IR-XY-PV: Infrared Multihop Communication
for XY-Coordinated PV Modules
Hideya Ochiai

Zhiqing Zhang

The University of Tokyo
ochiai@elab.ic.i.u-tokyo.ac.jp

Tsinghua University
zhangzq14@mails.tsinghua.edu.cn

Hiroto Kitamura

Hiroshi Esaki

The University of Tokyo
zun@hongo.wide.ad.jp

The University of Tokyo
hiroshi@wide.ad.jp

ABSTRACT
Photovoltaic (PV) power stations are rapidly increasing as an alternative energy resources for oil, natural gas, coal, and nuclear.
If each PV module has intelligence and the ability to report its
working status (e.g., voltage and temperature), we can manage thesystem status of such PV power stations with IoT systems. PV
modules are usually installed on an XY-grid, indicating that we
can apply an XY multihop routing for gathering such sensor readings with tiny wireless nodes. We propose the architecture androuting schemes of an infrared multihop communication for XYcoordinated PV modules (IR-XY-PV). This includes neighbor discovery and disruption tolerant packet forwarding, i.e., single-copy
forwarding and multi-copy forwarding schemes for packet propagation in the network. We have developed 20 node scale IR-XYPV network and confirmed that IR-XY-PV can provide practicallyenough performance regarding delivery success rate, delivery latency and memory usage.

CCS CONCEPTS• Networks → Layering; Network protocol design; Sensor networks;

KEYWORDS
Multihop Communication, IoT, PV, DTN
ACM Reference Format:
Hideya Ochiai, Zhiqing Zhang, Hiroto Kitamura, and Hiroshi Esaki. 2017.
IR-XY-PV: Infrared Multihop Communication for XY-Coordinated PV Modules. In AINTEC ’17: AINTEC ’17: Asian Internet Engineering Conference , November 20–22, 2017, Bangkok, Thailand. ACM, New York, NY, USA, 7 pages.
https://doi.org/10.1145/3154970.3154971

1 INTRODUCTIONManagement of photovoltaic (PV) power stations is one of the important applications of the Internet of Things (IoT) these days [3,Permission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others thanACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission
and/or a fee. Request permissions from permissions@acm.org.
AINTEC ’17, November 20–22, 2017, Bangkok, Thailand© 2017 Association for Computing Machinery.
ACM ISBN 978-1-4503-5551-3/17/11. . . $15.00
https://doi.org/10.1145/3154970.3154971

6, 13]. A PV power station has tens of, hundreds of, or even thousands of PV modules depending on its scale. Some of them sometimes become malfunctioned causing fatal performance degradation, which can be detected at the macro-level – but it is not easy
to identify the wrong module(s) without checking each PV module
one-by-one. If each PV has intelligence and the ability to report its
status (e.g., voltage and temperature), todays IoT systems can tellthe identified wrong PV panels to their remote operators [7, 9–11].
PV modules are usually installed in an XY-coordinated manner,indicating that we can deploy wireless tiny sensor nodes on each
PV module and can run an XY distance based routing for sensor
data gathering. We can probably design a sensor node using infrared LED as a transmitter and photodiode as a receiver assuming
to attach itself on a PV module in the manufacturing process.
In this paper, we propose the architecture and routing schemesof an infrared multihop communication for XY-coordinated PV
modules (IR-XY-PV). Each PV module has an addess on an XYgrid, e.g., (s x , sy ), and can send packets to a destination address(d x , dy ). The packet may contain the voltage information which
can be used for PV module diagnosis. In this work, we identify the
network architecture and study XY routing for a narrow-band infrared communication network. They include neighbor discovery
and disruption tolerant forwarding schemes.
“XY routing” performs packet routing by choosing the closer
node to the destination as the next hop node. Here, the distancemetric is defined on the XY-grid. It is basically stateless. An intermediate node (i.e., router node) does not have a stateful routing
table as the Internet protocol (IP) routers do. However, each node
still (1) needs to know the existence or availability of neighbors,(2) needs to confirm that their forwarded packet has been acceptedby the next hop node in order to perform disruption tolerant forwarding, and (3) needs to manage its packet buffer. In this paper,
we design the network architecture with considering these issues.
We also study the packet forwarding schemes – single-copy forwarding (SCF) and multi-copy forwarding (MCF). SCF just makes
one copy of a packet in the network during the packet delivery,
whereas MCF generates many copies in the network so as to havemultiple delivery paths for redundancy and faster delivery.
We have developed 20 IR-transceiver nodes and evaluated (1)delivery success rate, (2) delivery latency, (3) delivery paths, and(4) buffer occupancy for SCF and MCF with different distance cost
metrics and network parameters. Clock synchronization, configuration of node address, and packet broadcasting are also important
but we do not focus these issues in this paper.

