1. Overview of ‘Tiered Contention’ Multiple Access (TCMA)
doc.: IEEE /457
January 2001
Overview of ‘Tiered Contention’ Multiple Access (TCMA)
Mathilde Benveniste
AT&T Labs, Research
Relevant submissions: IEEE /375 (.ppt and .doc); -00/456; -00/457; -01/002; -01/003; -01/004
Mathilde Benveniste, AT&T Labs - Research

2. TCMA features January 2001 Basic Contention Resolution
A backoff counter is drawn from a random distribution (P-DCF, V-DCF)
Class Differentiation
By Initial Backoff Parameters: The parameters (offset, window size) of the starting backoff distribution vary by class (P-DCF, VDCF)
By Arbitration Time: The waiting time to start countdown after a transmission (DIFS for legacy) varies by class (No conflict)
By Retrial Backoff Parameters: The rules to update the backoff distribution following collision vary by class (No conflict)
By Dwell-Time Limit: The maximum time a packet may spend attempting transmission varies by class (No conflict)
Mathilde Benveniste, AT&T Labs - Research

3. TCMA features January 2001 Averting Packet Aging
Urgency Class Upgrade: Packets in certain classes are upgraded dynamically [as their age increases] to reflect their transmission urgency (No conflict)
Packet Expiration Packets are eliminated from queue when queueing times reach class-specific thresholds (No conflict)
Scheduling of Competing Traffic Streams at a Node
Parallel Queues: Class specific queues are maintained at a node with own backoff countdown (No conflict)
Adaptation to Traffic Intensity: Control Mechanism
Distributed: A node (STA or AP) updates traffic from local information as needed
Adaptation to Traffic Intensity: Frequency
On First Backoff: Backoff process is adjusted for traffic on first backoff (P-DCF, V-DCF)
On Retrial Backoff: Backoff process is adjusted on retrial, based on congestion estimates (P-DCF, V-DCF)
Continuous – Backoff Scaling Current backoff counter is scaled up or down if traffic intensity changes substantially
Mathilde Benveniste, AT&T Labs - Research

4. Backoff countdown procedure
January 2001
Backoff countdown procedure
BCUT
BCPT
For legacy stations:
BCPT=DIFS
BCUT=slot time
Example
All nodes are legacy stations with backoff counters m at equal to:
m(A)=3; m(B)=2; m(C)=1
Node C
Node B
Node A
transmission
Time
End of last
transmission
BCPT = backoff counter preparation time
BCUT = backoff counter update time
Mathilde Benveniste, AT&T Labs - Research

5. January 2001
Urgency Arbitration Time (UAT) Differentiation by BCPT Contention resolution through backoff
BCPT = backoff counter preparation time
BCUT = backoff counter update time
Time
Node C
Node B
Node A
transmission
BCUT
BCPT
End of last
TCMA Protocol Backbone
Each urgency class is assigned a different urgency arbitration time (UAT) whose length decreases with increasing urgency.
Arbitration Time = interval that the channel must be sensed idle by a node before decreasing its backoff counter.
In congestion:
Higher urgency packets dominate the channel, as lower urgency packets get less of a chance to transmit - decrease their backoff counters.
Lower urgency packets do not collide with higher urgency packets.
For stations with classification i= 0,1,...
Example
The backoff counter m at is equal to 1 for all nodes; m(A)= m(B)=m(C)=1
Transmit urgency ranking: (C, B, A); hence,
UAT(A) >UAT(B)>UAT(C)
Mathilde Benveniste, AT&T Labs - Research

6. ... Averting Packet Aging January 2001
doc.: IEEE /457
January 2001
Averting Packet Aging
There are nPC=8 priority classes defined for all traffic packets which are permanently assigned to a packet once generated, according to 802.1d Annex H.2
There are nUC=N urgency classes employed for channel contention
The urgency class of a packet is updated in real time so that it
reflects both the priority class of the packet and the speed with which packets of different traffic classes and ages (the time since arrival at the queue) must be transmitted
relies on the performance observed in real time
PC1
PCn
Priority
Classes
Urgency
Traffic
Re-classification
(t2)
UC1
UCm
...
Classification
(t1)
Mathilde Benveniste, AT&T Labs - Research

7. Scheduling of Competing Traffic Streams at a Node
doc.: IEEE /457
January 2001
Scheduling of Competing Traffic Streams at a Node
Parallel queues shall be maintained for each class at a node, each adhering to backoff principles consistent with that class
Packets will change queues when their urgency classifications are adjusted
A packet will leave its queue if its transmission is cancelled due to excessive latency; the age threshold, aAgeLimit, will be class dependent
When a packet’s backoff counter becomes 1 it shall be placed in the node’s access buffer; in case of a tie, the higher urgency packet is selected
Packets generated by stations with multiple traffic types will thus not be disadvantaged
Access Buffer
Packet Stream to Node A
CHANNEL
TRANSMISSIONS
Packet Stream to Node B
Contention
for access
Packet Stream to Node C
Mathilde Benveniste, AT&T Labs - Research

