Bluetooth Scheduling Reference Enhancing performance of asynchronous data

Bluetooth: Scheduling Reference: “Enhancing performance of asynchronous data traffic over the Bluetooth wireless ad-hoc network”; Das, A. ; Ghose, A. ; Razdan, A. ; Saran, H. ; Shorey, R. ; Proceedings, IEEE INFOCOM 2001, pp. 591 -600 (BTSche-2. pdf)

Introduction • Contribution – Propose two SAR policies with the aim of increasing link utilization and decreasing end-toend delay of data packets – MAC scheduling algorithm are needed to achieve fair sharing of bandwidth, high link utilization and low queue occupancy – Demonstrate that Round-Robin scheduling is unable to meet these requirements and proposed three new scheduling algorithms – Investigate the performance improvement provided by FEC and ARQ schemes – Compare the performance of different versions of TCP over Bluetooth 2

Bluetooth Protocol Stack 3

Design Issues in BT • 1. Segmentation and Reassembly schemes – Supporting a maximum transmission unit (MTU) size larger than the largest baseband packet – Note that the payload size (without FEC): Ø 5 slot packet: 339 bytes (67. 8 bytes/slot) Ø 3 slot packet: 183 bytes (61 bytes/slot) Ø 1 slot packet: 27 bytes – Define slot_limit as the maximum # of slots across which a baseband packet can be sent Ø May be less than 5 due to presence of SCO connections or due to very high bit error rate in the wireless channel Ø This parameter can be conveyed by the LMP to the L 2 CAP through a signaling packet 4

Design Issues in BT (cont) • Two SAR schemes – SAR-Best Fit (BF) – SAR-Optimum Slot Utilization (OSU) 5

Design Issues in BT (cont) • Scheduling algorithms in BT – Master-driven Round-Robin scheduling Ø Achieve fair sharing of bandwidth and high link utilization when each such connection has equal data flow Ø However, each slave in the piconet has varying data input rates Ø Consequently, numerous baseband slots are wasted by polling sources with low input rate, thereby decreasing link utilization, increasing queuing delay and leading to unfair sharing bandwidth 6

Design Issues in BT (cont) • Two basic methods for scheduling – 1. Queue Priority based on Flow Bit Ø Assign priority to per-slave baseband queues at the master (similar queues at the slave) based on the pending data in the L 2 CAP buffers, and use the flow bit field for this purpose Ø The flow bit is set when the number of packets in the L 2 CAP buffer for a particular slave is larger than a threshold buf_thresh Ø At the master, variable flow to quantify the traffic rate on the wireless channel, which is set when the flow bits for packets traveling in either direction is turned on 7

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Design Issues in BT (cont) – 2. Queue Stickness Ø To reduce mean queue occupancy, propose to transmit a number of baseband packets successively (quantified by a parameter num_sticky) for each queue having the flow parameter set • Three scheduling algorithms – AFP, Sticky. AFP 9

Design Issues in BT (cont) • 1. Adaptive Flow-based Polling (AFP) – Define P 0 the initial polling interval – AFP uses an adaptive polling interval P, whose value is changed based on the traffic rate Ø 1. If flow = 1 and the HOL packet is a data packet, transmit the data packet and set the polling interval P to P 0 (High rate for this slave) Ø 2. If flow = 0 and the HOL is a data packet, transmit the data packet and keep the polling interval unchanged Ø 3. If a polling packet is transmitted and a null packet is received, double the current polling interval P unless a threshold value Pthresh is reached 10

Design Issues in BT (cont) • 2. Sticky – 1. If flow = 1, a maximum of num_sticky packets are transmitted for that queue – 2. If flow = 0, one packet is transmitted for that queue, as in Round-Robin scheduling • 3. Sticky AFP – Similar to AFP except that when flow = 1 and the HOL packet is a data packet, a maximum of num_sticky packets are transmitted for that queue 11

Design Issues in BT (cont) • Error Handling – For ARQ scheme, quantify timeout by a maximum number of retransmissions tx_thresh – Using two state Markov channel model – Study the effect of using FEC on the baseband payload and of varying the parameter tx_thresh in the ARQ scheme on the data throughput – Propose Channel State Dependent Packet (CSDP) scheduling Ø Upon encountering a packet loss (NACK), CSDP policies defer the retransmissions to that slave till the next polling instant 12

Design Issues in BT (cont) • Number SCO connections – The master sends SCO packets at regular intervals, the so-called SCO intervals TSCO (counted in slots) – In the presence of one SCO link, 1 and 3 slot ACL packets can be supported • TCP variants – TCP Tahoe, Reno, New Reno, Sack 13

Simulation Model 14

Simulation Model (cont) • Performance Metrics – Throughput, end-to-end delay, link utilization • Fading Channel Model r. G= 437. 5 ms 6. 879 x 10 -5 r. B= 55. 8 ms 1. 263 x 10 -3 15

Simulation Results • Performance evaluation: SAR schemes – Use Round-Robin scheduling at the MAC level – Assume an error-free channel – Compare: SAR-BF vs. SAR-OSU – Figures 5 ~ 7 for slave 1 16

