27-08-2014, 10:58 AM
Medium Access Control with Token Approach in
Wireless Sensor Network
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ABSTRACT
Hybrid medium access control (MAC) design in Wireless
Sensor Network (WSN) brings a new research challenge
nowadays. Hybrid MAC gives significant improvement in
network performance especially in terms of energy efficiency
and reliability of the network. Some of the data are sensitive
to loss in the medium such as video data and data for
emergency application. In MAC protocol, a contention access
method which is Carrier Sense Multiple Access (CSMA)
encounters collision problem when the number of nodes in the
network increases. Meanwhile, the issue of slotted access
which is Time Division Multiple Access (TDMA) is a strict
synchronization problem. To avoid the weakness of both
access methods, a hybrid MAC layer is proposed with
unsynchronized TDMA, which is a token approach that calls
the HMAC-TA. Token approach will be used in this protocol
to avoid synchronization problems that can degrade network
performance in TDMA protocol. The performance analysis of
HMAC-TA shows 48% significant improvement in terms of
energy efficiency compared to MAC IEEE 802.15.4 standard.
The packet delivery ratio of proposed protocol also shows the
good performance.
General Terms
Wireless Sensor Network, Medium Access Control, Token
approach
Keywords
WSN, Hybrid MAC, Token, MAC IEEE 802.15.4
1. INTRODUCTION
Wireless sensor network (WSN) is designed based on
application specific. Different application poses different
quality of service (QoS) requirement. The applications in
WSN that send periodic data for monitoring such as
temperature, humidity and vibration can tolerate with QoS in
terms of reliability of the data. However, for emergency data
such as fire, enemy appearance and medical, the reliability of
the data should be achieved to make sure the data arrive at the
destination [1]. Besides, video application such as
transmission of low bit rate video which is MPEG-4 in WSN
environment requires reliable transmission medium [2].
Another critical issue in WSN design is energy efficiency. To
the best of our knowledge, most of the researchers or
academia take energy as a goal in designing any protocol in
WSN because the nature of the wireless device that is
dependent of battery power [3]. The deployment of the
wireless device that scattered wirelessly in geographical
structure makes it difficult for replacement and recharging the battery. For this reason, all the protocol designs maintain low
energy usage in the network.
The key layer in WSN protocol that can handle such issue is
Medium Access Control (MAC) due to its ability to control
the physical radio directly [4]. MAC layer has responsibility
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popular protocol in MAC layer is carrier sense multiple access
(CSMA) protocol. Even though, CSMA protocol achieves
high performance in scalability, but it suffered from the
collision problems. A collision might reduce the throughput,
increase the energy consumption because of retransmission of
the loss data and will increase the delay in data delivery. To
over
2. MOTIVATION OF TOKEN
APPROACH
The motivation of using token approach apart from
minimizing the network energy consumption is to provide
reliable data transmission. This is because every node that
intends to transmit the data should have the token first before
The motivation of using token approach apart from
minimizing the network energy consumption is to provide
reliable data transmission. This is because every node that
intends to transmit the data should have the token first before it senses the medium. This will reduce the probability of node
collision and avoid the node to sense in the busy network.
Besides, through this approach, no hidden terminal problem
will occur. For example, as shown in Figure 1, node A and
node C can hear FRPPXQLFDWLRQWKDWRFFXUVZLWKLQQRGH%¶V
range, but node A and node C cannot hear each other's
signaling. Without token approach, in CSMA protocol, each
node sense the channel and transmit the data if found that the
channel is idle. Hence, node A and node C might sense and
transmit data at the same time and cause a collision. However,
with token approach, only node that has the token can
transmit the signal. Node A holds the token and can transmit
the data without collision because node C will wait for the
token before it sends the data.
Fig
3. RELATED WORKS
In recent years, many hybrid MAC layer for WSN design
have been proposed in the literature. Most of them combine
CSMA protocol with TDMA protocol either using centralized
approach or distributed approach.
Zebra MAC (ZMAC) [10] is one of the well-known hybrid
MAC protocols in WSN. It selects the protocol either CSMA
or TDMA depends on the contention level of the channel.
Each of the node already knew its own timeslot at the
beginning of the ZMAC protocol using Distributed
Randomized Timeslot Assignment Algorithm (DRAND) [11].
