1 / 19

An E-DCF Proposal Using TCMA

An E-DCF Proposal Using TCMA. Mathilde Benveniste AT&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. EDCF proposed features. Basic Contention Resolution: Backoff

jodie
Télécharger la présentation

An E-DCF Proposal Using TCMA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. An E-DCF Proposal Using TCMA Mathilde Benveniste AT&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 Mathilde Benveniste, AT&T Labs - Research

  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 Mathilde Benveniste, AT&T Labs - Research

  3. Element ID Length CPM TxOp Limit UCI0 UCI1 UCI2 UCI3 24 ASCi CWPFactori CWSizei TLTi EDCF Parameter Set element CPMurgency class index for “contention” priority TxOp Limitlimit on transmission duration (μsec) UCIi contains parameters defining Urgency Class i Octets: 1 2 1 1 2 1 1 2 Octets: ASCi arbitration slot count CWPFactori contention window persistence factor (1/16) CWSizei contention window size TLTi transmit lifetime (TU) Mathilde Benveniste, AT&T Labs - Research

  4. 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. Mathilde Benveniste, AT&T Labs - Research

  5. time slot UAT Node C transmission Node B Node A Time T0 End of last transmission 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 i Example The 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) Mathilde Benveniste, AT&T Labs - Research

  6. Backward Compatibility • Backoff Time = (Random() + X) aSlotTime • whereX = 0 for all STAs and ESTAs with ASCi>1 • X = 1 for ESTAs with ASCi =1 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 urgencyabove 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 CWSizei values Mathilde Benveniste, AT&T Labs - Research

  7. 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 factor The 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 valueeffective now, 2 x 16. Mathilde Benveniste, AT&T Labs - Research

  8. Removal of Stale Packets Time-sensitive packets are obsolete if delayed excessively Discarding excessively aged packets relieves congestion without impact on QoS The 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 Mathilde Benveniste, AT&T Labs - Research

  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. Access Buffer Packet Stream to Node A CHANNEL TRANSMISSIONS Packet Stream to Node B Contention for access Packet Stream to Node C Mathilde Benveniste, AT&T Labs - Research

  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) Mathilde Benveniste, AT&T Labs - Research

  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 Mathilde Benveniste, AT&T Labs - Research

  12. Scenario I: Traffic Poisson arrivals 3 Mbps Start at T=0 • Frames size -- 1504 bytes • Includes 192 microseconds of PHY overhead • Independent packet arrivals • Exponential inter-arrival times 6 Mbps Start at T=30 9 Mbps Start at T=60 Mathilde Benveniste, AT&T Labs - Research

  13. Scenario I: Class Description *Factor CWPFactor multiplies CWSize before retrial Mathilde Benveniste, AT&T Labs - Research

  14. Scenario I: Simulation Results -- TCMA vs DCF Throughput (bits/sec) Dropped Packets (bits/sec) Delay (sec) TCMA DCF Mathilde Benveniste, AT&T Labs - Research

  15. Throughput (bits/sec) Class 0 Delay (sec) CWPFactor0=1.5 CWPFactor0=2 Scenario I: Simulation Results --TCMA CWPFactors Mathilde Benveniste, AT&T Labs - Research

  16. Scenario II: Traffic -- 1 Low Priority and 17 Voice calls Traffic Voice - 285 Kbps per call Fixed arrivals Frame size* - 356 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 Mathilde Benveniste, AT&T Labs - Research

  17. Scenario II: Offered Load Call Volume vs Time - With PHY Overhead Call Volume vs Time - No PHY Overhead 8 8 Total Total 7 7 Voice Voice Video Low 6 6 5 5 4 Load (bits/sec) 4 Load (bits/sec) Load (bits/sec) 3 3 2 2 1 1 0 0 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 Time (sec) Time (sec) Mathilde Benveniste, AT&T Labs - Research

  18. Voice Calls Voice Calls Low Priority Low Priority Scenario II: Simulation Results -- TCMA vs DCF Average End to End Delay - TCMA Average End to End Delay - DCF 9 7 8 6 7 5 6 4 5 End to End Delay (sec) End to End Delay (sec) 4 3 3 2 2 1 1 0 0 0 50 100 150 0 50 100 150 Time (sec) Time (sec) Mathilde Benveniste, AT&T Labs - Research

  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 Mathilde Benveniste, AT&T Labs - Research

More Related