ON SUPPORTING REAL-TIME COMMUNICATION OVER THE

IEEE 802.15.4 PROTOCOL

Hoai Hoang and Jerker Bengtsson

CERES - Centre for Research on Embedded Systems, Halmstad University, Halmstad, Sweden

Keywords:

Real-time communication, wireless network, QoS.

Abstract:

IEEE 802.15.4 is a new enabling technology for low data rate wireless personal networks. This standard was

not speciﬁcally designed for wireless sensor networks, but it has shown to be a good match with necessary

requirements on low data rate, low power consumption and low cost. Unlike the former 802.11 standard,

the MAC protocol speciﬁed in IEEE 802.15.4 can operate in two different modes: beacon-enabled mode or

non-beacon enable mode. In beacon-enabled mode, nodes can exclusively allocate a number of guaranteed

time slots, similar to a resource reservation scheme. Hence, the IEEE 802.15.4 MAC protocol have sufﬁcient

capabilities for supporting real-time communication. This paper presents the key features of IEEE 802.15.4

which makes it an attractive standard to use for real-time wireless sensor networks. Two real-time protocols

extending the IEEE 802.15.4 standard are reviewed. The purpose of this paper is to present the state of the

art on real-time support over IEEE 802.15.4 for wireless sensor networks and to discuss the possibilities on

improvements on both the standard and the real-time protocols extending the standard.

1 INTRODUCTION

Supporting real-time (RT) communication over Wire-

less Sensor Networks (WSNs) has attained an increas-

ing interest from both industry and academia. During

the recent decade, advances in WSN enabling tech-

nology has opened up for many proposals of new

innovative applications. However, many of the pro-

posed applications require support for RT commu-

nication; guarantees for data being delivered from a

source node to a destination node within its given end-

to-end deadline. For example, a surveillance system

needs to alert the detection of an intruder within sec-

onds for the authorities to be able to initiate pursuing

actions in time (He et al., 2006). Depending on the

speciﬁc application, different kinds of data have dif-

ferent deadline requirements due to its validity inter-

val. Data registering the movement of a tank in a mili-

tary surveillance system have shorter update intervals

than data registering transport pallets being moved in

and out from a goods storage. Similarly, the loca-

tion registration of an intruder have a shorter valid-

ity interval than the temperature measurement in au-

tomatic room temperature control system (Lu et al.,

2002). Thus, there is a need for WSNs to support

time-sensitive communication by guaranteeing pack-

ets end-to-end deadlines or minimizing the deadline

miss ratio, i.e., the number of packets that meet their

end-to-end deadline.

Supporting RT communication in WSNs is a great

challenge due to many reasons (Stankovic et al.,

2003). The communication in WSN per se is less

reliable and less predictable than in wired networks

because of its wireless nature. Moreover, a WSN

in general consists of an arbitrarily number of nodes

equipped with highly limited resources; at the one

hand batteries need to be small and at the other hand

they need to be operable during a long time. There-

fore a major factor that must be considered in the de-

velopment of a communication protocol for WSNs is

power-efﬁciency. Most of the earlier reported work

on RT communication protocols for WSNs have to

a large an extent been focused on the trade-off be-

tween energy efﬁciency and end-to-end delay guaran-

tees (He et al., 2003; Chipara et al., 2006). Still it re-

mains as one of the most important issues for research

community. Other characteristics that complicates RT

support is network topology. In many proposed ap-

plications, the topology is required to be highly dy-

namic: nodes can relocate, join or leave the network.

Besides making routing more complicated, the work-

loads in such dynamic topologies become highly un-

predictable. Clearly, there is a need for new schedul-

ing methods which can adapt efﬁciently to such dy-

Hoang H. and Bengtsson J. (2008).

ON SUPPORTING REAL-TIME COMMUNICATION OVER THE IEEE 802.15.4 PROTOCOL.

In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 82-87

DOI: 10.5220/0002023800820087

 SciTePress

namic environments. A more detailed view of RT

challenges in WSNs can found in (Stankovic et al.,

2003; Li et al., 2007).

