MAC Protocols for Wireless Sensor Networks overRadio-over-Fiber Links
Tiago P. C. de Andrade, Nelson L. S. da FonsecaUnicamp, Brazil
[email protected], [email protected]
Leonardo B. OliveiraUFMG, Brazil
Omar C. BranquinhoPUCC, Brazil
Abstract—In this paper, two Medium Access Control (MAC)protocols exclusively tailored to WSNs over RoF (RWSNs)namely SPP-MAC (Scheduled Priority Polling Medium AccessControl) (polling-based) and HMARS (Hybrid Medium AccessControl for Hybrid Radio-over-Fiber Wireless Sensor NetworkArchitecture) (hybrid-based) are proposed. They deal with themain problems in RWSNs i.e. the delay imposed by optical fiberand the existence of two collision domains: the wireless and thefiber optical links. The performance of these two protocols evincetheir effectiveness for the connection of WSNs by RoF links.
Index Terms—MAC Protocols; Radio-over-Fiber (RoF); Wire-less Sensor Networks (WSNs); Hybrid Systems; New Architec-ture.
I. INTRODUCTION
Wireless Sensor Networks (WSNs) are ad-hoc networks
comprised mainly of small sensor nodes (SNs) with limited
resources and one or more base stations (BSs), which connect
the sensor nodes to the rest of the world [1][2]. They are used
for monitoring environments and provide fine grained sensing
to users. Application areas range from battlefield reconnais-
sance and emergency rescue operations to surveillance and
environmental protection. As any other technology, however,
they are not a panacea and suffer from problems such as rapid
attenuation of signal strength [3].
Radio-over-Fiber (RoF), has been employed in network
infrastructures due to its large capacity and low signal attenu-
ation. For instance, RoF has been employed for monitoring
cities [4], mines [5], pipe of gas and oil. In this type of
infrastructure, there are dark fibers that can be used for the
transmission of radio signals [6].
In RoF, radio signals are transmitted on optical links by
Remote Antenna Unit (RAU) while more complex signal
processing is carried out at the Base Station Controller (BSC).
In this way, operational costs are reduced and coverage area
enlarged. In addition, it also implies on access with fine cap-
illarity. Moreover, the large amount of underutilized fibers in
the world makes the RoF technology of paramount importance
for new uses of underutilized fibers. Such idleness is caused
mainly by the wide spread of this type of infrastructure in the
past few years and by the advent of the Wavelength-Division
Multiplexing (WDM) [7] that has optimized the use of the
capacity of the optical fiber.
The use of RoF links for connecting WSNs implies on
low delay when compared with ad-hoc and mesh WSNs and
on high reliability when compared with traditional non-RoF
connectivity [4]. Moreover, the employment of RoF links to
connect WSNs reduces significantly the cost of deploying
WSNs in areas with underutilized fibers since there is no need
to install several base stations. However, the delay introduced
by the fibers and the existence of two collision domains (the
wireless and the optical one) makes such integration specially
challenging.
In this work, we propose two MAC protocols exclusively
tailored to WSNs over RoF (RWSNs) namely SPP-MAC
(polling-based) and HMARS (hybrid-based). They deal grace-
fully with two collisions domains and achieve high perfor-
mance by decreasing collisions, specially the HMARS pro-
tocol. It is also shown that pure collision based protocols
degrades considerably the performance of RWSN.
The remainder of this work is organized as follows. In Sec-
tion II, we discuss related work. In Section III, we present the
architecture concept. In Section IV and Section V, we present
the SPP-MAC and the HMARS MAC protocols respectively.
In Section VI we describe simulation setup and results. Finally,
in Section VII we draw conclusions.
II. RELATED WORK
There is a significant number of studies on MAC protocols
for conventional WSNs (e.g. [8], [9], [10], [11], [12], [13],
[14], [15], [16]). Ye et al., proposed the Sensor MAC (S-MAC)
protocol [8] that provides a tunable periodic active/sleep cycle
for energy conservation. During sleeping periods, nodes turn
off the radio to save energy and during active periods, nodes
turn on the radio to Tx/Rx frames. Active periods are of fixed
duration whereas the duration of sleep periods depends on a
IEEE ICC 2012 - Ad-hoc and Sensor Networking Symposium
978-1-4577-2053-6/12/$31.00 ©2012 IEEE 254
predefined duty-cycle parameter. Besides, neighboring nodes
can form virtual clusters to set up a common sleep schedule.
