Multi-Path Multi-Channel Data Aggregation

Scheduling in Wireless Sensor Networks

Miloud Bagaa∗, Mohamed Younis§, Adlen Ksentini‡ and Nadjib Badache∗

∗ Department of Theories and Computer Engineering, CERIST, Algiers, Algeria.

Emails:{bagaa,badache}@mail.cerist.dz§ Department of Computer Science and Electrical Engineering, University of Maryland Baltimore County.

Email:younis@cs.umbc.edu‡ IRISA, University of Rennes 1, Rennes, France Email: adlen.ksentini@irisa.fr

Abstract—In-network aggregation is employed to cut on re-dundancy and conserve the network resources. To meet thecriticality and responsiveness goals, the aggregated data are tobe disseminated to the base-station reliably while reducing thedelivery latency. In this paper, a novel approach is proposed forReliable Multi-channel Scheduling for timely dissemination ofAggregated data (RMSA). RMSA strives to form an aggregationtree such that there are k disjoint paths from each node tothe base-station and finds a collision free schedule for nodetransmissions so that the aggregated data reaches the base-stationin minimal time. RMSA is a cross-layer scheme that intertwinesthe formation of the multi-path structure and the assignmentof transmission slots to the individual node with the objectiveof increasing the simultaneity of transmissions and reducing thebuffering delay. The availability of multiple radio channels isfurther exploited in order to prevent colliding transmissions andboost the overall network throughput. RMSA is validated throughsimulation and is shown to outperform previously publishedschemes.

Index Terms—wireless sensor network, data aggregation,scheduling media access, multi-path routing, multi-channels.

I. INTRODUCTION

Wireless sensor networks (WSNs) can be used in many

applications such as security surveillance [2]. Responsiveness

in these applications is very important in order to prevent harm

from being inflicted, e.g., damages caused by some intruders.

This requires careful management of the limited bandwidth

in order to increase the data throughput and reduce packet

delivery latency. In addition, like most WSNs operating in

inaccessible setups the sensor nodes operate on small batteries

and energy-efficiency has to be instrumented at all layers

of the communication protocol stack, most notably at the

link and network layers. The use of Time Division Multiple

Access (TDMA) is deemed effective for achieving these design

goals since it avoids medium access collisions among nodes

and thus reduces the data latency and energy consumed in

failed-transmissions and in idle-listening [3]. In-network data

aggregation helps in limiting medium access contention by

reducing the number of transmitted packets, and thus it can

enable timely delivery of data and facilitate finding low-latency

medium access schedule [4].

While time-based medium access scheduling and in-network

data aggregation have individually received lots of attention

from the research community, few solutions have been recently

proposed in literature to reduce the data latency by combining

the formation of the aggregation tree with medium access

scheduling [5] [4]. These solutions assume that a sensor node

uses only one channel to communicate with its neighbors.

However, the new generation of sensor nodes is equipped with

multi-channel radio transceiver, which allows them to com-

municate on multi-channels. For example Micaz is equipped

with a CC2420 transceiver that provides 16 non-overlapping

frequency channel. Exploiting the availability of multiple

channels would increase the simultaneous of transmission, and

hence reduce more the time latency.

However, the harsh conditions that the nodes operate in and

the limited on-board energy supply make them susceptible

to failure. Moreover, the wireless links often suffer from

interferences and packets may get lost before reaching the

base-station. Therefore, the failure of nodes and links is

an inevitable, which can reduce dramatically the quality of

the service provided by the WSN to the applications. To

mitigate the effect of link and node failures, few solutions have

been proposed in [9] [10]. These solutions employ multi-path

routing, such that each node has more than one parent while

forwarding the aggregated data towards the base-station.

In this paper, we pursue a novel cross-layer approach for

low-latency and reliable delivery of aggregated data in WSN.