In December 2003, IEEE introduced the IEEE

802.15.4 standard (IEEE, 2003) - covering the

Medium Access Control (MAC) layer and the phys-

ical layer - for Low-Rate Wireless Personal Area Net-

works (LR-WPAN). The IEEE 802.15.4 protocol is

particularly suitable for WSNs because it is mainly

speciﬁed for: low data rates, providing reliable data

transfer, low cost and low power consumption. Espe-

cially interesting for RT applications is that the IEEE

802.15.4 MAC protocol has the capability of provid-

ing Guaranteed Time Slots (GTS) for time-sensitive

data, similar to traditional TDMA allocation. The

IEEE 802.15.4 protocol has been widely adopted as

one of the more interesting candidates for develop-

ing RT communication protocols for WSNs. The

GTS transmission scheme allows each node in the

network to send data, using a certain number of time

slots according to the ﬁrst-come-ﬁrst-served (FCFS)

algorithm. However, due to its simplicity, only static

schedules can be implemented using FCFS based

scheduling. This problem has motivated a number of

researchers to work on more efﬁcient RT scheduling

methods, enabling dynamic allocation of time slots.

This paper presents the most relevant characteris-

tics of the IEEE 802.15.4 - the de facto standard for

time-sensitive WSN applications - for RT scheduling

methods. Two of the more interesting RT schedul-

ing methods developed for this standard are reviewed

(Koubaa et al., 2006b; Mishra et al., 2007). The

rest of the paper is organized as follows: section

2 introduces to real-time concepts and the main re-

quirements for developing a RT communication pro-

tocol. Section 3 provides a brief description of the

IEEE 802.15.4 standard. In section 4, we discuss

two related allocation methods: one method based

on ﬁx-priority scheduling and one based on dynamic-

priority scheduling. Finally, we make a concluding

remark of our review ﬁndings and discuss possibili-

ties for future work in section 5.

2 REAL-TIME

COMMUNICATION

The characteristics of RT communication differ from

the characteristics of traditional best-effort communi-

cation. For example, in best-effort communication,

the performance measurement of a communication

protocol is the network throughput. In RT communi-

cation the performance of the protocol is measured by

the number of messages successfully delivered before

their deadlines. RT communication is usually divided

into two main categories: hard RT and soft RT. In soft

RT communication, some amount of packet loss can

be tolerated, while in hard RT communication there is

no toleration for any packet loss. In hard RT commu-

nication, all messages have strict timing constraints.

For example, if any packet is delivered late, it is con-

sidered as being lost. In hard RT communication, an

upper bound on end-to-end message delays or a de-

terministic bound is required. In contrast, for sup-

porting soft RT communication over the network, a

probabilistic end-to-end delay bound should be pro-

vided.

For WSNs, it is more realistic to offer a probabilis-

tic end-to-end guarantees, due to facts that: the net-

work conditions varies with time, the trafﬁc is unpre-

dictable and constraints on power-efﬁciency requires

i.e., limited transmit power. A common approach

taken for supporting end-to-end guarantees in multi-

hop networks in general, and in WSNs speciﬁcally, is

that the MAC protocol should provide a delay bound

for a single-hop, and the network layer should take the

responsibility for multi-hop delay bounds. Schedul-

ing methods are commonly used in the MAC protocol

(scheduled-based MAC protocols). Finding efﬁcient

scheduling algorithms for the MAC protocol is one of

the most important issues for RT communication in

WSN.

3 IEEE 802.15.4

The IEEE 802.15.4 protocol speciﬁes the MAC layer

and the physical layer for LR-WPANs. This proto-

col is often used in conjunction with ZigBee proto-

col (Zigbee Alliance, 2004) to provide a full protocol

stack. This section presents the relevant features of

the IEEE 802.15.4 standard related to RT communi-

cation over WSNs. Further details of the standard can

be found in (IEEE, 2003; Koubaa et al., 2005).

3.1 Network Devices and Topologies

Network Devices. The IEEE 802.15.4 standard

speciﬁes two types of network devices that are sup-

ported: a Full Function Device (FFD) and a Reduced

Function Device (RFD). An RFD is a simple de-

vice operating with a minimal implementation of the

IEEE 802.1.5.4 protocol. An FFD is a device that

has the capability of operating in two modes; either

as a coordinator, providing a synchronization service

through the transmission of beacons, or, as a simple

device but implementing the complete protocol set.