Vamdam et al. presented Timeout MAC (T-MAC) [9] that
follows up on the basic idea introduced by S-MAC. The
novelty of their solution is an adaptive duty-cycle in which
the duration of active periods is no longer fixed but varies
according to the traffic. The key idea is to make a node predict
the channel activity during an active period so that it can
switch its radio off before the active period ends, in case it
does not expect any traffic.
Polastre et al. came up with Berkeley MAC (B-MAC) [11],
which uses a technique based on outliers detection to improve
the quality of Clear Channel Assessment (CCA) comparing
with IEEE 802.15.4 [14] standard. In this technique, a node
searches for outliers in the received signal such that the
channel energy is significantly below the noise floor. If the
node detects outliers during channel sampling, then it declares
the channel is clear. If the node does not find any outliers in
fives samples, then it declares the channel to be busy.
The Framelet MAC (F-MAC) [15] essentially tries to reduce
collisions and interference. It decomposes a packet into several
framelets and sends them at a given frequency. As each
potentially interfering node operates at a different frequency,
the probability of collisions is reduced.
The Sparse Topology and Energy Management (STEM) [16]
protocol uses two channels: a wakeup channel and a data
channel. The wakeup channel is used to organize a meeting
between the transmitter and the receiver to avoid deafness,
whereas the data channel is used only for data exchange
once the meeting occurs. To ensure a meeting between the
transmitter and the receiver, nodes follow a preamble sampling
approach.
Among the works that consider RoF as a backhaul for
radio applications, Gomes et al. [17] present an analysis of
the use of RoF technology in IEEE 802.16 networks. The
propagation delay introduced by the fiber length impacts the
tuning of MAC and physical layers parameters. For an effec-
tive tuning, this work presents a comprehensive study of the
performance degradation of WiMAX networks employing RoF
infrastructure indicating the feasibility of RoF scenarios with
degradation bounded to 20% at the physical layer when using
fiber links with maximum length of 115km and degradation
bounded to 20% at the application layer for fiber length of up
to 80km.
Tang et al. [5] proposed a hybrid architecture that is
adequate to monitor temperature, humidity, gas and to locate
employees in environment such as mine. In this scenario,
the radio technology is not feasible, considering the high
attenuation of radio signals. Hossen et al. [4] pointed out
the advantages of RWSNs for metropolitan areas considering
radius of around 10 km to create a smart environment for
monitoring. The application, without the RoF as a backhaul,
would need several ad-hoc network hops to transmit from
distant nodes to the central station.
The work in [?] proposed solutions for the increase of
fiber extension based on the manipulation of the RTS/CTS
timeout. Simulation results show that fiber up to almost 8 km is
feasible, but leads to degradation of 15%. In [18], an extensive
analysis has been carried out using real testbed experiments,
simulations and analytical approximations. Results showed
that the insertion of fibers decreases the throughput less than
15%. However, fiber delays that exceed the defined timeouts
of acknowledgment and RTS/CTS mechanisms cause commu-
nication fails when using fibers longer than 8.1 km.
III. HYBRID RADIO-OVER-FIBER WIRELESS SENSOR
NETWORK ARCHITECTURE
A. Overview of RoF Systems
Optical fibers can provide infrastructures capable of trans-
mitting the whole usable RF signals and, therefore, it is the
preferable medium for the distribution of wide-area wireless
network data. When implementing an optical infrastructure
for wireless network all signal processing functions can be
performed in a centralized way. In this scenario, the antenna
system can cover several kilometers since fiber links have very
low losses (typically 0.2 dB/km) and simple and non expensive
electrical/optical (E/O) converters are available in the market.
In RoF systems, the optical link is an analogue transmission
medium that does not modify the nature of radio signal format,
the fiber links deliver the signals at the RAUs, which act as
transparent O/E converters.
B. Proposed Architecture
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Figure 1. Proposed Architecture
The proposed architecture considers the use of an optical
fiber pair to connect WSNs, using RoF to enlarge the coverage
areas, building a large sensor network.