Our approach, which we call RMSA, exploits the availability of

multi-channel links in order to minimize data delivery latency

and ensure data freshness. To mitigate the effect of link and

node failures, a multi-path routing topology is established

where each node can reach the base-station over k node-

disjoint paths. RMSA intertwines the formation of the multi-

path routing topology and the medium access scheduling

in order to minimize the buffering delay at the aggregation

nodes while ensuring the freshness of the data. Contemporary

schemes found in the literature pursue breadth-first ordering

when forming the aggregation tree, where a node in level Lwith respect to the base-station always sends its data to a node

(parent) in level L − 1. Unlike these schemes, in RMSA a

node that is L-hop away from the base-station (i.e., in level

L) selects its parents from nodes in levels L−1, L, and L+1and its time slot as the earliest one in all the channels.

978-1-4799-0543-0/13/$31.00 ©2013 IEEE

The rest of the paper is organized as follows. Related

work is summarized in section II. The problem formulation

and the basic idea of our approach are presented in section

III. The approach details are provides in section IV. The

simulation results are discussed in section V. Finally, the paper

is concluded in section VI.

II. RELATED WORK

It is important to avoid mixing data from different periods

in periodic applications so that the data aggregation will

not degrade the accuracy of the assessment process. This

requirement is often referred to as data freshness and imposes

ordering and latency constraints on the aggregation tree. In

other words, the data reading sent from a node to its parent

on the tree should be aggregated with readings made in the

same sampling period. To ensure that the time slot at which

each parent in the tree transmits data must be later than that

of all its children. In addition to the energy savings achieved

by data aggregation, reduced latency and increased medium

efficiency are the most popular performance objectives. Chen

et al. proved that scheduling transmissions on an aggregation

tree in order to achieve minimum delivery latency is a NP-hard

problem [5]. They further proposed a centralized heuristic to

assign time slots to reduce the latency on a given aggregation

tree. On the other hand, Bagaa et al. [4] combined the tree

construction and nodes scheduling process in order to reduce

the delay.

Both solutions proposed in [5] and [4] do not support

multi-channel links when scheduling network nodes. In order

to further reduce the data delivery latency, the approaches

of [3], [6], [7], and [8] exploit the capabilities of emerging

multi-channel transceivers. In [8] network nodes are grouped

into clusters. The cluster heads are organized into tree to

aggregate and forward data from whole the network to the

base-station. To reduce the latency, the branches of the tree

are assigned different channels. Meanwhile, a Receiver-Based

Channel Assignment (RBCA) protocol is proposed in [3], in

which the children of the same parent use the same channel

to transmit their data. A Tree-Based Multi-Channel Protocol

(TMCP) is proposed in [7]. In TMCP, the aggregation tree

is subdivided into a set of sub-trees such that the root of

each of them is a neighbor of the base-station. To increase

the network throughput, each sub-tree is assigned a different

channel. On the other hand, in JFTSS [6] the links of the

tree are sorted such that the link which conflicts with the

most number of other links has high priority for channel

reassignment. However, RBCA, TMCP and JFTSS do not

ensure the aggregation freshness when scheduling network

nodes.

A major drawback for the solutions discussed above is

that they do not consider links and nodes failure, which can

significantly affect the data throughput. EMDC [9] and RTAD

[10] are recently proposed to deal with potential link and

node failures by forming multi-path aggregation topologies.

Nevertheless, both solutions assume that only one channel is

used when scheduling network nodes.

III. PROBLEM FORMULATION AND BASIC IDEA

The problem addressed in this paper is: given C channels

how to schedule all the network nodes without collisions so

that the sensor data are reliability delivered to the base-station

in minimal time. Each node is assumed to have one half-

duplex multi-channel transceiver such that it cannot transmit

and receive simultaneously and cannot hear two messages at

the same time, even on different channels. In other words, this

transceiver can use at most C channels, and at given time,

each node can use only one channel to transmit or receive

data. As proved by Chen et al. in [5], achieving optimal data

aggregation scheduling is NP-hard problem. Therefore, we

pursue heuristics to schedule network nodes’ transmissions.

