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Abstract –Wireless Sensor Networks (WSNs) recently find exten-

sive applications in remote disaster and health monitoring, where

traffic prioritization is extremely essential for timely dissemination of

critical information to the first responders. Though several resource

control standards exist for general wireless networks, supporting dif-

ferentiated traffic services (Quality-of-Service (QoS)) at the Medium

Access Control layer for sensor networks need exclusive investigation

and significant improvements. Particularly, the network treatment

for disaster or medical emergencies needs to be exclusive (determinis-

tic guarantees) rather than probabilistic guarantees as in most wire-

less QoS standards.

In this paper, we introduce channel service preemptions during

random medium access in WSNs. In the context of time-critical sen-

sor applications, emergency traffic will have the privileges to inter-

rupt the services of other routine traffic in the network to guarantee

the lowest possible channel access latencies. We design our methodol-

ogy within the QoS framework of the popular 802.11e EDCA stan-

dard and demonstrate the importance of service preemptions for

emergency reporting. The performance analysis predicts close to

50% uniform decrease in emergency medium access delays when us-

ing our Channel Preemptive-EDCA (CP-EDCA) as compared to

EDCA. The results also depict that CP-EDCA guarantees the lowest

delay bounds and immunity to the presence of other lower priority

traffic, for emergency frames, even under high network loads. The

cost, however, is increased delays for routine traffic, which is accept-

able during emergency sensor reporting.

Key words: Wireless sensor networks, MAC, QoS

I. INTRODUCTION

Recent advancements in Micro-Electro-Mechanical Systems (

MEMS) [1] technology allow for compact and low-cost sensor

nodes [2] with embedded sensing, processing and short-range

wireless communication capabilities. These inexpensive sensors

can be generously deployed in target environments for autono-

mous disaster/health surveillance and real-time telecommunication

to remote user facilities. Embedded wireless sensor technologies

will undoubtedly enhance tracking reliability in time critical ap-

plications such as healthcare. The biggest challenge, however, is

the development of a contextual networking platform that permits

coherent and timely dissemination of emergency information to

the appropriate entities.

Wireless Sensor Networks (WSNs) find most of the applications

in data-centric network environments, where the deployed sensors

collaboratively work towards accomplishing a common applica-

tion task. In such cases, preferential traffic services (QoS) were

not perceived important, since every data was considered equal

for satisfying the application task. However, the exploration of

sensor technology usage in disaster surveillance and health moni-

toring [3] has emphasized the need to support traffic priorities and

differentiated resource allocation methods. Although major pro-

gress has been made in sensor networking, network QoS for

WSNs is at a premature stage [4].

A. Example Application

Telemedicine [5] refers to a field where remote health-monitoring

and communication to healthcare providers can enhance the qual-

ity of medical services in underdeveloped communities. Figure 1

shows the architecture of a prototype healthcare sensor network,

using Imotes [2], currently under-development in our labs. The

dense, multi-hop sensor network will leverage existing smart-

phone infrastructures to extend heath monitoring capabilities to

remote areas at minimal cost.

Large Healthcare Facility

In-body Network: In-body sensors

form a 1-hop wireless network. A head

collects data from the body sensors and

communicates it to the outside world

Healthcare Sensor Network: Multi-

hop ad-hoc network between in-body sensors and the nearest medical

provider (smart-phone).

Medical mobile-phone carried by healthcare providers for

real-time health monitoring and response advisory

Central Database of

Patient Records

WiFi

Paper Focus

Service preemptive MAC for

emergency medium access in dense,

multi-hop WSNs

Figure 1. Emergency Medium Access in Medical Sensor Networks

A robust QoS-supported communication platform is essential for

medical networks, and this paper addresses prioritized Medium

Access Control (MAC) techniques for emergency data diffusion in

such time-critical sensor applications.