All sensor nodes within the coverage area of the RAU form
a cluster, with the RAU as the Cluster Head (CH). Clusters
are distributed along an optical fiber link and the SNs in
each cluster transmit only to the BSC located in Data Center
(DC). Thus, all clusters are in the same collision domain. In
this architecture, the MAC protocol needs to deal with the
challenge of contention in both wireless and optical channels.
IV. PROTOCOLO SPP-MAC
The SPP-MAC combines polling and prioritization tech-
niques to allocate the right amount of slots to each sensor
node.
To save energy, SSP-MAC enables transceivers only when
nodes need to transmit frames. Another source of concern is
overhearing. SPP-MAC performs overhearing avoidance from
MAC headers alone. In the protocol, a receiver examines the
destination address of a frame as soon as it receives the MAC
255
header – even before completely receiving the frame. If it
is a unicast frame addressed to any other node, the receiver
immediately ceases the reception of the frame.
A. MAC Frame Format
Figure 2 shows the three types of SPP-MAC’s frames,
namely the Poll Frame, the Data Frame, and the ACK Frame.
• Frame Control field which carries the information about
the frame type and other control flags;
• Target Cluster and Target Node fields which stores the
target cluster and node IDs, respectively. These two fields
compose the Destination Address;
• Source Cluster and Source Node fields which carries
the source cluster and node IDs, respectively. These two
fields compose the Source Address;
• Sequence Number field specifies the sequence identifier
for the frame.
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Figure 2. The SPP-MAC’s frames format
B. Priority Assignment
In WSNs, nodes typically have different roles and thus
different levels of priority. As a result, a node might have
more or less opportunities to transmit than others. In other
words, nodes that play critical roles in the network must have
higher probability to transmit. To address this, SPP-MAC uses
a priority policy based on the role of the network nodes. Note
that these priorities are not related to QoS but actually with
the probability of transmission.
The BSC keeps up to date a list with nodes priorities. Based
on this list, the BSC then builds the polling list. Only nodes
in the list are eligible to receive polls.
C. The Polling Scheduler
The polling algorithm provides one more polling slot to
sensor nodes with priority i than those to sensor nodes with
priority i+ 1.
Slots are organized into cycles. For each cycle, nodes with
priority i (1 ≤ i ≤ m) will get m − i + 1 poll slots. This
means that nodes with priority m will get exactly one polling
slot during the cycle. Therefore, the total number of polling
slots in each cycle is
m∑
i=1
ni ∗ (m− i+ 1) (1)
The next step is to sort the poll slots. One way would
be to scan all nodes and assign each station m − i + 1consecutive poll slots. The strategy does not perform well
since nodes would have to wait a considerable amount of time
to retransmit. Instead, we divide a cycle into m rounds and
for each round i all sensor nodes with priorities from 1 to iwill be polled. The round counter is initially set to m and it
is monotonically decreased after a transmission round until it
reaches 0. Whenever this happens, i.e. the round counter is set
to 0, ending one transmission and the scheduling process.
Algorithm 1 Poll Scheduling Algorithm
Input: mOutput: list L of scheduling
round = 1while round ≤ m do
for each priority ∈ (1,m− round + 1) doadd in L all nodes with priority priority
end forround = round + 1
end while
D. Data Transference
Whenever a sensor needs to transmit data to the BSC, it
enables its transceivers and waits for a polling frame. As soon
as the frame is received, the node transmits. On the arrival
of a frame, the BSC checks if the sender has asked for an
acknowledgment and, if so, it sends back an acknowledgement
to the node. Otherwise, it directly sends a polling frame to the
next node on the L list.
V. HMARS PROTOCOL
By combining the reservation and the contention techniques,
the HMARS Protocol uses Time Division Multiple Access
(TDMA) to avoid collisions in the optical channel and Carrier
Sense Medium Access (CSMA) to avoid collisions in the
wireless channel.
This approach requires knowledge of the network topology
and network synchronization to establish a schedule that
allows each cluster to access the optical channel and commu-
nicate with the BSC. In TDMA, time is divided into frames
and each frame is divided into slots. During a frame, each
cluster is assigned a unique slot during which it has the right to
transmit. As a consequence, transmissions of different clusters
do not collide. In CSMA, a node having backlogged frames
first senses the wireless channel before actually transmitting
the frames. In case the node finds the wireless channel busy,
it postpones its transmission to avoid interfering with ongoing
transmission. In case the node finds the wireless channel idle,
256
it transmits the frame (after possibly having waited a random
time).
