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Dynamic Bandwidth Allocation Algorithm
With Proper Guard Time Management
Over Multi-OLT PON-Based Hybrid
FTTH and Wireless Sensor Networks
Monir Hossen and Masanori Hanawa
Abstract—A passive optical network (PON) is a highly
capable access network that effectively converges several
service providers, without suffering from any bandwidth
deficiency. However, a PON consisting of a single optical
line terminal (OLT) for multiple service providers increases
the computational complexity for data packet processing in
the OLT, resulting in a longer time delay and more packet
loss. A multi-OLT PON-based access network is an effective
solution for reducing the computational complexity of data
packet processing in a hybrid network of multiple service
providers. The most important issue concerning the multi-
OLT PON is the sharing efficiency of upstream channels
among multiple service providers having different packet
lengths and data rates. In this paper, we propose a dynamic
bandwidth allocation algorithm called adaptive limited dy-
namic bandwidth allocation for multi-OLT PON (ALDBAM).
The proposed scheme is a modified version of adaptive lim-
ited dynamic bandwidth allocation (ALDBA) algorithms
that we proposed before, where both the ALDBA1 and
ALDBA2 schemes are combined with proper guard time
management and a modified multipoint control protocol.
The simulation results show that the ALDBAM scheme pro-
vides lower packet delay with higher bandwidth utiliza-
tion, higher upstream efficiency, and higher throughput
than the conventional ALDBA1 and ALDBA2 schemes.
Index Terms—DBA algorithm; FTTH; Hybrid network;
Multi-OLT PON; Wireless sensor network.
I. INTRODUCTION
Aubiquitous city (u-City) is an autonomous city. A
significant number of service providers, such as
fiber-to-the-home (FTTH), wireless sensor networks
(WSNs), high-definition TV (HDTV) or video on demand
(VoD), and Femto networks (FNs), will comprise modern
u-Cities. Each home and business appliance in a future
u-City will be equipped with several sensor nodes remotely
monitored by owners and service providers. Usually, sensor
nodes will send notifications to the central office (CO)
concerning any abnormality of any household or commer-
cial device in a sensor network that is expected to be
deployed for all households and commercial systems in
the u-City, e.g., gas systems, temperature and pressure
monitoring systems, electric sparking and smoke detection
systems, automobile systems, and medical sensor nodes in
a hospital. Constructing a closed, specific-use network for
each individual application and accommodating several
users using different access terminals and servers require
an enormous amount of time and expense. To overcome the
enormous expense and deployment of several backbone
networks, passive optical network (PON)-based converged
networks have been proposed to connect the FTTH and
WSNs in a single optical network [1], because PON systems
can effectively share the upstream channel and the CO
equipment over high-speed and high-capacity bandwidth
demands [2].
One of the most critical issues for converging FTTH ac-
cess networks and several service providers, e.g., WSN,
HDTV/VoD, FNs, etc., in a single optical line terminal
(single-OLT) PON is the requirement of more computa-
tional complexity for data packet processing in the OLT.
Because all of these service providers in a u-City have sev-
eral features, e.g., device capacity diversity, application di-
versity, mobility, numbering and routing diversity, security,
and privacy, they significantly differ from conventional
access networks. This is why the current access network
architecture is not capable of integrating these service pro-
viders efficiently [3]. To mitigate this problem, some polling
algorithms have been proposed to allow additional time in
the OLT for computation and management in addition to
the guard time between every two successive optical net-
work units (ONUs) [4]. More recently, a multi-OLT PON-
based hybrid network combining FTTH access networks
and WSNs in a single PON has been proposed to reduce
the computational complexity in the OLT [5]. Even though
the single-OLT PON, as explained in [1], can be used to con-
nect multiple service providers in a u-City like an open-
access network, a multi-OLT PON can play a vital role
in alleviating the ascended problems in a single-OLT
during management and computing of data packets from
multiple service providers with less overhead in the up-
stream channel. In a multi-OLT PON, each OLT independ-
ently handles the control messages and data packets of
http://dx.doi.org/10.1364/JOCN.5.000802
Manuscript received January 7, 2013; revised April 18, 2013; accepted
May 22, 2013; published July 1, 2013 (Doc. ID 182841).
Monir Hossen is with the Interdisciplinary Graduate School of Medicine
and Engineering, University of Yamanashi, Kofu-shi, Yamanashi 400-8511,
Japan.
Masanori Hanawa (e-mail: hanawa@yamanashi.ac.jp) is with the Inter-
disciplinary Graduate School of Medicine and Engineering, University of
Yamanashi, Kofu-shi, Yamanashi 400-8511, Japan.
802 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
1943-0620/13/070802-11$15.00/0 © 2013 Optical Society of America
each service provider in a u-City. However, both an effective
network structure and an efficient dynamic bandwidth
allocation (DBA) scheme suitable for the multi-OLT PON
are essential for obtaining optimum services from the net-
works. In a time division multiple access PON, the system
performance depends on the sharing efficiency of the up-
stream channel. DBA plays a vital role in improving the
access network efficiency and bandwidth management of
the upstream channel.