B. Choice of QoS MAC for Sensor Networks

Several QoS MAC schemes exist for wireless networks. Priori-

tized services can be guaranteed with higher probability in conten-

tion-free or reservation-based MAC schemes (such as TDMA), as

compared to random medium access techniques, due to the pre-

allocation of channel times. However, sensor network deploy-

ments are generally dense (for reliability) and nodes are subject to

frequent failures, and TDMA-like schemes incur significant man-

agement overhead due to frequent re-configurations of slot as-

signments when nodes die or newly deployed. Node polling tech-

niques are also an attractive option for QoS provisioning due to

the centralized control, but is inappropriate for distributed sen-

sor/ad-hoc networks when the nature of traffic is unpredictable

[6]. Polling is not attractive for energy constrained sensor net-

works because head nodes would deplete energy rapidly as com-

pared to the slaves. In addition, polling schemes do not respond

well to urgent requests.

For dense, distributed and multi-hop sensor networks, random ac-

cess MAC techniques are the most suited, since (1) they are inher-

Service Preemptions for Guaranteed Emergency

Medium Access in Wireless Sensor Networks Manikanden Balakrishnan, Driss Benhaddou, Xiaojing Yuan and Deniz Gurkan

University of Houston

978-1-4244-2677-5/08/$25.00 ©2008 IEEE

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ently adaptive to node dynamics i.e no additional management

overhead is incurred due to node deaths/new deployments (2)

there would be no additional functionality for certain nodes (bi-

ased burden), which is critical for network energy balance (3) the

overhead of time synchronization among nodes is eliminated,

which is complex for multi-hop sensor networks.

Zigbee/802.15.4 [7] is a widely-used commercial protocol stack

for sensor technology, but supports traffic priorities only through

guaranteed time-slots (not during random channel access), which

is not sufficient for sensor networks [8]. The IEEE 802.11e stan-

dard already supports multiple traffic priorities and differentiated

services during random channel access through the Enhanced Dis-

tributed Channel Access (EDCA) [9] scheme. Since, the conten-

tion-based medium access (CSMA/CA) method is similar in both

802.11e and 802.15.4, we design our preemption methodology

over the established QoS framework of EDCA, rather than design-

ing a new priority-supported CSMA scheme for 802.15.4.

C. Motivation and Contributions

Providing guaranteed/deterministic traffic services is extremely

challenging in contention-based MAC schemes due to the random

nature of channel access. The EDCA provides faster channel ac-

cess to higher priority traffic only with high probability, but not

with certainty. The EDCA analysis in [10] shows that though, on-

average, the transmission delays of the highest priority traffic are

much lower, at least one flow could not meet the requested service

demands. In short, uniform QoS guarantees (across all flows at all

nodes) are extremely difficult to achieve with purely probabilistic

approach to prioritized medium control. However, such strict QoS

is required for emergency medium access in time-critical sensor

applications.

In this paper, we improve the current probabilistic service guaran-

tees in EDCA through the following major contributions:

1. Design of Channel Preemptive EDCA (CP-EDCA) protocol

that will provide exclusive, rather than probabilistic, channel

access privileges for emergency traffic in time critical sensor

applications such as disaster or health monitoring. Through

in-channel service preemptions, emergency traffic can in-

terrupt and replace the current channel occupancy of other

routine network traffic for guaranteeing the lowest possible

MAC latencies. The intention is to achieve deterministic and

uniform QoS guarantees even during random channel access.

CP-EDCA will be a significant contribution to QoS in emer-

gency-tracking WSNs, since it is essential that the rare emer-

gency cases get treated exclusively, when present.

2. Implementation of CP-EDCA in NS2 and performance inves-

tigation of the MAC delay distribution, relative to the stan-

dard EDCA. The results verify the effectiveness of service

preemptions in guaranteeing lowest delay bounds (QoS) for

emergency traffic, even under network overloads.

