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15-08-0044-00-004e. 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. 15-08-0044-00-004e. Ultra-reliable Wireless Network. Motivation

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15-08-0044-00-004e


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slide1

15-08-0044-00-004e

Low Latency Channel Access Scheme for Time Critical ApplicationsA modified IEEE 802.15.4 Super-frame structure

by

Zafer Sahinoglu, Ghulam Bhatti, Anil Mehta

ultra reliable wireless network

15-08-0044-00-004e

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
Current IEEE 802.15.4 Channel Access Scheme (CAS)

beacon

Superframe interval

beacon

Beacon interval

GTS

Inactive

CFP

CAP

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
Current IEEE 802.15.4 CAS – with retransmissions in CAP

beacon

Superframe interval

beacon

Beacon interval

GTS

Inactive

CFP

CAP

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
Improved CAS Approach

Superframe interval

beacon

Beacon interval

beacon

GTS

GTS

Listen

CFP

CAP

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 approach1
Improved CAS Approach

Superframe interval

beacon

Beacon interval

beacon

33.06 ms

GTS

GTS

Listen

CFP

CAP

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
Extended GTS

Extended

CFP for retries

GTS

GTS

Listen

CFP

CAP

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
cas schemes studied
CAS schemes studied
  • 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
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
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
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
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 settings1
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
Simulation results – GTS transmission delay vs CSMA load

less CSMA loadimplies 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
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
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 gts transmission delay
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
Simulation results – CSMA Queue Size

λGTS = 0.5 frames /sec /node, Pe = 0.1

simulation results csma transmission delay
Simulation results – CSMA Transmission Delay

λGTS = 0.5 frames /sec /node, Pe = 0.1

salient features of extended gts scheme
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