Intensive research has been conducted on DBA algo-
rithms over a PON, and among them, the popular schemes
are the limited service (LS) [4], early DBA (E-DBA) [6], ex-
cessive bandwidth reallocation (EBR) [7], and limited shar-
ing with traffic prediction (LSTP) [8] schemes. In the LS
scheme, the granted time slot length depends on the dy-
namic network traffic, and the maximum length of a time
slot is upper-bounded by the maximum transmission win-
dow Ωmax. The E-DBA scheme reduces the idle period in the
usual DBA scheme by analyzing the historical traffic man-
agement. The EBR scheme redistributes the available
bandwidth from the lightly loaded ONUs to the heavily
loaded ONUs that are also incorporated with the priority
scheduling of network traffic. In the LSTP scheme, data de-
lay is reduced by predicting the traffic that arrived during
the waiting time. The more recently proposed adaptive lim-
ited dynamic bandwidth allocation (ALDBA) [1] algorithms
are more suitable for multi-OLT PON-based hybrid net-
works, because these algorithms consider two different
maximum transmission windows, Wmax
FTTH for the FTTH ter-
minal and Wmax
WSN for the cluster head (CH) of a WSN, for
two different service providers to improve the bandwidth
sharing efficiency. However, there is a possibility of utiliz-
ing guard time savings for the heavily loaded ONUs, which
can improve the quality of services (QoSs) for a multi-OLT
PON. Moreover, all of the existing algorithms have been
proposed for a single-OLT PON, and without any modifica-
tion, they are not suitable for a multi-OLT PON. One of the
main reasons is that a single polling table in a multi-OLT
PON will be shared by all the OLTs that require modifica-
tion in the multipoint control protocol (MPCP) [9] and
control message scheduling algorithm.
In this paper, we propose a new DBA algorithm for a
multi-OLT PON called adaptive limited dynamic band-
width allocation for multi-OLT PON (ALDBAM) and ana-
lyze the performance of the multi-OLT PON using the
ALDBAM algorithm. Our proposed algorithm has the fol-
lowing characteristics. First, the conventional ALDBA1
and ALDBA2 schemes are combined in the ALDBAM
scheme with proper guard time management and the modi-
fied MPCP. Second, improved QoS provisioning is achieved
in a multi-OLT PON by utilizing the excess bandwidth sav-
ings from the reduced guard time for the heavily loaded
ONUs. Third, we provide a detailed network architecture
for a multi-OLT PON with upstream frame formats.
Fourth, a Gate message scheduling algorithm is modified
appropriately for the multi-OLT PON and ALDBAM algo-
rithm. We have conducted extensive theoretical and
numerical analyses of QoS provisioning in terms of the
packet delay, bandwidth utilization, time jitter, upstream
efficiency, and throughput. The analyses are conducted
for a multi-OLT PON consisting of two OLTs and two differ-
ent FTTH and WSN service providers. Compared with the
single-OLT PON, the multi-OLT PON provides better
performance.
The rest of this paper is organized as follows. The net-
work architecture with upstream and downstream frame
formats and the modified MPCP for a multi-OLT PON
are investigated in Section II. Guard time management
and the ALDBAM scheme with the Gate message schedul-
ing algorithm in a multi-OLT PON are presented in
Section III. In Section IV, we explain the simulation envi-
ronment in detail. Section Velucidates the simulation
results. Finally, our conclusions are presented in
Section VI.
II. NETWORK ARCHITECTURE AND THE MPCP FOR A
MULTI-OLT PON
In this section, we first explain the network structure of
the hybrid multi-OLT PON combining an FTTH access net-
work and WSN with upstream and downstream frame for-
mats. Then we explain the modified version of the MPCP
suitable for the hybrid multi-OLT PON and the proposed
ALDBAM scheme.
A. Network Architecture of a Multi-OLT PON
One of the main aspects of the PON architecture that
helped it become a popular network is its simplicity. The
OLT is the main element of the network and is usually
placed in the CO. ONUs serve as an interface between
the OLTand customers through a splitter/combiner and op-
tical fiber links. A tree-topology-based hybrid multi-OLT
PON consists of multiple OLTs that are connected to sev-
eral ONUs of the FTTH access network and WSN service
providers. In the multi-OLT PON, a cluster-based WSN is
considered where each cluster consists of a static CH con-
nected to the ONU through an optical fiber [10]. Most PON
systems consist of one OLT and NONUs connected to the
FTTH terminals with different round-trip-time (RTT) de-
lays. In contrast, the hybrid multi-OLT PON structure con-
sists of multiple OLTs and several ONUs from different
service providers, e.g., ONUs connected to FTTH terminals
and ONUs connected to CHs of a WSN. In the multi-OLT
PON, the number of OLTs depends on the practical sce-
nario, the number of service providers installed, in a u-City.
If a u-City comprises mdifferent service providers, then the
number of OLTs will be m, e.g., OLT1 for FTTH terminals,
OLT2 for WSNs, OLT3 for HDTV/VoD, and OLT mfor FNs.
Therefore, the number of OLTs and ONUs may vary; how-
ever, for simplicity, only two OLTs and four ONUs for both
services with both upstream and downstream packets are
shown in Fig. 1. Here, the single splitter can be divided into
two and a longer feeder optical fiber can be installed
between them, although it is not shown in Fig. 1.