D. Related Work

Protocol solutions for network QoS have been extensively ad-

dressed for WLANs, mobile communications and multimedia

(consumer electronics) networks, where user-centric services de-

mand differentiated traffic treatment. The current literature in

WSN QoS is limited due to the data-centric application focus; al-

though, the huge body of work in general wireless networks can

be tailored to WSN requirements. Recent standard specifications

in 802.11e [11], WiMedia [12] and 802.16e [13] substantiate the

effectiveness of supporting QoS at the MAC layer.

Authors in [14] study the QoS performance of 802.15.4 and

802.11e MAC protocols in body sensor networks and highlight

the need for significant improvements in design considerations.

Research in [8] states the inadequacy of the current QoS support

(through time-slot guarantees) for WSNs in IEEE 802.15.4, and

designs differentiated services through a modified CSMA/CA

back-off process. Though our views, in opting for random MAC,

are similar to [8], our work provides preemption services during

the actual channel access period in addition to the existing priori-

tized back-off methods of EDCA. In [15], channel resources are

reserved end-to-end for time-critical traffic, which works well for

their collaborative intruder detection sensor task. However, end-

to-end resource reservations would not be appropriate in several

applications such as medical sensor networks where each sensor

system independently tracks a particular patient (per-node fairness

is essential).

802.11e EDCA standard [11] provides a robust framework for

multiple traffic priorities (queues) and preferential random chan-

nel access, which can be employed for distributed WSNs. How-

ever, the traffic treatment and QoS requirements have to be han-

dled differently for time-critical sensor applications, particularly

during emergencies. Simulation results in [16] show (as a disad-

vantage) that the EDCA scheme can starve low priority traffic,

which could actually be exploited for emergency channel access.

The concept of in-channel preemptions has been discussed before

[17], but not exclusive for sensor networks. Also, researches that

address preemptions for contention-based MAC methods are very

few. In this paper, we design a service preemption algorithm, for

the EDCA (random medium access) protocol, that will provide

exclusive channel acquisition privileges for emergency traffic in

time-critical sensor applications. In summary, the significant con-

tribution of this work is the introduction of a methodology that

will provide guaranteed QoS bounds for emergency traffic during

random channel access.

II. EDCA CFB REVIEW AND PREEMPTIONS

EDCA supports multiple traffic priorities, with separate internal

queues for each traffic type at each node. The priority-queueing

and contention (binary exponential back-off) procedures are ex-

actly the same in CP-EDCA, except for the behavior during actual

channel access. Interested readers can refer to [9, 11] for a de-

tailed review of EDCA.

For bandwidth and energy constrained sensor networks, channel

contention per frame is extremely in-effective, particularly when

the nodes are deployed in-abundance and the application data

rates are high (real-time healthcare). Excessive contention, frame

collisions and retransmissions degrade the throughput drastically,

which was one of the reasons for employing hybrid [19] or con-

tention-free MAC [20] schemes for dense sensor networks. How-

ever, the negligible management overhead of contention-based

schemes is still attractive for highly dynamic WSNs, if the prob-

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lem of heavy frame contention/collisions is addressed.

EDCA supports an efficient performance enhancement mode

called Contention Free Bursting (CFB) [21], also called Trans-

mission opportunity (TXOP) bursting, which allows for a con-

tiguous transmission of multiple frames upon one successful

channel acquisition. The concept of CFB is shown in figure 2.

Channel busy

DataChannel busy Ack

AIFSContention

period

SIFS

Data Ack

SIFSSIFS

TXOP time.

Only one contention for multiple fragments (fragment bursting)

Figure 2. CFB in EDCA

Every node has to sense a channel free for at least an Arbitrary In-

terframe Space (AIFS) [9] time to start contention, and higher pri-

ority queues have shorter AIFS wait times (priority-based). Upon

winning channel contention, a node can use the channel for a

maximum of TXOP period, where multiple frame transmissions

occur without contention. During TXOP, each frame is acknowl-

edged and the next frame transmission is separated only by Short

Inter-frame Space (SIFS) [9] time, the shortest time for channel

access waiting in the 802.11 standards. The CFB mode has sev-

eral advantages, including

• Exponential improvement in throughput and reduced per

packet delays [21]

• Nullifies unfairness due to uneven frame sizes, since all nodes

are allowed fixed channel transmission time.

