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Congestion-free Routes for Wireless Mesh Networks
                                  Nemesio A. Macabale Jr.*†, Roel M. Ocampo†, and Cedric Angelo M. Festin†
                                                *
                                                  Central Luzon State University, Philippines
                                                  †
                                                    University of the Philippines, Philippines
                                              E-mail:{namacabale, roel, cmfestin}@up.edu.ph

   Abstract— Recently proposed wireless mesh routing metrics              neighbors. Consequently, we found out that the information provided
based on awareness of congestion, load or interference typically          by the MAC layer of a wireless node would be sufficient to achieve
employ queue occupancy of a node's wireless interface to                  our goals.
estimate traffic load. Queue occupancy, however, does not                    The rest of the paper is organized as follows: Section II elaborates
directly reflect the impact of channel contention from neighbor           further on the motivation for this work, and discusses similar work
nodes. We propose an alternative called the channel load-aware            found in the literature. Section III presents an analysis that leads to
(CLAW) routing metric that takes into consideration not only              CLAW's design and implementation, while Section IV discusses the
the traffic load within the node itself, but also the degree of           results of the preliminary evaluation. Finally, Section V concludes by
interference and contention within the channel. CLAW uses                 enumerating the contribution of this work.
local information from a node's MAC layer to estimate channel
busyness and contention levels. It does not require complex                                      II.    RELATED WORK
computations, nor the exchange of link-level statistics with
neighbors. Our preliminary results show that CLAW can                        In a multi-hop WMN, routing is more critical than in wired
identify congested regions within the network and thus enable             networks, because the wireless medium is shared and is highly
the determination of routes around these congested areas. We              dynamic [11]. Different packet flows may interfere with each other
present the results of simulations we conducted to evaluate the           even when they do not necessarily traverse the same path. Along a
use of CLAW in mesh-wide routing.                                         path, neighboring nodes that share a channel compete for its use
                                                                          forming a collision domain (see Figure 1). As more flows traverse
  Keywords - wireless mesh networks, routing, routing metric,             nearby paths and nodes, they compete for access to the shared
congestion awareness.                                                     channel, eventually congesting the path and lowering throughput
                                                                          significantly.
                        I.    INTRODUCTION                                   There have been several efforts to address this issue through the
                                                                          use of load-aware, interference-aware, and/or congestion-aware
   Wireless mesh networks (WMN) have attracted significant                routing metric either singly [12-23] or in combination with multiple
attention in recent years for flexible and rapid deployment of            metrics [24-32]. Load-aware routing algorithms such as DLAR [16]
wireless services in a wide variety of applications. These                and ALARM [30] measure load based on the number of packets
applications include broadband home networking and automation             buffered in the interface queue. However, a single node's internal
[1], [2], community mesh networking [3-5], in transportation              load as gauged from the state of its buffers cannot reliably estimate
systems [6], public safety and disaster scenarios [7], [8], and in        the level of congestion within a collision domain, because the queues
medical applications [9].                                                 of other nodes within that domain could be empty or lightly loaded.
   Mesh networks are composed of wireless nodes that participate          In this case, the heavily- and lightly-loaded nodes do not jointly paint
either as routers or clients of the network. The mesh routers are         a consistent picture of the channel. In other words, while interface
generally static or minimally mobile and serve either as dedicated        queue occupancy accurately measures load on nodes, it does not
forwarding nodes, access points for clients like desktop PCs, laptops     necessarily estimate the load on a region in a network.
and mobile devices, or both. Collectively, mesh routers form the             To measure loaded regions, many proposals either obtain the sum
backbone of the wireless network, enabling traffic to be transported      [13], [18], [20], [21], [28], [29], [31] or the average of queue length
and ensuring reachability between participating nodes.                    [14], [15], [19], [25] of nodes within a collision domain. This
   However, despite advances in the field, there are still many           approach requires the data to be collected or exchanged among
interesting research challenges in optimally routing traffic within a     neighbors, and thus generates additional overhead in terms of
wireless mesh network. Due to the shared nature of the wireless           bandwidth and route convergence time.