In the downstream direction of the multi-OLT PON, each
OLT will alternately broadcast data to the network
through a passive splitter. The destination ONU will
Monir Hossen and Masanori Hanawa VOL. 5, NO. 7/JULY 2013/J. OPT. COMMUN. NETW. 803
selectively extract the broadcasted data from the OLTs.
Figure 2(a) shows downstream data transmission in a
multi-OLT PON using the ALDBAM scheme.
In the upstream direction of the multi-OLT PON, data
packets from any ONU will reach both of the OLTs. How-
ever, data packets from an ONU will be accepted only by
the designated OLT. Other OLTs will discard the data pack-
ets from that ONU and will wait for data packets from the
next ONU. Figure 2(b) shows the upstream frame format of
the hybrid multi-OLT PON using the ALDBAM scheme.
Here, ONU1 is an ONU connected to an FTTH terminal
communicating with OLT1. In contrast, ONU2 is an
ONU connected to the CH of a WSN communicating with
OLT2. Each data packet of ONU1 contains a user identifi-
cation (ID) number with the payload of that user and may
be multiplexed with different users if the ONU consists of
multiple users. In contrast, each data packet of ONU2 con-
tains a node ID number, a service code (SC), and the pay-
load of the sensor node, which can be multiplexed with
different sensor nodes of different services. Here, the node
ID is a unique number for each sensor node, and the SC
indicates the type of service, i.e., gas, water, electricity,
etc., and a code for a service provider (for discrimination
if the same service is provided by different service provid-
ers) to recognize each sensor node uniquely.
B. MPCP for a Multi-OLT PON
The MPCP provides timing reference to synchronize
ONUs and allocate bandwidth or timeslots to ONUs to al-
low efficient transmission of data in the upstream direc-
tion. In the MPCP, timing synchronization among ONUs
is achieved by calculating the RTT and by maintaining a
polling table. The RTT depends on the physical distance
from the OLT to an ONU. On the other hand, the MPCP
uses a DBA algorithm to allocate the transmission window
or timeslots for every ONU and to share the single optical
fiber link with multiple ONUs [9]. Figure 3illustrates the
MPCP for a hybrid multi-OLT PON. In the downstream
transmission, the MPCP maintains a timestamp with its
local time and broadcasts a Gate message to all the ONUs.
In the upstream transmission, all the ONUs share a
common channel to transmit data to the OLTs. The up-
stream transmission window of each ONU also contains
a report message at the end of its timeslots to request
the desired transmission window in the next time cycle
Tcycle, depending on the ONU’s buffer occupancy. Upon re-
ceiving the report message at the OLT, the MPCP incorpo-
rated with the DBA algorithm determines the allocated
transmission window and recalculates the required over-
head and RTT to update the polling table. In a multi-
OLT PON, the conventional MPCP is modified, where both
OLTs share a common polling table to store the RTT of each
ONU that ensures timing synchronization among all ONUs.
Fig. 2. (a) Downstream data transmission and (b) upstream data
transmission in a hybrid multi-OLT PON.
Fig. 3. MPCP operation in a hybrid multi-OLT PON.
Fig. 1. Network structure and data transmission for a hybrid
multi-OLT PON.
804 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
III. GUARD TIME MANAGEMENT AND ALDBAM SCHEME
In this section, we first explain the guard time manage-
ment scheme in a multi-OLT PON, followed by the pro-
posed ALDBAM scheme. The proposed ALDBAM scheme
utilizes the total guard time savings by fair distribution
to the heavily loaded ONUs. Furthermore, the scheduling
algorithm of the Gate message is modified for the
ALDBAM scheme.
A. Guard Time Management in a Multi-OLT PON
Guard time is required to avoid the turn on/off delay of
an optical transceiver and fluctuation of the RTT and to
provide times for clock and data recovery (CDR). A typical
PON system has to cope with these constraints by provid-
ing enough space as a guard time between the data packets
of every two consecutive ONUs. Figure 4illustrates guard
time management in a conventional single-OLT PON
system. The guard time TGbetween every two consecutive
ONUs in the conventional PON is
TGToff TFRTT Ton TCDR;(1)
where Toff is the laser off time, TFRTT is the fluctuation of the
RTT, Ton is the laser on time, and TCDR is the time for CDR.
In a conventional PON, Ωmax is constant for each ONU.
The maximum granted transmission window to each ONU
by the OLT, GOLT, can be calculated as follows:
GOLT Ωmax Ton TCDR Tmax
DToff ;(2)
where Tmax
Dis the length of the maximum granted data
packets.
