IEICE TRANS. COMMUN., VOL.E89–B, NO.4 APRIL 2006
1252
PAPER
Support of Efficient Route Discovery in Mobile Ad Hoc Access
Networks
Chun-Yen HSU†a) , Student Member, Jean-Lien C. WU†† , and Shun-Te WANG†† , Nonmembers
SUMMARY
The Public Wireless Local Area Network (PWLAN) is an
emerging service for wireless access to the Internet. However, the service
coverage of the PWLAN is limited by the deployment of access points
(APs) because only those who stay near the AP can access the PWLAN. A
feasible way of extending the service coverage of a PWLAN is to deploy
mobile ad hoc access networks (MAHANs) so that users who are not in an
AP’s radio coverage area can send their packets to the AP in a multihop
manner. However, in a MAHAN, mobile nodes that intend to access the
Internet have to discover routes to the AP first, which may result in considerable bandwidth cost. In this paper, we propose the Appointed BrOadcast
(ABO) method to reduce the cost of route discovery in MAHANs. Using
the ABO method can achieve this goal on the basis of packet overhearing.
Functions that are necessary for network and data link layers to employ the
ABO method are also discussed. Simulation results show that using the
ABO method can significantly reduce the cost on route discoveries. Due
to the widespread use of legacy IEEE 802.11 nodes, the problem of how
ABO-enhanced and legacy IEEE 802.11 nodes can coexist in a MAHAN
is also discussed.
key words: mobile ad hoc access network (MAHAN), public wireless local
area network (PWLAN), IEEE 802.11, routing, packet overhearing, Internet
1.
Introduction
With the evolution of wireless devices as well as wireless
technologies, accessing the Internet via laptops or PDAs instead of desktop computers as we use today is expected to
be an everyday occurrence in the future. The public wireless local area network (PWLAN), which enables user to
access the Internet via WLANs ubiquitously or at least at
some hotspots such as airports, stations, libraries or museums, emerges with this trend [1]–[4]. However, the service
coverage of a PWLAN is limited by the deployment of access points (APs), i.e., only users that stay near an AP can
connect to the AP directly. In order to extend the service
coverage of PWLANs, the mobile ad hoc access network
(MAHAN), as shown in Fig. 1, can be a feasible solution because of the following two reasons. The first one is that there
is no need of infrastructures. In a MAHAN, each node acts
as an end point as well as a router for other nodes. Those
who are not in the radio coverage of the AP can rely on their
Manuscript received May 18, 2005.
Manuscript revised October 5, 2005.
†
The author is with the Department of Computer Science and
Information Engineering, National Taiwan University of Science
and Technology, Taipei, Taiwan, R.O.C.
††
The authors are with the Department of Electronic Engineering, National Taiwan University of Science and Technology,
Taipei, Taiwan, R.O.C.
a) E-mail: yen@nlhyper.et.ntust.edu.tw
DOI: 10.1093/ietcom/e89–b.4.1252
Fig. 1
The mobile ad hoc access network. (MAHAN)
neighbors to forward packets to the AP in a multihop manner. The second one is that using the MAHAN can mitigate
the effect of dead spot. This is valuable especially for the
urban environment where the shadowing effect could be a
problem.
The MAHAN is a special case of mobile ad hoc networks (MANETs) [5] that has two characteristics:
• There is a hot node (i.e., the AP) that all traffics flow
through it.
• This hot node is fixed and has no energy consumption
concerns.
While many routing protocols for MANETs also work
in MAHANs, they can be further optimized [6]–[12]. Because of the node mobility, route failures may occur abruptly
and frequently, therefore routing protocols must be adaptive.
However, the limited radio bandwidth as well as the imperfect channel quality dictate the amount of routing information exchanged and make routing in MAHANs a laborious
work. Routing protocols for such a dynamic network environment can be either proactive or reactive according to the
maintenance strategy of routing tables. A detailed description and comparison between proactive and reactive routing
protocols can be found in [13]. In this paper, reactive routing is considered. A typical reactive routing protocol is described as follows. When a packet arrives at the network
layer of a source node that has no valid route information
to the destination, a route request (RREQ) message will be
broadcasted. Having received the RREQ message, either
the destination node or an intermediate node that has valid
route information to the destination can reply a route reply
(RREP) message to the source node using unicast. When
the source node receives the RREP message, it can begin
to forward packets along the discovered route. If the route
fails because of link failures, the upstream node of the failed
link will initiate a route error (RERR) message to notify the
source node. The forwarding of RERR message can be either broadcast if it is used to notify multiple nodes, or uni-
c 2006 The Institute of Electronics, Information and Communication Engineers
Copyright HSU et al.: SUPPORT OF EFFICIENT ROUTE DISCOVERY IN MOBILE AD HOC ACCESS NETWORKS
1253
cast otherwise.
