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Energy Efficiency in Ethernet Passive Optical Networks (EPONs): Protocol Design andPerformance Evaluation
Yan, Ying; Dittmann, Lars
Published in:Journal of Communications
Link to article, DOI:10.4304/jcm.6.3.249-261
Publication date:2011
Document VersionPublisher's PDF, also known as Version of record
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Citation (APA):Yan, Y., & Dittmann, L. (2011). Energy Efficiency in Ethernet Passive Optical Networks (EPONs): ProtocolDesign and Performance Evaluation. Journal of Communications, 6(3), 249-261.https://doi.org/10.4304/jcm.6.3.249-261
Energy Efficiency in Ethernet Passive OpticalNetworks (EPONs): Protocol Design and
Performance EvaluationYing Yan and Lars Dittmann
Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby, 2800 DenmarkEmail: {yiya, ladit}@fotonik.dtu.dk
Abstract— As concerns about energy consumption grow,the power consumption of the EPON becomes a matterof increasing importance. In respect of energy efficiency,the current standard has no management protocols aimingto reduce power consumption in EPONs. In this paper,we propose an Energy Management Mechanism (EMM)for downlink EPON systems. The proposed mechanism isdesigned to enhance the standardized control scheme inEPON with the objective to increase energy efficiency whilesatisfying diverse QoS requirements. The main idea is toput an Optical Network Unit (ONU) into the sleep modeand determine a suitable wakeup time scheduler at theOptical Line Terminal (OLT). A generic EPON system isconsidered, which is composed of an OLT and several ONUsthat are EMM enabled. An energy consumption optimizationproblem aimed at saving energy is proposed and twoheuristic sleep mode scheduling policies are addressed tosolve it. The scheduling algorithms are tightly coupled withthe upstream bandwidth allocation and downstream trans-mission scheduling together through an integrated approachin which awake time in ONUs is minimized. There is atrade-off decision between maximizing the power saving andguaranteeing the network performance at the same time.Simulation results show that an EMM-based EPON withwell designed scheduling disciplines is essential to achievingsignificant energy saving while meeting the delay constraint.
Index Terms— EPON, Energy efficiency, MPCP, schedulingalgorithms.
I. INTRODUCTION
It has been widely recognized that reducing powerconsumption in data communication network becomes animportant issue to global environment and future humanlife [1]–[5]. Recent research has shown that, among allnetwork segments, broadband access networks are thehighest share of the network energy consumption, whichconsumes more than 75 percent of energy consumptionby all telecommunication equipments today. Due to therapid expansion of access network connectivity, increas-ing number of users and data rate are foreseeable in thefuture broadband access networks. As a result, energyconsumption of future broadband access networks would
Manuscript received Feb 22, 2011; accepted Mar 11, 2011.This paper is based on “Energy Efficient Ethernet Passive Optical Net-
work (EPONs) in Access Networks,” by Ying Yan and Lars Dittmann,which appeared in the conference proceedings for the 10th WSEASInternational Conference on Applied Informatics And Communications(AIC) in Taipei, Taiwan August, 2010
continue to rise, with a steady annual growth rate inthe near future. Therefore, it has raised attention in bothacademia and industry to design energy efficient networksystems. Commonly, sleep mode operation is introducedto allow that nodes (or stations) can switch to sleep whenthey are idle and wake up when they receive or transmitpackets. Such approach has been widely exploited inwireless networks, where saving battery power in mobilestations is of paramount importance due to finite powersupplies in wireless devices [6]–[11].
The EPON, a prevailing deployed broadband accesstechnology, provides cost efficiency and high data ratefor the last mile access. A typical EPON is a Point-to-Multipoint (PMP) network with a tree based topology,where an OLT connects multiple ONUs via optical links.The OLT plays a role of distributor, arbitrator and aggre-gator of traffic. In the upstream direction (from ONUs tothe OLT), multiple ONUs share a single link and trafficmay collide. The OLT distributes the fiber capacity usingan upstream bandwidth arbitration mechanism to avoidcollisions. In the downstream direction (from the OLT toONUs), data frames are broadcasted to all ONUs. ONUsfilter and accept data that are addressed to them. However,ONUs have to constantly listen and examine downstreamtraffic, which results in wasting significant energy in theONU. Shown in Fig. 1, in the traditional EPON, ONUsconsume energy to keep active when there is neitherupstream nor downstream traffic. As a result, minimizingpower consumption is a major factor driving the design ofEPON devices and of the protocols therein. An effectiveEnergy Management Mechanism (EMM) that schedulesthe sleep mode period to ONUs is a key to conservepower.