• Eliminating the need for contention per packet allows frag-

mentation of application frames into short packets on channel,

which eliminates the need for overhead RTS/CTS [11] pack-

ets. Also, short frames reduce the impact of channel errors

(crucial for sensor networks). Only the erroneous fragments

have to be retransmitted instead of the entire frame, which re-

duces channel capacity losses.

In contention-based MAC schemes, CFB mode should be strictly

employed to match the performance of reservation based MAC

schemes. Also, multiple frame transmissions without contention

can provide scope for dynamic adaptation of channel occupancy

periods (TXOP) in QoS-critical sensor applications.

The major problem with CFB is the high possibility of increased

MAC delays for emergency (highest priority) frames. In the plain

EDCA mode, the channel is relinquished after a single frame

transmission, after which the newly arrived emergency frames

contend with higher probability of channel acquisition (EDCA has

prioritized wait times and back-off). However, when CFB mode is

implemented the channel is not relinquished for TXOP interval.

Since emergency traffic is rare and potentially bursty, the prob-

ability that a channel is pre-occupied (by lower priority frames)

when new emergency frames arrive is high, and therefore emer-

gency traffic would suffer heavy delays when using the CFB

mode. Such inability to quickly acquire channel resources will ex-

tremely degrade the emergency QoS requirements in time-

sensitive sensor applications (medical or disaster reporting).

In this paper, we present an elegant solution that exploits the ad-

vantages of CFB mode, but also guarantees minimal MAC laten-

cies for emergency frames. We design in-channel service preemp-

tions for emergency medium access; the concept is depicted in

figure 3.

Routine Data

Ack

SIFS

Emergency TXOP bursting

SIFS

Service Preemption: Frames whose priority are emergency (E), upon arrival,

preempts the routine traffic bursting and transmits for TXOP period.

Channel is relinquished by emergency

queue for contention by all traffic

Figure 3. Channel TXOP Preemptions

Emergency traffic, when present at a node, is provided with privi-

leges to interrupt and seize the channel acquired by other lower

priority traffic for frame bursting. The interrupted lower priority

queue backs off and contends only after the emergency bursting.

We can predict that such channel preemptions would increase the

MAC delay of lower priority frames, but is essential in the context

of time-critical sensor applications1.

III. CHANNEL PREEMPTIVE EDCA (CP-EDCA)

This section describes the design of service preemptions within

the EDCA framework (CP-EDCA). Apart from the standard

EDCA parameters (contention window, AIFS), CP-EDCA defines

additional timing parameters, which are discussed below.

• SIFS: 192 µs

PHY and MAC processing delay + Receive/Transmit turn-

around time for sensor (Zigbee-compliant) radios [22])

• Emergency Priority SIFS (EPSIFS): 192 µs

Smallest wait time between frame transmissions, which is

equivalent to SIFS. During emergency bursting (TXOP), all

frames (data, ack) are separated by EPSIFS, so that an on-

going emergency CFB can never be interrupted.

• Emergency Priority AIFS (EPAIFS): 320 µs (EPSIFS +

CCA time)

Clear Channel Assessment (CCA) time is the time taken to de-

tect energy in the medium after a transmission initiation from

another node, which is 128 µs for sensor radios ([22]). New

emergency frames at a node have to detect a free channel for at

least EPAIFS to begin a new burst transmission. EPAIFS is

designed such that new emergency traffic can interrupt the

TXOP of routine (lower priority) queues, but not the TXOP of

an emergency queue, since EPAIFS > EPSIFS.

• Normal Priority SIFS (NPSIFS): 448 µs (EPAIFS + CCA)

CFB of routine frames are separated by NPSIFS, which per-

mits new emergency traffic to interrupt the current routine

TXOP and enough time for every node, including the inter-

rupted one, to detect the new transmission.