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand

y

(1,3)

(2,3)

Hideya Ochiai, Zhiqing Zhang, Hiroto Kitamura, and Hiroshi Esaki

IR

(…,3)

(tx,ty+1)

TX

(…,2)

TX

(tx,ty) (tx+1,ty)LED(tx-1,ty)

IR
TX

(2,2)

LED

LED

(1,2)

IR

LED

(2,1)

(tx,ty-1)

TX

(1,1)

(…,1)

x
(a) Infrared transceiver on XY-coordinated PV modules

IRRX(b) Infrared transceiver

Figure 1: IR-XY-PV deployment plan – IR transceiver addressing for XY-coordinated PV modules
This paper is organized as follows. Section 2 provides the related
work. We propose IR-XY-PV architecture and its routing schemes
in section 3. Section 4 provides our evaluation work with our network testbed. Section 5 gives discussion, and we conclude this paper in section 6.

2 RELATED WORK
Module level PV monitoring has been proposed with short-range
radio-based wireless communication [10, 11] and power-line communication [7, 9, 12]. In this paper, as an alternative method, weare proposing infrared communication for PV module monitoring.
XY routing, we propose in this paper, fits into the family of
position-based routing or geographical routing [8]. However, mostof the position-based routing were discussed in the context of vehicular ad hoc networks (VANETs) [1], where the nodes move on
XY-plane. Hybrid with delay/disruption tolerant network (DTN)[4, 14] were also for VANET.
A number of DTN routing schemes on narrow-band and intermittently connected links have been studied [2]. However, it is notstudied on PV module networking, especially with infrared and XY
routing.
Optical wireless (OW) communication [5] are well-studied, however, we have not found the work of multihop network with infrared sensor nodes.
The contribution of this paper is to propose the network architecture and routing protocol of an infrared based XY routing for
PV module networking.

3 INFRARED XY-ROUTING FOR PVSThis section describes the architecture and routing schemes forinfrared multihop communication for XY-coordinated PVs (IR-XYPV). As Fig.1, we attach an infrared communication node on thesurface of each PV module, assigning the address of a XY-grid (i.e.,
Z × Z). The communication node has sensors with which it canmeasure the status of the PV module (e.g., voltage and temperature). Each node, for sending those sensor readings, periodicallygenerates a packet p, which contains:• p.dst: The destination node of the packet, which we denoteby (d x , dy ) ∈ Z2 .
• p.src: The source node of the packet, which we denote by(s x , sy ) ∈ Z2 .
• p.time: The time of generating the observation values.
• p.values: The observation values (e.g., voltage and temperature of the PV module).
• p.crc: The checksum of this packet.
The source node sends p to the routing layer, which delivers p
to the destination p.dst. Then, the destination node will receive pand use the contents for their application after verifying p withp.crc. In IR-XY-PV, we considers p0 and p1 are identical if all theparameters of p0 and p1 are the same.
In the following discussion, with Fig.2, we focus on the routing
scheme of IR-XY-PV. It contains the design of (a) IR transceivernode, (2) neighbor discovery, (3) disruption tolerant XY routing
with SCF and MCF.
We introduce the concept of “this” node (which we denote byt = (t x , ty ) ∈ Z2 ) for desiging such routing schemes. We discuss,focusing on node t, for discovering neighbors and for forwarding
a packet to the next hop.

3.1 IR Transceiver NodeIn our design, the transceiver has four IR-LEDs for transmission
and one photodiode for receival as Fig.1(b). These LEDs can becontrolled separately. For example, only the left LED can be usedfor (t x , ty ) to transmit data frame to (t x − 1, ty ). Though there may
be several design choices for IR physical communication protocols,in this paper we consider to use 38kHz carrier based IR signal, as
they are widely used as worldwide standard for short-range andeasy-to-use communication. It encodes bit 0 and 1 for different duration of signal transmission as Fig.2. The starting mark has longer
duration for the receiver to identify the start bit.