The major sources of energy waste of shared resourceEPON system include: overhearing, control packet over-head, and idle listening. Overhearing represents that anONU receives and decodes packets that are not destinedto it. Control packet overhead means the transmissionof control messages, which are necessary to coordinatetransmission between the OLT and ONUs, and unfortu-nately increases time and energy consumption. At last,idle listening means active time and energy ONUs spenton waiting for the next upstream and downstream trans-mission. It is clear that an energy efficient scheduler isdesigned aiming to eliminate these sources of useless
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1:K
splitterOLT
ss1
1 2 k 1
1 1
2
k
1 k 1 2 1 2 k
Uplink
Downlink
1 1 1
2 2
k k
ONU 1
ONU 2
ONU k
ss2
ssk
Figure 1. EPON upstream (multi-point to point) and downstream (pointto multi-point) transmission.
energy consumption.To reduce power consumption, ONUs are designed to
enter sleep mode when they do not need to either receiveor send traffic. During a sleep period, ONUs turn offthe transceiver in order to save energy. In case there isincoming data for a sleeping ONU, data is queued inbuffers at the OLT. Obviously, it is better to keep an ONUto stay in sleep mode as much as possible to conserve itsenergy. However, a power management mechanism withefficient scheduling for sleep and wake-up periods amongmultiple ONUs is a challenging task due to:
- ONUs should wake up and exchange traffic with theOLT. However, arrival traffic profile is not alwaysknown by the device, because the downstream andupstream traffic can be either periodically or aperi-odically arrived. Therefore, the sleep period shouldbe carefully assigned in order to avoid missing anyincoming packets.
- During sleep period, data is buffered and presched-uled for upstream and downstream transmission inboth the OLT and ONUs. Due to QoS constraints interms of delay and latency, the sleep period shouldbe carefully scheduled, so target ONUs can wakeup and complete transmission without violating QoSrequirements.
As discussed in next Section II, several studies havebeen proposed to analyze the power consumption forEPON, while applying the approach of allowing ONUsto sleep mode. Although Different aspects have beeninvestigated on how to minimize energy consumption inPON systems, there have been few studies focusing onthe protocol design for supporting sleep mode ONUs inEPON. To implement EMM in EPON, the legacy controlscheme requires modifications and extensions. Moreover,very few have explicitly addressed the trade-off betweenenergy consumption and network performances. To thebest of our knowledge, our work is the first to proposethe assignment of sleep period to ONUs taking intoaccount of the traffic transmission in both upstream anddownstream directions.
The remainder of this paper is organized as follows.We first give a brief description of related works inSection II. The introduction of the system model isaddressed in Section III. The design of EMM based EPONwith two different downstream schedulers is introduced
in Section IV. Simulation environments and results areoutlined and discussed in Section V. Finally, conclusionsand future work are drawn in Section VI.
II. RELATED WORKS
Issues of energy efficiency have been studied in EPONsas well as in other optical access networks. In this section,we present related works and compare their models withours.
Authors in [12] propose a management protocol forONUs to request entering in sleep mode. The OLT reservea minimal bandwidth for ONUs in sleep mode. During asleep period, no connection is established and no trafficis delivered. The sleep mode can be terminated by eitherthe OLT or the ONU by using an administration message.This protocol eliminates conventional requirements for acomplete activation procedure in ONUs. The authors ad-dress the impact of sleep mode protocol for re-activationprocess in GPON.
In [13], the authors present implementation challengesof sleep mode operation in PONs, such as achieving clocksynchronization and reducing the clock recovery over-head. The process of regaining network synchronizationafter waking up from sleep mode is studied in both GPONand EPON system. Two novel ONU architectures areproposed and compared with current ONU architectureson power consumption and wakeup overhead. As alreadyindicated, we focus on protocol design and do not addressthe physical implementation issues in our work. Thearchitectures as well as the overhead values proposedin [13] can be used for this purpose.
Another paper [14] proposes a power saving mecha-nism for 10 Gigabit class PON, where ONUs are im-plemented with a sleep and periodic wakeup regime. Thesleep mode is triggered based on negotiations between theOLT and the ONU, and a variable sleep time is allowed.The authors use a prediction model of traffic profiles todetermine the initiation and the duration of sleep mode.A probabilistic analysis of the presence or absence ofdownstream traffic is required in order to ensure theONU can wake up and the packet is delivered on time.This model differs from ours in that our model does notuse estimation and the energy management mechanism ismapped in EPON MAC layer.
Author in [15] presents the EPON power saving issueon the IEEE 802.3az meeting in September 2008. Thefunctions for power saving are proposed to be imple-mented in both the OLT and ONUs. The initiation andtermination of sleep mode by using handshake messagesare drafted. The attention of energy efficiency is raised inthe IEEE standardization, however, there are considerablyfew research works in saving energy consumption of opti-cal network management systems. Our work investigatesthe current control mechanism for EPON and proposescontrol protocol for sleep mode operation.
The relationship between energy and data rate in gen-eral optical IP networks was also studied in a differentcontext. For example, [16] presents power saving of
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different network architectures, including all-optical net-works, first-generation networks and multi-hop networks.Power consumption of electronic versus optical compo-nents in optical WDM networks is investigated underdifferent traffic loads. In [4] this problem is generalizedby considering the power consumption in an optical IPnetwork. A new network-based model is established,which includes core, metro and access networks.
III. SYSTEM MODEL
In this paper, we consider an EPON system consistingof an OLT, 1 : K splitter and multiple ONUs. Theupstream and downstream traffic are separated in differentwavelength, typically 1310 nm for the upstream trans-mission and 1550 nm for the downstream transmission.TDMA is used in the physical layer where bandwidth isdivided in time slots. Each ONU maintains an upstreambuffer and sends upstream data to the OLT during as-signed time slots. For simplicity, we assume in this paperthat the upstream data rate is fixed for all ONUs. In thedownstream direction, when the arrival rate at the OLTexceeds the output data rate, data are queued in the OLTand transmitted when downstream bandwidth is available.
