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

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

3. 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

4. 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 MAC
Consider a small change to allow retransmitting failed GTS frames in CAP

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Simulation settings Platform: OPNET 11.0 Simulated CAS schemes
Standard IEEE 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

13. 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

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

15. 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.

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

17. 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 = frames/ sec/ node

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

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

20. 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