3.2 Neighbor Discovery
As Fig.2, node t has a neighbor map at the routing layer. In IR-XYPV, they run a neighbor discovery protocol and manages the listof neighbors. This includes the statistics calculation.

IR-XY-PV: Infrared Multihop Communication
for XY-Coordinated PV Modules

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand

Instead of broadcasting a discovery frame and waiting for responses from all the possible neighbors, IR-XY-PV takes master/slave
approach – node t sends the discovery request frame for potential
neighbors one-by-one. This is possible because nodes are deployedon a XY-grid and neighbor’s addresses can be calculated.
The following algorithm describes neighbor discovery routine
at node t:

SENSOR

CLOCK

V
GENERATE

USE

Application Layer
Node ID+

RECV

Function NeighborDiscovery()
For i = 1 to MAXRAD do
NDReq((t x − i, ty ))NDReq((t x , ty − i))
NDReq((t x + i, ty ))NDReq((t x , ty + i))
End ForEnd Function

timevaluesSEND

Routing Layer
Neighbor Map

Here, MAXRAD is a constant parameter that represents the maximum radius for single hop transmission. In each NDReq(n) process, node t sends neighbor discovery request to node n. Node nthen returns neighbor discovery response to node t. If node t does
not receive the response within a certain duration (e.g., within 1
second), t sends the request again. t stops sending the request after
receiving the response or after trying three times.
Each node carries out this algorithm periodically, for example,
with 300 second interval. Based on the responses of neighbor discovery requests, node t generates the neighbor map in the following manner. Neighbor map consists of the list of (1) neighbor node,(2) link cost, and (3) node cost.
Let LC (n, t) be a cost of link that represents the link quality fromt to n, and TryCount (n, t) be the tried count of neighbor discovery
request from t to n. After NDReq(n) is finished, node t updates
LC (n, t) with the following rule:
LC (n, t) := a · LC (n, t) + (1 − a) · TryCount (n, t)

USER

(1)

Here, a is a smoothing parameter, which is given as a constantvalue between 0 and 1. If NDReq(n) is finished with failure (i.e.,
the neighbor response is not received in the three trial), we set
TryCount (n, t) to be five as a special case.
The neighbor map also manages the cost of node n (which wedenote by NC (n, t)) from the observation by t,bif Freject received,
NC (n, t) := (2)
c · NC (n, t) otherwise.
Here, b is a positive constant value which represents the maximum cost of the node. NC (n, t) will be set to b when Freject isreceived from n at t (as for Freject – see Section 3.3). c is a smoothing parameter, which is given as a constant value between 0 and
1.

3.3 Disruption Tolerant XY Routing
As Fig.2 shows, node t has a packet buffer and a history buffer.
The packet buffer stores packets for achieving disruption tolerant
store-and-forward scheme. The history buffer manages the footprint of the packets in order to tell to the previous hop node that
this node has already had the packet in the near past.
The packet buffer is a table of

Packet Buffer

Packetdst src time values crc

MAC Layer

Forward Request

Neighbor Discovery Request

Forward Confirmed

00 nbr

10 next this dst src time values crc CRC
11 prev this

this CRC

Neighbor Discovery Response01 ognHistory Buffer

this CRC

hash

CRC

Forward Rejected
12 prev this hash

CRC

RX

TX

Infra-Red Layer
start

0

1

0

0

1

0

Figure 2: IR-XY-PV protocol stack: (1) application layer generates and uses packets, (2) routing layer delivers packets tothe destination node, (3) MAC layer manages the communication with neighbors, and (4) infrared layer modulates anddemodulates data bits.
• packet: key of the table,• lst: last sent time of the packet,• fn: forwarded nodes of the packet,• fr: forward retry count remains for the packet,• at: timestamp of the packet arrival.
Those parameters are bound to each packet, so we describe themfor instance of packet p as p.lst, p.f n, p.f r , p.rt in the following
discussion. When a new packet p arrives at t, the packet buffer
will add a new record for p initializing that p.lst = 0, p.f n =NULL,
p.f r =INFINITE, and p.at =time(). Here, time() gives the current
timestamp.
For forwarding a packet between nodes, IR-XY-PV defines thefollowing three data frames as Fig.2: (1) forward request (Frequest ),(2) forward confirmed (Fconfirmed ), and (3) forward rejected (Frejected ).
Node t sends a packet on Frequest to the specified next n, and thenn replies back Fconfirmed or Frejected to the previous node, i.e., t.
Fconfirmed means that the packet has been accepted to the node,
whereas Frejected means the packet has not been accepted by thenode. Fconfirmed and Frejected do not contain the packet itself. Instead, they contain the hashed value of the packet, i.e., hash(p).
To forward a packet in disruption-tolerant manner, we define
two algorithms for single-copy forwarding (SCF) and multi-copy
forwarding (MCF).