channel, routing based on metrics traditionally used in wired                Other proposals measure channel load based on radio-frequency
networks such as hop counts do not take into account interference         (RF) channel interference [18], [23] and delay [33], [34]. However,
and contention within the channel shared among neighboring mesh           in most wireless environments there are other potential sources of
nodes. As a result, routing algorithms that use such "congestion-         interference and delay aside the load in the channel, such as physical
agnostic" metrics may tend to direct multiple traffic flows naively       layer impairments and bad channel conditions [11]. Hence, there
along known best paths, eventually congesting wireless channels           should be a way to both measure and differentiate channel and node
along the path and causing significant drops in network throughput.       load. The interference awareness and load-balancing metric in [26],
In contrast, a routing algorithm that is able to veer the traffic flow    [27] requires probe packets and neighbor-wide gathering of link-state
towards calmer regions of the network would be less likely to suffer      statistics, which likewise generate overhead in the bandwidth and
from such a scenario.                                                     time needed to calculate the metric.
   To address this issue, we propose a routing metric called the             Some proposal that truly measure congestion, interference, and
channel-load aware routing metric (CLAW) designed to take into            load include LWR [12] and C2WB [17]. LWR however combines
account congestion, interference and load-imbalance issues found in       multiple metrics to achieve its goal, requiring more calculations than
wireless mesh networks. Our design goal is to come up with a simple       CLAW, which relies on a single metric. In addition, LWR collects
yet accurate congestion / interference / load-aware routing metric        information from neighbors. Similarly, C2WB requires probing
that can be incorporated into a more general concept of capacity          packets, neighbor information, and a complex computation. In
awareness [10]. To accomplish this task, in CLAW's design, we             addition, it requires a change in the MAC layer protocol,
avoided the need to advertise and collect link-level statistics between
We proposed CLAW to address the issues mentioned, through the                                    Ch _ load=T sensedEnergyT blockedForAccess                        (1)
use of node-local information, and by requiring only simple
computations. In our investigation, we found that the MAC layer has
all the information needed to accurately estimate channel load,                      where :
                                                                                     T sensedEnergy is that fraction of time that a node is transmittinga packet to
interference, and node load. CLAW can be used by routing protocols
as an alternative to existing congestion awareness mechanisms either                                thechannel , is receiving a packet from the channel , issensing
in single channel or multi-channel environments.                                                    transmissionenergy beit collision , interference , or noise
                                                                                                    in thechannel
                  III.   DESIGN   AND IMPLEMENTATION
                                                                                     T blockedForAccess isthat fraction of time that anode
   Our analysis begins by looking at a node j's collision domain. It is                             is backing - off or deferring
comprised of all nodes within j's carrier sensing range that operate on
the same channel. Transmissions of these nodes may interfere with
transmissions from j. This is illustrated in Fig. 1, with the                                        01         02       03         04      05
simplifying assumption that the carrier sensing range is circular. The                                                                           Node i's collision
nodes in this diagram are furthermore assumed to operate using the                                                                               domain
IEEE 802.11b wireless standard.                                                                      06         i        08         09      10
   Because of the shared nature of the channel, the load on a node




                                                                                                                     k
affects all the neighbor nodes that can sense its transmission. That is,
an idle node will respond to a new traffic flow request like a busy or                               11         12       j          14      15
loaded node if a neighbor within its carrier sensing range is in fact
busy or loaded. Hence, identifying busy regions, rather than busy
                                                                                                     16         17       18         19      20
nodes, is a more effective approach in avoiding congestion,
preventing interference, and distributing traffic loads. The routing                                                                             Node j's collission
                                                                                                                                                 domain
protocol may then assign a lower cost to the next-hop node that has                                  21         22       23         24      25
the least busy collision domain. This is the basic intuition behind,
and our motivation for, the development and use of the CLAW                                   Figure 1: A Wireless Mesh Network with 25 nodes
metric.