The node will remain using its own timeslot if the channel is
in high contention level, but if the channel is in low
contention level, it will use the CSMA protocol. This protocol
achieves high adaptability with the channel condition and
fairness but leak with scalability in timeslot assigning. ZMAC
assigned timeslot using DRAND protocol in an offline
process at the time of deployment. The problem will occur
when there is a new node that joins the network in the
transmission time, it would not have timeslot and only can
content in low contention level. For synchronization issue,
ZMAC just broadcasts a synchronization message in high
contention level, but it requires high overhead to make sure
that the node is synchronized (need at least 10
synchronization messages to resynchronize 30 nodes).
4. SYSTEM DESIGN
4.1 Network Model
The proposed protocol is designed based on MAC IEEE
802.15.4 standard. This protocol aims to UHGXFH WKH QRGH¶V
energy consumption by avoiding the unnecessary carrier sense
and providing a reliable path for data transmission in wireless
mediums. In this paper, hybrid MAC was proposed that
consists of combination the CSMA and TDMA in one
protocol design. To avoid tight synchronization problem that
can degrade the network performance in TDMA protocol,
unsynchronized TDMA is used, which is a token approach
and named it as a hybrid MAC with token approach (HMACTA). In the token approach, any node that has the token can
transmit the data for token holding time (THT). When THT
expired, they hand over the token to the next node that
controls the token. Figure 2 shows the frame structure for of
the proposed hybrid MAC design.
4.2 Initialization Phase
Level implementation and neighbour discovery are done
during initialization phase by using CSMA protocol. At the
beginning, each node in the network is set to default level (Li)
value. A node that generates the event will act as source node
and it is set with level 0. Source node also has the
responsibility to generate the token in data transmission
phase. When the event occurs, source node starts the
initialization phase by broadcasting Token Level Message
(TLMsg). TLMsg contains level value (Li) and address of a
node. The nodes within the source node coverage which is
neighbouring nodes receive the message and update their level
by 1. Then, each node that has already updates the level will
broadcast TLMsg that contains its new level id. Each node
only broadcasts one TLMsg during the initialization phase
When a node receives a TLMsg from its neighbours, it will
not only update its level but also store its one hop neighbour
id. The level (Li) value at each node indicate the number of
hops that node from the source node. For example, if node A
has Li equal to 3, that means node A is three hops away from
the source node. In HMAC-TA protocol, level id is very
important and will be used in transmission phase.
4.3 Data Transmission Phase
In data transmission phase, two protocols are used depends on
the type of data. CSMA protocol used to send control message
such as routing message (RTLD control packet), token
message and acknowledgement message. Nodes run a backoff
timer and scan the network before sending the control
message. For data message, token approach will be used. Any
node that intends to transmit the data message should have
token first. Token just like a ticket, a station that has the token
can use the channel. When the node gets the token, it also has
to sense the channel first before transmit the data message to
make sure that the channel is free.
For token message, there are three different types of message
as shown in Figure 4 which is send request to join (RTJ)
message, Token Announce (TA) message and Token passing
(TP) message.
SendID is a node id for sending node, Li is level value, FT is
flag token either 0 or 1, NCT is a node's control token at that
time, and NHT is the next node that holds the token. TST is
the token scheduler table. Inside TST, there is a list of
neighbour nodes that joining the token. These neighbour
nodes are in closed-loop ring that constructs by NCT.
In data transmission phase, source node that triggers the event
will generate token and assign itself as the first NCT in the
network. Then NCT will broadcast TA Msg to acknowledge
its neighbour about token appearance inside its coverage area.
Neighbour node that receives TA Msg will send a RTJ
message if there is any data to be sent. After THT expired,
NCT will pass the TP Msg in a unicast way to next NCT in
the network. The next NCT will check the request token table
(TRT), if there is a list of neighbour nodes that request the
token, NCT will construct a ring structure and broadcast TA
Msg. After NCT finishes using the token, it passes the token
to the next holding token (NHT) based on the list of
neighbours in TST. This process continues until the token
reaches back at NCT. Then NCT will choose next NCT in the
network. The selection of next NCT will be explained in
section 4.4. Example of token operation is shown in Figure 5.
