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Analytical Performance Evaluation of IEEE 802.15.4 with Multiple Transmis-sion Queues for Providing QoS under
Non-Saturated Conditions
APCC 201031 October - 3 November 2010
Langham Hotel, Auckland, New Zealand
Youn-Soon Shin, Kang-Woo Lee and Jong-Suk AhnDongguk Univ.
Table of Contents
Ⅱ. Multiple Queues for IEEE 802.15.4
Ⅲ. Analytical Performance Model
Ⅳ. Experiments
Ⅴ. Conclusion
Ⅰ. Introduction
2/17
Contributions and Motivations
• Two contributions– proposes multiple transmission queues for IEEE 802.15.4 – investigates the effects of adopting the multiple queues
for improving Quality of Service(QoS)• Motivations
– 802.15.4 cannot prioritize important frames such as GTS requests
– When a node wants to send data over a CFP• it should reserve the CFP time slots by sending a GTS
request frame with CSMA/CA during CAP intervals in advance
– Our multiple queue model is one of the feasible platform candidates for time-sensitive applications such as medical/health-care systems
CFP (Contention Free Period)GTS (Guaranteed Time Slot)
3/17
Introduction(1/2)
CAP (Contention Access Period)CSMA/CA (Carrier Sensing Multiple Access/Collision Avoidance)
IEEE 802.15.4 Standard
• alternates the sleep period and active period to minimize wasting energy
• Active period is further divided into – Contention Access Period
which each node randomly chooses the sending time by CSMA/CA with BEB algorithm
– Contention Free Periodis slotted into small fixed GTS which a future sender should reserve in advance
BEB (Binary Exponential Backoff) 4/17
Introduction(2/2)
• Differences from the previous models– Our model abstracts the multi-queue system faithfully
• where each node runs a scheduler resolving virtual collisions among its multiple queues
– Our model further augments the conventional models by adding two important features of 802.15.4 such as
• maximum transmission retries limitIf a frame collides after max. number of retransmissions, it should be dropped.
• deferment behaviorsWhen a frame’s transmission cannot be completed within a given CAP, the transmission should be delayed to the next CAP
• Previous models– can only evaluate networks where each node is assumed to
keep sending frames of one traffic class among available traffic classes
– do not include the deferment and transmission retries behaviors
Differences from the Previous Models
5/17
Multiple Transmission Queues for IEEE 802.15.4(1/4)
Queue 0 Queue N-1
backoffmoduleCon-
tention Window
Class N-1Class 0
Schedulerto resolve virtual colli-
sions
backoffmodule
Classifier
MAC
PHY
[Fig.1] Multiple Queue System
Multiple Priority Queue Algorithm
• Once a frame arrives at the head of queue– the associated backoff module is
executed • Each queue’s transmission priority
– is distinguished by associating different contention window parameters
• When the backoff timeouts of two or more different priority queues on the same node are the same, – the scheduler resolves this virtual
collision by offering the right of channel access
to the frame in higher priority queue• For the other frames with the equal
backoff delay,– scheduler invokes corresponding
backoff modules to recalculate their new backoff delaybecause they are considered to collide virtually
6/17
Multiple Transmission Queues for IEEE 802.15.4(2/4)
Complete Markov Chain Block Diagram
(1-pg,c)successful
(1-pg,c)successful
0-th CSMA/CA with BEB and
Deferment
frame dis-card
frame dis-card
frame dis-card
…
a new frame arrival
Idle
qsg
…
1-qsgqeg
1-qeg
pg,c
colli-sion
i-th CSMA/CA with BEB and
Deferment
R-th CSMA/CA with BEB and
Defermentpg,c
colli-sion
transmission
transmission
(1-pg,c)successful
pg,c
colli-sion
transmission
[Fig.2] The Block Diagram of IEEE 802.15.4 Markov Chain with Trans-mission Retries
• abstracts CSMA/CA of 802.15.4 running on class g with transmission retries limit
• When a frame of class g is generated with probability qeg from the idle state– node tries to transmit the frame by
CSMA/CA with BEB algorithm• A CSMA/CA with BEB and
Deferment algorithm has three possible outcomes – Successful transmission– Collision
• runs the next CSMA/CA until the number of retransmissions reaches R where R represents the max. number of retransmissions
– Frame discard• A frame from class g should be
dropped ▫ after max. of channel capture failure▫ after (R +1)-th collision
7/17
Multiple Transmission Queues for IEEE 802.15.4(3/4)
[Fig.3] Discrete Markov Chain of IEEE 802.15.4 with Deferment Algorithm
g,j,-1 g,j,Wg,j-2g,j,0 g,j,Wg,j-1g,j,1 …1 1
g,j-1,-1
1/Wg,j
Df
(1- pd)αg
pd
1
(1- pd)(1-αg)
1-βg
βg
1-βg
βg
Df
g,j,0,Rt-1
g,j,0,Rt-2
g,j,0,0
…
pd
1
1
1
Tx
Tx
(b)(a)
• [Fig. 3] presents a discrete Markov chain describing the detail behaviors of each small box – labeled as i-th CSMA/CA with BEB and deferment in [Fig. 2]