An FFD can act as both a simple coordinator and

ON SUPPORTING REAL-TIME COMMUNICATION OVER THE IEEE 802.15.4 PROTOCOL

a PAN-coordinator. A Coordinator is a network de-

vice that provides synchronization services through

the transmission of beacon frames. If a coordinator

has the function of identifying and controlling the net-

work, it is called the PAN coordinator. A network

should include at least one FFD performing as PAN-

coordinator.

Network Topologies. The IEEE 802.15.4 protocol

supports networks organized in both star-topology

peer-to-peer topology. In star topology, communi-

cation is established between network devices (FFD

and RFD) and the PAN-coordinator. After an FFD

is activated the ﬁrst time, it may establish its own

network and become the PAN coordinator acting as

the network’s controller. A PAN coordinator can al-

low other devices to join its network. The communi-

cation paradigm in star network topology is central-

ized. In peer-to-peer topology, any network device

can communicate with any other devices within its

radio range. There is also a PAN coordinator in the

peer-to-peer topology. In contrast to the star topol-

ogy, a device can be nominated as PAN coordinator

by another device in the peer-to-peer topology. A net-

work with peer-to-peer topology can be ad hoc, self-

organizing, self-healing, and allows multiple hops to

route data from any source node to any destination

node.

3.2 IEEE 802.15.4 MAC Layer

The MAC protocol speciﬁed in IEEE 802.15.4 can

operate in two different modes: beacon-enabled

mode and non beacon-enabled mode.

In beacon-enabled mode, the coordinatorgenerate

beacons periodically in order to synchronize all nodes

in the PAN and to identify the PAN. Data exchanged

between all nodes and the PAN coordinator has a spe-

cial format, which is deﬁned by the PAN coordina-

tor, called superframe structure. The length of a super

frame is equal to the interval between two consecutive

beacons, and it is divided into 16 equally sized time

slots. Figure 1 illustrates some parameters embedded

in a superframe.

A superframe can either include both an active-

and an inactive period, or just an active period. Dur-

ing the inactive period, the coordinator do not have

any interaction with other nodes and it stays in a low

power-mode. The ﬁrst slot in a superframe is used for

the beacon frame.

• CAP. The contention access period (CAP) starts

immediately after the beacon frame. Transmis-

sions occurring during the CAP period must fol-

low a slotted-CSMA/CA mechanism. Nodes

wishing to transmit must compete for the com-

munication medium based on CSMA/CA with

some back-off period, aligned with superframe

slot boundaries.

• CFP. In difference to other IEEE standards

for wireless communication, such as the IEEE

802.11, the IEEE 802.15.4 do not only offer the

CSMA/CA mechanism for the MAC protocol.

The IEEE 802.15.4 MAC protocol may also in-

clude a contention free period (CFP) within the

superframe. CFP is an optional ﬁeld, which has

been deﬁned for QoS requirements on for exam-

ple low-latency or a speciﬁc transmission band-

width. A PAN coordinator can allocate a cer-

tain number of time slots called Guaranteed Time

Slots (GTS), during the CFP upon on request. All

transmissions occurring during the CFP do not

follow the CSMA/CA mechanism. The PAN co-

ordinator can assign up to 7 GTSs and can only be

used for communication between a node and the

PAN coordinator.

• GTS. A GTS scheme is a kind of resource reser-

vation in WPANS. It is a portion of the super-

frame, which is allocated exclusively to a given

node in the PAN. When a node have data with

timing constraints to transmit, it sends a request

frame to the PAN coordinator asking for GTSs.

The PAN coordinator will do admission control to

check whether the request can be accepted or not.

The GTSs are allocated following the basic ﬁrst-

come-ﬁrst-served (FCFS) rule. A node is assigned

GTSs if there are resources available, i.e. if the

number of requested slots are less than or equal

to the number of free slots in the superframe du-

ration. Node with allocated GTSs can access the

communication medium without contention dur-

ing the CFP. A node transmitting during a GTS

must guarantee that its transmission end before

the next GTS start. There are maximum seven

GTSs that can be used at the same time.