Most of published solutions in the literature create tree

structure then schedule nodes transmissions by allocating slots

and channels, such that the collisions are prevented. Therefore,

if a node loses its parent, these solutions should be rerun to

connect whole network nodes with base-station and devise new

transmission schedule. Our proposed scheme overcomes this

limitation by forming k paths from each node to the base-

station. RMSA also opts to reduce time latency by increasing

the time slot reuse and by selecting for each node the earliest

time slot in all channels. Time slot reuse in this context means

that multiple nodes are able to transmit at the same time

without causing an interference at the receivers. Moreover,

in RMSA the formation of aggregation multi-path structure

and the medium access scheduling process are intertwined

in order to increase simultaneity among the transmissions.

For each node u, RMSA uses a subset of k neighbors of uto act as parents in order to simultaneously reach the next

hop on k distinct-paths. Let us denote by γ(u) and ρ(u)the set of neighbors and parents of node u, respectively (i.e.,

ρ(u) ⊆ γ(u)).

For each channel i the neighbors γ(u) of a node u can be

subdivided into three disjoint subsets according to a time slot

τ . The set δτi (u) of node u consists of nodes which are: (a) not

selected as parents at time slot τ on channel i, and (b) cannot

overhear other transmutations at this time slot using channel

i. Formally, each node in δτi (u) has at least one neighbor (i.e.,

scheduled node) which has already selected its parents, and

sets its time slot and its channel to τ and i, respectively; The

set τi (u) has nodes which are selected as parents by other

scheduled nodes at time slot τ and channel i. The neighbors

of nodes in this set are prevented from using τ on channel idue to collision. The set ϕτi (u) has the nodes which can be

selected as parents of node u at time slot τ on channel i and

can be formally defined as:

ϕτi (u) = γ(u)− (δτi (u) ∪ ξτ (u)

k⋃

j=1

τj (u)) (1)

such that ξτ (u) is the set of u’s neighbors which are (i)scheduled and (ii) their time slot is lower or equal to τ . We

remove ξτ (u) from ϕτi (u) in order to ensure data freshness

and prevent the creation of cycles. If τi (u) 6= ∅ or ϕτi (u) = ∅,

node u should select a time slot that is higher than τ in order

to avoid introducing collision with nodes in τi (u).

In RMSA, to ensure the communication between a node uand its parents ρ(u) only one channel CHu and one time

slot TSu are sufficient. In order to increase the network

throughput, CHu and TSu should be selected such that the

time latency is reduced. To do that, TSu should be selected as

the earliest available time slot in all channels, whereas CHu

is the channel which allows more time slot reuse. While all

neighbors of u cannot receive another packet when u transmits

at time slot TSu on channel CHu, a node v ∈ (γ(u)− ρ(u))can transmit to a node w ∈ γ(v) at TSu on channel CHu as

long as w /∈ γ(u) and v is not neighbor of ρ(u). Moreover,

if v uses a channel different than CHu, it can transmit to any

node in w ∈ (γ(v)− ρ(u)) at TSu. RMSA takes advantage of

this observation to boost the simultaneity of transmissions, and

consequently minimize the data delivery latency, by increasing

the chance of finding u’s parents ρ(u) for which u can use a

smallest time slot.

IV. RELIABLE MULTI-CHANNEL SCHEDULING FOR

TIMELY DISSEMINATION OF AGGREGATED DATA (RMSA)

In order to facilitate the presentation of RMSA, Figure 1

will be referenced throughout the discussion. Figure 1 shows

a detailed example of RMSA execution using two channels

and for k = 2. In this figure, a dashed arrow between

two nodes a and b indicates that a has chosen b as one of

its parents when the arrow becomes solid it indicates that

the link (a, b) is scheduled. The dotted lines represent the

graph connectivity, i.e., the presence of a communication

link between a pair of nodes. The gray number besides the

dashed and solid arrow (a, b) represents a’s transmission slot.

The black number besides the dashed and solid arrow (a, b)represents a’s transmission channel.