• Deferment scheme – [Fig. 3-(a)] adds a small box labeled as Df – When a frame’s transmission cannot be completed
within a given CAP with probability pd,• the transmission of that frame should be delayed to the next CAP
– It loops back to the current stage to pick up a new backoff delay when the subsequent CAP starts
8/17
Multiple Transmission Queues for IEEE 802.15.4(4/4) Discrete Markov Chain including Deferment Be-haviors
Equation (13),(14)
Probabilities of Frame Discards and Collisions
1 1, _ ,
1 1,1
, _ 1,
( (1 ))
1 ( (1 ))
1 (1 )
M Rg discard collisison g c g
M Rg c gM
g discard channelcapturefail g Mg c g
p p p
p pp p
p p
11
,0 1
1 (1 ) (1 )h h
g Nn n
g c h hh h g
p
the collision probability
two frame discard probabilities
• We modified the second term,
to accommodate the effect of virtual collision resolution
to exclude lower priority queues than class g in the same node
from the competition over underlying physical channel. 9/17
Analytical Performance Model(1/3)
11
,0,
1 (1 ) (1 )g h
Nn n
g c g hh h g
p
previous models
Equation (10),(11),(12)
Delay Model for 802.15.4 with Multiple Queues
[ ] [ ] [ ] [ ]g g g gE DSS E DS E DC E DCD
( 1) 1, , ,
0 0
( 1) 1, ,
0
( 1) ( 1),
[ ] (1 )(1 ) [ ]
[ ] (1 )(1 )
(1 ( (1 )) )
[ ] [ ] [ ]
R ii M i
g g c g c g g CSMAi h
Ri M i
g g c g c gi
M Rg c g
g g g
E DS p p p E D
E DC Tc i p p p
Ts p p
E DCD E DCDB E DCDC
( 1) ( 1), ,
0
[ ] ( (1 )) [ ]
[ ] ( 1)
RM R
g g c g CSMAi
g
E DCDB p p E D
E DCDC Tc R
the total delay taken to send a frame successfully including the delay wasted for discarded frames due to transmission retries
10/17
Analytical Performance Model(2/3)
Equation (15),(16)
Throughput Model for 802.15.4 with Multiple Queues
11
,0 1
(1 ) (1 ) (1 )(1 )h h
g Nn n
g success g g h h g gh h g
P n
,
[ ]g success payload
g
P LS
E slot
The successful transmission probability differs from the previous ones since they did not consider virtual collision resolution Higher priority frames should be transmitted before lower ones in
the same node even though they have the same backoff delay Lower priority classes' transmission will not affect the throughput of
higher priority class.
The throughput of class g : the fraction of the time that spent on the channel for successful
transmission of frames in a unit time span
11/17
Analytical Performance Model(3/3)
previous models
11
,0,
(1 ) (1 )h h
Nn n
g success g g h hh h g
P n
System Parameters
Experiments
Parameter Value
Data, GTS request Frame Pay-load
70 Bytes
PHY + MAC header 13 Bytes
ACK 11 Bytes
Channel Bit Rate 250 Kbps
duty cycle 100%
macMinBE 1,2,3,4
macMaxBE 5
macMaxCSMA 4
aMaxFrameRetries 3
Superframe Order 2,3,9
The simulation modules for 802.15.4 are developed using ns-2 2.33Each class’s priority is adjusted with macMinBE which determine the size of the contention window
12/17
Experiments(1/4)
[Fig.4] delay (the effet of deferment)
Delay Variations due to Deferment
[Fig. 4] illustrates the effect of deferments to the average channel access delay when the number of nodes and SO varyThe value of SO determines the size of CAP intervalThe effect of deferments becomes apparent when SO is set to 3 or 2, leading to 14.5% difference of access delays at maximum from the model without defer-ments
13/17
Experiments(2/4)
14.5%
[Fig.5] delay (the effect of dual queue)
Access Delays of Single Queue, Class0, Class1
[Fig. 5] shows the effect of dual queues employed in 802.15.4, where the class 1 is for GTS request frames with higher priority and the class 0 is for data frames with lower prioritymacMinBE for class0and1 are set to 3 and 1, respectivelyThis graph illustrates that class 1 takes much less average channel access delays by around 45% at maximum at the expense of 5% of delay increase over class 0 compared to the delays of the single queue system
14/17
Experiments(3/4)
45%
5%
[Fig.6] throughput(the effect of dual queue)
Throughput of Single Queue, Class0, Class1
[Fig. 6] plots the throughputs as a function of offered load λg when the nodes with dual queues contend for the wireless channelThe throughput of the class 1 exceeds that of class 0 as the offered load be-comes heavy while the throughputs of two classes are mostly the same when the load is lightThe throughputs evaluated by the analytical model are different by 7% at maxi-mum from those of simulations 15/17
Experiments(4/4)
Summary and Future Plan
• This paper– expands the traditional 802.15.4 model to incorporate
multiple queues for supporting QoS services– proposes an enhanced analytical model for 802.15.4
by including deferment behaviors and transmission retries
– proves that • the deferment and transmission retries behaviors should be
included for accurate forecast of delays
• the multi-queue system can deliver data of higher priority classes rapidly without sacrificing the delays of lower priority classes
• Future research– develop the performance model applicable to bursty
traffic 16/17
Conclusion
