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CCR: A Novel MAC Scheme with Constant-Time Contention Resolution for WLAN Zakhia G. Abichar and J. Morris Chang Computer Systems Laboratory Dept. of Electrical and Computer Engineering Iowa State University Ames, IA 50011 December 10, 2004
Motivation • WLAN ubiquitously employed • DCF: Basic access scheme • Used for PCF and the proposed HCF • Based on the CSMA/CA algorithm • Contention Window (CW) management based on the binary exponential backoff (BEB) algorithm • High collision rate for a large network size • CCR addresses the issue of high-collision rate in DCF
Objective • Propose a scheme that: • Is distributed • Exhibits a low collision rate (even at large network sizes) • Resolves contention in a constant time
Outline • Overview on Wireless MAC Protocols • The IEEE 802.11 Standard • The CCR Scheme • Mathematical Analysis • Performance Evaluation
Overview • Wireless networks • Shared medium • Basic MAC scheme • The most delicate component of wireless network architecture
Overview • Research in wireless MAC schemes started in the 1970s: • Pure ALOHA • A station transmits whenever a packet is ready • No carrier-sense • Medium utilization: 18% • Slotted ALOHA • Time is divided into discrete slots • Transmissions occur only at slot boundary • Medium utilization: 36% • CSMA protocols: MACA, CSMA/CA • Carrier-sense capability • Avoid colliding with ongoing transmissions • Use of RTS/CTS
Blackburst scheme Overview • Wireless MAC schemes (cont’d) • Fast Collision Resolution (FCR) • Allowing bursts of packets • Issue of fairness • Applying SCFQ (Self-Clocked Fair Queuing) algorithm to mitigate the fairness metric • Blackburst • Adopted in HIPERLAN • STAs jam the medium to indicate their delayed time • Low throughput for large network size
Overview • CCR is based on the Binary Countdown Mechanism • Contention runs for d slots • d is the number of bits in stations’ address • A station jams for a ‘1’ and senses for a ‘0’ • Station with higher address always wins • Based on a mathematical analysis CCR determines: • A proper number of contention slots • Optimal probability of choosing ‘1’ in a given time slot
Overview • Binary tree schemes • Close to the binary countdown mechanism • Allow collisions to occur and then split colliding stations into two groups • Not a good idea for wireless networks since collisions cannot be detected • Difficult to apply in wireless networks • Need multi-channel setting and feedback • CCR is free of the above limitations
The IEEE 802.11 Standard • All of the above schemes rely on the efficiency of the CSMA/CA algorithm DCF – Basic access mode CSMA/CA algorithm PCF – Contention-free access mode Alternates with DCF HCF – (under study, not yet standardized) Alternates with DCF
CSMA/CA Algorithm • Two steps to access the medium • Wait for inter-frame space (IFS) to expire • Decrease back-off timer
CSMA/CA Algorithm • Reasonable performance for best-effort packets • Large decline in throughput for a large network size • Most wasted time slots attributed to backoff window slots and collisions E [Nc] Average number of collisions per transmission E [Bc] Average number of back-off slots per transmission ts Value of aSlotTime
CCR Scheme – The Rationale • As shown in the previous slide, most of the wasted time in the DCF scheme is due to: • Collisions (dominant factor) • Backoff slots • The main goal of CCR is to reduce the number of collisions
CCR Scheme • Allows a channel access after a constant number of time slots • Exponentially reduces the number of contending stations every contention slot t = 2 t = 1
1 0 0 1 0 1 1 0 1 0 1 0 CCR Scheme Operation • Each station chooses a random value for its try-bit (stations divided into active group and passive group) • Active group stations jam the medium if they want to transmit while passive group stations must sense the medium • If the medium has been jammed, passive group retires from the contention. Otherwise, passive group remains • Remaining stations refresh their try-bit and new groups are formed • Contention time is of logarithmic complexity Passive Group Active Group
CCR Access Example Low probability for p Higher probability for p – Choose a value of 1 with probability p –Run CCR for k slots
Choosing a try-bit of one with probability ½ is not the best choice Better have a low value at the early stages of the contention, then a higher value later on Contending stations need to know the number of time slots to run CCR Run the scheme for the same number of slots for 10, 50, 100 stations CCR Scheme – In Practice
Mathematical Analysis • Objective of the analysis, threefold: • For how many contention slots to run CCR? • Choosing a value of one for the try-bit with what probability? • Estimate the throughput of CCR
Try-bit equals to one with proba. p • Probability that i stations move to the next contention slot: • Expected value of i:
Number of Contention Slots • Number of contending stations converges to one after 3 time slots • Perfect knowledge assumed (figure 4) • Simulation studies: 6 slots are used with probabilities: p = [0.07, 0.2, 0.25, 0.33, 0.4, 0.5] Note: Values of p remain unchanged when the number of stations varies
Throughput Estimation • Transmission interval • Proba. of a transmission (Markov Chain) • Proba. any STA transmits:ps = n . t . (1 – t)n-1 • Proba. of a collision: k: number of slots Note: collision is the opposite of successful transmission plus the nothing to transmit.
Throughput Estimation • In a transmission interval, there could be several collisions and one successful transmission • Mean length of an interval: • Where: p [Nc = j] = (pc)j . ps • Throughput is given by: where Ts is the transmission time of a packet. j: number of collisions
Throughput Estimation • From the model, the throughput of CCR reaches 95%
Throughput at 2 Mbps • Throughput of CCR reaches: 92%, 91%, 90% • Throughput of DCF reaches:82%, 66%, 58%
Throughput at 11 Mbps • Throughput of CCR reaches: 87%, 86%, 85% • Throughput of DCF reaches:79%, 64%, 57%
Throughput Observation • At low packet sizes and small network size (~10 STAs): • DCF outperforms CCR • Average access time of CCR is larger than DCF’s • The lower collision rate of CCR does not materialize • Packet sizes are small • Observation holds for packets: • Smaller than 50 bytes at 2 Mbps, (300 mirco-s) • Smaller than 475 bytes at 11 Mbps, (300 micro-s)
Collision Rates • Collision rate of CCR ranges from: 4.37% to 6.47% • Collision rate of DCF ranges from: 16% to 40% • Main reason why CCR outperforms DCF • The high collision rate of DCF impedes other metrics, i.e. throughput, delay, fairness
Fairness Metric • Longer time range for DCF to exhibit fairness • In the short-run, colliding stations are penalized
Questions? Thank You