• EPSlotTime: 320 us (EPSIFS + CCA time)

The slot time for emergency priority, which is different from

the slot time for routine priorities (unlike standard EDCA).

Emergency queues have to sense an idle channel for EPSlot-

Time before decrementing their back-off counter [9].

• NPSlotTime: 576 µs (NPSIFS + CCA time)

1 Imagine an ambulance on road. It is required that the medical emergency gets

exclusive road access as compared to other traffic, which is the concept employed

by CP-EDCA for channel access.

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Time that routine priority queues (all priorities other than

emergency) have to sense an idle channel before decrementing

their back-off counter. NPSlotTime allows enough time to de-

tect an on-going routine CFB.

EPSIFS

CCA EPSIFS

EPAIFS CCA

NPSIFS CCA

SIFS for emergency bursting

EPAIFS = AIFS [0] = EPSlotTime

NPSIFS (SIFS for routine frame bursting)

NPSlotTime

Figure 4. CP-EDCA Timing Design

TXOP

Data Fragment ACK

EPSIFS

DATA Fragment ACK

EPSIFS EPSIFS

DATA Fragment ACK ACK

NPSIFS

TXOP

NPSIFS NPSIFS

DATA Fragment

TXOP

NPSIFS

EPAIFS

NPSIFS

DATA Fragment ACK

EPSIFS EPSIFS

Back-off

TXOP

DATA Fragment ACK

TXOP

NPSIFS NPSIFS

Back-off

EPSlotTime

DATA Fragment ACK

NPSIFS

EPSlotTime

NPSIFS

Preemption block

Preemption block

DATA Fragment

New emergency frame

arrival at any node

ACK

Data-Ack sequence of any frame

will never be interrupted (NAV).

New arrival to emergency queue,

starts back-off

Since EPSlotTime < NPSIFS, emergency

queue counts down back-off slots even

during routine frame bursting

Back-off freezes

Back-off resumes and counts down to 0 (emergency

queue will transmit). Preemption possible even when

emergency queue is backing off

Routine bursting (TXOP) interrupted. The

sender relinquishes channel and starts back-off.

Emergency queue starts frame bursting

a. Emergency-frame bursting

b. Routine-frame bursting

c. Routine-frame bursting: Preemption by an emergency frame from any node

d. Emergency queue back-off behavior during routine-frame bursting

TXOP

Data Fragment ACK

SIFS

DATA Fragment ACK

SIFS SIFS

EDCA: CFB for all frames

CP-EDCA

Figure 5. Frame Bursting in CP-EDCA (Preemptive Channel Access)

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Figure 4 depicts the timing relations, which are crucial for our

preemption design. The basic concept behind this formulation is

similar to the IFS design of 802.11 schemes, which uses the wait

times to allow for precedence in acquiring a free channel. For ex-

ample, in 802.11e EDCA an on-going frame bursting sequence

(Data-Ack-Data) is separated by SIFS, which implies that it takes

the highest precedence irrespective of the frame priority. How-

ever, in CP-EDCA, emergency traffic takes the highest prece-

dence for acquiring a free channel, even if it is required to inter-

rupt an on-going lower priority burst.

As per figure 4, current emergency TXOP bursting has the highest

channel access priority (EPSIFS), followed by new emergency

frames (EPAIFS) and finally other routine frames (NPSIFS). Each

IFS time is separated by CCA time, which is the minimum period

required to detect a new transmission on channel. It has to be

clearly noted that this IFS and slot prioritization methods are ex-

clusively for providing preemption privileges for emergency traf-

fic. Thus, all priorities other than emergency will fall under the

“normal/routine” category, in the context of channel preemptions.

However, prioritized channel access still exists between all traffic

categories, since the contention mechanism of CP-EDCA is still

the same as the standard EDCA. Figure 5 emphasizes the actual

enhancement of CP-EDCA; guaranteeing medium access for

emergency priority through TXOP preemptions.