In a multi-OLT PON, two OLTs alternately receive data
from two consecutive ONUs, and the laser on and off times
can be easily avoided. When the data of ONU1 is received
by OLT1, OLT2 is in a sleeping condition at that time and
can wake up early to provide enough time for Ton and re-
ceive data during Toff of OLT1. In contrast, when the data
of ONU2 is received by OLT2, OLT1 is in a sleeping condi-
tion at that time and can wake up early to compensate for
Ton and receive data during Toff of OLT2. Figure 5shows
the guard time management in a multi-OLT PON system
where only the CDR and RTT fluctuation times are used as
the guard time. TG M is the guard time between every two
consecutive ONUs in a multi-OLT PON in the following:
TG M TFRTT TCDR:(3)
The total guard time savings TGS Tin a multi-OLT PON
can be calculated as follows:
TGS TNTG−TG MNTon Toff . (4)
In a multi-OLT PON, the maximum transmission win-
dow for each ONU of an FTTH terminal is Wmax
FTTH, while
the maximum transmission window for each ONU of a
WSN is Wmax
WSN. The maximum granted transmission win-
dows to the ONUs of the FTTH terminals and WSN by
OLT1 and OLT2 can be expressed, respectively, by
GOLT1Wmax
FTTH TCDR Tmax
DFTTH;(5)
GOLT2Wmax
WSN TCDR Tmax
DWSN;(6)
where GOLT1is the granted window by OLT1, GOLT2is the
granted window by OLT2, Tmax
DFTTHis the length of the
maximum granted data packets to the ONU of the FTTH
terminal, and Tmax
DWSNis the length of the maximum
granted data packets to the ONU of the WSN.
B. ALDBAM Scheme
In this scheme, we consider a hybrid multi-OLT PON-
based access network with two OLTs and NONUs. Here,
Nis divided into two groups and NNFTTH NWSN,
where NFTTH is the number of ONUs connected to the
FTTH terminals and NWSN is the number of ONUs con-
nected to the CHs of the WSN. Usually the packet size
of the WSN is smaller, and the data rate is lower than
the FTTH access network. This is why the usual maximum
transmission window of the WSN will be smaller than the
maximum transmission window of the FTTH terminals,
i.e., Wmax
WSN <W
max
FTTH. Owing to these packet length and data
rate differences, the total available bandwidth savings in
the proposed scheme, WTS, is calculated as in the ALDBA1
scheme [1]:
WTS NWSNWmax
FTTH −Wmax
WSN:(7)
Fig. 5. Guard time management in a multi-OLT PON.
Fig. 4. Guard time management in a conventional PON.
Monir Hossen and Masanori Hanawa VOL. 5, NO. 7/JULY 2013/J. OPT. COMMUN. NETW. 805
WTS is divided by Nto calculate the average available
bandwidth savings for each ONU (i.e., Wavg WTS∕N), and
this average bandwidth savings is used to provide some
transmission window to the deferred data during the wait-
ing time between the transmission of the Gate and Report
messages. Usually, the waiting time in a PON is equal to
the RTT of each ONU and delay of the Gate starting time
from the OLT. The OLTs predict the amount of deferred
data during the waiting time for each ONU and allocate
the additional bandwidth up to Wavg in addition to the
granted window GOLT1or GOLT2. Prediction of the deferred
data during the waiting time depends on the current queue
occupancy, RTT of each ONU, and Gate starting delay from
the OLTs:
Wpred
i; j WR
i; jRTTiTGD
Tacq
i; j
;(8)
where Wpred
i; j is the predicted window size for ONU iat time
cycle j,Tacq
i; j is the acquisition time of the present data in the
queue, WR
i; j is the requested window by ONU iat time cycle
j,TGD is the Gate starting delay, and Wpred
i; j ≤Wavg.
Owing to the bursty nature of the network traffic [11],
some ONUs might have traffic demand less than Wmax
FTTH
or Wmax
WSN, called lightly loaded ONUs, while other ONUs
might have traffic demand higher than Wmax
FTTH or Wmax
WSN,
called heavily loaded ONUs. This results in some amount
of excessive bandwidth from the lightly loaded ONUs. The
total excessive bandwidth in the hybrid multi-OLT PON, as
in the ALDBA2 scheme [1], is calculated by
Wexcess
Total;j X
LFTTH
m1Wmax
FTTH;j
−WR
m; jX
LWSN
n1Wmax
WSN;j
−WR
n; j;
(9)
where Wexcess
Total;j is the total excessive bandwidth at time cycle
j;LFTTH and LWSN are the number of lightly loaded ONUs
connected to the FTTH terminals and CHs of the WSN, re-
spectively; and WR
m∕n; j is the requested window size of a
lightly loaded ONU m∕nat time cycle j.
In the ALDBAM scheme, this total excess bandwidth
from the lightly loaded ONUs is incorporated with TGS T
in Eq. (4). These two excess bandwidth savings from Eqs. (4)
and (9) can be fairly distributed to the heavily loaded
ONUs, without changing the length of Tcycle. The following
equation is used to fairly distribute the total excessive
bandwidth in Eq. (9) and the total guard time savings in
Eq. (4) among the heavily loaded ONUs to solve the conges-
tion problem in the hybrid multi-OLT PON:
Wexcess
i; j Wexcess
Total;j TGS TWR
i; j
PH
k1WR
k; j
;(10)
where Wexcess
i; j is the excessive bandwidth for ONU iat time
cycle jand His the number of heavily loaded ONUs.
The bandwidth allocation formulas for the ALDBAM
scheme in a multi-OLT PON are as follows:
Gi; j
OLT1(WR
i; j Wpred
i; j For lightly loaded ONUs
Wmax
FTTH Wexcess
i; j Wpred
i; j For heavily loaded ONUs;
(11)
Gi; j
OLT2(WR
i; j Wpred
i; j For lightly loaded ONUs
Wmax
WSN Wexcess
i; j Wpred
i; j For heavily loaded ONUs ;
(12)
where Gi; j
OLT1is the granted window to ONU iof the FTTH
terminal by OLT1 at time cycle jand Gi;j
OLT2is the granted
window for ONU iof the CH of the WSN by OLT2 at time
cycle j.