In this paper, we propose the Appointed BrOadcast
(ABO) method to minimize the route discovery cost. The
ABO method achieves this goal on the basis of packet overhearing. Details of the ABO method are provided in Sect. 3.
Because of the widespread use of legacy IEEE 802.11 nodes
[14], the coexistence situation of both ABO-enhanced nodes
and legacy IEEE 802.11 nodes in a MAHAN is also discussed. The rest of this paper is organized as follows. Section 2 provides the concepts of packet overhearing in IEEE
802.11 MAC protocol. The proposed ABO method and its
performance evaluation are provided in Sects. 3 and 4, respectively. Finally we conclude this paper in Sect. 5.
2.
Packet Overhearing in IEEE 802.11 MAC Protocol
A route discovery process is necessary in ad hoc networks
but it may involve large amount of control messages (i.e., the
RREQ and consequent RREP messages) and thus introduce
considerable amount of overhead. To reduce the overhead
on route discovery, one approach is to allow nodes to overhear packets sent by their neighboring nodes. Routing information may be obtained in advance by analyzing the overheard packets and hence the frequency of route discoveries
is reduced. It has been shown in the literature that packet
overhearing can significantly reduce the number of control
messages produced for route discovery [15], [16]. In the Adhoc On-demand Distance Vector with Backup Routes routing protocol (AODV-BR) [17], backup routes are used to recover from route failures. By overhearing packets, backup
routes can be constructed in the process of route discovery
without producing extra control messages.
Packet overhearing is helpful to reduce route discovery
cost, however, in current IEEE 802.11 MAC design, frames
that are not targeted to the node will be discarded. If a node
needs to receive packets that are not targeted to it, it is essential to set the node’s network interface into promiscuous
mode. In this mode, the MAC address filtering function that
discards frames not targeted to the node is disabled. The
drawback of using promiscuous mode is that, once it is set,
all frames that can be correctly received will be passed to
the upper layer and the node may eventually be drowned by
overheard packets.
We propose the ABO method as an alternative to
achieve packet overhearing. The concept of the ABO
method is that, packet overhearing should be performed in a
controlled manner, in other words, the network layer should
be able to actively select critical packets that need to be disseminated to neighbors. In this way, nodes only overhear
critical packets, and then the load on processing overheard
packets is reduced while the benefit of packet overhearing is
retained.
3.
The Proposed ABO Method
In the ABO method, a special type of frame called the ABO
frame, which is a modified IEEE 802.11 data frame and can
http://folk.uio.no/paalee/
Fig. 2
The IEEE 802.11 frame format in ad hoc mode.
Fig. 3
The ABO frame structure.
coexist with legacy IEEE 802.11 data frames, is introduced.
The ABO method takes advantage of the ABO frames to
achieve packet overhearing. At the MAC layer, the packets
that need to be disseminated to neighbors are carried by the
ABO frame. Necessary functions in the logic link control as
well as network layers in order to obtain maximum benefit
available from the ABO method are also described in the
following subsections.
3.1 MAC Layer Design
The IEEE 802.11 data frame structure in ad hoc mode is
shown in Fig. 2, where the duration field is used to update
the network allocation vector (NAV), address 1 holds the address of the receiver, address 2 holds the address of the transmitter and address 3 holds the basic service set identification
(BSSID) of the network [14]. An ABO frame, as shown
in Fig. 3, is transmitted in the way of IEEE 802.11 MAC
layer broadcast. However, broadcast frames in IEEE 802.11
MAC design are acknowledge-free and thus unreliable. To
keep the transmission to the target node reliable, the target
node’s MAC address is recorded in the intended receiver address (IRA) appended in the frame body. Upon receiving an
ABO frame, the node whose MAC address matches the IRA
will reply an ACK frame to the sender. The ABO frame,
as depicted in Fig. 3, therefore has the following differences
compared to legacy IEEE 802.11 data frame:
1. The IRA field is appended at the end of the frame body.
2. Address 1 holds the broadcast address.
3. The duration field contains the short interframe space
(SIFS) time plus the ACK frame transmission time.