No preemptions are allowed within emergency TXOP, since the

emergency frame sequence is separated by the shortest wait time,

EPSIFS (figure 5a). New emergency frames start channel acquisi-

tion or contention if they sense the channel free for an EPAIFS

period, which is smaller than NPSIFS. Figure 5c shows the sig-

nificance of this design; allowance for emergency preemption dur-

ing routine TXOP. In this initial work, the data-ack sequence is

never interrupted and can be achieved by setting the Network Al-

location Vector (NAV) [11] to the end of ack transmission.

Figure 5d shows the behavior when an emergency queue backs-off

due to energy detection on channel (assume routine frame trans-

mission). At the end of NAV period, the emergency queue re-

sumes back-off and even counts down slots, since EPSlotTime is

lesser than NPSIFS. With this design, emergency queues count

down back-off slots even during routine traffic TXOP and eventu-

ally execute preemption. Back-off procedure is still required for

emergency queues, since there can be simultaneous attempts of

emergency frame transmissions from multiple nodes. Without slot

randomization, repeated collisions would occur when multiple

nodes attempt to send emergency frames at the same time. Finally,

since EPSIFS < EPSlotTime, emergency queues never count

down back-off slots during an on-going emergency CFB i.e under

any condition emergency CFB cannot be preempted.

This CP-EDCA design achieves both in-node preemption (inter-

ruption of routine TXOP within a same node) and inter-node pre-

emption (interruption of routine TXOP of other nodes), since the

IFS and slot time design applies to all priority queues in all nodes.

The events preceding the channel bursting (priority queueing and

contention procedures) are similar to the standard EDCA and

readers can review [9] for details. Within the scope of this paper,

we detail only the modifications of CP-EDCA.

IV. PERFORMANCE ANALYSIS

We modified the existing ns-2 EDCA model [23] to implement

the new timings (figure 4) and the preemption procedure (figure

5) of CP-EDCA. In this section we present the results of investi-

gation under two traffic categories: Class 0 Emergency and

Class 1 Routine. Operating the network under overloaded routine

traffic conditions will be sufficient to test the efficiency of CP-

EDCA in providing guaranteed emergency medium access. Add-

ing more priorities would be redundant (since they would be pre-

empted anyway) and would not change the conclusion of this

analysis, and hence only the most relevant results are presented.

Table 1. Simulation Parameters

General Pa-

rameters

Channel speed = 250 Kbps (typical for current sensor

technologies [2])

Number of competing nodes (neighbors) = 5

Slot Time (Tslot) = SIFS + CCA time = 320 µsec (used

by EDCA, CP-EDCA uses NP/EPSlotTimes)

Application frame size = 1000 bytes (fixed)

Maximum frame size on channel = 200 bytes

TXOP limit = 100 msec

Class 0 Class 1

Contention

Parameters

AIFS[0] = EPAIFS (for CP-

EDCA)

AIFS[0] = 2 slots† (for EDCA)

minimum contention window

size = WMin[0] = 2 slots

maximum contention window

size = WMax[0] = 8 slots

Smaller back-off slots, since

emergency frame arrivals are

rare and bursty

AIFS[1] = 4 slots

WMin[1] = 8 slots

WMax[1] = 64

slots

†slot size is same (Tslot) for all priorities in EDCA. In CP-EDCA, slots

imply EPSlotTime and NPSlotTime for class 0 and 1 respectively

Apart from the parameters discussed before, table 1 shows the

simulation parameters used in this experiment. RTS/CTS ex-

change is eliminated due to the short frame sizes on channel (one

of the advantages of CFB mode). The research in [24] models the

traffic generation rate in medical applications as CBR traffic,

which reaches up to thousands of bps (for ECG waveforms).

Thus, the maximum frame generation rate at a node in this analy-

sis can be in the order of tens of frames/sec.