A.    Channel Load                                                                                                              busy _ count
                                                                                                                Ch _ load=                                             (2)
   From the point of view of a node, the channel is in use, i.e. busy,                                                          scan _ count
when the node is either transmitting or receiving a packet from the
channel, or if it senses any transmission energy that hinders                                 CLAW j t =1−×CLAW j t −1×Ch _ load j                            (3)
successful transmission such as those resulting from collisions,
interference, or other forms of noise. In addition, the channel may                                 where :
                                                                                                    CLAW j t The value of CLAW at timet
likewise be considered busy when the node is blocked from
accessing the channel, such as due to the back-off and defer periods                                α isa tunable parameter :0≤ α ≤ 1,here 0.5is used
in the distributed coordination function (DCF) in the IEEE 802.11                                   Ch _ load j isthe current observed channel load at node j
standard [35]. If all these events can be classified into one of two                                CLAW j t−1 isthe previous CLAW
fractional components of time, called TsensedEnergy and                                             t refers tothe current measuring period
TblockedForAccess, then channel load is the total fraction of time that a
node is busy due to any of these contributing events. Equation (1)                                             CLAW P t = ∑ CLAW j t 
expresses this definition of channel load.                                                                                                                             (4)
                                                                                                                                j∈P
   We derived this definition from the result of a simple experiment
with three IEEE 802.11b nodes placed within a single collision                                 where :
domain. In the experiment, a node Node0 sent packets to another                                CLAW P t  isthe equivalent path metric based on CLAW
node Node1 until channel saturation, while a third node Node2
                                                                                              1.2
silently observed. Although the physical layer of all three nodes
sensed the channel with the same degree of actual utilization (i.e.
amount of time packets occupied the channel), the sender Node0 was                             1
loaded/busier (see Fig. 2) than the the receiver Node1 and the
                                                                                                                     Estimated
observer Node2, all the way through saturation, because of the                                                       Channel Load
blocking time (back-off and defer periods) in the DCF functionality                           0.8
                                                                            percent of time




                                                                                                                     Estimated
of IEEE 802.11b[35]. At saturation, although the sender viewed                                                       Packet In the Air
                                                                                                                     Mac Load of
channel load to be 100% the receiver and observer only viewed the                             0.6                    Node 0
channel as around 78% loaded. It is interesting to note that the 78%                                                 Mac Load of
load approximated the ratio of time the packets propagating in the air                                               Node 1
occupied the channel. This is comparable to the throughput                                    0.4                    Mac Load of
                                                                                                                     Node 2
saturation encountered at around 80% channel busyness by others
[36]. Generally, saturation throughputs have not been achieved at
100% busyness [36], [37] as may be intuitively expected from such a                           0.2
metric, because the back-off and defer periods in the IEEE 802.11
MAC protocol were not taken into account. In contrast, by taking                               0
these into account, the CLAW metric is able to account for the
                                                                                                           0             1               2         5          8
missing ~20% busyness. Thus, not only can CLAW effectively
                                                                                                                              input traffic (Mbps)
identify busy regions, in addition, it can discriminate between loaded
and non-loaded nodes within such busy regions.                                                                   Figure 2: Channel Load Measurement
It is also worth noting that we do not make any assumption about                                                  V.      CONCLUSIONS
the operating channel of a collision domain. Our analysis only
require that i and j's collision domain operate on the same channel. If      We propose the channel-load aware (CLAW) routing metric to
some collision domains operate over different channels the analysis       address issues in congestion, interference and load-imbalance
will follow the same process. In addition, the analysis (TsensedEnergy    problem in wireless mesh networks. CLAW does not require
and TblockedForAccess) will still be valid had a different mac layer      complex computations, nor any exchange or collection of neighbor-
technology been used. Thus, CLAW is suitable for both single- and         wide link-level statistics. Its simplicity allows it to be easily
multi-radio and multi-channel mesh networks.                              integrated, if necessary, with other capacity-aware routing metrics
                                                                          with minimal overhead. Analysis also shows it is suitable to both
B.   Implementation                                                       single-and multi-channel or multi-radio mesh networks.