A. Algorithm description

To reduce data latency, the time slot reuse should be

increased as much as possible. To do so RMSA addresses

the following issues: (i) how the nodes select their parents

in multi-path structure; (ii) how time slots and channels

are distributed over nodes, such that the time slot reuse is

maximized. In addition to CHu, TSu, ρ(u), γ(u), δτi , τi , ϕτidefined earlier, the following notation will be used to explain

RMSA:

• SCu: Set of u’s scheduled neighbors, i.e., the neighbors

which have already selected their parents and set their

channel and time slot,

• SCu: Set of u’s unscheduled neighbors i.e., the neighbors

which have not yet selected their parents. Formally,

SCu = γ(u)− SCu,

• T ji : Time slot j on channel i,• ηi: The node whose identifier is i,• ψi(u): The parents of node u on channel i,

1) Temporal ordering of transmissions: During the execu-

tion of RMSA, each node can be in one of the following three

states:

i) Unready state : a node in this state is not yet ready to

select its parent. Initially, all nodes except the leaves are

in an unready state. A node in this state is represented by

a gray circle.

ii) Ready state : a node in this state is ready to select its

parent. Initially, all leaf nodes are in this state. A node in

this state is represented by a white circle.

iii) Scheduled state : a node in this state has already se-

lected its parents and set its time slot and channel. Ini-

tially, all nodes are not scheduled. When RMSA completes

its execution all nodes except the base-station should be

scheduled. A node in this state is represented by a black

circle.

A non-leaf node in RMSA will not change its state from

unready to ready until all its neighbors in level L + 1 are

scheduled. Therefore, nodes in RMSA will be scheduled

bottom-up, level by level in a breadth-first order. Each node

in level L will not be ready until all its L + 1 neighbors

are scheduled. For example in Figure 1(a) η1, η2, η3 and

η4, which are in level 3 should be scheduled before nodes

η5, · · · , η11 in level 2. The scheduling of nodes in a breadth-

first order does not mean that the aggregated data is also

routed on a breadth-first tree.

2) Assigning parents and time slots: When a node u is

ready to schedule (i.e., whose state is ready), it should be

assigned CHu, TSu and ρ(u) which reduce the time latency,

ensure the aggregation freshness and maximize the time slot

reuse. To do so for each channel i we compute the earliest

time slot T τi that ensures the aggregation freshness and avoids

medium access collision. Formally, T τi is defined as the

earliest time slot in channel i which is higher than the slots

of all u’s children, and which ensures that τi (u) = ∅ and

ϕτi (u) 6= ∅. Moreover, a set of k parents ψi(u) for each

channel i are selected from ϕτi (u) which reduces the time

latency, and increases time slot reuse and network reliability.

To provide more flexibility in picking parents in ψi(u) that

meet the criteria above. In RMSA, the parents ψi(u) of a node

u, which is in level L, can be selected from levels L − 1,

L and L + 1, respectively. If u is an already-scheduled node

and w is one of its parents (w ∈ ψi(u)), all w’s neighbors

in SCw will be forbidden from using τ on channel i when

setting their schedule since ∀v ∈ SCw, τi (v) 6= ∅. If a node

selects parents which have the highest number of unscheduled

neighbors, many nodes will be prevented from using TSu in

subsequent iterations. In order to increase the time slots reuse,

for each channel the k parents ψi(u) should be selected such

that |⋃

w∈ψi(u)

SCw−({u}∪ψi(u))| is minimized and the fault-

tolerance goal is achieved. If there is more than k nodes in

(u) which fulfill the above conditions, the k parents ψi(u) for

channel i are selected as the ones whose ID is the smallest.

Obviously, if u’s node degree is less than k, all neighbors are

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used as parents and reliability goal will not be met at that

node.