The main contribution of CP-EDCA would be the ability to guar-

antee certain QoS for every emergency frame generated. Thus, per

frame statistics would also be studied in this section apart from

average node statistics. We consider reporting-delay as a crucial

metric for time-critical sensor applications, and MAC delay is one

of the primary components of the overall network latency. In this

work, we measure the MAC delay per frame at a node, which is

the delay suffered by a single application frame at a node for suc-

cessful one-hop channel transmission (includes queuing, channel

contention and retransmission delays).

Figure 6 depicts the average MAC delay at a node for both class

frames, under varying λ1 and a neighborhood density of 5 (reason-

able for short-range radios [2]).

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0

0.04

0.08

0.12

0.16

0.2

0.24

0.28

0 0.4 0.8 1.2 1.6 2 2.4 2.8

(frames/sec/node)

Ave

rag

e M

AC

De

lay (

se

c/f

ram

e/n

od

e)

CP-EDCA, Emergency Class

CP-EDCA, Routine Class

EDCA, Emergency Class

EDCA, Routine Class

λ1

λ0= 0.1 frames/sec/node

Figure 6. CP-EDCA: Steady Emergency Delay

λ0 and λ1 represent the emergency and routine frame generation

rates, respectively, at a node. The results are shown with 90%

confidence intervals [25] for the simulation mean value. Figure 6

substantiates the efficiency of our channel preemption method in

providing exclusive channel access precedence for emergency

traffic. The average MAC delay of emergency frames is perceiva-

bly insensitive to the amount of routine traffic in the network.

Even when the network is overloaded (λ1=2.8), TXOP preemp-

tions guarantee the lowest possible MAC delay for emergency

traffic with certainty.

In EDCA, though the average delay suffered by the emergency

frames is lower than routine frames, it still increases with increas-

ing λ1, unlike CP-EDCA. With probabilistic prioritization, in-

creased contention from routine traffic decreases the probability

of channel acquisition, thereby increasing the MAC delays, of

emergency traffic. The influence of other network traffic on emer-

gency medium access should be eliminated for meeting the strict

QoS demands of time-critical sensor applications, and CP-EDCA

provides such immunity. For the given configuration, CP-EDCA

decreases the MAC delay of emergency frames by 47 percent (%)

at the cost on increasing the MAC delay of routine traffic by 30%,

which is acceptable during emergency reporting.

The delay of emergency class increases marginally as λ1 increases

(not perceivable in graphs), since the probability that a new emer-

gency frame will find a busy channel increases as the network

traffic increases. In such cases, the emergency queue is pushed to

back-off, but still counts down slots during the TXOP of routine

traffic (see figure 5) and eventually acquires the channel with

marginal back-off delay.

CP-EDCA aims to provide uniform QoS across all network nodes,

and average MAC delay might not be an accurate indicator of uni-

form QoS (since certain nodes experiencing extremely low delays

pull the average value down). Figure 7 shows the 99th

percentile

of the delay/frame at a node, the delay value below which 99% of

the measurements fall. The 99th

percentile values for emergency-

frame MAC delays in EDCA is in close agreement with the aver-

age values, implying that the emergency traffic at any node can

have guaranteed bounds on MAC delay.

CP-EDCA guarantees the lowest possible channel access laten-

cies for emergency traffic, irrespective of routine traffic presence,

which is a significant QoS improvement from 802.11e EDCA and

required for emergency monitoring sensor networks.

0

0.04

0.08

0.12

0.16

0.2

0.24

0.28

0 0.4 0.8 1.2 1.6 2 2.4 2.8

(frames/sec/node)

99th

Perc

entile

fo

r M

AC

Dela

y

(se

c/fra

me/n

ode)

CP-EDCA, Emergency Class

CP-EDCA, Routine Class

EDCA, Emergency Class

EDCA, Routine Class

λ0= 0.1 frames/sec/node

λ1

Figure 7. CP-EDCA: Uniform performance across all nodes

To emphasize the stability offered by CP-EDCA, figures 8 and 9

show the MAC delay distribution, under overload condition, of all

the emergency frames (irrespective of nodes) generated in one

simulation, without any average statistics.