   To estimate the channel load, we simply monitor how the MAC               Initial simulation results demonstrated its ability to effectively
layer views the channel. The MAC layer senses the busyness of the         estimate channel busyness and enable flows to avoid congested
channel through carrier sensing (provided by the physical layer) and      regions.
virtual carrier sensing through its NAV (network allocation vector)          Although it shows promise, our initial comparison with hop-count
[35]. Within a defined observation period the MAC layer is queried        routing merely demonstrates CLAW's basic ability to support
whether it senses the channel to be busy, backing-off, or deferring.      congestion-free routing. A more comprehensive performance
The number of times where the MAC layer reports any of these three        comparison with similar congestion-aware metrics is therefore in
conditions (busy_count), divided by the number of times the MAC           order. Ultimately, the usefulness of this metric can only be fully
layer is queried (scan_count) becomes the estimated channel load as       realized through actual, working implementations, rather than
defined in Eq. (2). It is interesting to note that the channel load       through theoretical simulations. We will hopefully address all of
computed using Eq. (2) consistently matched the estimated channel         these in our future work.
load (for Node0) and actual fractional packet-in-the-air time (for
Node1 and Node2) as observed and presented in Fig. (2).                                                              ACKNOWLEDGMENT
   To account for sudden changes in traffic and the dynamic
                                                                            This work has been supported by the Engineering Research and
behavior of the wireless channel, we employ a moving average for
                                                                          Development for Technology (ERDT) Consortium, Department of
the channel load using a tunable parameter α. We initially used
α=0.5, although further experimentation and study may suggest other       Science and Technology – Science Education Institute (DOST-SEI),
                                                                          Republic of the Philippines.
values. The CLAW metric is thus defined in Eq (3) as the moving
average of the estimated channel load. Equation (4) is the equivalent                                       20       21        22         23       24
path metric based on CLAW.

                IV. SIMULATION AND DISCUSSION                                                                        16        17         18       19
                                                                                                            15
   We performed preliminary qualitative and quantitative
experiments to evaluate the performance of our proposed routing
metric using ns-2 [38] with the OLSR extension as used in [39]. We                                          10       11        12         13       14
wanted to quickly test whether our metric would in fact avoid busy
regions of the network, and whether it would achieve better
throughput compared to hop count-based routing.                                                             05       06        07       08         09
   The first set of simulations were designed to show whether the
CLAW metric could steer flows away from loaded regions of the
network. The set-up shown in Fig. 3, similar to that in [36] involved
                                                                                                           00        01       02        03         04
25 nodes uniformly distributed in a grid of 800 x 800 square meters.
Data-rates between nodes is set to 11 Mbps. For the main traffic               Figure 3: Node 00 originates an FTP flow towards Node 24.
flow, FTP bulk traffic over TCP was used, with the packet size set of                  CBR traffic between Node 11 and Node 12 form an
1040 bytes (NS2 default size [38]). Constant bit rate (CBR) is used                    interference flow. While hop-count based routing would
for the interference flow. To simplify the simulation both                             result in the straight-line path 00-06-12-18-24, CLAW
transmission range and sensing range were set to 250m, while the                       routes the flow through 00-01-02-03-09-14-19-24,
distance between nodes was set to 176 m. At the start of the                           avoiding busy regions
simulation, the interference flow between nodes 11 and 12 was
initiated, creating the busy region indicated by the two circular areas                          600                                                         CLAW
in Fig. 3. With a traditional hop count metric, packets traversed the                                                                                        HopC
path 00-06-12-18-24. With CLAW, packets followed the path 00-                                    500
01-02-03-09-14-19-24, effectively avoiding the busy region in the
                                                                                                 400
                                                                             Throughput (kbps)




network.