Note that a legacy IEEE 802.11 broadcast frame has a
zero value in the duration field. Hence, when a node receives
a frame, it can check whether the frame is an ABO frame
or not. The node examines if the address 1 field holds a
broadcast address and whether the duration field contains a
nonzero value. If all positive, the frame is recognized as
an ABO frame, otherwise it is recognized as a legacy IEEE
802.11 frame.
Figure 4 depicts the corresponding processes when an
IEEE 802.11 node or an ABO-enhanced node receives a data
frame. When an ABO-enhanced node receives a data frame,
the node first checks the address 1 and the duration fields to
IEICE TRANS. COMMUN., VOL.E89–B, NO.4 APRIL 2006
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Fig. 4
Processing of received data frames.
see if it is an ABO frame. If true, the following steps will be
triggered, otherwise, the received frame is processed in the
legacy way.
Step 1: Check if the IRA field matches its own MAC address, if true, prepare the ACK frame and transmit after
a SIFS time period.
Step 2: Trim the IRA field.
Step 3: Send the MSDU to the upper layer.
In generating an ABO frame, the IRA field must be included
in the frame check sequence (FCS) calculation to prevent the
receiving node from mistaking the ABO frame as a frame
error. Similarly, if encryption is needed, the IRA field is
treated as a part of the plaintext. In the IEEE 802.11 MAC
design, two functions are prohibited, namely, fragmentation
and the RTS/CTS transaction, for broadcast frames. The
transmissions of ABO frames also obey these rules. The
no-fragment rule avoids the potential problem that an overhearing node fails to receive all fragments and then requests
for the lost ones. This rule is convenient because in most
systems, the maximum transmission unit (MTU) is set to be
1500 bytes, which is smaller than the frame body length limitation of 2312 bytes in the IEEE 802.11 standard. The noRTS/CTS rule potentially increases the probability of collisions, so a short ABO frame is preferred.
3.2 Logical Link Control and Network Layers
The ABO method gives routing protocols the flexibility to
selectively disseminate packets to neighbors. Packets that
are to be disseminated to neighbors are carried by the ABO
frame. This can be done by sending these packets via a specific service access point (SAP) to the logical link control
(LLC) layer and then to the MAC layer as shown in Fig. 5.
The way to select packets that are to be disseminated to
neighbors could be a strategy. Routing protocol can determine the critical messages to be disseminated to neighboring
nodes based on its own routing metrics and strategies. For
example, in AODV-BR [17], nodes just need to overhear the
Fig. 5
Protocol stack of the ABO method.
RREP messages, so only RREP messages are transmitted
using the ABO method while other packets are transmitted
with original processes.
On receiving an overheard packet, a node should first
extract the routing information embedded in the packet and
then discard the packet. According to the send/receive operations particularly in the ABO method, the network layer
will be enhanced in two aspects in advance to take advantage of the ABO method. The first one is the awareness of
the specific ABO SAP described above. The second one is
that, because the next hop address in the overheard packet
will not match the current node’s address, the routing protocol must be able to extract the routing information embedded in the packet rather than just trigger an error and then
discard it. Some routing protocols, such as the Dynamic
Source Routing (DSR) [18], can handle the overheard packets and benefit from it. Other protocols, e.g., AODV [19],
however, are not designed to handle overheard packets and
thus enhancement is required. In most protocol designs, the
network layer, LLC functions and network interface card
drivers are implemented using software, so they can be enhanced easily.
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3.3 The Reply Storm Problem
Many routing protocols allow intermediate nodes that have
valid route information to the destination node to reply
RREP messages upon receiving the RREQ message in order
to reduce the number of broadcast RREQ messages. However, if the number of intermediate nodes that have a valid
route to the destination node is large, a considerable number
of RREP messages will be sent. Because all the RREP messages are sent to the source node, congestion may occur at
or near the source node and hence the reply storm problem
occurs [18], [20]. The reply storm problem may appear in
MAHANs because all nodes have the same destination node
(i.e., the AP) rather than peer-to-peer communications as in
MANETs. The reply storm problem can be even worse if
all RREP messages are transmitted using the ABO method
because more nodes have route information to the AP. Besides, since RREP messages are carried by the ABO frame,
which can be received by all the neighboring nodes, a large
number of RREP messages can introduce considerable processing load to the network layer. Therefore, schemes to
eliminate the reply storm problem are desired.