We also assume that the OLT and ONUs can regainsynchronization successfully so that ONUs can entersleep mode and return back to awake mode. The powerconsumption value in the awake state includes energyconsumed in listening to the OLT and in receiving ortransmitting data. We neglect the transition delays andthe energy consumption when an ONU changes states.We assume that the packets are able to be served withinan integer number of time slots. During the observationperiod, the number of ONUs is also fixed. Next we listsome notations (Table I) used in the rest of this work andthen formulate the energy saving scheduling problem.
Assuming that the overall transmission period is di-vided into N time slots, ONUs are either in awake mode
Notations ValueK The number of ONUs registered in the OLT.
i The index of ONUs in EPON. i ∈ K
N The total time slots within the overall observedtransmission period.
n The index of time slot. n ∈ N
sni ONU i state indication (awake modeor sleep mode) in time slot n.
Tni Period of time slot n in ONU i.
Pawake The average amount of energy (Watt)consumed during the awake period.
Psleep The average amount of energy (Watt)consumed during the sleep period.
TABLE I.NOTATIONS IN ENERGY EFFICIENCY MECHANISM
or in sleep mode during the time slot n. The total energyexpenditure model is formalized as follows:
E =K∑
i=1
N∑
n=1
Tni · [sni · Pawake + (1− sni ) · Psleep] (1)
A. Analysis of Sleep Mode OperationsWithin a period, the total energy consumption includes
energy consumed by M ONUs, which are in activestate. As expressed in our energy expenditure model(Equation 1), the total energy consumed by each ONUconsists of two parts: in either the awake or sleep mode. Inthe awake mode, there are three different states: receivingpackets, sending packets and idle listening. The idle stateis defined as the time when an ONU is active without anyreceiving or transmitting action. In the sleep state, ONUsswitch off transceiver functions and keep a timer on tocount down the wakeup time. Nodes consume much lesspower in the sleep mode than in the awake mode. Forinstance, regarding to different architectures for a sleepmode enabled ONU [4], the expected power consumptionsrequire 2.85 W for active and 750 mW - 1.28 W forsleeping. When ONUs enter sleep state, ONUs are inthe absence of traffic at the optical transmission interface.However, some functions, such as clock and data recoveryand back-end digital circuit, are still powered on. In thissense, the sleep mode differs from power down, which isusually an indication of any equipment fault or discon-nection in EPON. If an ONU is reported as deactivationafter a power down, the ONU has to register itself to theOLT again via the periodic discovery processing. On theother hand, using the sleeping mode, ONUs can wakeup, listen to, and communicate with the OLT without theregistration process, when the sleep period is finished.
As shown in Fig. 2, a timer is utilized in the sleepmode control. Upon receiving notification of enteringthe sleep mode, the timer counts down and the ONUwakes up when the timer is expired. When an ONUwakes up from the sleep mode, there is an overheadtime window (Tov) for recovering the OLT clock (Trecv)and retrieving network synchronization (Tsync). The clockrecovery time is various in different implementations ofONU receiver architectures. For synchronization process,a fixed Start Position Delimiter (SPD) is embedded inthe EPON control message header, which allows ONU togain synchronization with the start of the EPON frame.The synchronization time in EPON is up to 125 µs [13].
Following our model, it is obvious that energy depletionis determined by the time the ONU spend in awake mode.An optimal EMM solution is to schedule ONUs intosleep mode whenever there is no transmission in orderto minimize the overall power consumption. However,since there are QoS requirements in terms of delay andthroughput, it requires a scheduling policy to improveenergy efficiency without violating the QoS requirements.Therefore, this work deals with the trade-off between theallocation of wakeup frequency and the queuing delay inthe EPON.
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Front-end
Analog Circuit
sleep Trecv Tsync
t
Enter sleep mode Wake up
Recover
OLT clock
Sync with
network timing
Dataup & down
ONU k
Sleep
control
Optical
Interface
Back-End
Digital
Circuit
Queue
OLTUser
Interface
ONU Receiver with sleep control
Dataup & down
Figure 2. ONU receiver with sleep control function.
B. Analysis of Energy Consumption and Delay Perfor-mance
When energy management mechanism is implementedin EPON, ONUs turn their optical communication in-terface on and off to minimize energy consumption.Therefore, in order for an ONU to communicate withthe OLT, it must be in active mode. As a centralizedcontrol station, the OLT schedules active duty cycles andinforms connected ONUs about its assignments. An ONUmust comply with the noticed sleep period and wakeuptime. When an ONU is in sleep mode, the downstreamtraffic destined to the ONU is queued in the OLT. Thiscommunication model imposes a clear trade-off betweenthe delay encountered by a packet and the time duringwhich ONUs are in active mode. Solutions for addressingthis trade-off depend, to a large extent, on the followingaspects:
1) The expected amount of data to be received andtransmitted.
2) The polling scheme and upstream bandwidth allo-cation scheme: for example, the sequence of polledONU and the size of bandwidth granted to theONU.
3) Whether the OLT process the packets and send toONUs with priority discrimination.
With respect to 1, if the data arrival rate is high andcontinuously delivered, data are queued and the OLT canschedule transmission. With respect to 2, if the pollingscheme and upstream bandwidth allocation are flexibleenough, it can determine the order of ONUs to commu-nication according to the expected delay constraint. Withrespect to 3, if the OLT can process packets, it may bufferand merge several packets for aggregation.