In non beacon-enabled mode, there are neither

beacons nor superframes. Nodes communicate with

each other freely according to the unslotted CSMA

mechanism. Both the non beacon-enabled and the

beacon-enabled mode use the CSMA/CA with back-

off periods, but in the former the back-off period for

each node is independent from other nodes back-off

periods.

WINSYS 2008 - International Conference on Wireless Information Networks and Systems

Contention

Free Period

Contention Access

Period

Inactive

Period

Beacon frame

Figure 1: Structure of a supperframe.

4 REAL-TIME PROTOCOLS

OVER IEEE 802.15.4

The GTS scheme in beacon-enabled mode is more in-

teresting when it comes to supporting RT communi-

cation over the IEEE 802.15.4. However, there are

still some limitations that might make it less efﬁ-

cient (Koubaa et al., 2006b). Firstly, since the GTS

allocation is managed using simple FCFS, there is

no priority support. All types of trafﬁc are treated

equally. Secondly, it might also happen that a node

with low arrival rate will be assigned unnecessary

high bandwidth and thereby reducing the network uti-

lization. Moreover, the GTS allocation is made ex-

plicitly, which means that the number of nodes that

can have access to GTSs is limited to the maximum

number of GTSs.

During the last two years, there are few works re-

ported on improving the GTS allocation scheme in or-

der to provide a more efﬁcient way of utilizing GTS to

support RT communication. The most common way

is to apply some kind of scheduling method, either

static scheduling algorithms or dynamic scheduling

algorithms. In this section, we review two of the more

recent and interesting GTS allocation methods. One

method uses static scheduling based on round-robin

scheduling algorithm (called i-GAME ) (Koubaa et

al., 2006b) and the other applies dynamic scheduling

based on the earliest deadline ﬁrst (EDF) scheduling

algorithm (Mishra et al., 2007) (called GSA).

4.1 i-GAME

Motivated by having a more efﬁcient GTS alloca-

tion in IEEE 802.15.4, A. Koubaa et. al proposed

an implicit GTS Allocation Mechanism (i-GAME)

for time-sensitive wireless sensor networks (Koubaa

et al., 2006b), instead of using explicit GTS alloca-

tion. The i-GAME method allows several nodes in

the network to share the same GTS under the round-

robin scheduling algorithm. The node that wants to

have a guaranteed time slot, sends its trafﬁc speciﬁca-

tion and its delay requirement to the PAN coordinator.

The PAN coordinator will do the admission control

to check whether the amount of available GTSs sat-

isfy the communication requirements. The authors in

(Koubaa et al., 2006b) have adopted the (b,r) trafﬁc

model, in which b denotes the maximum burst size,

r denotes the average arrival rate. The network is a

star topology network with a unique PAN coordinator.

Each node i in the network generates a ﬂow F

repre-

sented by three parameters (b

, r

, D

), where D

is the

delay requirement of ﬂow F

. It has been addressed

in (Koubaa et al., 2006a; Koubaa et al., 2006b) that,

N ﬂows F

, i = 1, 2, .., N are allowed to share a GTS

allocation consisting of k slots if

• the sum of all arrival rates is less than or equal

to the bandwidth of k time slots:

∑

i=1

≤ k × R

where R

is bandwidth guaranteed per time slot

• the delay bound guaranteed by the allocation,

i,max

, does not exceed the delay requirement:

i,max

≤ D

, ∀1 ≤ i ≤ N

The calculation of guaranteed delay bounds per time

slot for original explicit GTS allocation and for the

implicit method has been presented in (Koubaa et al.,

2006a) and (Koubaa et al., 2006b) respectively. Net-

work calculus have been used in both cases. By ap-

plying a round-robin scheduling algorithm, i-GAME

allows each ﬂow to equally share the bandwidth of k

time slots in GTS.

Although the round-robin algorithm provides a

fair GTS allocation scheme, it is not effective when

different trafﬁc ﬂows have different arrival rates.

Thus, it would be even more interesting to ﬁnd a more

ﬂexible allocation scheme that can support different

classes of trafﬁc.

