Int. J. Communications, Network and System Sciences, 2010, 3, 925-933
doi:10.4236/ijcns.2010.312126 Published Online December 2010 (http://www.SciRP.org/journal/ijcns)
A Synchronou s and Deterministic MAC Protocol for
Wireless Communications on Linear Topologies*
Daniele De Caneva1, Pier Luca Montessoro2
1Pervasive Technologies Laboratory, Agemont, Italy
2Department of Electrical, Managerial and Mechanical Engineering, University of Udine, Udine, Italy
Received September 30, 2010; revised October 31, 2010; accepted November 22, 2010
Abstract
Linear topology is useful in several pervasive application scenarios. Even though a linear topology can be
handled by unspecific routing algorithms over general purpose MAC protocols, better performance can be
obtained by specialized techniques. This paper describes a new communication scheme called Wireless Wire
(WiWi), which builds up a bidirectional wireless communication channel with deterministic properties in
terms of throughput and latency over a strip of pervasive devices with short-range transmission capabilities.
The system is synchronous and fault tolerant. With low cost and extremely simple devices, WiWi builds up a
“wire-like” dielectric link, but its applications are not limited to end-to-end communications. For example,
WiWi can be used to collect data from sensors along the path, thus acting as a virtual conveyor belt.
Keywords: Wireless Sensor Networks, Linear Topology, Pervasive Devices, Ad-Hoc Network
1. Introduction
Many routing protocols have been designed for Wireless
Sensor Networks (WSNs) considering nodes that operate
in a mesh topology. For specific application scenarios,
however, a mesh topology may not be appropriate or
simply not correspon ding to the natural node deplo yment.
Bridge [1] or pipeline [2] monitoring applications are
examples where the position of sensor nodes is prede-
termined by the physical structure and application re-
quirements. In this applications, where it is clearly pre-
sent a privileged dimension, it is quite natural to take
advantage of it.
So far, little focus has been given to efficient MAC
protocols for low-power, wireless communications over
linear topologies. This paper presents WiWi (Wireless
Wire): a contention-free MAC protocol based on syn-
chronous multi-hop transmission along a chain of inde-
pendent nodes.
The original and main purpose of WiWi is to perform
the virtualization of a wired link by means of an ad-hoc
network, made of a chain of tiny short-ranged transceiv-
ers with limited power capabilities. Nonetheless, WiWi
is not limited to end-to-end communication but it can be
profitably used to collect data along the path.
In the WiWi architecture, devices are displaced in or-
der to build up a linear (or curvilinear) strip. WiWi does
not require any routing table or complex calculation for
message delivery: this makes it feasible to apply the
protocol even on very simple transceivers, with limited
memory and processing capabilities. Moreover, it is
ready for hardware implementations that could be real-
ized on tiny pervasive devices.
The issue regarding synchronization of nodes along
the network is addressed by choosing fixed-size mes-
sages.
WiWi takes advantage of linear topology and syn-
chronous communication to provide deterministic and
predictable latency and throughput in both directions. As
will be later described, they can be configured by modi-
fying protocol parameters in order to fulfill the nodes’
capabilities and the app lication requirements.
The paper is organized as follows. After this introduc-
tion, Section 2 presents interesting related work. The
WiWi architecture, protocol, performance and applica-
tions are discussed in Section 3, whereas Section 4
shows the prototypal implementation. A fault-tolerant
WiWi node architecture is presented in Section 5, and
Section 6 draws some conclusions tracing some expecta-
tions for future wor k.
*This work has been partially funded by Project A-LEAP/L.R. 26/2005
(art. 21), Regione Friuli Venezia Giulia.