We first formulate our scheduling problem: find anoptimal scheduling discipline that minimizing the averageenergy consumption of EPONs while guaranteeing anupper bound on the queuing delay. This problem isformalized as an optimization problem that an optimalassignment of wakeup time to ONUs is an assignmentthat guarantees an upper bound Dmax on the maximumdelay while minimizing the total energy spent by theONUs in awake mode. Let K be a set of ONUs that are
awake and communicate with the OLT during the wholeN observation periods. Let Tn
i and Pawake be the awakeperiod and the average energy consumption for each ONUi, respectively. Then, the goal is to:
minimize : E =1
K
K∑
i=1
N∑
j=1
T ji · Pawake (2)
subject to:dmi < Dmax (3)
where dji is the delay experienced by every connectionsin AG i, which should deliver qualified services.
IV. ENERGY MANAGEMENT MECHANISM (EMM)
In this section, we introduce the implementation ofEMM together with EPON MAC protocol, MultipointControl Protocol (MPCP) [17]. The OLT is in full controlof the bandwidth allocation in both downstream andupstream. An ONU can associate with the OLT eitherin normal mode, referred to as Power Ignoring Scheme(PIS), or in EMM mode. When an ONU is in normalmode, it constantly stays awake and consumes power, i.e.,never goes into sleep mode. In the following, we assumethat all ONUs are EMM-enabled.
In order to minimize power consumption, ONUs re-main in a sleep mode most of the time while adheringto the assignment from the OLT. We show how beingin EMM yields considerable power saving for an ONUas compared to PIS case. In addition, we introduce howthe sleep mode scheduling scheme has an impact onthe energy and delay performance of the EMM ONUs.The sleep period and wakeup time can be calculated andassigned using either an Upstream Centric Scheduling(UCS) algorithm or a Downstream Centric Scheduling(DCS) algorithm.
A. Energy Efficient Scheduler Design
In EPONs, the downstream and the upstream trans-mission are separated, and the OLT plays a central roleto reserve bandwidth and to serve traffic transmission.In this paper, a subframe period (SP), Tsp, refers tothe time assigned to an ONU to send upstream data tothe OLT (SP-UL) or to receive downstream data fromthe OLT (SP-DL). In this work, we will analyze theenergy consumption and determine the wakeup time byconsidering both the upstream and downstream data sincethe instants of terminating sleep mode by upstream anddownstream transmission are different.
- Downstream bandwidth allocation: In the MPCPspecified in IEEE 802.3ah, the OLT broadcasts adownstream packet with destination MAC addressto all ONUs. ONUs that are awake to listen ifthe OLT has packets to deliver to them check theheader for destination identification. Since there isno retransmission mechanism in the EPON down-stream, it takes a long latency if the ONU misses
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packets. Therefore, ONUs keep active all the timein order to listen to the OLT and receive packets ifthe destination identification is matched. Since thepower saving mechanism is ignored traditionally,time and energy are wasted when an ONU isactive to listening to the OLT and decoding packetsdestined to other ONUs.
- Upstream bandwidth allocation In the MPCP, theOLT maintains a polling list and a polled ONU hasthe right to transmit data within its assigned up-stream time slots. The OLT allocates upstream timeslots to each ONU in order to avoid that multipleONUs transmit to the uplink simultaneously. Theupstream bandwidth information such as the starttime and the length of granted transmission windowis carried in the GATE message. The OLT pollseach ONU by sending them GATE message andONUs transmits traffic within the assigned upstreamtime slots after receiving the GATE message. Theupstream bandwidth is distributed among all ONUsby using either a fixed allocation scheme or a dy-namic bandwidth allocation (DBA) scheme [18]. Inthe former case, each ONU is scheduled with a fixedamount of upstream transmission period. In thelatter case, the bandwidth is allocated dynamicallywith a bounded size according to ONUs’ request.A cycle (Tcycle) refers to an interval between twosuccessive polling messages to an ONU. Since thepower saving mechanism is ignored traditionally,the time is wasted when an ONU is active to waitingfor its next upstream transmission period.
The EPON supports energy management at the MAClayer, and ONUs need to negotiate with the OLT inorder to decide the power saving parameters. The powersaving parameters include the time to sleep and wake up.Theoretically, the power saving control can be initiatedeither at the OLT or at the ONUs, if they are in an idlestate, where no packet to be received or sent. However,as the ONU is an aggregation node that collects trafficfrom low level networks and relays to the OLT, it iscommonly assumed that ONUs always have queued datafor upstream transmission. In this work, we consider onlythe OLT triggered power saving mechanism. We focus onprotocol design of EMM and the scheduling schemes tomaximize power saving subject to the constraint of packetdelay.