0

0.1

0.2

0.3

Emergency frames in one simulation

MA

C D

ela

y (

s)

Delay Curve

λ0= 0.1 frames/sec/node

λ1= 2.2 frames/sec/node

Figure 8. EDCA: Channel Access Delay Distribution

0

0.1

0.2

0.3

Emergency frames in one simulation

MA

C D

ela

y (

s)

λ0= 0.1 frames/sec/node

λ1= 2.2 frames/sec/node

Delay Curve

Figure 9. CP-EDCA: Channel Access Delay Distribution

As expected, the delay curve (two-point moving average line plot)

for EDCA shows significant variations in the frame MAC delay

due to probabilistic channel access. The introduction of channel

preemptions improves the delay guarantees significantly as dis-

played by the relatively very stable CP-EDCA curve. CP-EDCA

substantiates the effectiveness of preemptions in providing tight

and deterministic QoS bounds even during random channel ac-

cess. However, the QoS for emergency traffic is enhanced at the

cost of increasing the routine traffic delays (biased channel time

allocation), but is acceptable and necessary for time-critical appli-

cations.

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In this paper, the analysis under varying λ0 is not presented, since

it is not relevant to the paper’s goal, which is to substantiate the

advantages of channel preemptions. Increasing λ0 would surely in-

crease the emergency access delays (no need of proof), but it is

more important to verify the effects of preemptions on emergency

traffic delays under heavy routine traffic competition, which was

analyzed here. Even if we use more granular routine traffic classi-

fication (multiple lower category traffic), the conclusions of this

paper would not change, since all lower priority classes would

anyway be preempted by emergency traffic. Our preemption de-

sign makes the emergency delay performance independent of the

number of lower priority classes, competition or neighborhood

density, which is the main goal of CP-EDCA.

V. CONCLUSIONS AND FUTURE WORK

In this paper, we introduced the concept of in-channel service pre-

emptions for providing exclusive medium access precedence for

emergency traffic in time-critical sensor applications. Contention-

based MAC schemes are attractive for distributed sensor net-

works, and CP-EDCA addressed the challenge of guaranteeing de-

terministic QoS bounds during random channel access. By allow-

ing for emergency preemptions, CP-EDCA enhanced the usage of

CFB (TXOP bursting) mode for medical sensor networks, which

has considerable performance gains. The performance analysis

substantiated that CP-EDCA, under the experimented conditions,

was able to reduce the MAC delays of emergency traffic by ap-

proximately 50%, as compared to the standard 802.11e EDCA,

uniformly for all flows and even under heavy network traffic.

The results also depicted that, in CP-EDCA, the probability of

channel acquisition for emergency frames is insensitive to the

presence of other routine network traffic. Such service starvation

for low priority traffic might not be preferred in general wireless

applications [16], but is extremely essential for emergency report-

ing sensor networks. For example, medical emergencies have to

be treated exclusively, rather than probabilistically, and CP-

EDCA is designed with that intent. We expect that our continued

work in this methodology would lead to a significant enhancement

to the existing wireless QoS literature for applicability in health or

disaster monitoring sensor networks.

Though we experimented with only two priorities, the CP-EDCA

method will extend to any number of priorities and the emergency

delay performance would change only negligibly. Our future work

includes: (1) The integration of CP-EDCA with priority aging

techniques, which refers to the evolution of traffic priorities to

emergency or higher status in proportion to the delay suffered in

queue. In CP-EDCA, routine traffic could suffer exaggerated de-

lays due to multiple preemptions, and priority aging methods

could alleviate these unfairness problems by elevating the priori-

ties of multi-preempted frames for faster channel access (2) Study

hidden-node effects, since, with the current CP-EDCA design,

preemptions by hidden nodes will go unnoticed and result in colli-

sions (3) Investigate the impact of channel errors on QoS guaran-

tees, since frame errors and re-transmissions will have a major in-

fluence on delay performance.

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