HMARS suppresses the RTS/CTS exchange and employs
random backoff to reduce overhead and collisions. It also
suppresses acknowledgment frames.
A. MAC Frame Format
Figure 3 shows the two types of HMARS’s frames, the Data
Frame and the Schedule Frame.
• Frame Control field which carries the information about
the frame type and other control flags;
• Target Cluster and Target Node fields which stores the
target cluster and node IDs, respectively. These two fields
compose the Destination Address;
• Source Cluster and Source Node fields which carries
the source cluster and node IDs, respectively. These two
fields compose the Source Address;
• Sequence Number field specifies the sequence identifier
for the frame.
In the Schedule Frame, the Start field contains the time
when the first active period will begin, the Active field contains
the duration (in seconds) of the active period, the Sleep field
contains the duration (in seconds) of the inactive period and
the Schedule field contains the time when the next scheduling
will occur.
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Figure 3. The HMARS’s Frames format
B. CSMA access to the Wireless Channel
Three variables are maintained: CW, NA, and MAXAttempt.CW is the contention window size, which defines the number
of backoff periods without channel activity before transmission
can start; its value is initialized to 2 before each transmission
attempt and reset to 2 each time the channel is busy. NA is
the number of attempts of transmission; its value is initialized
to 2 before each transmission. MAXAttempt is the maximum
number of transmission attempts.
The MAC sublayer delays transmission for a random num-
ber of complete backoff periods in the range 0 to 2 (Figure 4,
Step 2) and then requests the physical layer to perform a Clear
Channel Assessment (CCA) (Figure 4, Step 3).
If the channel is busy, NA is incremented by one and CWis set to two (Figure 4, Step 5). If NA is less than or equal
to MAXAttempt, a new transmission is attempted after 0 to 2
backoff periods later (Figure 4, Step 2). If NA is greater than
MAXAttempt, the MAC sublayer reports a transmission error
to the layer above.
If the channel is idle, the MAC protocol makes sure the
contention windows has expired. To do that, CW is decre-
mented monotonically (Figure 4, Step 4) and, subsequently, it
is checked whether its value is zero. If so, the MAC sublayer
immediately requires a new CCA to the physical layer. If this
value is null, transmission is restarted. By doing that, we limit
the number of simultaneous transmissions and decrease the
number of intra-cluster collisions.
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Figure 4. The HMARS’s CSMA algorithm
C. TDMA access to the Optical Channel
We assume that the BSC knows the network topology and
the delays to each cluster, which are realistic assumptions.
As the optical channel is the common collision domain to
all nodes in the clusters, the TDMA protocol tries to avoid
collisions among them.
Time is divided into frames and each frame is divided into
slots. The number of slots is the number of clusters in the
network and each slot has the same duration. The number of
frames depends on the synchronization interval.
During a frame, each cluster is assigned a unique slot
according to its location along the optical fiber, during which
257
the clusters has the right to transmit. As a consequence, colli-
sions among clusters are avoided, which guarantees finite and
predictable scheduling delays and also increases the overall
network throughput under heavy loaded networks.
Scheduling is the crucial part of the protocol. As it requires
great computational power, it is carried out by the BSC.
VI. SIMULATION AND PERFORMANCE EVALUATION
A. Simulation Setup
The NS-2 Simulator with the CMU Wireless extension was
used in the simulations. An RoF module was implemented as
well as the SPP-MAC and HMARS protocols.
Physical radio characteristics of each sensor node and the
BSC (such as antenna gain, transmit power and receiver
sensitivity) are shown in Table I.
In our simulation, each cluster contains 10 sensor nodes
distributed along the optical fiber link separated by 5 km from
one another. Each node in the cluster is 10 meters away from
the RAU and sends their data only to the BSC. Impairments
caused by RoFs has not been implemented in the simulation
and only the attenuation of RF in the optical fiber link has
been considered, which was set to 0.4 dBm/km.
Each sensor generates one Poisson flow to the BSC; the
packets size was 18 bytes.