An illustrative example of bandwidth allocation in the
ALDBAM scheme for heavily loaded ONUs is shown in
Fig. 6. The bandwidth allocation conditions in Fig. 6follow
Eqs. (11) and (12) for heavily loaded ONUs. Here, Tcycle;j is
the length of a polling cycle at time cycle j. The maximum
transmission window Wmax
FTTH or Wmax
WSN and excessive band-
widths Wexcess
1;j ;Wexcess
2;j ;…;Wexcess
i; j ;…;Wexcess
N;j with predicted
windows Wpred
1;j ;Wpred
2;j ;…;Wpred
i; j ;…;Wpred
N;j are alternately al-
located by OLT1 or OLT2 to the heavily loaded ONUs
1;2;…;i;…;N of both service providers at time cycle j.
In contrast, the requested windows WR
i; j with Wpred
i; j are al-
located by OLT1 or OLT2 to the lightly loaded ONU iat
time cycle j, as shown in Eqs. (11) and (12).
As network complexity has increased with the history of
Internet development due to the inclusion of more diverse
and new inconsistent functions [3], the ALDBAM scheme
also requires more computational complexity than the
LS scheme [4]. Because the ALDBAM scheme needs to cal-
culate predicted traffic, access bandwidth, and lightly
loaded and heavily loaded ONUs, this requires a larger
number of summation and multiplication operations than
the LS scheme. However, these complexities do not heavily
affect the online bandwidth allocation. Moreover, deploy-
ment of multiple OLTs can share the overall complexity
to reduce the computing time more than the single-
OLT PON.
The main differences between the proposed ALDBAM
scheme and the existing ALDBA1 and ALDBA2 schemes
in [1] are as follows:
1) Consideration of multiple OLTs for multiple service
providers in a single PON.
2) Calculation of the total guard time savings by proper
guard time management in a multi-OLT PON and uti-
lization of this guard time savings for heavily
loaded ONUs.
3) Appropriate modification of the MPCP for a multi-
OLT PON.
4) Provision of detailed analysis of upstream and down-
stream frame formats with different maximum trans-
mission windows for different OLTs and service
providers.
5) Consideration of the Gate starting delay to calculate
the predicted traffic in the waiting time.
806 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
6) Modification of the Gate message scheduling algorithm
for a multi-OLT PON and the ALDBAM algorithm.
C. Gate Message Scheduling Algorithm in a
Multi-OLT PON
In upstream transmission, a scheduling algorithm for
Gate messages in a multi-OLT PON is very important to
prevent data collision due to multiple ONUs transmitting
at the same time. Figure 7shows the Gate message sched-
uling algorithm in a multi-OLT PON for the ALDBAM
scheme. As the scheduling of Gate messages depends on
the RTT and granted window sizes of different ONUs, a
starting Gate message can be sent by any of the OLTs. Gate
messages for different ONUs in a multi-OLT PON are
scheduled using the following formulas:
TGi1;j
2TGi;j
1RTTiTFRTTTCDR TDFTTH
−RTTi1TFRTT;(13)
TGi2;j
1TGi1;j
2RTTi1TFRTTTCDR
TDWSN−RTTi2TFRTT;(14)
where TGi;j
1and TGi1;j
2are the time epochs for OLT1 and
OLT2 when Gate messages are transmitted to ONU iand
ONU i1, respectively, at time cycle j, and TGi2;j
1is the
time epoch for OLT1 when the Gate message is transmitted
to ONU i2at time cycle j.
IV. PERFORMANCE EVALUATION BY SIMULATION
In this section, the performance of the proposed
ALDBAM scheme for a hybrid multi-OLT PON isevaluated
in terms of the average packet delay, bandwidth utilization,
jitter, upstream efficiency, and throughput. All these
parameters are evaluated by simulation results. The evalu-
ation was performed using laboratory-made computer
simulation programs. We considered a hybrid PON
architecture with two OLTs and 32 ONUs in a tree topol-
ogy. In the ALDBAM scheme, we have considered two dif-
ferent maximum transmission windows for two different
service providers for upstream transmission and two differ-
ent maximum transmission windows for two OLTs for
downstream transmission. We have also incorporated the
impact of guard time savings by fairly distributing to
the heavily loaded ONUs. The downstream and upstream
channel speeds were considered at 1 Gbps. The distance
from an ONU to the OLT is assumed to be random and
in the range of 10–20 km. All the data packets were as-
sumed to have the same priority, meaning the service pol-
icy was on a first-in first-out (FIFO) basis with an infinite
buffer size for each ONU. A highly bursty self-similar
Fig. 7. Scheduling diagram for a Gate message in a multi-OLT
PON.