With respect to bandwidth allocating, since the MPCPis central control protocol, the OLT has full knowledgeto assign transmission bandwidth in both upstream anddownstream directions. This implies that the order ofgranted ONUs and the amount of granted bandwidth aredetermined by the OLT. An intuitive solution for EMMscheduler is to aggregate the downstream traffic in order tominimize the time allocated to ONUs for idle states. Thegeneral design of EMM based EPON proposes followingfunctions in the OLT and ONUs:
1) OLT operation:The OLT can buffer downstream traffic and sched-
ule the appropriate transmission period to eachONU. To meet the QoS requirement of delay sensi-tive traffic, the downstream bandwidth is allocatedsuch that every packet can be served before itsdeadline. The original control message GATE ismodified with additional fields indicating the as-signed sleep time and wakeup time. To assign thewakeup time for the next upstream transmission,the OLT needs to calculate the time for an ONUto upload and download data packets. Consideringthe wakeup time to transmit the upstream packet,the OLT determines and allocates the upstreamtransmission windows for all ONUs. As for thewakeup time to receive the next downstream packet,since the number of buffered downstream data iscompletely know by the OLT, it can calculate andschedule downstream transmission.
2) ONU operation:After scheduling the sleep period, the OLT sendsa control message to the ONU for the permissionto transit into sleep mode. Upon receiving thismessage with parameters start time for sleep andwakeup time from sleep, the ONU enters into sleepmode. After a sleep mode, the ONU transits backto the awake mode again. Scheduled ONUs shouldwake up according to the assigned wakeup timeand check the sleep-mode GATE message (Gs).Upon receiving the Gs message, the ONU derivesthe sleep period and obeys the assigned upstreamand downstream bandwidth allocation. As for theupstream subframe period announcement, the as-signed wakeup time implies its next access timeto the upstream transmission. For a Gs messageannouncing the downstream subframe period, theONU should awake at the notified wakeup time toreceive the buffered data and then back to sleepafterwards.
B. Upstream Centric Scheduling (UCS) Algorithm
The main idea of UCS scheme is that the OLT assignsthe awake period to ONUs according to their correspond-ing upstream allocation. The OLT grants the upstreambandwidth by polling each ONU. During the grantedupstream subframe period, the ONU is awake. Once anONU transits into the awake state, the OLT with UCSalgorithm only transmits downstream packets to the awakeONU, and queues those packets destined for ’sleep’ ONUsinto a buffer.
In this subsection, we give a description of the UCSalgorithm. For simplicity of illustration, we consider asystem of an OLT and three ONUs for two upstreamtransmission periods. For more ONUs and transmissionperiods, the same logic can be applied. Fig. 3 shows theprocess of UCS protocol. In this work, the t denotesfor the instant time and T represents for a period. TheOLT maintains an entry table, which contains 1) thestart transmission time (tstartni ) and granted bandwidth(BWn
i ) for allocating upstream bandwidth; 2) the sleep
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time (tsleepni ) and the wakeup time (twakeni ) for assigning
sleep period. The table is updated every cycle n forONUi. Downlink data are stored in subqueues for eachONU.
As shown in Fig. 3, the ONU1 wakes up at t0and receives sleep mode GATE message (Gs
11) at the
time t1. After learning the granted upstream subframeperiod (Tsp−ul
11), ONU1 generates REPORT message
(R1) and starts upstream transmission (dul11) complyingwith the assignment. During Tsp−ul
11, the OLT derives
downstream data (ddl11) from the subqueue for the ONU1
and transmits them to the ONU1. Notice that the data ddlrepresents either one data packet or a series of integrateddata packets. The start time of sleep mode (tsleep11)for ONU1 is assigned so that the ONU1 enters intosleep mode immediately when the upstream subframeperiod is completed at t2. The calculations of upstreamtransmission period and the time to turn into sleep modeare listed in Equation 4 and Equation 5.
Tnsp−ul,i =
BWni
Ro, i ∈ K,n ∈ N (4)
tnsleep,i = tnstart,i + Tnsp−ul,i, i ∈ K,n ∈ N (5)
where BWni is the allocated upstream bandwidth for
the ith ONU during the nth cycle. Ro is the transmis-sion rate of the optical upstream and Tg is the guardtime between two successive upstream transmissions. Thewakeup time (twakeup
11), which is same as the length of
sleep period, is calculated based on the period before theONU1 is polled again (Tcycle
11). In the Fig. 3, the next
wakeup time for the ONU1 is t3. The value of wakeuptime is computed at the OLT and assigned to each ONU.The polling cycle is computed using Equation 6.
Tncycle,i =
K∑
i=1
(Tnsp−ul,i + Tg)
=K∑
i=1
(BWn
i
Ro+ Tg), i ∈ K,n ∈ N
(6)
After a period of Tcycle, the interval between twoadjacent polling messages, an ONU is polled again at t4(t1 + Tcycle
11). The calculation of ONU wakeup time needs
to take the overhead period into account due to the timeof recovering the OLT clock and retrieving the networksynchronization. The computation of the overhead period(Tov) and the time to wake up are listed as Equation 7and Equation 8, respectively.
Tov = Trecv + Tsync (7)
tnwakeup,i = tnstart,i+Tncycle,i−Tov, i ∈ K,n ∈ N (8)
During the observation period, the total awake time foreach ONU is the sum of allocated upstream time slotsand an ONU enters the sleep mode otherwise. During thesubframe period, the ONUi transmits upstream data dul
ni
Start
Check upstream polling
timer
Time to send
GATE to ONUi
Allocate upstream bandwidth for
the ONUi based on
RTTi and BWni
Calculate the time for next
polling mesage to the ONUi
(Tcycleni)
Assign sleep period
info to the ONUi
(tsleepni and twakeup
ni)
Setup timer to poll the ONUi
after Tcycleni
Generate and transmit
sleep mode GATE
message to the ONUi
Time to send
DATA to ONUi
Check upstream transmission
period to find awake ONUi
Update OLT entry table with
tstartni
Available data
for ONUi in the buffer
Transmit DATE
message to the ONUi
Y
N
Y
Y
N
N
Figure 4. Flowchart of the UCS algorithm designed in the OLT.
and receives downstream data ddlni , within the granted
time slot n. Notice that the size of the uploaded anddownloaded data must equal or less than the upstreambandwidth, i.e. size(dulni )≤ BWn
i and size(ddlni )≤ BWni .