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand

Hideya Ochiai, Zhiqing Zhang, Hiroto Kitamura, and Hiroshi Esaki

3.3.1 Single-Copy Forwarding Algorithm. When node t has packet
p, t chooses a single next hop node for forwarding p. In the following discussion, we denote the next hop node chosen by single-copy
strategy by nextSCF (t, p). For choosing this, we define a cost function for sending p for a next node n at node t as follows:
cost (n, t, p) = D (n, p.dst ) + αLC (n, t) + βNC (n, t).

(3)Here, α and β are positive constants. D (n, p.dst ) gives the distance on the XY-grid from the next n to the destination p.dst. We
included LC and NC in the cost function because the network status cannot be simply determined by the distance function but avaliability of the links and the nodes are also important for choosing
the next hop node. Here, we can consider Manhattan distance D M
or Euclidean distance D E , which are formally defined as:
D M (n, d)=

D E (n, d)=|n x − d x | + |ny − dy |√(n x − d x ) 2 + (ny − dy ) 2

(4)

y
14

24

34

44

54

13

23

33

43

53

12

22

32

42

52

11

21

31

41

51

x

(5)

In SCF, node t chooses the lowest cost node as the next. Moreformally,
nextSCF (t, p) = argminn∈nbr(t) {cost (n, t, p)}.

(6)

When nextSCF (t, p) is determined, node t sends Frequest with p
to nextSCF (t, p). If the nextSCF (t, p) receives the Frequest , it replies
back Fconfirmed or Frejected .
If node t receives Fconfirmed for p, it removes p from the packet
buffer, calculates hash(p) and pushes it into the history buffer. If
the history buffer is full, it overrides the oldest one.
If t does not receive Fconfirmed or does receive Frejected , it will
update p.lst as:
p.lst := time()(7)
Especially if t receives Frejected , it also changes the neighborbuffer based on Eqn. 2.
Node t tries forwarding of p if p.lst+RFI < time(). Here, RFI isa re-forward interval, which is defined in disruption tolerant forwarding context.
When node t receives packet p with Frequest from another node,
it first searches the packet buffer whether the same packet is already managed, if so, it just responds Fconfirmed . If p is not contained in the packet buffer, it calculates hash(p) and searches thehistory buffer. If hash(p) is contained in the history buffer, it also
just responds Fconfirmd . If hash(p) is not contained in the history
buffer, and the packet buffer has a space for accepting, it responds
Fconfirmed and store p into the packet buffer as a new packet. It responds Frejected if the forwarding decision is not appropriate, forexample, if it has no memory space for the packet.
3.3.2 Multi-Copy Forwarding Algorithm. Making multiple-copies
in the network will potentially improve the delivery latency andthe probability of reachability. Instead, it will have more chance of
encountering dead situation. Dead packets will remain in the network, consuming buffer and transmissions. To avoid this, we allow
the nodes to intentionally delete the packets which have gone tosuch situations.
Using the cost (n, t, p) defined in section 3.3.1, in MCF, node tchooses the lowest N nodes as the next nodes (where N > 1);
we denote them by nextMCF (t, p). Then, it sends Frequest to allof nextMCF (t, p) one-by-one except the nodes listed in p.f n. The

Event Log
Collection Server

Figure 3: IR-XY-PV experiment with 5×4 deployment configuration. Each node had a management interface throughwhich we collected experiment logs for analysis.

nodes that received Frequest return Fconfirmed . Node t add the responded nodes to p.f n. If the count (p.f n) reaches N , node t removes packet p from the packet buffer, pushing hash(p) to the history buffer.
In MCF, we also define intentional packet deletion scheme asfollows.
(1) Node t removes p if the packet buffer is full and p is the oldest in the buffer when adding a new packet into the buffer.
This time, t does not return Frejected .
(2) Node t removes p if t has received Frejected five times fromthe next nodes.
(3) Node t removes p if no more appropriate next candidates
available. For example, deдree (t) is just one or two.
(4) Node t sets p.f r = 3 if t has received Fconfirmed from at least
one of the next nodes. p.f r decrements at every RFI. If it
reaches 0, node t removes p.
When node t removes p from the packet buffer, it adds hash(p)to the history buffer.