ONU1
OLT1
Data from ONU1 (FTTH)
pred
j1,
W
R
j1,
W
1
R=
RTT1 +T
GD1
ONU N
R
jN,
W
N
R=
Data from ONU2 (WSN)
pred
j,2
W
T
GD1
T
GD2
Tim e c ycle j-1 Time cy cle j+1
Tim e c ycle j, Cycle time = T
cycle, j
RTT2+T
GD2
G
1/2
= Gate message from OLT1/OLT2,T
GD1/2
= Gate sta rting dela y of OLT1/OLT2, RTT+T
GD
= Wait ing tim e, a n d R = Repor t messa ge
EO
GI
EOGI
pred
j1,
excess
j1,
max
FTTH1 WWWG ++=
OLT2
ONU2
EO
GI
R
j2,
W
2
R=
pred
j2,
excess
j2,
max
WSN2WWWG ++=
Data from ONUN (WS N)
T
GD2
pred
jN,
excess
jN,
max
WSN2WWWG ++=
EO
GI
pred
j,N
W
RTTN+T
GD1
max
D(FTTH)
T
max
D(WSN)
Tmax
D(WSN)
T
max
D(FTTH)
T
Fig. 6. Illustrative example of the ALDBAM scheme for heavily loaded ONUs.
Monir Hossen and Masanori Hanawa VOL. 5, NO. 7/JULY 2013/J. OPT. COMMUN. NETW. 807
network traffic model, as most network traffic can be
characterized by self-similar and long range dependence
[11], was used to generate the data packets for both the
FTTH terminals and sensor nodes of the WSN. This traf-
fic model generated traffic from 0 to multiple packets in
each active ONU in every time cycle, and the total re-
quested window size of an ONU depended on the number
of packets multiplied by the maximum length of a
packet, PBmax
FTTH for FTTH terminals and PBmax
WSN for the
CH of the WSN. The maximum packet lengths for the
FTTH terminals Bmax
FTTH and WSNs Bmax
WSN were 1500 bytes
[12] and 1024 bytes [13], respectively. The processing
time was assumed to be 10 μs for the proposed ALDBAM
scheme, as used by Hwang et al. [6]. All analyses were
performed for a nonuniform offered load in the range
of 0–1.0 with a variable cycle time in the range of
0.5–3 ms. Our simulation took into consideration the
queuing delay, transmission delay, congestion delay,
and processing delay, without taking into consideration
any priority scheduling. The simulation parameters are
summarized in Table I.
The hybrid multi-OLT PON consists of sensor networks,
and the data of some sensor nodes, e.g., hospital and fire
alarm sensor systems, are delay sensitive. One of the main
objectives of the proposed ALDBAM scheme is the reduc-
tion of the end-to-end packet delay by allocating a larger
transmission window to the heavily loaded ONUs from
the lightly loaded ONUs and guard time savings. Reduc-
tion of packet delay is also achieved by reducing the data
processing time and guaranteed scheduling of Gate
messages in the multi-OLT PON.
The bandwidth utilization BWU of a multi-OLT PON
using the ALDBAM scheme is expressed by
BWU NFTTHGOLT1NWSN GOLT2
NFTTHGOLT1NWSN GOLT2NTC
;(15)
where TCis the summation of BR∕Ru,BE∕Ru,TFRTT, and
TCDR. The proposed scheme can achieve better bandwidth
utilization by utilizing excessive bandwidth from lightly
loaded ONUs and guard time savings for the heavily loaded
ONUs.
The burst network traffic and DBA in a PON provide
variation in Tcycle in every time cycle. Due to this variation
in Tcycle, the arrival times of data packets fluctuate in dif-
ferent time cycles. To measure the variation in the data
packet arrival time, the jitter performance of the proposed
ALDBAM scheme was analyzed. The jitter may be calcu-
lated by
Jitter 1
n
XTj
avl −Tj−1
avl 2
q;(16)
where nis the total number of time cycles, Tj
avl is the data
packets’arrival time at time cycle j, and j1;2;3;…;n.
The ratio between the successful upstream transmission
and the total generated traffic in the network is called the
upstream efficiency. The expression for the upstream
efficiency of a PON system is
UE NFTTHGOLT1NWSN GOLT2
NFTTHPBmax
FTTH NWSNPBmax
WSN NTC
;(17)
where UE is the upstream efficiency.
V. SIMULATION RESULTS
In this section, the system performance of the proposed
ALDBAM scheme for a multi-OLT PON is compared with
that of the existing ALDBA1 and ALDBA2 schemes for a
single-OLT PON. All the performance parameters are an-
alyzed for nonuniform burst traffic in both the upstream
and the downstream directions. All the results are pre-
sented using contour plots over a wider range of offered
loads and cycle times. Lighter colors signify better perfor-
mance in all of the contour plots in this paper.
Figures 8(a)–8(c) show the end-to-end average packet de-
lay of the existing ALDBA1 and ALDBA2 and the proposed
ALDBAM schemes, respectively, for NFTTH ∶NWSN 16∶16
using contour plots for different offered loads and cycle
times. From these three contour plots, the ALDBAM
scheme clearly provides a wider area of lowest packet delay
in both directions of offered loads and cycle times. On the
other hand, the highest packet delay in the ALDBAM
scheme is 1.8 ms, whereas the highest packet delays in
the ALDBA1 and ALDBA2 schemes are 3 and 2.5 ms, re-
spectively. Figure 8(d) shows a comparison of the average
packet delay among the three schemes for a 2 ms cycle
time. The ALDBAM scheme provides approximately 75%
and 30% less delay than the ALDBA1 and ALDBA2
schemes, respectively, at an offered load of 1.0. However,
the effectiveness of the ALDBAM scheme becomes more
significant at higher data rates and a larger number of
service providers and OLTs.