   In the second set of simulations, the interfering traffic was varied                          300
from 0, 0.5 Mbps, 1 Mbps, 1.5 Mbps, …, 5 Mbps in order to observe
network behavior and performance with varying degrees of                                         200
busyness. Fig. 4 compares the throughput attained by the main flow
with hop count and CLAW routing metrics. Each data point in the                                  100
graph represents the average from 10 simulation runs. The dramatic
decrease in the throughput of the network that used hop count                                      0
routing, especially around 2-2.5 Mbps interference traffic, was due to                                 0   0.5   1     1.5     2    2.5        3   3.5   4    4.5   5
packet drops within the busy region. In contrast, CLAW was able to                                                    Interfering Traffic (Mbps)
avoid the busy region, resulting in significantly better end-to-end
throughput even with high levels of busyness within the network.                                 Figure 4: Throughput comparison between CLAW and Hop-
                                                                                                                          count metrics
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Congestion Free Routes for Wireless Mesh Networks

  • 1. Congestion-free Routes for Wireless Mesh Networks Nemesio A. Macabale Jr.*†, Roel M. Ocampo†, and Cedric Angelo M. Festin† * Central Luzon State University, Philippines † University of the Philippines, Philippines E-mail:{namacabale, roel, cmfestin}@up.edu.ph Abstract— Recently proposed wireless mesh routing metrics neighbors. Consequently, we found out that the information provided based on awareness of congestion, load or interference typically by the MAC layer of a wireless node would be sufficient to achieve employ queue occupancy of a node's wireless interface to our goals. estimate traffic load. Queue occupancy, however, does not The rest of the paper is organized as follows: Section II elaborates directly reflect the impact of channel contention from neighbor further on the motivation for this work, and discusses similar work nodes. We propose an alternative called the channel load-aware found in the literature. Section III presents an analysis that leads to (CLAW) routing metric that takes into consideration not only CLAW's design and implementation, while Section IV discusses the the traffic load within the node itself, but also the degree of results of the preliminary evaluation. Finally, Section V concludes by interference and contention within the channel. CLAW uses enumerating the contribution of this work. local information from a node's MAC layer to estimate channel busyness and contention levels. It does not require complex II. RELATED WORK computations, nor the exchange of link-level statistics with neighbors. Our preliminary results show that CLAW can In a multi-hop WMN, routing is more critical than in wired identify congested regions within the network and thus enable networks, because the wireless medium is shared and is highly the determination of routes around these congested areas. We dynamic [11]. Different packet flows may interfere with each other present the results of simulations we conducted to evaluate the even when they do not necessarily traverse the same path. Along a use of CLAW in mesh-wide routing. path, neighboring nodes that share a channel compete for its use forming a collision domain (see Figure 1). As more flows traverse Keywords - wireless mesh networks, routing, routing metric, nearby paths and nodes, they compete for access to the shared congestion awareness. channel, eventually congesting the path and lowering throughput significantly. I. INTRODUCTION There have been several efforts to address this issue through the use of load-aware, interference-aware, and/or congestion-aware Wireless mesh networks (WMN) have attracted significant routing metric either singly [12-23] or in combination with multiple attention in recent years for flexible and rapid deployment of metrics [24-32]. Load-aware routing algorithms such as DLAR [16] wireless services in a wide variety of applications. These and ALARM [30] measure load based on the number of packets applications include broadband home networking and automation buffered in the interface queue. However, a single node's internal [1], [2], community mesh networking [3-5], in transportation load as gauged from the state of its buffers cannot reliably estimate systems [6], public safety and disaster scenarios [7], [8], and in the level of congestion within a collision domain, because the queues medical applications [9]. of other nodes within that domain could be empty or lightly loaded. Mesh networks are