4 EVALUATION
We have evaluated IR-XY-PV regarding to (1) packet delivery success rate, (2) delivery latency, (3) delivery path, and (4) buffer occupancy. We have tested with 20 IR transceivers for SCF and MCF,
and for different distance metrics.

IR-XY-PV: Infrared Multihop Communication
for XY-Coordinated PV Modules
Table 1: Experiment settings
Type

Neighbor
Discovery

PacketForwarding

Parameterabc
MAXRAD
INTERVAL
MAX_RETRYRETRY_INTERVALβCHECK_INTERVALRFIRETRY_INTERVALBUF_SIZE
HASH_BUF_SIZE

Value
0.9
3
0.9
1
300
3
2-2.8
0.33
2
15
2-2.8
20
16

Delivery Probability[%]

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand
100

100

80

80
M 0.0160

E 0.0140

M 0.0160

M 0.33

M 0.33
E 0.0140

E 0.3320

E 0.33

20

0

0
0

10

20

30

40

50

Delivery Latency[sec](a)Single copy forwarding

0

10

20

30

40

50

Delivery Latency[sec](b)Multi copy forwarding

Figure 4: Packet delivery latency for SCF and MCF. We evaluated with Manhattan(M) and Euclidean(E) distance, and
α = 0.01 and α = 0.33.

Table 2: Delivery success rate
Case
Manhattan
SCF
Euclidean
Manhattan
MCF
Euclideanα = 0.01
α = 0.33
α = 0.01
α = 0.33
α = 0.01
α = 0.33
α = 0.01
α = 0.33

Sent
118
118
118
118
118
118
118
118

Received
118
118
118
118
117
118
118
118

4.1 Experiment Settings
We developed 20 IR transceivers with Arduino Mega 2560, and deployed them on 5×4 grid as shown in Fig.3. Each node had four
LEDs (for left, right, up and down transmissions) and one photodiode for receival. Each node also had an Ethernet port throughwhich we collected log of events for analysis.
We evaluated (1) delivery success rate, (2) latency, (3) paths, and(4) buffer occupancy for Manhattan distance and Euclidean distance, and α = 0.01 and α = 0.33 for both SCF and MCF schemes. If
α = 0.01, the distance metric becomes more dominant for choosing
the next. If α = 0.33, it is more awared of link status for choosing
the next. In this experiment, node (5,4) sent packet to (1,1) every
60 second, and we studied the delivery features of the packet. Table 1 shows the other configuration parameters. TX-LED direction
control was not applied.
We conducted the experiments in the following manner – (1)we installed software into the 20 nodes, (2) we reset all the nodes,(3) we waited for 15 minutes for the network to be stabilized, (4) 2hour experiment followed, and (5) we waited addtional 15 minutes
for all the packets in the network to be delivered. And, we analyzed
for the 2 hour duration.

4.2 Delivery Success RateAs Table 2 summarizes almost all the packets were delivered in
all the cases except one packet was lost in MCF with Manhattan

M

E

M

SCF

E

MCF

Figure 5: 4 samples of packet delivery paths for SCF and
MCF. These samples were taken with Manhattan(M) and Euclidean(E) distance and α = 0.33. The arrow label shows thetime from the packet generation at node (5,4).

distance and α = 0.01. In our observation, this loss was causedby the intensional deletion defined in MCF scheme. All the packet
copies were considered to be faced with dead situation and deleted
before arriving at the destination.

4.3 Delivery Latency
Fig.4 shows the cumulative distribution function of the delivery latency for all the cases. In general, MCF performed better compared
to SCF, especially for the last 20% of slow-delivery packets. In SCF,
Manhattan+α = 0.33 relatively performed well for all the latency
situations. There are no clear differences in MCF. This is probably

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand

Hideya Ochiai, Zhiqing Zhang, Hiroto Kitamura, and Hiroshi Esaki(a)SCF, Manhattan, 𝛼=0.33(b)SCF, Euclidean, 𝛼=0.33(c)MCF, Manhattan, 𝛼=0.33(d)MCF, Euclidean, 𝛼=0.33

Figure 6: Aggregated delivery paths from (5,4) to (1,1). The arrow label shows the number of packets successfully forwardedon this link.
because, in MCF as described in section 4.4, almost all the nodes
got the copies of all the packets – the distance metrics and α did
not practically make a difference for determining nextMCF (t, p).

the diagonal paths e.g., (2,2) and (3,3). In MCF, we cannot observe
the differences between Manhattan and Euclidean as they copied
the packets to basically all the nodes.