To increase the time slot reuse and also reduce the time

latency, CHu is set to be the channel i which offers the

earliest time slot T τi . If there is many channels which qualify,

CHu is picked as the one which enables the selection of

the parents ψi(u) with the fewest number of unscheduled

neighbors. If there is more than one channel which fulfill the

above conditions, CHu is set as the channel i which allows

the selection of the highest number of parents ψi(u) in order

to increase the network reliability. If there is more than one

channel which fulfill all the above conditions, the channel with

smallest ID will be selected. Meanwhile, ρ(u) and TSu will

be selected according to CHu value. ρ(u) and TSu are set to

ψCHu(u) and T τCHu

, respectively, in order to further enable

more simultaneity transmissions,

RMSA allows each node u to select its parents from sched-

uled nodes, i.e., {ρ(u)} ∩ SCu 6= ∅. From equation 1 a node

u in RMSA selects its parent w from ϕτi ∩ SCu if and only

if TSu < TSw, thus the data freshness is ensured and the

creation of cycles are prevented.

Figure 1(a) shows how node η2 choses CH2, TS2 and

ρ(2). We note that #11 (2) = ∅ and ϕ#1

1 (2) = {η7, η8, η9}.Therefore, slot #1 is the earliest available time slot on channel

1 that can be assigned to η2. Node η2 has to select two

nodes among its neighbors as parents ψ1(2) on channel 1 from

ϕ#11 (2). The choices are:

1) Nodes η7 and η9: If node η2 selects node η7 and η9 as its

parents ψ1(2) on channel 1, the number of unscheduled

neighbors that will be prevented from using the same

time slot is |SC7 ∪ SC9 − {η2, η7, η9}| = 11. Therefore,

if node η7 and η9 are selected as parents, eleven nodes

will be prevented from using time slot #1 on channel 1.

2) Nodes η8 and η9: If node η8 and η9 are selected as

parents, eight nodes will be prevented from using time

slot #1 on channel 1.

3) Nodes η7 and η8: Only 7 nodes will be prevented from

using time slot #1 on channel 1.

Obviously the best choice is {η7, η8} since it imposes the least

constraints and it is likely to enable the reuse of time slot #1on channel 1 and consequently reduce latency. For the same

reason in channel 2 the earliest available time slot that can be

assigned to η2 is #1 and its parents ψ2(2) = {η7, η8}. As the

two channels offer the same time slot, whose parents have the

same number of unscheduled neighbors and |ψ1(2)| = |ψ2(2)|,η2 selects the channel which has the smallest ID, which is 1.

Thus, CH2, TS2 and ρ(2) will be set to 1, #1 and {η7, η9},respectively.

Meanwhile, in Figure 1(b) #11 (3) = {η10}, which means

that the time slot #1 with channel 1 is reserved by the children

of η10, and if η3 uses this time slot on channel 1 a collision will

occur at η10. As #21 (3) = ∅ and ϕ#2

1 (3) = {η8, η9, η10}, the

earliest time slot that can be assigned to η3 on channel 1 is #2.

Also, it has to select two nodes among its neighbors as parents

ψ1(3) on channel 1 from ϕ#21 (3). The choice {η8, η9} prevents

7 nodes to use time slot #2 on channel 1, whereas the choices

{η8, η10} and {η9, η10} prevent only 6 nodes to use time slot

#2 on channel 1. Obviously the best choice are {η9, η10}or {η8, η10} . So, η3 has to choose between {η9, η10} and

{η8, η10}. As nodes in {η8, η10} have the smallest identifier

compared to {η9, η10}, ψ1(3) sets to {η8, η10}. In the same

Figure, #12 (3) = ∅ and ϕ#1

2 (3) = {η8, η9}, the earliest time

slot that can be assigned to η3 on channel 2 is #1 and its

parents ψ2(3) = {η8, η9}. As channel 2 offers the earliest time

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Fig. 2: Comparison of the data delivery latency of RMSA and the baseline approaches versus: (a) the number of nodes (N),(b) the network density (ψ), (c) the number of channels (C), and (d) the number of parents (k).

slot, CH3, TS3 and ρ(3) will be set to 2, #1 and {η8, η9},respectively. In both scenarios, η3 which is in level L selects

its parent ρ(3) from L− 1.