Figures 9(a) and 9(b) compare the average packet delay
among the ALDBA1, ALDBA2, and ALDBAM schemes for
TABLE I
SIMULATION PARAMETERS
Symbol Quantity Value
NTotal number of ONUs 32
NOLT Number of OLTs 2
DDistance between OLTs and ONUs 10–20 km
Ton Laser on time 1.5 μs
Toff Laser off time 1.5 μs
TFRTT Fluctuation of RTT 1.5 μs
TCDR CDR time 0.5 μs
Tcycle Cycle time 0.5–3ms
Tproc Processing time 10 ms
RuTransmission speed 1 Gbps
BRLength of report message 576 bits
BELength of Ethernet overhead 304 bits
Bmax
FTIH Maximum packet length of the
FTTH terminal
1500 bytes
Bmax
WSN Maximum packet length of the WSN 1024 bytes
PNumber of generated packets 0–10
808 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
a 2 ms cycle time by changing the ratio of the number of
ONUs connected to the FTTH terminals and to the CHs
of the WSN. The delay characteristics of the three schemes
in Figs. 9(a) and 9(b) are also similar to those in Fig. 8(d).
However, the average packet delays for all three schemes
are far less when the number of ONUs connected to the
CHs of the WSN is larger than the number connected to
the FTTH terminals, as shown in Fig. 9(b), where
NFTTH∶NWSN 8∶24. The first reason is that a larger num-
ber of ONUs from the WSN provides less aggregated traffic
in the network as the data rate is lowered, and the packet
size is smaller than those of the FTTH terminals. The
second reason is that a larger number of ONUs from the
WSN provides more bandwidth savings that is utilized
by the deferred data.
The contour plots of Figs. 10(a)–10(c) show the band-
width utilization of the ALDBA1, ALDBA2, and ALDBAM
schemes, respectively. From the analysis of these three con-
tour plots, it is clear that the ALDBAM scheme provides far
superior bandwidth utilization than both the ALDBA1 and
ALDBA2 schemes. The highest bandwidth utilization in
the ALDBAM scheme is 0.95. In contrast, the highest band-
width utilization in the ALDBA1 and ALDBA2 schemes is
0.8 and 0.9, respectively. Moreover, the ALDBAM scheme
provides higher bandwidth utilization from a much lower
value of the offered load. From the comparison of the band-
width utilization at a 2 ms cycle time in Fig. 10(d), the
bandwidth utilization in the ALDBAM scheme exceeds
0.85 at an offered load of 0.12. In contrast, the bandwidth
utilization exceeds 0.85 at an offered load of 0.45 and 0.85
in the ALDBA2 and ALDBA1 schemes, respectively. Sim-
ilarly, if we draw a horizontal line at a bandwidth utiliza-
tion of 0.85, as shown in Figs. 11(a) and 11(b), we can then
see that the ALDBAM scheme continually provides similar
performance for different ratios of the number of ONUs
connected to the FTTH and WSNs.
Usually, in a DBA scheme, we cannot avoid jitter because
of bursty network traffic. The contour plots in Fig. 12 show
the jitter performance of the existing ALDBA1, ALDBA2,
and proposed ALDBAM schemes. From the contour plots, it
is clear that the ALDBAM scheme provides less jitter than
the ALDBA1 and ALDBA2 schemes. The comparison of
jitter for a 2 ms cycle time with different numbers of ONUs
from two service providers is shown in Figs. 12(d),13(a),
and 13(b). Jitter characteristics at a 2 ms cycle time for
every combination of ONUs from the FTTH terminals
and WSN are similar in all the three schemes.
Upstream efficiencies are compared among the three
schemes by the contour plots in Figs. 14(a)–14(c). In this
case, the proposed ALDBAM scheme provides better
Fig. 9. Comparison of average packet delay for a 2 ms cycle time.
Fig. 8. Average packet delay in milliseconds for
NFTTH∶NWSN 16∶16.
Fig. 10. Bandwidth utilization for NFTTH∶NWSN 16∶16.
Fig. 11. Comparison of bandwidth utilization for a 2 ms cycle
time.
Monir Hossen and Masanori Hanawa VOL. 5, NO. 7/JULY 2013/J. OPT. COMMUN. NETW. 809
upstream efficiency than the ALDBA1 and ALDBA2
schemes. The highest upstream efficiency area in the
ALDBAM scheme is broadened in the directions of both
maximal cycle times and offered loads. This means that
the ALDBAM scheme can provide better performance from
a lower offered load and cycle time to a higher offered load
and cycle time. From the analysis of Figs. 14(d),15(a), and
15(b), we can say that the ALDBAM scheme is also consis-
tent for maintaining higher upstream efficiency for every
combination of numbers of ONUs from two different service
providers. To compare the results of the upstream effi-
ciency among the three schemes more efficiently, we draw
a horizontal line at the 80% upstream efficiency level in
Figs. 14(d),15(a), and 15(b). From these three figures, it is
clear that the ALDBAM scheme provides about a two times
greater offered load than the ALDBA2 scheme for both
NFTTH∶NWSN 16∶16 and NFTTH ∶NWSN 8∶24 with an
upstream efficiency higher than 80%. Moreover, the ALD-
BAM scheme provides four times more offered load than
the ALDBA2 scheme for the NFTTH∶NWSN 24∶8case
with an upstream efficiency higher than 80%. However,
the ALDBA1 scheme never provides an upstream efficiency
higher than 80%.
