Zafer Sahinoglu, Ghulam Bhatti, Anil Mehta Low Latency Channel Access Scheme for Time Critical Applications A modified IEEE 802.15.4 Super-frame structure by Zafer Sahinoglu, Ghulam Bhatti, Anil Mehta

Ultra-reliable Wireless Network Motivation Current IEEE 802.15.4 Channel Access Scheme (CAS) Improved CAS Approach Simulation settings Simulation results – latency and reliability Theoretical validation Future work

Current IEEE 802.15.4 Channel Access Scheme (CAS) beacon Superframe interval beacon Beacon interval GTS CAP CFP Inactive CAP GTS GTS GTS ~= Beacon Interval DATA ACK This is latency only for one hop You can at the earliest transmit here Single failure in GTS frame transmission results in large delay Once failed, there is no way to re-transmit in CAP of SAME active region

Current IEEE 802.15.4 CAS – with retransmissions in CAP beacon Superframe interval beacon Beacon interval GTS CAP CFP Inactive CAP GTS GTS GTS ~= Beacon Interval DATA ACK This is latency only for one hop Allowing retransmissions in CAP in 802.15.4 MAC Consider a small change to allow retransmitting failed GTS frames in CAP

Improved CAS Approach Superframe interval beacon Beacon interval beacon GTS GTS CFP CAP Listen CFP CAP GTS GTS GTS GTS DATA ACK DATA ACK DATA ACK 3.84 ms 1st retransmission in the CAP 2nd retransmission (successful) Consider a small change to allow retransmitting failed GTS frames in CAP Now lets flip ‘CFP’ and ‘CAP’ regions

Improved CAS Approach Superframe interval beacon Beacon interval beacon 33.06 ms GTS GTS CFP CAP Listen CFP CAP GTS GTS GTS GTS DATA ACK DATA ACK DATA ACK 3.84 ms 1st retransmission in the CAP 2nd retransmission (successful) Two suggested modifications for reduction in latency and increase in reliability are Allow for retransmissions in CAP FLIP CFP and CAP

Extended GTS 1st retransmission CFP for retries GTS GTS CFP CAP Listen CFP CAP GTS GTS GTS GTS DATA ACK DATA ACK 1st retransmission Dynamically allocate new GTS slots to nodes with failed GTS transmissions Use ‘GACK’ frame at end of every CFP period to maintain sync Provides 2 collision free and 1 contention based transmission period

Class of CAS schemes studied CAS with no GTS retransmission in CAP CAS with GTS retransmissions in CAP CAS with XGTS and GTS retransmissions in CAP All above schemes drop a GTS frame if it has failed transmission for one Super-Frame and a new GTS frame awaits transmission We study GTS transmission delay vs. CSMA load; Channel error probability GTS frame drop vs. CSMA load; Channel error probability

Theoretical Analysis – Variables Defined Δ - average GTS frame transmission delay Pe – average channel packet error; we keep it constant β – Probability of Collision free transmission (includes probability of successful channel access) γ – Probability of successfully transmitting a frame in CAP which starts competing for channel at beginning of CAP ζ – average maximum number of transmission attempts for a frame in a CAP BI – length of the Beacon Interval in seconds ε – GTS frame transmission time, including ack frame reception time and L/SIFS δ – average delay for sending a GTS frame in CSMA period λGTS – average frame arrival rate for GTS frames per node Q1 – Probability of number of frames in queue ≤ 1

Theoretical Analysis – GTS Frame Transmission Delay Transmission Delay for scheme with no CAP retransmission Transmission Delay for scheme with CAP retransmission Transmission delay with packet drop included

Theoretical Analysis – GTS Frame Drop Packet loss rate for schemes without retransmission of GTS frames in CAP Packet loss rate for schemes with retransmission of GTS frames in CAP

Simulation settings Platform: OPNET 11.0 Simulated CAS schemes Standard IEEE 802.15.4 MAC After swapping CFP and CAP periods After enabling GTS retransmissions in CAP period (2) and (3) combined Key assumptions: Arrivals are Poisson distributed All packets have equal length If a new GTS frame arrives before retransmission of a GTS frame, the retransmission is cancelled and the frame is dropped Long buffers to prevent buffer overflow

Simulation settings WPAN Settings: Beacon Order = 5, Superframe Order = 3 Star network 27 End Nodes and 1 PAN Coordinator Node All 7 GTS allocated to 7 of the 27 nodes, hybrid nodes GTS and CSMA traffic sources are independent All traffic is ‘acked’ CSMA/CA Setting Minimum Backoff Exponent – [2 - 5] Maximum Backoff Number – 4 CCA Window – 2 Max Frame Retries – 3

Simulation results – GTS transmission delay vs CSMA load less CSMA load implies HIGHER GTS latency for Standard 802.15.4 MAC with retransmissions λGTS = 0.5 frames /sec /node, Pe = 0.1

Simulation results – GTS frame drop rate vs CSMA load λGTS = 0.5 frames /sec /node, Pe = 0.1 Extended GTS shows dedicated slots provide guaranteed results than leaving re-transmission to CAP period.

Probability of Channel Error vs GTS drop rate λGTS = 0.5 frames /sec /node and λCSMA load = 0.125 frames/ sec/ node

Simulation results – Probability of channel error vs Simulation results – Probability of channel error vs GTS Transmission Delay λGTS = 0.5 frames /sec /node and λCSMA load = 0.125 frames/ sec/ node

Simulation results – CSMA Queue Size λGTS = 0.5 frames /sec /node, Pe = 0.1

Simulation results – CSMA Transmission Delay λGTS = 0.5 frames /sec /node, Pe = 0.1

Salient Features of Extended GTS scheme Major reduction in GTS transmission delay Significant reduction in GTS frame drop rate GTS drop rate and transmission delay nearly independent of CSMA load Equal or better performance in increasing channel error than other schemes Tolerable increase in CSMA queue size and queuing delay due to resource re-allocation