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The System Issues of Rate Adaptation

This article explores the issues of rate adaptation in wireless networks, focusing on the impact on system performance. It discusses the limitations of link-based rate adaptation and the need for system-based rate adaptation. Examples and models are provided to illustrate the importance of optimizing data rates based on network traffic and the interaction between links. The article concludes with suggestions for improving rate adaptation abstraction.

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The System Issues of Rate Adaptation

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  1. The System Issues of Rate Adaptation Hujun Yin hujun_yin@vivato.net Hujun Yin,Vivato

  2. Motivation • Traditionally, rate adaptation is done at link level • Ignoring the impact of other links at MAC • Ignoring the impact of link adaptation on other links • Understand the impact of rate adaptation on system performance • Considering the synergy of multiple links • Identify the issues of link based rate adaptation • Provide insight into system based rate adaptation Hujun Yin,Vivato

  3. Assumptions • Network is operating in infrastructure mode • One AP and multiple clients • Communication only between AP and clients • No multiuser reception capability • If collision happens, at most one packet can be successfully received • No non-802.11 interference • Packet error is only introduced by low SNR or packet collision Hujun Yin,Vivato

  4. Link Based Rate Adaptation • SNR based rate adaptation • Chooses data rate from a rate-SNR lookup table • PER based rate adaptation • Monotonic relation between data rate and PER • Adjust data rate so that PER keeps constant • Decreases data rate when PER raises • Increases data rate when PER drops • Rate adaptation is done from link perspective without considering the impact on system performance Hujun Yin,Vivato

  5. System Performance Criteria • Maximize total throughput under certain constraints • Only consider PER constraint here S : total throughput; N: number of nodes; Ri: data rate of node i; pi: PER of node i; : utilization time of node i; : channel idle time; : channel busy time; p0: PER constraint Hujun Yin,Vivato

  6. System PER Model • The packet error is contributed by two sources: • pe: wireless channel & noise (SNR related) • pc: MAC collisions • pe is a function of data rate and SNR • pc is a function of packet arrival rate and channel access mechanism Hujun Yin,Vivato

  7. Rate Adaptation Criteria Revised • Optimal data rates are not only a function of SNR, but also a function of network traffic • Data rate adaptation is link dependent • Traditional link adaptation only considers SNR is suboptimal • Data rate shall be optimized with regard to interaction between links (collision) Hujun Yin,Vivato

  8. Examples: TDMA MAC • Each node is assigned to unique time slots • No MAC collisions: pc=0 • Links are isolated • Maximize overall throughput equivalents to maximize individual link throughput • Link based rate adaptation is optimal Hujun Yin,Vivato

  9. Examples: ALOHA Random Access • Total throughput • Probability of collision • If N>>1 nodes are identical with aggregated traffic rate l and data rate R • Pc decreases with data rate R • pe increases with data rate R • PER=pe+pc is no longer a monotonic function of rate R Hujun Yin,Vivato

  10. Instability of Link PER Based Rate Adaptation • Consider two rates: • R1=R, pe1=10% • R2=R/10, pe2=0 • Network operates at 20% PER • Pc=10%, l=R/10, S=0.081R • pe1->15% • Switch to R2: pe2=0, pc=63%, PER=63%, S=0.037R • Stay at R1:pe1=15%, pc=10%, PER=24%, S=0.081R • Rate adaptation to improve PHY PER may result in much worse system performance Hujun Yin,Vivato

  11. Tx Starts Sensing Packet 2 Ack Packet 1 DIFS SIFS DIFS + Backoff DIFS: Distributed Interframe Space SIFS: Short Interframe Space Examples: 802.11 DCF MAC • CSMA/CA • Exponential Backoff Hujun Yin,Vivato

  12. 802.11 Packet Collision Probability • 802.11a initial backoff window only has 16 slots • The probability of collision is close to 10% with two saturated nodes • The probability of collision is close to 30% with 5 saturated nodes* • The PER caused by packet collision can not be ignored in CSMA based MAC like 802.11a • Rate adaptation in 802.11 is an involved system problem** • High packet collision probability • Variable packet size *G. Bianchi, ”Performance analysis of the IEEE 802.11 distributed coordination function”, IEEE JSAC, March 2000. ** H. Yin, “Multirate 802.11a Networks: System Performance Evaluation”, CISS, March 2002. Hujun Yin,Vivato

  13. Conclusion • Link adaptation depends on: • Channel condition (SNR) • Activity of other links • Rate adaptation considering PHY only is sub-optimal in random access MAC • Rate adaptation in PHY simulation may provide biased results to random access MAC • Limiting PHY PER at low level may be artificial if the nature of MAC contention is not considered • Data rate adaptation has to be considered at system level • The black-box approach can not provide accurate PHY-MAC interface abstraction for all possible MAC proposals Hujun Yin,Vivato

  14. Ways to Improve Rate Adaptation Abstraction • Partition rate adaptation • Coarse rate adaptation at MAC • Deals the network aspect of rate adaptation • Fine rate adaptation at PHY • Deals the channel aspect of rate adaptation • Link adaptation localization • Other links only impacted if the rate of one link varies significantly • Coarse adaptation at MAC sets constraints on the fine rate adaptation at PHY Hujun Yin,Vivato

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