Two schemes can be employed at the network layer
to eliminate the reply storm problem. The first one is the
ABO frame prohibition (AFP) scheme, which reduces the
frequency on using the ABO frame. If an intermediate node
has learned a new route, it is allowed to transmit an RREP
message using the ABO frame for once to disseminate the
new route information. Otherwise, the use of ABO frame
is prohibited. The second one is that, similar to [18], when
an intermediate node decides to send a RREP message to
the source node, the RREP message will be deferred for a
time period of DRREP before sending. During the deferring period, if the node receives a RREP message heading
to the same source node, the node will discard the RREP
message it prepares. DRREP can be determined according
to the routing protocol’s major concern. For example, in the
Load-Balanced Ad hoc Routing (LBAR) [21], traffic load is
the major concern, so the deferring delay can be determined
according to the load along the route. In routing protocols
such as ADOV and DSR, route length is the major concern.
It is intuitive to assign a smaller DRREP for more preferable
routes. In this way, transmission of RREP messages for minor route will have larger deferring delay and is more likely
to be discarded.
3.4 Coexistence with Legacy IEEE 802.11 Nodes
Because of the widespread use of the IEEE 802.11 devices,
the coexistence of both the ABO-enhanced and legacy IEEE
802.11 nodes in one MAHAN must be considered. In the
ABO method, the ABO frame has the same frame control
values as that in IEEE 802.11 frame, so upon receiving an
ABO frame, a legacy IEEE 802.11 node will take it as a normal broadcast frame. In this case, the IRA field will not be
trimmed and will be passed to the upper layer as part of the
Fig. 6
Handling of the legacy IEEE 802.11 and ABO frames.
frame body. This is safe because the IRA field will be simply trimmed by the packet length verification at the network
layer [22], thus the higher layer transactions is prevented
from error. How legacy IEEE 802.11 and ABO-enhanced
nodes handle their respective frames are depicted in Fig. 6,
where the dotted lines between the NSDU and NPDU implies that they are present only at the source node and the
destination node. Note that legacy IEEE 802.11 nodes transmit standard IEEE 802.11 frames only, thus the higher the
proportion of ABO-enhanced nodes in a network, the better
the dissemination efficiency.
In case an ABO-enhanced node transmits an ABO
frame to a legacy IEEE 802.11 node, no IEEE 802.11 ACK
will be returned even if the ABO frame is received correctly. In this case, the transmitter will time out and take it
as a transmission error, then enter the retransmission phase.
However, the transmitter will eventually receive no ACK
frame from the legacy IEEE 802.11 node even in retransmissions. This issue is solved as follows. If an ABO frame
experiences a transmission error, the subsequent retransmissions of this frame should use the legacy IEEE 802.11
frame. In this way, the desired information is disseminated
to neighbor nodes in the transmission of the ABO frame
while the subsequent retransmissions ensure reliable transmissions to the target node.
The ABO method solves the coexistence problem at
the cost of more MAC layer transmissions. Because larger
proportion of ABO-enhanced nodes implies more efficient
route information dissemination, the additional MAC layer
load can be compensated by the reduction of control messages. The simulation results on the coexistence scenario
presented in the following section will verify this point.
4.
Simulation Study
In this section, we evaluate the performance of the ABO
method using simulation.