composed of wireless nodes that participate In this case, the heavily- and lightly-loaded nodes do not jointly paint either as routers or clients of the network. The mesh routers are a consistent picture of the channel. In other words, while interface generally static or minimally mobile and serve either as dedicated queue occupancy accurately measures load on nodes, it does not forwarding nodes, access points for clients like desktop PCs, laptops necessarily estimate the load on a region in a network. and mobile devices, or both. Collectively, mesh routers form the To measure loaded regions, many proposals either obtain the sum backbone of the wireless network, enabling traffic to be transported [13], [18], [20], [21], [28], [29], [31] or the average of queue length and ensuring reachability between participating nodes. [14], [15], [19], [25] of nodes within a collision domain. This However, despite advances in the field, there are still many approach requires the data to be collected or exchanged among interesting research challenges in optimally routing traffic within a neighbors, and thus generates additional overhead in terms of wireless mesh network. Due to the shared nature of the wireless bandwidth and route convergence time. channel, routing based on metrics traditionally used in wired Other proposals measure channel load based on radio-frequency networks such as hop counts do not take into account interference (RF) channel interference [18], [23] and delay [33], [34]. However, and contention within the channel shared among neighboring mesh in most wireless environments there are other potential sources of nodes. As a result, routing algorithms that use such "congestion- interference and delay aside the load in the channel, such as physical agnostic" metrics may tend to direct multiple traffic flows naively layer impairments and bad channel conditions [11]. Hence, there along known best paths, eventually congesting wireless channels should be a way to both measure and differentiate channel and node along the path and causing significant drops in network throughput. load. The interference awareness and load-balancing metric in [26], In contrast, a routing algorithm that is able to veer the traffic flow [27] requires probe packets and neighbor-wide gathering of link-state towards calmer regions of the network would be less likely to suffer statistics, which likewise generate overhead in the bandwidth and from such a scenario. time needed to calculate the metric. To address this issue, we propose a routing metric called the Some proposal that truly measure congestion, interference, and channel-load aware routing metric (CLAW) designed to take into load include LWR [12] and C2WB [17]. LWR however combines account congestion, interference and load-imbalance issues found in multiple metrics to achieve its goal, requiring more calculations than wireless mesh networks. Our design goal is to come up with a simple CLAW, which relies on a single metric. In addition, LWR collects yet accurate congestion / interference / load-aware routing metric information from neighbors. Similarly, C2WB requires probing that can be incorporated into a more general concept of capacity packets, neighbor information, and a complex computation. In awareness [10]. To accomplish this task, in CLAW's design, we addition, it requires a change in the MAC layer protocol, avoided the need to advertise and collect link-level statistics between
  • 2. We proposed CLAW to address the issues mentioned, through the Ch _ load=T sensedEnergyT blockedForAccess (1) use of node-local information, and by requiring only simple computations. In our investigation, we found that the MAC layer has all the information needed to accurately estimate channel load, where : T sensedEnergy is that fraction of time that a node is transmittinga packet to interference, and node load. CLAW can be used by routing protocols as an alternative to existing congestion awareness mechanisms either thechannel , is receiving a packet from the channel , issensing in single channel or multi-channel environments. transmissionenergy beit collision , interference , or noise in thechannel III. DESIGN AND IMPLEMENTATION T blockedForAccess isthat fraction of time that anode Our analysis begins by looking at a node j's collision domain. It is is backing - off or deferring comprised of all nodes within j's carrier sensing range that operate on the same channel. Transmissions of these nodes may interfere with transmissions from j. This is illustrated in Fig. 1, with the 01 02 03 04 05 simplifying assumption that the carrier sensing range is circular. The Node i's collision nodes in this diagram are furthermore assumed to operate using the domain IEEE 802.11b wireless standard. 