Figure 1(d) shows an example to illustrate the selection

of a parent of η9 from scheduled nodes and level L + 1.

As shown in Figure 1(d), ξ#1(9) = {η2}, #11 (9) = ∅,

ϕ#11 (9) = {η3, η11, η14, η15, η16, η17} and {η3, η15} have the

fewest unscheduled neighbors, η9 selects {η3, η15} as its

parents on T#11 . Note that, η3 is selected from the scheduled

nodes without creating cycle as shown in Figure 1(e).3) Scheduling the transmitting nodes: Assigning CHu,

TSu and ρ(u) to a ready node u does not mean that uis automatically scheduled as there might be another ready

node v and a collision would be created if both nodes

are scheduled simultaneously. Figure 1(a) shows the conflict

which can happen between nodes η1 and η2 if they are

scheduled simultaneously. Based on the parent selection and

time slot assignment procedure discussed above, we have: (i)ρ(1) = {η5, η6}, CH1 = 1 and TS1 = 1; (ii) ρ(2) = {η7,

η8}, CH2 = 1 and TS2 = 1. Therefore, if η1 and η2 are

scheduled simultaneously a collision will occur at η7 and η8.

RMSA resolves such a conflict by considering every pair for

ready nodes u and v in the level that is being scheduled. A

conflict exists between nodes u and v if one of the following

conditions holds:

1) A collision: If TSu = TSv and

a) ρ(u) ∩ ρ(v) 6= ∅ or

b) CHu = CHv and (ρ(u) ∩ γ(v) 6= ∅ or ρ(v) ∩ γ(u) 6=∅).

2) Violation of data freshness or creation of cycle: If both

u and v are ready nodes and u selects v as a parent or

the other way around, i.e., v ∈ ρ(u) or u ∈ ρ(v).

To resolve a collision, RMSA picks the one among the

two nodes u and v whose parents collectively have the

least number of unscheduled neighbors and reschedule its

transmission in order to enable the most reuse. In case of

a tie, the node, either u and v, that has the least number

of unscheduled neighbors will keep the earliest slot in order

to allow as many nodes as possible to be parent in this slot

with channel CHu, i.e., reduce the size of δTSu

CHu. In the case

both u and v, fulfill the above conditions, the node with the

smallest ID will be scheduled. Meanwhile, the violation of

data freshness or creation of cycles are resolved as follows:

If v ∈ ρ(u) or u ∈ ρ(v) then the node which has the earliest

time slot, say u, should be scheduled first. That is if node vsets u as one of its parents and v is scheduled first, node ushould change its time slot to be later than TSv in order to

ensure the data freshness. If TSu = TSv , the conflict will be

resolved using the same approach.

Coming back to the example in Figure 1(a), let us consider

η1 and η2. If η1 is scheduled first, the set #11 of the four

unscheduled neighbors of η1’s parents (i.e., SC5 ∪ SC6 −{η1, η5, η6} = {η7, η12, η13, η14}), will not be empty and will

be prevented from transmitting at T#11 . Moreover, two nodes

(i.e., γ(1)−ρ(1) = {η7, η8}) will be prevented from serving as

parents at T#11 . Assigning slot #1 to η2 makes {η7, η8} locked

and will disallow: (i) seven unscheduled neighbors (i.e.,

SC7∪SC8−{η2, η7, η8} = {η1, η3, η6, η12, η13, η14, η15}) to

transmit at T#11 and (ii) one node (i.e., γ(2) − ρ(2) = {η9}

) to serve as a parent. As η1 prevents less number of nodes

to use T#11 , η1 is to be allocated T#1

1 to transmit before η2.

Afterwards, based on the parent selection criteria discussed

above, η2 recomputes its CHu, TSu and ρ(u). As shown in

Figure 1(b), CH2 = 2, TS2 = 1 and ρ(2) = {η7, η8}. Then

scheduled as shown in Figure 1(c).