The total energy consumed by the ONU i is illustrated inEquation 9:
EUCS =
K∑
i=1
N∑
n=1
[Tnsp−ul,i + Tov] · Pawake
+K∑
i=1
N∑
n=1
[Tncycle,i − Tn
sp−ul,i − Tov] · Psleep
(9)
The UCS based scheduler is simple because the sleepperiod is determined based on the upstream transmission.An OLT with the UCS based scheduler uses the awakeONU, which is assigned to the upstream transmissionas destination to retrieve buffered data. The algorithmis presented in the flowchart in Fig. 4. However, theperformance of downstream data latency and bandwidthutilization may not be satisfied due to the dependencyon the upstream subframe period, the upstream pollingsequence, and the total number of active ONUs. Onedisadvantage of this scheme is that it is not suitablefor delay-sensitive traffic due to longer queuing delayexperienced in the OLT.
C. Downstream Centric Scheduling (DCS) Algorithm
The second scheduling policy presented in this paper isthat the OLT stores downstream traffic in a First-In First-Out (FIFO) buffer and the ONU has to be awake and
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dul11
t1
ONU1
t
R1
t2 (tsleep 11)
t4
t3 (tstart 21)
t
ONU2
ONU3
ddl1
1Gs11
ddl21
dul2
1R1
dul12R2
ddl1
2
dul2
2R2
ddl22
dul1
3R3
ddl13
R3
Tsleep
Tsleep
Tsleep
Tsleep
Tsleep
t5
REPORT
Sleep mode GATEGs
R
t
txrx
tx
rx
txrx
ONU RTTGranted
BWtsleep
tsleep n
11
2
3
Trtt 1 BW1
Start Tx
Time
tstart n1
tstart n2
tstart n3
Trtt 2
Trtt 3
OLT Entry Table
twakeup
twakeup n
1
twakeup n2
twakeup n3
OLTtxrx
Gs1
1
1
2
3
ddl 1
OLT DL Queue
ddl 2
ddl 3
Gs2
1
dul11R1
BW11
Tg dul12R2
BW12
dul13R3
BW13
Tg
ddl11G1 ddl
12G2
Gs1
2
ONU Ddownlink
Gs12
Gs1
3
Gs21
Gs22
Gs2
3
Uplink datadul
Downlink dataddl
Tg Guard time
t
t0
BW2
BW3
tsleep n2
tsleep n3
tstart 11
Tsp-ul 11
Tsp-dl 11
Tcycle11
Tov
Figure 3. Sleep mode operation with Upstream Centric Scheduling (UCS) scheme.
receive downstream traffic whenever the OLT sends one.The awake periods are assigned to favor both upstreamand downstream traffic. The DCS scheme is illustratedin Fig. 5. The sleep mode GATE message, Gs
ni , is
used for both upstream bandwidth allocation and sleepperiod assignment. The upstream transmission period iscalculated using the same equations 4- 8 as in the UCSscheme. For example, the Gs
11 at time t1 indicates that the
next polling time is at time t8 and the ONU1 needs towake up at time t7. In the downstream, the OLT servesdata in a first-in first-out order. For instance, shown inFig. 5, during the upstream subframe, dul11, the OLT sendspackets to both ONU1 (data ddl
11) and ONU2 (data ddl
12).
As shown in Fig. 5, the admission information for bothupstream and downstream transmission is notified by thesleep mode GATE message. The Gs
11 informs ONU1
the allocated upstream transmission of dul11 and assignedsleep period during t3 to t7. After transmitting ddl
12 to
the ONU2, the OLT gets data ddl11 from the head of its
downlink queue and sends to the ONU1 together withGs
21. Since there is data ddl
21 in the buffer, the ONU1 is
notified to wake up at t4 instead of t7. The sleep periodis assigned if there is neither upstream nor downstreamtransmission scheduled. If the OLT has queued data forthe ONU, the wakeup time for the next data can becalculated. For example, the period between t3 and t4is the idle period for ONU1. Otherwise, the OLT sends amessage to inform the ONU of remaining in awake modein order to avoid missing any downstream traffic. In the
following, the assignment of sleep period is discussed indetails.
Under the condition that the downstream traffic ter-minates the sleep mode and the OLT assign the sleepperiod for the ith ONU, we distinguish four possibilitiesas shown in Fig. 6. The sleep period is determined byfavoring both the successful reception of upstream anddownstream data. Accordingly, we denote Gs as theGATE control message carrying the information of sleepperiod, such as the start time of sleep period (tsleep) andthe wakeup time (twakeup). Let Tsp−ul
ni and Tsp−dl
ni be
the nth subframe period of uplink and downlink transmis-sion, respectively. In addition, the Tov period representsan overhead time for clock recovery and synchronization.