926 D. DE CANEVA ET AL.
2. Related Work
Design issues and tradeoffs that need to be considered
for power-constrained WSNs with low data rate links
have been addressed and studied in noteworthy works
[3-5].
A series of studies on routing in ad hoc networks and
WSNs face the problem of optimization on behalf of
higher layer parameters, such as efficient localization,
propagation, resiliency, and so on, proposing a wide va-
riety of algorithmic solutions. Some approaches also
examine cross-layer issues that aim at minimizing energy
consumption and computation [6]. It is likely to observe
that topology in most studies is often in a second order
matter, since nodes are expected to be mobile or to be
deployed taking random positions in a field. More re-
cently, deployment of wireless sensors has been studied
and optimized to achieve coverage and connectivity [7].
Topology is important for any type of network be-
cause it has great impact on the performance of the sys-
tem. Limited research has been conducted on the effect
that well-defined topologies have on protocols for wire-
less networking [8]. The focus, however, has been on
mobile networks rather than the ones with regular to-
pologies or with a fixed node placement. The case of
patterned WSNs is known in literature and well de-
scribed in [9].
On the other hand, most works are interested in dem-
onstrating how the topology of a WSN impacts on the
performance of a given MAC or routing protocol. Our
perspective, instead, aims at simplifying protocol re-
quirements and device complexity starting from a very
specific application, in order to propose a reliable and
efficient solution for this and similar problems.
Quite common methodologies in WSN-related proto-
col development [10] recommend that protocols should
reduce the number of contentions to improve power sav-
ing, as well as using shorter packet lengths. The receiver
usage time, however, tends to be higher for protocols that
require the mobile nodes to sense the medium before
attempting a transmission. In our system, the protocol
has been optimized on behalf of these main goals.
Moreover, devices’ link layer is capable to perform and
keep synchronization during the whole lifetime of the
network.
In literature we can find some examples of algorithms
and protocols that are specifically aimed for linear to-
pologies. MERR [11] is a routing protocol whose refer-
ence scenario is a network made by sensors deployed
over a linear topolog y. MERR deals with the problem of
finding the best route from every node to a common con-
trol center. With MERR, Zimmerling et al. propose a
distributed protocol where each node independently
chooses the best relay node among its neighbors.
In [12] is presented an algorithm whose aim is to
minimize the rou ting path and, at the same time, balance
the load. In particular that work covers the special case
of a network where nodes are located in a narrow strip
with a width at the most 32 times the communica-
tions range of each node.
Both [11,12] are focused on routing problems without
considering the underlying MAC protocol, which could
become a real bottleneck for network performances.
In [13] is presented DiS-MAC (Directional Scheduled
MAC). This protocol has been developed for wireless
sensor networks that show a linear topology. It reaches
the considerable channel utilization of 1/2, but requires
every node could direct the radiation beam of its antenna
and suffers from being unidirectional. WiWi recalls
some DiS-MAC features; in particular both protocols
avoid interferences between simultaneous transmissions
by alternating transmissions between adjacent nodes.
However, WiWi presents many important advantages: it
does not require directional antenn as, it provides bidirec-
tional communication over a single RF channel (thus
providing support for an end-to-end acknowledgement),
and it can be configured in order to make the bandwidth
and latency fit the application n eeds.
3. WiWi Architecture and Communication
Protocol
The development of WiWi was originated by the need
for a system able to emulate a wired link by means of an
ad hoc network constituted by nodes distributed along a
strip. The purpose of this emulation is to handle scenar-
ios where a single hop wireless link is not feasible and a
wired link is not practical. An example could be given by
a speleologist going deep down into the bowels of the
Earth, who can deploy the wireless network while it goes
further with the exploration in order to maintain a com-
munication channel with the outside world. Other exam-
ples can be found in all those situations where a multi-
hop link is required, in particular those bounded to
monitoring applications.
The design of WiWi architecture is based on the ob-
servations about wireless communications presented by
Min and Chandrakasan in [3]. In particular the authors of
[3] showed that even if a power law often describes the
radiated power necessary to transmit over a distance d
and path loss n, this term alone fails to consider the en-
ergy overheads of the hardware. Min and Chandrakasan
suggested an energy consumption model with an addi-
tional distance-independent term in order to take account
of overheads caused by transmitter and receiver elec-

D. DE CANEVA ET AL.
927
d
tion pattern is presented in the following sub section.
tronics (such as PLLs, VCOs, LNA, bias currents, etc.).
The proposed model is the following:
3.1. Basic Communication Pattern