Finally, all simulations were run independently using 5
different seeds.
TABLE IFIXED MODEL PARAMETERS
PHY Module ParametersData Rate 250kbpsFrequency 915 MHzTransmitter Power 10 dBmCarrier Sense Sensitivity -95 dBm
Propagation ModelModel ShadowingAttenuation Factor (β) 3.41Standard Deviation (σ) 5Reference Distance (d0) 1
Others Module ParametersTransmit Power 90 x 10−3WReceive Power 45 x 10−3WIdle Power 15 x 10−3WSleep Power 20 x 10−6W
B. Performance Metrics
Three metrics to evaluate the performance of the protocols
were assessed, (i) Delivery Ratio which is the ratio of
successfully delivered packets to the total packets originating
from all sources; (ii) Aggregate Throughput which is the
aggregate throughput of the traffic that goes to the BSC;
and (iii) Successful Poll Rate which is the percentage of
successfully polls over the total number of polls.
If the BSC sends a poll frame to node i and receives
data from i before polling another node j, the poll is then
considered a successful one; otherwise, it is considered a
missed poll.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2 3 4 5 6 7 8 9 10
Del
iver
y R
atio
Number of Clusters
AlohaCSMA 0.85 persistent
HMARSSPP-MAC
Figure 5. Delivery Ratio
40
60
80
100
120
140
160
2 3 4 5 6 7 8 9 10
Agg
rega
te T
hrou
ghpu
t (kb
ps)
Number of Clusters
25 frames/second50 frames/second
250 frames/second500 frames/second
Figure 6. HMARS Aggregate Throughput
C. Simulation Results
Figure 5 compares the Delivery Ratio of SPP-MAC and
HMARS with two widely deployed protocols; the Aloha and
the CSMA p-persistent. Flows with 500 frames per second
were generated to all protocols.
The delivery ratio of the Aloha and CSMA degrades with
the increase of the number of clusters due to collisions.
Conversely, both HMARS and SPP-MAC can achieve 100%
delivery ratio since these protocols decrease collisions intra-
clusters.
Figure 5 shows that nodes using HMARS or SPP-MAC can
obtain almost dedicated bandwidth, achieving delivery ratio
equals to one, regardless of the number of nodes which does
not happen with the contention based protocols.
HMARS has a constant aggregate throughput regardless of
the number of clusters (Figure 6). This is because the protocol
is TDMA-based and each node and cluster issue the same
amount of traffic under high loads.
As the number of clusters increases the distance between
the BSC and the last cluster also increases. As a consequence,
258
45
50
55
60
65
70
75
80
85
90
2 3 4 5 6 7 8 9 10
Agg
rega
te T
hrou
ghpu
t (kb
ps)
Number of Clusters
25 frames/second50 frames/second
250 frames/second500 frames/second
Figure 7. SPP-MAC Aggregate Throughput
the optical fiber delay gets longer and the duration of timeouts
increases. The larger the delay, the lower is the number of poll
frames transmitted by the BSC and, therefore, there are less
transmissions of data frames, which decreases the aggregate
throughput (Figure 7). With the load increase, the polling
overhead has a lower impact on the throughput. However,
such overhead prevents the SPP-MAC of achieving the same
throughput achieved by HMARS.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2 3 4 5 6 7 8 9 10
Suc
cess
ful P
oll R
ate
Number of Clusters
25 frames/second50 frames/second
250 frames/second500 frames/second
Figure 8. SPP-MAC Successful Poll Rate
Figure 8 shows the percentage of successful polls as a
function of the number of clusters. Successful poll rate in-
creases as the number of active nodes increases. Note that
successful poll rate is important since it increases the wireless
medium utilization. A missed poll means waste of bandwidth
and medium resources.
VII. CONCLUSION
In this work, we proposed two MAC protocols exclusively
tailored to RWSNs, namely SPP-MAC and HMARS. They
deal with the main problems of RWSNs i.e. the delay im-
posed by the optical fiber and the existence of two collision
domains. The performance of different MAC protocols were
evaluated. Enhanced delivery ratio and aggregate throughput
were achieved by two proposed protocols. Besides, results
indicate that SPP-MAC and HMARS perform considerably
better than the others MAC protocols as Aloha and CSMA in
the analyzed scenarios.
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