8. Fast adaptation to traffic intensity
January 2001
Fast adaptation to traffic intensity
Bayesian stabilisation: the backoff counter is scaled up or down based on collisions or success feedback
Accounts for PCF, TCS/CTS reservation, ACKs
The age of packets is thus respected
Leading to low latency jitter, which is desirable for real-time and isochronous traffic
Example: Backoff counters (1, 1, 1, 2, 2, 3) become : (1, 2, 1, 3, 4, 5) after drawing random numbers from the range [0,1]: (0, 1, 0, 0, 1, 0) while (2, 4, 7) become (1, 2, 3)
Mathilde Benveniste, AT&T Labs - Research

9. Simulation Study Overview
January 2001
Simulation Study Overview
TCMA and V-DCF are compared for two network configurations and various traffic loads
Three priority classes are considered: High, Medium, and Low
Packets from legacy stations are treated the same as medium priority packets
The following statistics are plotted for stream aggregates referred to as “calls”:
Total throughput for all calls (bits per sec)
Goodput by call (bits per sec)
Goodput/Application load by call
Data sent by call (bits per sec)
Dropped traffic by call (bits per sec)
Media access delay (queueing plus contention time) by call (sec)
Retransmissions by call (packets per sec)
Mathilde Benveniste, AT&T Labs - Research

10. Compared Features January 2001 Features simulated for TCMA
doc.: IEEE /457
January 2001
Compared Features
Features simulated for TCMA
Differentiation by UAT (urgency arbitration time)
… by backoff parameters (offset and contention window) used on first transmission attempt
… by persistence coefficient used on retrial
Absent from simulation
‘Slow’ adaptation to traffic (initial and retrial backoff parameters)
‘Fast’ adaptation to traffic (during backoff)
Differentiation by contention time limit
Urgency class upgrade
Differentiation by limit on queueing time
Refs: IEEE /375; -00/456; -00/457; -01/002; -01/003
Features simulated for VDCF
Differentiation by backoff offset* used on first transmission attempt
… by backoff contention window
Absent from simulation
‘Slow’ adaptation to traffic (initial and retrial backoff parameters)
Refs: IEEE /398; -00/399; -00/412r1; -00/429; -01/011
*Offset can be used only for priorities below legacy-equivalent
Mathilde Benveniste, AT&T Labs - Research

11. Class Description January 2001 doc.: IEEE 802.11-00/457
*Factor multiplying contention window following collision
Mathilde Benveniste, AT&T Labs - Research

12. OPNET Simulations Network Configuration WLAN Parameters
doc.: IEEE /457
January 2001
OPNET Simulations
Network Configuration
Pairs of stations generate bi-directional traffic streams
One direction generates twice the volume of the other
Note: Under certain scheduling assumptions, a node with streams destined to different stations can be modeled approximately as a cluster of stations
WLAN Parameters
DS, 11 Mbps channel rate
Buffer size -- 2,024,000 bits
RTS/CTS suppressed
Maximum retry limit -- 7
[See Notes below for OPNET Model settings]
Traffic Statistics
Frames of fixed size bytes
Inter-arrival times exponentially distributed
Packet arrival rate determined from scenario traffic load
Adjustment for PHY overhead of 192 microsecs must be made
OPNET Simulation Assumptions
Specified on Transmitter:
Modulation bpsk
Transmission delay model dra_txdel
Receiver Group Model wlan_rxgroup
Closure Model dra_closure_all
Channel Match Model wlan_chanmatch
Transmission Gain Model dra_tagain
Propagation Delay Model wlan_propdel
Specified on Receiver
Modulation bpsk
Noise figure 1.0
ECC Threshold 0.
Receiver Gain Model dra_gain
Receiver Power Model wlan_power
Background Noise Model dra_bkgnoise
Interference Noise Model dra_noise
Signal to Noise Ratio Model dra_snr
Bit Error Rate Model dra_ber
Error Model dra_error
Error Correction Control Model wlan_ecc
__________________
dra_ are OPNET default radio models.
wlan_ are specific models.
Mathilde Benveniste, AT&T Labs - Research

13. Network Configuration A
doc.: IEEE /457
January 2001
Network Configuration A
Traffic
8 stations generate 4 bi-directional streams;
stream load split 1-to-2 between two directions
WLAN Parameters
DS, 11 Mbps channel
buffer size=2.024 Mbits; no fragmentation
RTS/CTS suppressed; max retry limit=7
Mathilde Benveniste, AT&T Labs - Research