En d

  (1)
where α is the distance-independent term. Additionally,
Min and Chandrakasan collected the estimated values of
the model’s term for a set of short-range radios having
maximum output power up to +20 dBm.
The communication between WiWi nodes is synchro-
nous, based on fixed size packets, and follows a stag-
gered pattern like the model presented in DMAC [15].
However, unlike the DMAC protocol, which is designed
to handle tree topologies, in WiWi the synchronism is
intrinsic in the communication model and bidirectional
data flows are supported.
Starting from these data, authors of [3] noted that for
short-range radios typically used in MANET research,
the value of α dominates the value of the path loss term.
This has severe consequences on multi-hop wireless
communications that try to reduce energy consumption
by adding intermediate relay nodes in order to redu ce the
path loss term. This strategy affects only the path loss
term (βdn) limiting the value of d, however this is useful
only for long range radio links where that term is domi-
nant. The energy model presented in [3] shows that a two
hop link consumes less energy than one hop link when
Figure 2 shows the communication model and the dif-
ferent handling of downstream and upstream data flows.
In this and following diagrams the vertical axis is de-
picted downward, according to the direction of the
downward flow that provides synchronization to all the
nodes. The master node, source of synchronization, is at
position n = 0.
The downstream data flow is generated by the head of
the chain which acts as master end point. This data flow
(the gray one in Figure 2) proceeds downwards from the
head of the chain to the tail follo wing a strictly staggered
pattern: a node sends a packet to the next one, which in
turn immediate ly fo rward s the pack et furthe r down along
the chain. This stream is responsible of maintaining
overall network synchronization too: every node resyn-
chronizes its clock upon the start of the incoming down-
stream packets.

1
12n
n
d 



 
(2)
Min and Chandrakasan noted that with the exception
of the μAMPS-1 custom radio, this inequality never
holds for typical path losses.
Similar considerations where made in [14] where
Bhardwaj et al. introduced the concept of characteristic
distance of a transceiver. This distance is strictly bounded
to the transceiver characteristics and minimizes power
consumption in a multi-hop communication. In fact, the
characteristic distance represents the optimal tradeoff
between distance-independent and distance-dependent
terms in power consumption relation.
The upstream flow follows the same principle of pass-
ing messages along the chain, but between the reception
of a packet and its forwarding, the node waits four time
slots in order not to collide with the downstream one. In
Figure 2, the upstream flow is depicted in white blocks,
while the arrows of different patterns follow the ad-
vancement of different upstream packets.
WiWi has been designed as a synchronous multi-hop
communication scheme where nodes are intended to be
deployed with mutual distances approximately close to
the characteristic distance (Figure 1). No other assump-
tion is made over node deployment: the chain of nodes is
not required to strictly follow a straight path as it can
bend. Moreover, in order to minimize the possible pro-
duction costs, we assumed that pervasive devices could
be absolutely identical one to each other, in the sense that
no factory-set unique ID is required.
This scheme combines very good performance with
high regularity that makes its implementation easy. In
fact, once a node is synchronized with the downstream
flow, its activity pattern is receive-transmit-idle-transmit-
receive-idle (R-T-I-T-R-I) regardless its position in the
chain.
Moreover, in WiWi, nodes require no explicit ad-
dressing because within the range of transmission there
is only one receivin g node, i.e. th e destination of the n ext
hop for t he pa cket.
A synchronous architecture is not very common in
typical sensor networks described in literature, neverthe-
less it is our opinion that its deterministic beha vior better
suites the aim of WiWi. It is worth noting that the transmission scheme is de-
signed to avoid interferences among different nodes. In
A detailed description of the synchronous communica-
Figure 1. WiWi topology: The linear strip as a chain of nodes.