1) Case0: GATE0 is used to inform the allocated up-stream bandwidth for the ONUi. Because the OLThas full knowledge about the upstream bandwidthallocation, the sleep interval is determined, includ-ing both the sleep time (tsleep0) and the wakeuptime (twakeup0).
2) Case1: The transmission of downstream data(Tsp−dl
1i ) is completed within the allocated up-
stream subframe period. If there is other queueddata in the OLT, such as the payload with GATE2,the sleep period is assigned as same as in Case0. Ifthe next received data is the payload with GATE3,the time to wake up is reassigned as same astwakeup2. If there is no more queued data in theOLT for the ONUi, the original assignment, tsleep0,
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REPORT
Sleep mode GATEGs
R
ONU RTTGranted
BWtsleep
tsleep n11
2
3
Trtt 1 BW11
Start Tx
Time
tstart n1
tstart n2
tstart n3
Trtt 2
Trtt 3
OLT Entry Table
twakeup
twakeup n1
twakeup n2
twakeup n3
OLTtxrx
Gs1
1
OLT DL FIFO
Queue
Gs4
1
dul11R1
BW11
Tg dul12R2
BW1
2
dul13R3
BW1
3
Tg
Gs1
2
Uplink datadul
Downlink dataddl
Tg Guard time
t
BW12
BW13
tsleep n2
tsleep n3
t1 t2 t4t3 t5 t6 t7 t9t8
Gs11 ddl
12
Gs12
ddl11
Gs2
1
Gs2
2
Gs22
ddl22 Gs
31 ddl
21
Gs21 ddl
13Gs
13
Gs1
3t
t
t
Tsleep Tsleep
Tsleep
Tsleep
ddl11
dul12R2
dul13R3
ddl13 G3
Tsleep
dul23R3
dul22R2
dul21R1tx
rx
tx
rx
txrx
ONU1
ONU2
ONU3
ddl31
ddl12
ddl32
Tsleep
Gs1
1 Gs2
1 Gs4
1
Gs12
Gs51
Gs22 Gs
32
Gs13
dul11R1
ddl22
ddl21Gs
31
Gs4
1 ddl32Gs
32
ddl31
Gs51
ddl12
ddl11
ddl22
ddl21
ddl13
ddl32
Tsleep
Figure 5. Sleep mode operation with Downstream Centric Scheduling (DCS) scheme.
is removed, because ONUi should keep awake inorder to avoid missing future arrival data.
3) Case2: In this case, the downstream data (Tsp−dl2i )
can not be finished before the start time of thesleep period as assigned in Case0 and Case1. There-fore, the start time of sleep period is postponed totsleep2. The OLT examines its downstream queueand schedules the transmission of the next down-stream data (Tsp−dl
3i ) for the ith ONU. Moreover,
the wakeup time (twakeup2) is calculated in orderto ensure a successful transmission.
4) Case3: In this case, there is downstream data re-ceived during the sleep interval. In GATE3 controlmessage, the new time of entering the sleep modeis updated, tsleep3, which is calculated based on thedownstream subframe period (Tsp−dl
3i ).
5) Case4: As showing in the last GATE4 in this figure,there is downstream data arrived during the sleepinterval and lasted till the next upstream subframeperiod. Upon the sleeping period is completed, theONU transits into the awake mode and will receivethe GATE4 message with the information of up-stream bandwidth allocation, such as the start timeand length of Tsp−dl
4i . Thus, the GATE4 message
contains bandwidth assignments for both upstreamand downstream subframe period.
The flowchart of the DCS algorithm implemented inthe OLT is described in Fig. 7. Compared to the UCSalgorithm, the OLT checks the available data in the bufferand update the sleep period for the destination ONUwith a sleep mode GATE message. The wakeup time isprecisely calculated and assigned, so that the ONU canbe active to carry out the next upstream or downstream
transmission. Since the operations of the above schedulingmechanism requires the OLT to locate the next packet inthe buffer in order to calculate the wakeup time, ONUswhich have no more queued packets cannot enter the sleepmode. In this case, the OLT assigns the sleep period aszero.
In order to calculate the total energy consumption in theDCS algorithm based EMM. We define the awake periodfor upstream transmissions (Tawake−ul) and downstreamtransmissions (Tawake−dl) separately. First, we computethe awake period and sleep period in the upstream direc-tion (shown in Equation 10 and Equation 11), which issimilar to Section IV-B.
Tnawake−ul,i = Tn
sp−ul,i + Tov, i ∈ K,n ∈ N (10)
Tnsleep−ul,i = Tn
cycle,i − Tnsp−ul,i − Tov, i ∈ K,n ∈ N
(11)Next, assuming that there are M downstream trans-
missions to the ith ONU. For the downstream, the awakeperiod is calculated (in Equation 12) differently in eachcase mentioned as Fig. 6. In Case1, ONUi is in awakestate, so that the awake period for receiving downstreamdata is zero. From Case2 to Case4, if the time forwaking up and sleeping is assigned, the period that theONU has to wake up from its sleep mode is calculatedas the interval between tsleep and twakeup. Illustratedin Equation 13, the sleep period is the upstream sleepperiod minus the awake period used for the downstreamtransmission, which occurs during the upstream sleepperiod.