14. Legacy Only Traffic Loading January 2001 30 90 5.0 Mbps 2.5 Mbps 150
doc.: IEEE /457
January 2001
Legacy Only
Time (sec) 30 90
5.0 Mbps
2.5 Mbps
150
Traffic
Loading
BL2
Mathilde Benveniste, AT&T Labs - Research

15. Legacy Only Traffic Loading January 2001 30 90 5.0 Mbps 2.5 Mbps 150
doc.: IEEE /457
January 2001
Legacy Only
Time (sec) 30 90
5.0 Mbps
2.5 Mbps
150
Traffic
Loading
BL2
Mathilde Benveniste, AT&T Labs - Research

16. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 5.0 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
5.0 Mbps
2.5 Mbps
150
Priority
QoS-Enhanced MAC
TCMA
TCMA A7
VDCF B7
V-DCF
Mathilde Benveniste, AT&T Labs - Research

17. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 5.0 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
5.0 Mbps
2.5 Mbps
150
Priority
QoS-Enhanced MAC
TCMA
TCMA A7
VDCF B7
V-DCF
Mathilde Benveniste, AT&T Labs - Research

18. Network Configuration B
January 2001
Network Configuration B
Traffic
60 stations generate 30 bi-directional streams;
stream load split 1-to-2 between two directions
WLAN Parameters
DS, 11 Mbps channel
buffer size=2.024 Mbits; no fragmentation
RTS/CTS suppressed; max retry limit=7
Mathilde Benveniste, AT&T Labs - Research

19. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.2 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.2 Mbps
1.6 Mbps
150
Priority
QoS-Enhanced MAC
TCMA
TCMA D4a
VDCF E4
V-DCF
Mathilde Benveniste, AT&T Labs - Research

20. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.2 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.2 Mbps
1.6 Mbps
150
Priority
QoS-Enhanced MAC
TCMA
TCMA D4a
VDCF E4
V-DCF
Mathilde Benveniste, AT&T Labs - Research

21. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.2 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.2 Mbps
1.6 Mbps
150
Priority
QoS-Enhanced MAC
Goodput / Application Load
0.2
0.4
0.6
0.8 1
1.2
1.4 20 40 60 80
100
Time (s)
Call 1
Call 2
Call 3
Call 4
Percent Retransmissions 0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100% 20 40 60 80
Time
Call 1
Call 2
Call 3
Call 4
TCMA
TCMA D4a
VDCF E4
Goodput / Application Load
0.2
0.4
0.6
0.8 1
1.2
1.4 20 40 60 80
100
Time (s)
Call 1
Call 2
Call 3
Call 4
Percent Retransmissions 0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100% 20 40 60 80
Time
Call 1
Call 2
Call 3
Call 4
V-DCF
Mathilde Benveniste, AT&T Labs - Research

22. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.0 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.0 Mbps
1.5 Mbps
150
Priority
QoS-Enhanced MAC
TCMA
TCMA D3
VDCF E3
V-DCF
Mathilde Benveniste, AT&T Labs - Research

23. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.0 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.0 Mbps
1.5 Mbps
150
1.0 Mbps
Priority
QoS-Enhanced MAC
TCMA
TCMA D3
VDCF E3
V-DCF
Mathilde Benveniste, AT&T Labs - Research

24. QoS-Enhanced MAC Priority TCMA V-DCF January 2001 30 90 3.0 Mbps
doc.: IEEE /457
January 2001
Time (sec) 30 90
3.0 Mbps
1.5 Mbps
150
1.0 Mbps
Priority
QoS-Enhanced MAC
TCMA
TCMA D4a
VDCF E4
V-DCF
Mathilde Benveniste, AT&T Labs - Research

25. Common Scenario TCMA V-DCF January 2001 doc.: IEEE 802.11-00/457
TCMA D4a
VDCF E4
V-DCF
Mathilde Benveniste, AT&T Labs - Research

26. Comparison of TCMA and VDCF
doc.: IEEE /457
January 2001
Comparison of TCMA and VDCF
Similarities
Both protocols differentiate between different priorities
Both protocols transmit comparable amounts of low priority traffic
Differences
TCMA transmits more of the high priority traffic
TCMA causes lower delay to the high priority traffic
TCMA results in fewer collisions
Mathilde Benveniste, AT&T Labs - Research

27. doc.: IEEE /457
January 2001
Conclusions
TCMA encompasses the other two proposals, adding more features
UATs prevent collisions by packets of different priorities in congestion (high priority goes first)
Fewer collisions, greater throughput
Reduced delay
Provides greater flexibility for class differentiation in presence of legacy (offset can be used only with UATs)
Scaling permits fast adaptation to traffic without latency jitter
Class upgrades enable better compliance with QoS and fairness objectives
Mathilde Benveniste, AT&T Labs - Research