928 D. DE CANEVA ET AL.
Figure 2. Bidirectional staggered transmission.
fact, from the receiver point of view, when an adjacent
node is transmitting, the closest other transmitting node
is three hops away. So, considering the common assump-
tion that the interference radius is twice the nominal
transmission one [13] this three-hop safe distance grants
a good resiliency from inter-node interference.
3.2. Performance Evaluation and Advanced
Patterns
A performance evaluation of this scheme of the commu-
nication pattern is straightforward. Let N be the number
of hops of the link, B the maximum raw bit rate and S the
time slot associated with a single packet transmission
(which will also include a communication safety time in
order to handle clock drifts between consecutive nodes).
Both downstream and upstream flows use one time slot
every six, therefore each one provides a bit rate of nearly
B/6 and the overall channel utilization is equal to 1/3 (1 /6
for each stream). Actually, the bit rate will necessarily be
lower than B/6 due to non-idealities of hardware (e.g.
clock drifts). It is worth noting that a channel utilization
equal to 1/3 is the optimum for a bidirectional commu-
nication, being the maximum allowed by the minimum
three-hops safe distance chosen to avoid interference
between simultaneous transmissions. Additionally, the
throughput of both streams equal to 1/6 of the transceiver
rate has to be considered a good tradeoff between per-
formance and latency, since in [16] the max imu m oper a-
tional limit of a unidirectional wireless chain is called to
be equal to 1/4.
The downstream flow has the minimum latency al-
lowed by a store-and-forward pattern, being the delay
equal to NS. On the other hand the upstream flow delay
is five times larger: since a packet can be sent every six
time slots, there is, in average, an additional constant
delay of three time slots per hop.
However, WiWi architecture is flexible: the receive-
transmit pattern can be adjusted to fit the desired trade-
off between latency, bandwidth and symmetry of up-
stream and downstream flows, at the cost of a less-
than-optimal channel utilization. Even the distance be-
tween simultaneously active transmitters could be changed
depending on the RF layer requ irements. Figure 3 shows
a pattern similar to the previous one but providing
asymmetrical throughput (B/5 downstream and B/10

D. DE CANEVA ET AL.
929
Figure 3. Bidirectional, staggered transmission with asymmetric throughput and latency over a WiWi link.
upstream) and asymmetrical latency (NS downstream
and 10 NS upstream, plus in average 2.5 time slots and 5
time slots respectively to wait for the transmission win-
dow).
Figure 4 shows another pattern where both bandwidth
and latency are symmetrical: a transmission occurs every
eight time slots in each direction, providing an overall
channel utilization of 25%, whereas the latency is 2NS.
The block arrows show the travel, in space and time, of
downstream and upstream packets.
Unfortunately, in this case the receive (R), transmit (T)
and idle (I) pattern is no longer independent from the
cluster position along the chain: even-order clusters run
R-R-T-T-I-I-I-I, whereas odd-order clusters behave
R-I-T-R-I-T-I-I, thus requiring a bit more complex chain
setup.
WiWi does not provide acknowledgment to guarantee
the correct packet exchange between nodes: datagram
transmission is much more flexible in the scenarios in
which WiWi is supposed to operate. This choice implies
that the nodes could be very simple because they do not
have to store more than two packets at a time in their
internal memory. Anyhow, error correction codes could
be adopted to increase the reliability of communications
along the WiWi chain. In addition, an acknowledgement
system could be implemented by higher layer protocols
between the head and the tail of the strip. In scenarios
where the end points are many hops away from each
other and the packet error rate is particularly severe, this
solution could be responsible of poor latency perform-
ance due to frequent retransmissions. This problem could
be mitigated by displacing intermediate endpoints, which
corresponds to deploy many WiWi strips in sequence
instead of one single strip.
3.3. Strip of Sensors: WiWi as a Conveyor Belt
So far, WiWi has been presented as a protocol for
end-to-end communication on a strip of short-range
wireless devices. But the WiWi synchronous nature
makes it a powerful tool to collect data from a strip of
sensors. Conventionally, in routing-oriented sensor net-
works, a sensor places its reading in a packet that finds
its way through the network, fulfilling at each hop the