06 i 08 09 10 Because of the shared nature of the channel, the load on a node k affects all the neighbor nodes that can sense its transmission. That is, an idle node will respond to a new traffic flow request like a busy or 11 12 j 14 15 loaded node if a neighbor within its carrier sensing range is in fact busy or loaded. Hence, identifying busy regions, rather than busy 16 17 18 19 20 nodes, is a more effective approach in avoiding congestion, preventing interference, and distributing traffic loads. The routing Node j's collission domain protocol may then assign a lower cost to the next-hop node that has 21 22 23 24 25 the least busy collision domain. This is the basic intuition behind, and our motivation for, the development and use of the CLAW Figure 1: A Wireless Mesh Network with 25 nodes metric. A. Channel Load busy _ count Ch _ load= (2) From the point of view of a node, the channel is in use, i.e. busy, scan _ count when the node is either transmitting or receiving a packet from the channel, or if it senses any transmission energy that hinders CLAW j t =1−×CLAW j t −1×Ch _ load j (3) successful transmission such as those resulting from collisions, interference, or other forms of noise. In addition, the channel may where : CLAW j t The value of CLAW at timet likewise be considered busy when the node is blocked from accessing the channel, such as due to the back-off and defer periods α isa tunable parameter :0≤ α ≤ 1,here 0.5is used in the distributed coordination function (DCF) in the IEEE 802.11 Ch _ load j isthe current observed channel load at node j standard [35]. If all these events can be classified into one of two CLAW j t−1 isthe previous CLAW fractional components of time, called TsensedEnergy and t refers tothe current measuring period TblockedForAccess, then channel load is the total fraction of time that a node is busy due to any of these contributing events. Equation (1) CLAW P t = ∑ CLAW j t  expresses this definition of channel load. (4) j∈P We derived this definition from the result of a simple experiment with three IEEE 802.11b nodes placed within a single collision where : domain. In the experiment, a node Node0 sent packets to another CLAW P t  isthe equivalent path metric based on CLAW node Node1 until channel saturation, while a third node Node2 1.2 silently observed. Although the physical layer of all three nodes sensed the channel with the same degree of actual utilization (i.e. amount of time packets occupied the channel), the sender Node0 was 1 loaded/busier (see Fig. 2) than the the receiver Node1 and the Estimated observer Node2, all the way through saturation, because of the Channel Load blocking time (back-off and defer periods) in the DCF functionality 0.8 percent of time Estimated of IEEE 802.11b[35]. At saturation, although the sender viewed Packet In the Air Mac Load of channel load to be 100% the receiver and observer only viewed the 0.6 Node 0 channel as around 78% loaded. It is interesting to note that the 78% Mac Load of load approximated the ratio of time the packets propagating in the air Node 1 occupied the channel. This is comparable to the throughput 0.4 Mac Load of Node 2 saturation encountered at around 80% channel busyness by others [36]. Generally, saturation throughputs have not been achieved at 100% busyness [36], [37] as may be intuitively expected from such a 0.2 metric, because the back-off and defer periods in the IEEE 802.11 MAC protocol were not taken into account. In contrast, by taking 0 these into account, the CLAW metric is able to account for the 0 1 2 5 8 missing ~20% busyness. Thus, not only can CLAW effectively input traffic (Mbps) identify busy regions, in addition, it can discriminate between loaded and non-loaded nodes within such busy regions. Figure 2: Channel Load Measurement
  • 3. It is also worth noting that we do not make any assumption about V. CONCLUSIONS the operating channel of a collision domain. Our analysis only require that i and j's collision domain operate on the same channel. If We propose the channel-load aware (CLAW) routing metric to some collision domains operate over different channels the analysis address issues in congestion, interference and load-imbalance will follow the same process. In addition, the analysis (TsensedEnergy problem in wireless mesh networks. CLAW does not require and TblockedForAccess) will still be valid had a different mac layer complex computations, nor any exchange or collection of neighbor- technology been used. Thus, CLAW is suitable for both single- and wide link-level statistics. Its simplicity allows it to be easily multi-radio and multi-channel mesh networks. integrated, if necessary, with other capacity-aware routing metrics with minimal overhead. Analysis also shows it is suitable to both B. Implementation single-and multi-channel or multi-radio mesh networks. To