4.1 Simulation Model
In the simulation, we tested two typical routing protocols
based on two different concepts: DSR [18] and AODV [19],
and three different MAC protocols: legacy IEEE 802.11,
ABO and promiscuous mode. DSR is a source routing protocol that the source node knows the complete hop-by-hop
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route to the destination node while AODV is a destinationoriented routing protocol that each node only knows the next
hop to the destination node. In DSR, the route is specified
by the source node in the packet header while in AODV, the
destination node address is the only guidance to which the
packet should be forwarded. Both the routing protocols are
enhanced with functions described in Sects. 3.2 and 3.3 to
employ the ABO method and to handle overheard packet
properly. Note that the AFP scheme described in Sect. 3.3
is applicable to the ABO method but is ineffective to the
other two MAC protocols. A variation of the ABO method
by releasing the AFP scheme will have similar performance
as the promiscuous mode because all control messages will
be overheard by neighboring nodes. The only difference
between using the AFP-released ABO method and using
promiscuous mode is that nodes using promiscuous mode
overhear data packet while nodes using the AFP-released
ABO method do not. We employ the expending ring search
mechanism, which is specified in both AODV and DSR, to
reduce the number of RREQ messages in the network.
Unicast control messages, including RREP and RERR
messages, are selected to transmit using the ABO method.
The deferring time DRREP is based on the following function proposed in [18]:
DRREP = H × (h − 1 + r)
where H is a small constant delay, h denotes the number of
hops along the route being replied, and r is a random number
uniformly distributed between 0 and 1. It is suggested that
H is at least twice the maximum wireless link propagation
delay [18]. We evaluated the effects of different H values as
shown in Fig. 8 and find that when H is 10 ms, the control
cost is the lowest.
Similar simulation configurations are used as described
in [12], where the channel bit rate is 2 Mb/s and the radio
range is 250 meters. An AP is placed at the center of the
simulation area where 50 mobile nodes are placed randomly.
Three scenarios are studied: 300 × 1500 m2 , 600 × 1500 m2
and 1000 × 1000 m2 . We find that similar results can be
drawn and here we only show the results using the first scenario.
The data packet size is 512 bytes and the inter-arrival
time of packets is 0.4 second. The simulation time is 900
seconds. The random waypoint model [23] is used for the
mobility model. Each mobile node is independent and no
group movement is presented. Mobile nodes are stationary
(i.e., no mobility is presented) for a period of time t pause
at the beginning of simulation. t pause is exponentially distributed with a mean pause time. Larger mean pause time
means more stationary nodes and lower topology-changing
rate. When t pause times out, the node will move to a randomly selected new location with a moving speed uniformly
distributed with a range of (0, 20) m/s. When the node
reaches the new location, it picks a new t pause value and
remains stationary before t pause times out. The pause time
ranges from 0 to 900 seconds. Note that a zero pause time
implies that mobile nodes are always moving and the net-
work topology is highly dynamic, while pause time of 900
seconds implies that all nodes are stationary during simulation. For fair comparisons, the mobility and traffic patterns
are the same among the compared protocols. The presented
simulation results come from a mean of 50 runs.
4.2 Performance Comparisons
Table 1 summarizes performance metrics that are used for
comparisons. Since the AP is assumed to have no energy
concerns, the cost spent by the AP to send or receive packets and frames is not included in the simulation results. In
this way, we emphasize on the costs incurred in mobile
nodes. Power consumption in memory access is not considered since it is insignificant compared with that of CPU operations and wireless transmissions [24]. We use the model
developed in [25] to measure the CPU time required at the
network layer. Pradhan and Chiueh [25] investigated CPU
time required for four packet lengths: 100, 192, 500 and
1000 bytes. We supplement the CPU time required for other
packet lengths by linear regression as depicted in Fig. 7. The
model developed by Feeney [16], where the corresponding
parameters are given in Table 2, is used to measure the enTable 1 The IEEE 802.11 frame format in ad hoc mode.
Performance metrics
Description
Normalized network
The number of control packets (i.e., the
layer control cost
RREQ, RREP and RERR packets, but
not data packets) sent or received at the
network layer per data packet delivered
to the AP.
Network layer cost
The total number of packets (including
data and network layer control packets)
sent and received at the network layer.
MAC layer cost
The number of frames (including data
and MAC layer control frames) sent and
received at the MAC layer.
Normalized CPU time
Required CPU time at the network layer
per data packet delivered to the AP.
Normalized energy conEnergy consumed at the MAC layer per
sumption
data packet delivered to the AP.
Packet delivery ratio
The number of data packets received by
the AP to the number of packets generated by all traffic sources.
Average end-to-end data The average end-to-end delay of data
packet delay
packets.
Fig. 7
CPU time required with respect to packet length.
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Table 2
Parameter
btranctl
brecvctl
btran
mtran
brecv
mrecv
bdiscard
mdiscard
brecv
prom
mrecv
prom
Parameters for energy consumption computation [16].