As shown in Figure 5, source node passes the token directly to
node 2 (N2) after THT expired, then N2 also passes the token
to node 5 (N5). When the token reaches N5, it constructs TST
because there is a list of nodes inside TRT. N5 will send TP
Msg to next NHT according to TST after THT expired
4.4 Next Node Control Token
The selection of the next nodes that control the token in the
network is based on the next hop forwarding node that is
selected in the routing protocol. In our approach, the token
will be passed to a node that has data. Through this approach,
each node that has the data obtains the token and transmits the
data towards the destination. Based on the assumption state in
section 1, RTLD routing protocol is implemented in this
protocol. In RTLD routing protocol, next hop forwarding
node is chosen based on optimal forwarding decisions that
consists of three parameters. The parameter selections are
remaining battery power, max packet velocity and link
quality. The equation can be found in this reference [20].
4.5 Token Generation
In our protocol, the token will be generated at the source node
after two hops neighbour to avoid collision. The network is
assigned to a level and node that two hops from the source
node is set with level 2 at initialization phase. As mentioned
before, TP Msg will be sent in a unicast way to NCT after
THT expired, but for a node with level 2, it will broadcast TP
Msg as shown in Figure 6. Node with level 1 that receives the
message will acknowledge the source node with a notification
message to generate a new token into the network. The token
flag will be set either 0 or 1. The generation of token after two
hops neighbours is to make sure that there is no token
collision inside the medium, and data can be transferred faster
towards the destination.
5. PERFORMANCE ANALYSIS
Simulation studies of the proposed hybrid MAC which is
HMAC-TA protocol is carried out using NS-2 simulation tool.
The performance of HMAC-TA is compare with MAC IEEE
802.14.5standard. There are two modes of operation in MAC
IEEE 802.15.4 standard, which are beacon enable and nonbeacon enable mode[21]. Even though, beacon enable mode
offers hybrid MAC with implementation of guarantee time
slot (GTS), it is only designed for star topology [22]. HMACTA is designed for distributed topology and should be
compared with non-beacon enable mode.
5.1 Simulation Parameters
Table 2 describes the simulation parameters that are used
throughout this analysis. HMAC-TA is an improved version
of non-beacon enable mode in MAC IEEE802.15.4 with token
implementation. The protocol stack of IEEE 802.15.4 is
already available in NS-2. The network performance for both
protocols is compare in terms of energy efficiency and packet
delivery ratio
5.2 Energy Consumption
The performance of energy consumption is evaluated for both
protocols by varying the traffic contention (packets per
second). Traffic contentions are varied from 2 to 50 packets
per second. The simulation runs for 100 second. Figure 7
shows that energy consumption of the network increases with
respect to the packet rate. Energy consumption for the
proposed protocol (HMAC-TA) shows average decrement at
48% for 8 to 50 packet rates and 28% for packet rates below
than 6 compared to IEEE 802.15.4.
5.3 Packet Delivery Ratio
To evaluate the packet delivery ratio, the total number of
packets that are received at the destination is divided with the
total number of packets that are sent by the sender. As shown
in Figure 8, for low traffic contention which is below 8
packets per second, IEEE 802.15.4 standard shows better
performance compare d to HMAC-TA protocol. This is due to
the fact that the CSMA protocol can send the data if the
channel is idle. Meanwhile, in HMAC-TA, nodes have to wait
for token before they can transmit the data. However, when
the packet rate increases above 10 packets per second, the
proposed protocol which is HMAC-TA shows good
performance compared to the standard. Traffic contention is
improved as the packet rate is increased in the network.
Figure 9 shows the performance of packet delivery ratio in
terms of time. For this analysis, packet rate is set for 10
packets per second. Packet delivery ratio increases with
respect to time and from this figure, it can be seen that
HMAC-TA performs better than standard.
6. CONCLUSION AND FUTURE
WORKS
In this paper, we have presented the improve version of MAC
IEEE 802.15.4 standard with token approach. The main
intention of the proposed protocol is to improve energy
efficiency and to provide reliable communication. This
protocol is suitable for priority application and emergency
scenarios that require the data to arrive safely. We also
introduce new idea which is token will generated after two
hops neighbour. Through this approach, data can be
transmitted faster compared to the transmission that only used
one token that circulated inside ring structure. Besides, ring
structure is only constructed upon request and token only
given to the nodes with data to be transmitted. From the
result, it can be seen that high energy efficiency is achieved
by utilizing this method. For future work, the proposed
protocol which is HMAC-TA will be simulated for the large
topology network and network performance in terms of delay
will be analyzed. Furthermore, to increase the scalability of
the network, this protocol will be extended to support two
modes of operation, which are fully CSMA and CSMA with
token approach.