930 D. DE CANEVA ET AL.
Figure 4. Bidirectional, staggered transmission with symmetric throughput and latency over a WiWi link.
chosen MAC rules. In WiWi each node can host one or
more sensors. The data flow that continuously traverses
the strip can be seen as a conveyor belt on which the
sensors place their readings. Several application-level
protocols can be adopted to handle possible conflicts: for
example, a flag that marks each time slot when vacant, or
a more sophisticated token-based protocol.
The WiWi application as conveyor belt for a strip of
sensor has been tested using accelerometers in our pro-
totypal implementation, as described in the next section.
4. Testing and Prototyping
WiWi has been validated in both fault-free and faulty
scenarios (see Section 5) by simulation using the Om-
net++ platform [17], but the obtained results are limited
by the validity of the transmission model that, in the real
world, should include RF hardware performance, battery
levels, environment, interferences, etc.
In order to verify the feasibility of the proposed ap-
proach and show the effectiveness of the system, we de-
veloped a prototype for two demo applications: end-to-end
communications and strip of sensors.
The prototype (Figure 5) accommodates a Microchip
microcontroller (PIC16F689), I/O hardware for debug-
ging purposes, and a transceiver. Two different trans-
ceiver modules by Aurel [18] have been tested; their bit
rates and transmission ranges are reported in Table 1.
Table 1. Transceivers performance.
TransceiverModulation Bit rate
Transmission
range
XTR-CYP-2
.4 GHz
GFSK,
78 channels
2.4 GHz band
up to
64 Kbit/s 45 m
XTR-VF-
2.4 LP
GFSK,
98 channels
2.4 GHz band
up to
1 Mbit/s 10 m

D. DE CANEVA ET AL.
931
Figure 5. Hardware prototype (5 cm × 6 cm).
The prototype is equipped with a switching step-up
voltage regulator in order to reduce power consumption
and make possible the use of two AA batteries as power
supply. USB connectivity has been included in order to
expedite firmware debugging and enable WiWi end-
points to exchange data with any common PC.
The first application developed with this prototype
emulated a chat application between two computers, us-
ing WiWi as channel to deliver messages from one user
to the other (Figure 6).
In the second application WiWi has been used to ag-
gregate and transport data coming from 3-axis acceler-
ometer sensors connected to WiWi nodes (Figure 7).
Data coming from sensors were preprocesses by nodes in
order to filter noise and obtain the maximum acceleration
for each axis in 10 milliseconds time windows. Since this
kind of application doesn’t need an extremely high
throughput, 500 ms time slots have been used, much
longer that the 1 ms packet length. This way proper
throughput and consumption performance have been
achieved since each node had more time to spend in low
power mode. At each step along the chain, data meas-
urement pertaining to the traversed node was inserted
into a specific field within the current packet. Since this
data-collecting application is essentially unidirectional,
the downstream flow was delegated to remote control
and management of the nodes. This flow was much less
intense, therefore downstream packets were shorter (one
quarter of upstream packets), resulting in additional en-
ergy saving, since idle time has been further increased.
5. Designing a Fault-Tolerant WiWi Node
An obvious weakness of chain topologies is that a single
node failure can block the whole communication channel.
Nevertheless, if the topology of the physical environment
supposed to host the chain is strictly linear, neither backup
paths nor meshed topologies can be deployed. Since
tunnel via WiWi
Figure 6. Chat demo application.
3- axis
acceler o meter
Figure 7. Sensoristic demo application.
multiple nodes within the same transmission range are
not suitable for the WiWi synchronous protocol due to
interferences and performance loss, a fault-to lerant WiWi
node has been designed.
A fault tolerant WiWi node is made of multiple trans-
ceiver modules (Figure 8), each one able to handle the
whole communication protocol. Their activity is sched-
uled on behalf of a round robin policy. Just one module
at a time is effectively involved in packet forwarding,
while the other ones act as backup modules. The mod-
ules are ordered, and the first one is the module on duty,
responsible for p a cket forwarding .
During the receive time slot every module receives
and stores the packet coming from the previous node in
the chain. In the subsequent transmission slot, the mod-
ule on duty forwards the packet, while at the same time
all backup modules sense the transmission to be sure it
occurs. If the first backup module perceives the loss of
the module on duty, after a short “sense time slo t” it for-
wards the packet. The remaining backup modules keep
listening to the channel in order to be sure that a backup
module has reacted. The process is iterated until a
backup module forwards the packet or there are no more
backup modules left. Figure 9 shows this technique giv-
ing an example of redundancy and failure management
over four modules: at the beginning of the transmit slot
the module on duty (number 0) should forward the
packet (time t1). Modules 1, 2 and 3 start sensing the
transmission for the duration of the s1 sen sing win dow. If
no transmission occurs, at time t2 node 1 should backup
module 0 transmitting the packet. Remaining modules 2
and 3 continue to sense, in order to backup module 1
(with module 2 at time t3) and, if still needed, module 2
(with module 3 at time t4). In the example the packet is
forwarded by the last backup module.
After a module failure, active backup modules rede-
fine their order. This way, the same technique can be
used to provide fault tolerance and to schedule sleep-times