Description
Fixed energy consumption on
transmitting a control frames.
Fixed energy consumption on
receiving a control frames.
Fixed energy consumption on
transmitting a data frame.
Incremental energy consumption on transmitting a data
frame.
Fixed energy consumption on
receiving a data frame.
Incremental energy consumption on receiving a data frame.
Fixed energy consumption on
discarding a frame.
Incremental energy consumption on discarding a frame.
Fixed energy consumption on
receiving a data frame in
promiscuous mode.
Incremental energy consumption on receiving a data frame
in promiscuous mode.
Value
120 mW·s
29 mW·s
246 mW·s
1.89 mW·s/byte
56.1 mW·s
0.494 mW·s/byte
97.2 mW·s
-0.49 mW·s/byte
Fig. 8 Different deferring delay. (pause time=300 s, CBR sources=25,
MAC=ABO)
136 mW·s
0.388 mW·s/byte
ergy consumption at the MAC layer† :
At the transmitter,
btranctl + brecvctl + mtran × size + btran + brecvctl .
At the destination node of the data frame,
Fig. 9
brecvctl + btranctl + mrecv × size + brecv + btranctl .
Packet delivery ratio. (pause time=300 s)
At non-destination node that discards the frame,
bdiscardctl +
bdiscardctl
n∈T
+
n∈D
(mdiscard × size + bdiscard ) +
n∈T
bdiscardctl .
n∈D
At non-destination node that operating in promiscuous
mode,
bdiscardctl +
bdiscardctl
n∈T
+
n∈T
n∈D
(mrecv
prom
× size + brecv
prom )
+
bdiscardctl ,
n∈D
where T and D denote respectively the set of neighboring
node of the transmitting and destination nodes.
Figure 8 shows the normalized network layer control
cost using different H values. H=0 implies that nodes do
not defer before replying RREP message back to the source
node. An underestimated H value cannot prevent the network from the reply storm problem. On the contrary, an
overestimated H value may result in that the source node
route discovery times out, and then route discovery is initiated again, which introduces more number of control messages. It is interesting that AODV and DSR have similar
performance while they are different in routing concepts.
The power of DSR is that nodes can learn routes to other
Fig. 10
Average end-to-end data packet delay. (pause time=300 s)
nodes from the packet header for future use. This is useful in peer-to-peer communications scenes [16]. However,
in MAHANs, all source nodes aim at the same destination
node, i.e., the AP, learning routes to nodes other than the AP
does not bring extra benefit. In the following, only the result
of AODV is presented.
The tags 802.11, ABO and Prom in Figs. 9 to 16 indicate the corresponding results of using legacy IEEE 802.11
†
The measurement of power consumption is based on an assumption that an ABO-enhanced network interface card consumes
equivalent power compared with a legacy one.
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MAC, the ABO method and promiscuous mode, respectively. The delivery ratio and end-to-end delay subject to
different number of CBR sources are depicted in Figs. 9 and
10, respectively. Promiscuous mode can achieve the highest
performance in terms of delivery ratio and end-to-end delay because that it consumes the least bandwidth on routing
control, thus more bandwidth is left for data transmissions
and consequently the performance is improved. Using the
ABO method can also achieve remarkable performance im-
Fig. 11
Packet delivery ratio. (CBR sources=25)
Fig. 13
provement compared with that of legacy IEEE 802.11 MAC.
However, using the ABO method does not give as much improvement as that of using promiscuous mode because the
AFP scheme causes the efficiency degradation on disseminating route information. Similar results can be observed
when using different pause time as shown in Figs. 11 and
12, respectively.
Figure 13 compares the number of control and data
packets sent and received using the three MAC proto-
Fig. 12
Average end-to-end data packet delay. (CBR sources=25)
Network layer costs. (CBR sources=25)
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cols. Note that even when all nodes are stationary (pause
time=900 s), a small amount of RERR messages are issued.
This is because, sometimes, the MAC layer transmits a
frame but receives no ACK because of collisions. The MAC
layer will retry until an ACK is received or the retry limit
is reached. If the latter occurs, the MAC layer will signal
the network layer that the wireless link is not available. The
network layer then recognizes this as a route failure and initiates an RERR message to the corresponding source nodes.
