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Doc.: IEEE 802.11-01/144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 An E-DCF Proposal Using TCMA Mathilde Benveniste AT&T Labs,

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Presentation on theme: "Doc.: IEEE 802.11-01/144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 An E-DCF Proposal Using TCMA Mathilde Benveniste AT&T Labs,"— Presentation transcript:

1 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 An E-DCF Proposal Using TCMA Mathilde Benveniste AT&T Labs, Research Relevant submissions: IEEE /375 (.ppt and.doc); -00/456; -00/457; -01/002; -01/004; -01/019; -01/117

2 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 2 EDCF proposed features Basic Contention Resolution: Backoff A 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 Traffic AP updates CWSize as needed Enable finer CWSize adjustment upon retrial Elimination of stale packets Packets can be discarded based on age limit that depends on class

3 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 3 EDCF Parameter Set element CPM urgency class index for contention priority TxOp Limit limit on transmission duration (μsec) UCI i contains parameters defining Urgency Class i Element IDLengthCPMUCI 0 UCI 1 UCI 2 UCI 3 TxOp Limit Octets: ASC i CWPFactor i CWSize i TLT i Octets: ASC i arbitration slot count CWPFactor i contention window persistence factor (1/16) CWSize i contention window size T LT i transmit lifetime (TU)

4 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 4 Priority to Urgency Class mapping There are 4 urgency classes, 0, …, 3 [another number can be specified, if desired] IEEE E allows ten values: the integers between and including 0 and 7 as well as the values allowed by IEEE IEEE allows two values: Contention or ContentionFree. CPM, the urgency class index for priority Contention, is specified in the EDCF Parameter Set Element.

5 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 5 Example The backoff value m at T 0 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,... UAT i = aSIFSTime + aASC i x aSlotTime aASC i = arbitration slot count for class i Time Node C Node B Node A transmission time slot UAT End of last transmission T0T0

6 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 6 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 aASC i >=2 (UAT>=DIFS ), and X=0 For classes with urgency above legacy aASC i = 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 whereX = 0for all STAs and ESTAs with ASC i >1 X = 1for ESTAs with ASC i =1

7 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 7 Slow Adaptation to Traffic (SAT) Using CWSize i 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 CWSize i, ESTAs set their CWSize i 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 factor The new CW used for retrial of a failed transmission, is obtained by multiplication with the persistence factor aCWPFactor i. new CW i = ((current CW i + 1) x (aCWPFactor i /16) - 1 SAT, which chooses a window appropriate for current traffic, obviates the need for large retry adjustments. High urgency classes benefit from smaller aCWPFactor i values. One can still use the aCWPFactor i value effective now, 2 x 16.

8 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 8 Removal of Stale Packets Time-sensitive packets are obsolete if delayed excessively Discarding excessively aged packets relieves congestion without impact on QoS The TLT i (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. TLT i = 0 indicates no restriction

9 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 9 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 CHANNEL TRANSMISSIONS

10 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 10 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)

11 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 11 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

12 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 12 Scenario I: Traffic Poisson arrivals Start at T=0 Start at T=30 Start at T=60 6 Mbps 9 Mbps 3 Mbps Frames size bytes Includes 192 microseconds of PHY overhead Independent packet arrivals Exponential inter-arrival times

13 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 13 Scenario I: Class Description * Factor CWPFactor multiplies CWSize before retrial

14 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 14 DCF TCMA Throughput (bits/sec) Dropped Packets (bits/sec) Delay (sec) Scenario I: Simulation Results -- TCMA vs DCF

15 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 15 Scenario I: Simulation Results --TCMA CWPFactors CWPFactor0=2 CWPFactor0=1.5 Throughput (bits/sec) Class 0 Delay (sec)

16 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 16 Scenario II: Traffic -- 1 Low Priority and 17 Voice calls Traffic Voice Kbps per call Fixed arrivals Frame size* bytes Low Priority - 3,318 Kbps Frame size* - 1,728 bytes Fixed arrivals 12 ms ON/88 ms OFF WLAN Parameters DS, 11 Mbps channel buffer size=2.024 Mbits; no fragmentation RTS/CTS suppressed; max retry limit=7 * Includes 192 microsec PHY overhead

17 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 17 Scenario II: Offered Load Load (bits/sec) Total Voice Video Call Volume vs Time - No PHY Overhead Time (sec) Load (bits/sec) Total Voice Low Call Volume vs Time - With PHY Overhead Time (sec) Load (bits/sec)

18 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 18 Scenario II: Simulation Results -- TCMA vs DCF Average End to End Delay - DCF Time (sec) End to End Delay (sec) Average End to End Delay - TCMA Time (sec) End to End Delay (sec) Voice Calls Low Priority Voice Calls Low Priority

19 doc.: IEEE /144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 19 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


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