Simulation Results (cont) For slave 1 17

Simulation Results (cont) For slave 1 18

Simulation Results (cont) 19

Simulation Results (cont) • Observations – 1. Higher throughput, higher overall link utilization, and lower end-to-end delays can be obtained by using SAR-OSU over SAR-BF – 2. The frequent fluctuations are due to the bursty sources (simulated by intermittent CBR traffic) – 3. Since two TCP connections are active between 10 s ~ 20 s, fair sharing of bandwidth leads to a drop in the individual throughput of TCP connection to slave 1, while the overall link utilization is seen to increase 20

Simulation Results (cont) • L 2 CAP Buffer size – Use Round-Robin scheduling and assume an error-free channel – Figures 8 & 9 – Observations Ø 1. The average TCP throughput becomes almost constant for a buffer size greater than four for the persistent TCP connection Ø 2. Conclude that a buffer size of four to six will optimally satisfy the memory requirements of a generic BT device (set L 2 CAP buffer size = 5 ) 21

Simulation Results (cont) Long TCP: slave 1 Short TCP: slave 2 22

Simulation Results (cont) 23

Simulation Results (cont) • Scheduling Algorithms – Figure 10 Ø AFP and Sticky algorithms give significantly improved performance compared to RR Ø The throughput of Sticky increases with increase in the value of num_sticky and is approximately the same as AFP for num_sticky=16 – Figure 11 Ø The throughput of Sticky. AFP with num_sticky=16 is better than that of AFP and Sticky. AFP with num_sticky=4, but not very significant – Figure 12 Ø High link utilization is obtained for Sticky. AFP (num_sticky=16), Sticky (num_sticky=16) and AFP, as compared to Round-Robin 24

Simulation Results (cont) AFP, Sticky(16) Sticky(4) Sticky(2) RR For slave 1 25

Simulation Results (cont) Sticky. AFP (16) AFP, Sticky. AFP(4) For slave 1 26

Simulation Results (cont) Sticky. AFP(16) AFP Sticky(16) RR 27

Simulation Results (cont) – Figure 13 Ø The Sticky algorithm is found to have the lowest endto-end delay while Sticky. AFP has the highest Ø 1. By increasing the polling interval, AFP decreases the number of poll packets (for those queues that have less data) which otherwise cause underutilization of available bandwidth, and hence increases link utilization Ø 2. Sticky reduces queue occupancy by transmitting multiple packets consecutively from queues with a high backlog, hence preventing queue overflow and reducing end-to-end delay 28

Simulation Results (cont) Ø 3. Sticky. AFP causes a marked increase in the end-toend delay of intermittent CBR traffic because flow is set infrequently for such bursty sources Additionally, each cycle has a larger duration due to other slaves being served num_sticky times – Authors infer that AFP and Sticky(16) result in the best overall performance 29

Simulation Results (cont) Sticky. AFP(16) RR AFP Sticky(16) 30

Simulation Results (cont) • Effect of Error Correction Schemes – For different values of tx_thresh (max. # of retransmissions of baseband packets) – AFP vs. CSDP-AFP (w/ and w/o FEC) – Observations (Figures 14, 15, 16) Ø Performance degradation in the presence of errors Ø Additional reduction in link utilization and increase in end-to-end delay due to the use of FEC Ø When FEC is added, the performance is independent of tx_thresh, and hence of the ARQ scheme 31

Simulation Results (cont) Ø ARQ leads to efficient error recovery and for values of tx_thresh > 4, the performance does not vary significantly set tx_thresh = 5 Ø Figure 16, CSDP versions of the proposed scheduling algorithms do not give a significant performance improvement and their relative performance is the same as that in the error-free channel condition Ø CSDP versions do not improve the performance in the presence of a link level ARQ scheme 32

Simulation Results (cont) AFP (error-free channel) CSDP-AFP, AFP CSDP-AFP (with FEC) 33

Simulation Results (cont) CSDP-AFP (with FEC) CSDP-AFP, AFP (error-free channel) 34

Simulation Results (cont) CSDP-Sticky. AFP(16) CSDP-AFP, AFP CSDP-Sticky(16) 35

Simulation Results (cont) • Varying voice connections – Using AFP – Figures 17 & 18 – Observations Ø The throughput decreases and end-to-end delay increases as the number of SCO connections increase Ø Higher throughput and lower end-to-to-end delay is obtained for slot_limit = 3 than for slot_limit = 1 36

Simulation Results (cont) NSCO=0, slot_limit=5 TSCO=6, NSCO=1, slot_limit=3 37

Simulation Results (cont) TSCO=6, NSCO=2, slot_limit=1 TSCO=4, NSCO=1, slot_limit=1 TSCO=6, NSCO=1, slot_limit=3 NSCO=0, slot_limit=5 38

Simulation Results (cont) • TCP variants (Fig. 19) – The difference in throughput is insignificant which clearly illustrates that the efficient link layer ARQ scheme of BT eliminates the need for modifications at the transport layer for error recovery 39

Simulation Results (cont) 40