In Fig. 13(a), compared to legacy IEEE 802.11 MAC, using the ABO method can significantly reduce the number of
control packets. Using promiscuous mode can obtain even
more improvements. As shown in Fig. 13(b), using promiscuous mode results in a large number of control packets arrived at the network layer since the MAC layer does not filter out frames. On the contrary, the network layer receives
the least number of control packets when the ABO method
is employed, because when the ABO method is used, only
those packets that are carried by ABO frames (unicast RREP
and RERR messages in this case) can be overheard by the
network layer. Note that the AFP scheme limits the frequency on using the ABO frame, so when the ABO method
is used, the number of control packets arrived at the network
layer is less than that when promiscuous mode is used. On
the other hand, compared with legacy IEEE 802.11 MAC,
the ABO method issues 50% less control packets as shown
in Fig. 13(a). The reduction of control packets can compensate the increased number of received control packets caused
by the use of the ABO frames. The number of control packets arrived at the network layer of the ABO method is less
than that of the legacy IEEE 802.11 MAC.
On the number of data packets sent/received at the network layer, although using promiscuous mode results in the
least cost on sending data packets as shown in Fig. 13(c),
a large number of data packets are received at the network
layer—about 20 times as many as that of using the legacy
IEEE 802.11 MAC and the ABO method as depicted in
Fig. 13(d). This is inevitable because using promiscuous
mode, the MAC layer receives data frames and passes all
Fig. 15
of them to the upper layer without filtering. The result indicates that nodes using promiscuous mode have to spend
much more computation power to deal with the overheard
packets, as shown in Fig. 14.
Figure 15 depicts the simulation results on the number of frames transmitted/received at the MAC layer. The
number of frames transmitted is directly related to the number of packets sent at the network layer, and the number of
frames received is directly related to the number of frames
transmitted. Because promiscuous mode has the least number of control packets issued, it introduces the least number
of frames transmitted/received at the MAC layer. However,
less number of transmitted/received frames does not always
imply lower energy consumption. It is reported that, for a
typical wireless LAN card such as the Lucent WaveLAN
IEEE 802.11 PC card, receiving a frame is more expensive
than discarding it because the network interface card can
enter a reduced energy consumption mode during the frame
transmission that will be discarded by the node [16]. The
normalized energy consumption of the three MAC protocols
is depicted in Fig. 16, where energy consumption is further
classified according to 1) overheard frames in promiscuous
mode (Overhear), 2) accepted frames that received by the
node and then the MSDU is sent to the upper layer (Accept)
and 3) discarded frames that are received and then discarded
MAC layer costs. (CBR sources=25)
Fig. 14
Normalized CPU time.
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by the node (Discard). In promiscuous mode, data frames
are not discarded and thus more energy is consumed compared with the other two MAC protocols.
Figures 17 to 21 show the simulation results of a MAHAN that consists of both ABO-enhanced and legacy IEEE
802.11 nodes. The pause time is 300 seconds and the number of CBR sources is 25. CBR sources are evenly distributed among all mobile nodes. At the network layer,
Fig. 16
Normalized energy consumption. (CBR sources=25)
Fig. 18
performance metrics including packet delivery ratio, endto-end delay, number of control/data packets sent/received
and CPU time have better improvement when more ABOenhanced nodes are present, as shown in Figs. 17 to 19. Note
that the MAC layer cost as well as the energy consumption show a minor improvement when 20% of mobile nodes
are ABO-enhanced, as shown in Figs. 20 and 21, because
that the ABO method solves the coexistence problem at the
Fig. 17 Delivery ratio and delay. (coexist, pause time=300 s, CBR
sources=25)
Network layer costs. (coexist, pause time=300 s, CBR sources=25)
HSU et al.: SUPPORT OF EFFICIENT ROUTE DISCOVERY IN MOBILE AD HOC ACCESS NETWORKS
1261
cost of introducing more MAC layer transmissions. The reduction of network layer control packets can compensate
the increased MAC layer load, however, when the proportion of ABO-enhanced nodes is small, the compensation effect is not obvious. As a result, the improvement on MAC
layer cost is minor. For larger proportion of ABO-enhanced
nodes, the effect of MAC layer cost compensation is more
apparent.
5.