estimate the channel load, we simply monitor how the MAC Initial simulation results demonstrated its ability to effectively layer views the channel. The MAC layer senses the busyness of the estimate channel busyness and enable flows to avoid congested channel through carrier sensing (provided by the physical layer) and regions. virtual carrier sensing through its NAV (network allocation vector) Although it shows promise, our initial comparison with hop-count [35]. Within a defined observation period the MAC layer is queried routing merely demonstrates CLAW's basic ability to support whether it senses the channel to be busy, backing-off, or deferring. congestion-free routing. A more comprehensive performance The number of times where the MAC layer reports any of these three comparison with similar congestion-aware metrics is therefore in conditions (busy_count), divided by the number of times the MAC order. Ultimately, the usefulness of this metric can only be fully layer is queried (scan_count) becomes the estimated channel load as realized through actual, working implementations, rather than defined in Eq. (2). It is interesting to note that the channel load through theoretical simulations. We will hopefully address all of computed using Eq. (2) consistently matched the estimated channel these in our future work. load (for Node0) and actual fractional packet-in-the-air time (for Node1 and Node2) as observed and presented in Fig. (2). ACKNOWLEDGMENT To account for sudden changes in traffic and the dynamic This work has been supported by the Engineering Research and behavior of the wireless channel, we employ a moving average for Development for Technology (ERDT) Consortium, Department of the channel load using a tunable parameter α. We initially used α=0.5, although further experimentation and study may suggest other Science and Technology – Science Education Institute (DOST-SEI), Republic of the Philippines. values. The CLAW metric is thus defined in Eq (3) as the moving average of the estimated channel load. Equation (4) is the equivalent 20 21 22 23 24 path metric based on CLAW. IV. SIMULATION AND DISCUSSION 16 17 18 19 15 We performed preliminary qualitative and quantitative experiments to evaluate the performance of our proposed routing metric using ns-2 [38] with the OLSR extension as used in [39]. We 10 11 12 13 14 wanted to quickly test whether our metric would in fact avoid busy regions of the network, and whether it would achieve better throughput compared to hop count-based routing. 05 06 07 08 09 The first set of simulations were designed to show whether the CLAW metric could steer flows away from loaded regions of the network. The set-up shown in Fig. 3, similar to that in [36] involved 00 01 02 03 04 25 nodes uniformly distributed in a grid of 800 x 800 square meters. Data-rates between nodes is set to 11 Mbps. For the main traffic Figure 3: Node 00 originates an FTP flow towards Node 24. flow, FTP bulk traffic over TCP was used, with the packet size set of CBR traffic between Node 11 and Node 12 form an 1040 bytes (NS2 default size [38]). Constant bit rate (CBR) is used interference flow. While hop-count based routing would for the interference flow. To simplify the simulation both result in the straight-line path 00-06-12-18-24, CLAW transmission range and sensing range were set to 250m, while the routes the flow through 00-01-02-03-09-14-19-24, distance between nodes was set to 176 m. At the start of the avoiding busy regions simulation, the interference flow between nodes 11 and 12 was initiated, creating the busy region indicated by the two circular areas 600 CLAW in Fig. 3. With a traditional hop count metric, packets traversed the HopC path 00-06-12-18-24. With CLAW, packets followed the path 00- 500 01-02-03-09-14-19-24, effectively avoiding the busy region in the 400 Throughput (kbps) network. In the second set of simulations, the interfering traffic was varied 300 from 0, 0.5 Mbps, 1 Mbps, 1.5 Mbps, …, 5 Mbps in order to observe network behavior and performance with varying degrees of 200 busyness. Fig. 4 compares the throughput attained by the main flow with hop count and CLAW routing metrics. Each data point in the 100 graph represents the average from 10 simulation runs. The dramatic decrease in the throughput of the network that used hop count 0 routing, especially around 2-2.5 Mbps interference traffic, was due to 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 packet drops within the busy region. In contrast, CLAW was able to Interfering Traffic (Mbps) avoid the busy region, resulting in significantly better end-to-end throughput even with high levels of busyness within the network. Figure 4: Throughput comparison between CLAW and Hop- count metrics
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