice
Low
Call Volume vs Time - With PHY Overhead
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Time (sec)
Lo
ad
(b
its
/se
c)
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 18
doc.: IEEE 802.11-01/144
Submission
Scenario II: Simulation Results -- TCMA vs DCF
Average End to End Delay - DCF
0
1
2
3
4
5
6
7
0 50 100 150
Time (sec)
En
d t
o E
nd
Del
ay (
sec)
Average End to End Delay - TCMA
0
1
2
3
4
5
6
7
8
9
0 50 100 150
Time (sec)
En
d t
o E
nd
Del
ay (
sec)
Voice Calls
Low Priority
Voice Calls
Low Priority
March 2001
Mathilde Benveniste, AT&T Labs - Research
Slide 19
doc.: IEEE 802.11-01/144
Submission
Conclusions
• TCMA differentiates effectively between classes of different priority– Delay is negligible for top priority traffic; and – remains small under overload conditions
• TCMA achieves greater total throughput – UATs prevent collisions by packets of different priorities in
congestion
• TCMA can coexist with legacy stations