4.2 GTS Allocation with On-line

Scheduling Algorithm - GSA

A. Mishra et. al (Mishra et al., 2007) recently

presented a new GTS allocation scheme in IEEE

802.15.4, with an on-line scheduling algorithm sup-

porting both periodic and aperiodic real-time trafﬁc

(named GSA). GSA is applied in the same network

conﬁguration as i-GAME, in beacon-enabled mode,

but a different trafﬁc model has been used. Each node

i in the network generates its trafﬁc ﬂow F

, which is

ON SUPPORTING REAL-TIME COMMUNICATION OVER THE IEEE 802.15.4 PROTOCOL

represented by three parameters: time constraint d

total payload p

and the number of requested slots r

For the GSA method, the authors have presented

a formula to calculate the estimated end-to-end delay

required for each node to send one frame to the PAN

coordinator. Base on this delay estimate, the PAN co-

ordinator will ﬁnd the minimum GTSs needed to be

assigned for each node, in such way that the estimated

delay must be less than or equal to the its requested

delay. All the trafﬁc ﬂows in the network will be

sorted according to the order of increasing deadlines

(EDF scheduling). A set of trafﬁc ﬂows is schedula-

ble with the GSA method, if for each ﬂow, the cumu-

lative transmission delay is less than or equal to its

upper bound. The GSA can be described as follows:

1. Input: S = F

, p

, r

)

2. Calculate estimated delay ED

3. Assign minimum number of GTSs s

for each ﬂow

4. Do a feasibility analysis of the set S

5. If not feasible the last ﬂow will be rejected

The biggest advantages of GSA is that it applies

an on-line scheduling algorithm, providing a better

support for different trafﬁc classes. However, GSA

is composed of a complicated analysis which cause

overhead for the PAN coordinator. It is obvious that

the analysis used in GSA could be improved. One of

the very well known techniques used for EDF feasi-

bility analysis - that have been widely adopted in the

research community, processor demand test (Baruah

et al., 1990), could be applied in GSA. The authors

in (Mishra et al., 2007) also showed the improvement

of GSA in comparison with basic FCFS scheduling.

However, the complexity of the algorithm has been

overlooked. Another drawback of GSA is that it may

waste a portion of the GTSs since it always allocates

the GTSs from the ﬁrst slot of the CFP.

5 CONCLUDING REMARKS

In this paper, we have presented the most relevant

characteristics of the IEEE 802.15.4 standard for sup-

porting RT communication in WSNs. We have cho-

sen to review two of the more interesting RT protocols

that have been proposed for IEEE 802.15.4: one pro-

tocol applying a static scheduling method (i-GAME)

and the other applying a dynamic scheduling method

(GSA). The i-GAME protocol has low complexity, is

easy to implement and introduces fairly small over-

head for the PAN coordinator. Moreover, iGAME

provides a fair allocation for all nodes in the network,

which is very efﬁcient in the speciﬁc case where all

trafﬁc ﬂows have a similar arrive rate. On the other

hand, the GSA protocol has signiﬁcant advantages

compared to iGAME since scheduling is dynamic and

performed on-line. Thus, a broader range of traf-

ﬁc classes are supported, such as for applications re-

quiring communication with unbalanced arrival rates.

The trade-off in the GSA compared to iGAME, is the

higher complexity of the scheduling algorithm and a

higher overhead for the PAN coordinator. It should

also be mentioned that iGAME has been implemented

in a real sensor network while GSA has been evalu-

ated by a simulation study.

As future work, we aim to investigate improved

scheduling methods to be applied for the IEEE

802.15.4. We are in favor of dynamic scheduling

methods, such as in the GSA, and we believe that

scheduling complexity can be reduced, for example

by applying a more simple feasibility analysis such

as the processor demand function. We are also inter-

ested in investigating some speciﬁc application such

as voice or media over WSNs, where QoS is highly

important.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Anis Koubaa and

Dr. Elisabeth Uhlemann for valuable input to this pa-

per. This work has been funded by the CERES re-

search proﬁle grant from The Knowledge Foundation.

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ON SUPPORTING REAL-TIME COMMUNICATION OVER THE IEEE 802.15.4 PROTOCOL