932 D. DE CANEVA ET AL.
Figure 8. Hardware prototype.
Figure 9. Redundancy mec hanism.
for load balancing and energy saving.
This fault tolerance extension to the basic WiWi pro-
tocol grants a redund ancy equal to the number of backu p
modules. The slot time upon which WiWi is based must
be extended to make room to the sensing time windows,
leading to a minor loss in throughput and latency per-
formances.
The extended time slot can be modeled as in Figure
10. Being Nmax the maximum number of nodes belonging
to a pool, the duration of a slot is represented by three
components:
 TP: time needed to receive the packet, depending
only on packet size and transceiver bit-rate;
 TS: sensing time window: sum of the sensing time
and the time needed to commute the radio module
from sensing state to transmitting state;
 TG: the inter-packet gap, a period comprehensive of
the time needed to switch from the receiving state
to the transmitting state, the time needed to store
and elaborate the packet, and a safety margin to
counteract clock drifts.
Hence, with Nmax modules in each node, the time dedi-
cated to packet transmission is equal to

max 1 S
TN T
P


and the total duration T of a slot is:

max 1
P
P
TTNT T
G


(3)
Thanks to the decoupling of the tiers composing the
architecture, the other performance relations remain valid.
In fact throughput is equal to

6
pck S
TT
while latency
is equal to NTS for the downstream flow and (N + 3)TS
for the upstream flow.
Figure 10. Time slot components.
The backup technique requires modules within each
pool to be dynamically enumerated and ordered. Option-
ally, the architecture could include a set of algorithms for
modules deployment and management that do not require
any manual configuration or the assignment of unique
node ID at firmware/hardware level. This aspect is not
discussed here because several algorithms for cluster
organization can be found in literature and easily app lied
to WiWi.
6. Conclusions and Future Work
A new MAC protocol for linear topologies, called WiWi,
has been presented. It is based on a chain of short-range
communication devices. Predictable latency and band-
width are provided by the synchronous communication
scheme and by specific transmit-receive patterns that
have been shown in Section 3. Fault tolerant node design
has been discussed as well. WiWi has been simulated
and tested on hardware prototypes.
Strategies to use WiWi in different scenarios are cur-
rently under study, in particular to use the strip as a fun-
damental brick for more complex topologies. Moreover,
different channels can be used to handle overlapping
strips, thus allowing a variety of topologies that com-
bines TDMA and FDMA techniques. In tree topologies,
an asymmetric configuration of the protocol can be ex-
ploited to benefit the links from the root to the leaves,
making WiWi a challenging solution to control and de-
liver data to a large number of devices in widespread
applications.
As a synchronous technique, WiWi is well suitable to
include strategies, already known in literature, to sched-
ule stand-by periods and wake-up events to save energy.
Since each strategy may be optimal only for one or few
kind of applications, another current research direction is
to define some detailed application scenarios and select
for them the optimal energy-saving schemes.
7. Acknowledgements
Authors wish to thank Giampietro Tecchiolli from Euro-
tech S.p.A. for his encouragements and his interest in

D. DE CANEVA ET AL.
933
WiWi application scenarios. We also wish to thank
Francesco Cossettini and Manuel Nobile for their con-
tribution in the simulations of the protocol and the de-
velopment of the hardware prototype.
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