V. SIMULATION RESULTS

In this section, we validate the performance of RMSA using

simulation and compare it to that of DAS-UT [4], EMDC [9],

JFTSS [6], TMCP [7] and RBCA [3] and RTAD [10]. The

following metrics are used:

• Time latency (latest time slot): is defined as the time

required for the base-station to receive the aggregated

data from all sensor nodes;

• Robustness: This metric reports the percentage of nodes

that have k parents. Recall that RMSA strives to maintain

k paths to the base-station. However, it may not be

possible to do so since some nodes may have fewer

than k neighbors, and most importantly due to the fact

that fewer than k neighbors would be able to receive

a transmission as a result of the employed transmission

scheduling approach.

In the simulation experiments, we assume that all nodes

have the same transmission range. We generate a topology of

N nodes and network density ψ in a square area according to

a uniform random distribution. The algorithms evaluation is

0

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50

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120

50 100 150 200 250 300 350 400 450 500 550 600

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EMDCRTADRMSA

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EMDCRTADRMSA

(d)

Fig. 3: Assessing the robustness of RMSA compared to the baseline approaches versus: (a) the number of nodes (N), (b) the

network density (ψ), (c) the number of channels (C), and (d) the number of parents (k).

performed by varying the number of nodes, network density,

the required number of parents k and the number of channels

C. We chose these metrics due to their impact on the time

latency and robustness. We conduct four sets of experiments:

(i) Vary N , and fix (i) ψ to 50, (ii) C to 8 and (iii) k to 5;

(ii) Vary ψ, and fix (i) N to 300, (ii) C to 8 and (iii) k to

5; (iii) Vary C, and fix (i) N to 300, (ii) ψ to 50 and (iii)k to 5; (vi) Vary k, and fix (i) N to 300, (ii) ψ to 50 and

(iii) C to 8.

As JFTSS, TMCP and RBCA do not meet the data freshness

requirement and in order to fairly compare RMSA with them,

we have slightly modified their implementations so that the

data freshness would be ensured. Basically, when the parent

node is ready to be scheduled, it is assigned a time slot later

than all its children. In our simulation results, each plotted

point represents the average of 35 executions. We plot the

95% confidence interval on the graphs.

A. Time latency

Figure 2 shows time latency as a function of N , ψ, C and

K. Time latency depends on the required number of parents

k, number of channels used by each node C, the flexibility in

choosing parents from the same, preceding and succeeding

levels, and the effectiveness of time-slot reuse mechanism.

When the value of k is high and/or value of C is low, the

size of δτi (u) and τi (u) will significantly grow, and then only

few nodes will succeed in reusing existing time slots. Although

RMSA uses multi-path structure, it outperforms all the baseline

solutions in term of data latency. RMSA has better results

due to: (i) intertwining the multi-path structure formation and

transmission scheduling; (ii) allowing nodes to select their

parents from three levels and from already scheduled nodes;

(iii) using multi-channels when scheduling network nodes.

B. Robustness

Figure 3 shows the percentage of nodes which select kparents as a function of N , ψ, C and k. All the nodes, in

DAS-UT, RBCA, TMCP and JFTSS, select only 1 (i.e., less

than k) parent. As shown in this Figure, in EMDC less than

%35 of nodes end up with k parents or more, whereas in RTAD

and RMSA more than 80% of nodes do so. This implies that

RMSA achieves high dependability.

VI. CONCLUSION

In this paper, we have presented RMSA which lowers the

data collection latency while taking into account possible

nodes and links failures. RMSA achieves its objective by: (i)intertwining the multi-path structure construction and nodes

scheduling, (ii) allowing each node to use more than one

channels so that an optimal time-slot and channel assignment

can be achieved, (iii) dissemination of sensor data over at

least k-node disjoint paths, (iv) employing criteria for parent

selection for nodes on the multi-path structure that maximize

time slot reuse. The simulation results have shown that RMSA

outperforms competing solutions in the literature with respect

to the number of nodes, the network density, the number of

channels and the required number of parents.

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