Conclusion
Reactive routing protocols typically rely on broadcast-based
route discovery messages (i.e., RREQ) to discover routes.
When the network is highly dynamic, route failures would
occur frequently and introduce considerable costs on routing control. Packet overhearing can help reduce the number
of control packets and leave more bandwidth for data transmissions. Promiscuous mode is generally used to achieve
packet overhearing. In promiscuous mode, the MAC layer
transfers all received packets to the upper layer without filtering. Although it can achieve the highest delivery ratio and
the lowest end-to-end delay while producing minimal number of control messages, using promiscuous mode will make
the network interface card always in active mode and results in large amount of packets arrived at the network layer.
Thus the node is forced to consume more energy and take
Fig. 19 Normalized CPU time. (coexist, pause time=300 s, CBR
sources=25)
Fig. 20
more CPU time on processing overheard packets. Consider
that wireless devices are usually equipped with lower level
CPU and are powered by batteries, using promiscuous mode
may cause disinclined performance degradation at the system level.
In this paper, we propose the ABO method as an alternative to achieve packet overhearing. In the ABO method,
packet overhearing is performed in a controlled manner that
routing protocols can determine which packet to be disseminated to neighbors. The determination of disseminated
packets is dependent on the routing protocol’s strategy. We
have shown that system and network performance, including
CPU time, energy consumption, packet delivery ratio and
end-to-end delay, can be significantly improved when only
the route reply (RREP) and route error (RERR) messages
are disseminated using the ABO method. Besides, considering the widespread used legacy IEEE 802.11 standard, the
coexistence problem of the ABO-enhanced and legacy IEEE
802.11 nodes is also discussed. A solution to the coexistence
problem is provided and evaluated in this paper.
Acknowledgments
The authors acknowledge the very useful feedback provided
during discussions with the members of Computer Network
Laboratory, NTUST. This work was supported by National
Science Council under Contract Nos. NSC 93-2213-E-011007 and NSC 94-2213-E-011-009.
Fig. 21 Normalized energy consumption. (coexist, pause time=300 s, 25
CBR sources)
MAC layer costs. (coexist, pause time=300 s, 25 CBR sources)
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Chun-Yen Hsu
received the B.S. and M.S.
degrees from National Taiwan Ocean University
in 1997 and 1999, respectively. He is working toward the Ph.D. degree in the Department
of Computer Science and Information Engineering at the National Taiwan University of Science and Technology. His research interests include ad hoc networks, wireless LAN and wireless MAN.
Jean-Lien C. Wu
received the BSEE from
National Taiwan University, Taiwan, in 1970,
and the Ph.D. degree in EE from Cornell University, Ithaca, NY, in 1976. From 1978 to 1984,
she was Member of the Technical Staff, Bell
Laboratories, Holmdel, NJ. Since 1984, she has
been a professor in the Electronic Engineering
Department, National Taiwan Univ. of Science
and Technology (NTUST), Taipei, Taiwan. She
was Head of the department from 1987 to 1990,
Dean of the College of Electrical and Computer
Engineering from 1998 to 2001 and Dean of the Academic Affairs from
2003 to 2005. She is currently Vice President of the university. Dr. Wu
is the coordinator of the Communication Engineering Program of the Engineering and Applied Sciences Dept. of the National Science Council
(NSC), Taiwan. She is also a member of the advisory board of the Science
and Technology Advisory Office of the Ministry of Education, Taiwan. Her
current research interests are in wireless networks, broadband Internet, and
mobile multimedia networks. Dr. Wu is a member of the IEEE communication society. She is also a member of the Chinese Institute of Engineerings
society and the Chinese Institute of Electrical Engineerings society.
Shun-Te Wang
received his B.E. degree in
mechanical engineering from National Taiwan
University of Science and Technology, Taipei,
Taiwan, in 1996. He received the M.S. degree in
engineering science from National Cheng Kung
University, Tainan, Taiwan, in 1999. He has
been a software engineer at Institute of Information Science, Academia Sinica, Taiwan, since
September 1999. Currently he is working toward a Ph.D. in the Department of Electronic
Engineering at National Taiwan University of
Science and Technology. His main areas of interest include pervasive
computing, sensor networks, ad hoc networks, wireless networks, mobile
computing, communication protocols, component software, software warehouse, authoring system, and Java collaborative computing.