Finally, we compare the improvement in throughput
when the ALDBAM scheme is used for a hybrid multi-
OLT PON. Figures 16(a)–16(c) show the contour plots of
the throughput for different offered loads and cycle times
for the existing ALDBA1, ALDBA2, and proposed
ALDBAM schemes, respectively. As expected, ALDBA1
has the lowest throughput, and the maximum through-
put achieved in the contour plot of Fig. 16(a) is 0.6. On
the other hand, the maximum throughput achieved
by the ALDBA2 scheme is 0.8, as shown in the contour
plot in Fig. 16(b). Ultimately, the ALDBAM scheme
achieves the highest throughput of 0.9 in Fig. 16(c).
Fig. 12. Jitter in milliseconds for NFTTH∶NWSN 16∶16.
Fig. 13. Comparison of jitter for a 2 ms cycle time.
Fig. 14. Upstream efficiency for NFTTH∶NWSN 16∶16.
Fig. 15. Comparison of upstream efficiency for a 2 ms cycle time.
Fig. 16. Throughput for NFTTH∶NWSN 16∶16.
810 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
The main reason for the low throughput for the ALDBA1
scheme is less utilization of the upstream channel due
to the lightly loaded ONUs, whereas the ALDBAM
scheme gains the utilization of excess bandwidth from
the lightly loaded ONUs and guard time savings for the
heavily loaded ONUs. From the comparison of throughput
among the three schemes for the 2 ms cycle time, the
ALDBAM scheme achieved more than 15% and 35% higher
throughput than the ALDBA2 and ALDBA1 schemes,
respectively, for every case of the ratio of ONUs from
two different service providers, as shown in Figs. 16(d),
17(a), and 17(b).
Even though the existing ALDBA2 scheme [1] can
achieve better results in a single-OLT PON than the con-
ventional LS scheme [4], it still has its limitations, because
the ALDBA2 scheme does not consider the utilization of
guard time savings in a multi-OLT PON. In a multi-OLT
PON, the computation time for data packet processing in
the OLT is also reduced by dividing the upstream traffic
from the ONUs of different service providers among multi-
ple OLTs. The proposed ALDBAM scheme copes with all
the limitations of the ALDBA1 and ALDBA2 schemes
and provides enhanced performance. Moreover, the effec-
tiveness of the proposed scheme will be more significant
if the analyses are repeated for a larger number of OLTs
and service providers.
VI. CONCLUSION
Our proposed ALDBAM scheme enhances the perfor-
mance of a multi-OLT PON by the reduction of data
processing time, fair distribution of excess bandwidth from
the lightly loaded ONUs to the heavily loaded ONUs,
proper guard time management, and the perfect schedul-
ing algorithm of Gate messages from multiple OLTs. The
proposed ALDBAM scheme outperformed the ALDBA1
and ALDBA2 schemes in terms of the average packet delay,
bandwidth utilization, jitter, upstream efficiency, and
throughput. The main contribution of the proposed ALD-
BAM scheme is that it can provide better bandwidth shar-
ing efficiency and utilization due to smaller cycle times and
lower offered loads. The ALDBAM scheme utilizes guard
time savings in a multi-OLT PON and provides better
QoS than the ALDBA1 and ALDBA2 schemes. The ALD-
BAM scheme provided 75% less delay with 35% higher
throughput and 30% less delay with 15% higher through-
put for a 2 ms cycle time at an offered load of 1.0 when
compared with the ALDBA1 and ALDBA2 schemes,
respectively.
ACKNOWLEDGMENT
This work was supported in part by the JSPS-NRF
bilateral joint research project.
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Monir Hossen received a B.Sc. degree in
electrical and electronic engineering from
Khulna University of Engineering & Tech-
nology (KUET), Bangladesh, in 2002. He
joined KUET as a faculty member in the
Electronics and Communication Engineer-
ing Department in 2004. He completed
his M.Sc. in electronics engineering at
Kookmin University, South Korea, in 2010.
Currently, he is working toward a Ph.D.
at the Interdisciplinary Graduate School of
Medicine and Engineering at the University of Yamanashi, Japan.
His present research focuses on PON-based hybrid networks com-
bining FTTH and wireless sensor networks and their bandwidth
allocation algorithms.
Masanori Hanawa received B.E., M.E.,
and Ph.D. degrees from Saitama University,
Japan, in 1990, 1992, and 1995, respectively.
In 1995, he joined the University of
Yamanashi, Japan, as a Research Associate.
Since 2002, he has been an Associate Profes-
sor at the university. His research interests
include optical signal processing and optical
fiber communications, including optical code
division multiplexing.
812 J. OPT. COMMUN. NETW./VOL. 5, NO. 7/JULY 2013 Monir Hossen and Masanori Hanawa
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