4.4 Packet Delivery Path

4.5 Buffer Occupancy

Fig.5 shows 4 samples of packet delivery paths, i.e., transmission
traces, for SCF and MCF with Manhattan distance and Euclidean
distance, and α = 0.33. This visualization is based on the receival of
Fr equest. The label on the arrow indicates when this transmission
has been made (in second) from the generation of the packet. We
can observe that (1) packet delivery with SCF is straight-forward– directly sending the packet to the destination with the minimum
copies, and that (2) packet delivery with MCF has made too many
copies in the network – distributing to all the nodes sometimes
causing backward-forwarding. The average number of copies madein the experiments are 7.00 for SCF and 18.4 for MCF.
Fig.6 shows the aggregated delivery paths for all the packets, i.e.,
118 packets generated at node (5,4). The label shows the numberof packets successfully forwarded between the two nodes – duplicated transmissions on the same link are not counted. From thethese graphs, we can observe that in SCF, Manhattan distance has
allowed the packets to take the route based on the cost of the links,
and that Euclidean distance has concentrated the packets aroundFig.7 shows the average buffer occupancy, i.e., the average numberof packets contained in the packet buffer, during the experimentfor SCF and MCF. There are clear differences between them. In
SCF, the average buffer occupancy were around 0.05 packets, but
in MCF, we observed much larger occupancies and variance among
the nodes. These results indicates that in SCF, a very few numberof packets have been remained in the network, where as, in MCF,
many packets have been remained in the network in order to makere-transmission of packets to the next hop nodes.

5 DISCUSSION
We have proposed the architecture and routing schemes of IRXY-PV. During the discussion, we have identified several design
choices such as single-copy forwarding (SCF) and multi-copy forwarding (MCF), Manhattan distance and Euclidean distance, and α
and other parameters. With the 20-node scale experiment, we havestudied the features of these design choices regarding to packet delivery success rate, delivery latency, paths, and buffer occupancy.

IR-XY-PV: Infrared Multihop Communication
for XY-Coordinated PV Modules

3

0.05

0.05

0.04

0.05

0.05

6.1

9.3

12.6

17.4

9.2

6.0

15.0

12.0

15.4

6.7

7.3

3

4

0.7
2

0.05
3

0.7
12.7

0.7

3

0.04

1
2

4

0.04

2

1

0.7

0.05

0.04 0.06

4

0

AINTEC ’17, November 20–22, 2017, Bangkok, Thailand

1.1

1
4

5

0

(a)SCF, Manhattan, 𝛼=0.33

6
0

0.6

1

2

0.7
5

(b)MCF, Manhattan, 𝛼=0.33

Figure 7: Average occupancies of packet buffers for SCF and MCF with Manhattan distance and α = 0.33. The destination (1,1),
which did not use the packet buffer, is specially marked.
According to the result, MCF performed better than SCF when wejust consider the latency for one packet delivery. However, MCF
consumed much larger buffer spaces and transmissions. MCF may
also potentially lose packet by deleting all the propagated packet
copies by the intensional deletion scheme defined in section 3.3.2– which has actually happened in the experiment.
In the future, we need to consider the case where all the nodes
generate packets to a sink node, i.e., to the same destination. Congestion in transmissions and buffer overflow will probably happen
especially in MCF – causing slower delivery and larger packet loss
than SCF. However we cannot simply use SCF, because SCF also
has a chance of packet loss. Thus, we consider that more study isnecessary for MCF algorithm.

6 CONCLUSION
In this paper, we have proposed the architecture and routing schemesof an infrared multihop communication for XY-coordinated PV
modules (IR-XY-PV). We proposed “XY routing” with several design choices on (1) IR transceivers, (2) neighbor discovery algorithm, (3) disruption tolerant forwarding, (4) single-copy and multicopy, (4) Manhattan distance and Euclid distance, (5) link cost management and other configuration parameters. We have developed
20 node scale IR-XY-PV testbed network and studied the delivery
success rate, delivery latency, delivery paths, and memory usage
for those several design choices. With the experiment, we confirmed that IR-XY-PV can provide practically enough performancewith our proposed architecture and routing scheme.

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