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Tsp-ul 1i Tsp-ul
2i
Tsp-dl1iGs Gs Tsp-dl
2iGs Tsp-dl
3iGs Tsp-dl
4i
Gs
GATE1
tsleep0 (tsleep1)
tsleep2 tsleep3twakeup2
twakeup0
(twakeup1)
GATE0 GATE2 GATE3 GATE4
tsleep4
tONUi
tx
rx
Tov
twakeup3
Tov Tov Tov
Figure 6. Downstream transmission in the sleep period model.
Tnawake−dl,i =
M∑
m=1
Tmawake−dl,i
=M∑
m=1
0 , if Case1tmsleep2 − tmsleep1 , if Case2tmsleep3 − tmsleep2 , if Case3tmsleep0 − tmsleep3 , if Case4
(12)
Tnsleep−dl,i = Tn
sp−ul,i−M∑
m=1
Tmawake−dl,i, i ∈ K,n ∈ N
(13)After analyzing the active behavior of an ONU in both
the upstream and downstream transmissions, the totalawake and sleep periods are concluded as Equation 14and Equation 15:
Tnawake−total,i = Tn
awake−ul,i+Tnawake−dl,i, i ∈ K,n ∈ N
(14)
Tnsleep−total,i = Tn
sleep−dl,i, i ∈ K,n ∈ N (15)
The total energy consumed is calculated in Equation 16.The DCS sleep mode scheduler transmits downstreamtraffic in a flexible way, unlike in the UCS that onlythe active ONU scheduled with an upstream transmissionperiod can receive data from the OLT. The OLT triesto schedule the queued downstream traffic in a wayto minimize the delay. The UCS is simple where thesleep period and wakeup time is determined only bythe upstream bandwidth allocation. On the other hand,the DCS scheduler requires to keeping track of bothdownstream and upstream transmission windows.
EDCS =K∑
i=1
N∑
n=1
[Tnsp−ul,i · Pawake]
+K∑
i=1
N∑
n=1
[Tnsleep−total,i · Psleep]
(16)
V. SIMULATION RESULTS
A. Simulation Scenario
In this section, we study performances and features ofour proposed energy management mechanism compared
to the fixed power control, which is represented as PowerIgnoring Scheme (PIS). Simulations have been carriedout by means of OPNET network simulator [19] to mea-sure the performance metrics. The performed simulationsaimed to show the diverse effects of system parameters,such as the upstream scheduling scheme, the number ofconnected ONUs, and the arrival packet size. Then thebenefits that can be obtained while utilizing the proposedEMM in EPON. The following parameters are monitored,such as the power consumption, the network throughput,and the average queuing delay. The average awake timein ONUs is used as a merit for energy efficiency perfor-mances. The less time an ONU is awake, the less energyan ONU consumes.
We have assumed that the OLT is connected to KONUs (K=16 or 32)., which are enabled with power sav-ing functionality. The optical link rate is 1 Gbps for bothupstream and downstream transmission. The guard timebetween two consecutive transmission slots is 5 us. Forallocating upstream bandwidth among ONUs, both fixedscheme (e.g. TDM) and dynamic scheme (e.g. IPACT)are implemented at the OLT. The maximum transmissioncycle is set to 2 ms. As the proposed power saving mecha-nism with two scheduling schemes cooperates the opticaldownstream bandwidth scheduler at the OLT, it is rea-sonable to monitor the performance of downstream traffic.Traffic is generated following Poisson distribution and thelength of packets is generated independently between 64bytes to 1200 bytes. Destinations of downstream trafficare uniformly distributed among connected K ONUs.The MPCP control messages, GATE and REPORT, areformatted according to the IEEE 802.3ah specification.The sleep mode GATE message, Gs, is an extension ofthe standard GATE message. The additional two 4 bytesfields are embedded as assigned sleep time and wakeuptime. The total size of Gs message is set to 41 bytes.
B. Validation of EMM with the fixed upstream bandwidthallocation
In Section IV, we analyze the energy efficiency mecha-nism with two proposed scheduling approaches and derivethe consumed power accordingly. The simulation scenarioconsists of 16 ONUs and the OLT grants fixed upstreambandwidth to each ONU. The results in Fig. 8 refer tothe system behavior in the presence of an fixed upstreambandwidth allocation scheme deployed at the OLT. Theyshow the energy consumption, represented by the ONU
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Start
Check upstream polling
timer table
Time to send
GATE to ONUi
Transmit DATE
message to the ONUi
Y
N
Y
Y
N
NONUK is awake
Available data Dk
in the buffer
Get data destination K;
Check sleep period table;
Available next data
in the buffer for ONUK
Y
Calculate the time for next
DATA to the ONUk
Generate GATE with
downstream transmission info.
(SP_DL-starttime, SP_DL-length)
Assign new sleep period info (tsleepni and twakeup
ni) according
to the current active status in the ONUi (ref to sleep period
decision figure)
Transmit GATE
message to the ONUi
Set sleep period is none
N
Allocate upstream bandwidth for
the ONUi based on
RTTi and BWni
Calculate the time for next
polling mesage to the ONUi
(Tcycleni)
Assign sleep period
info to the ONUi
(tsleepni and twakeup
ni)
Setup timer to poll the ONUi
after Tcycleni
Update OLT entry table with
tstartni
Generate and transmit
sleep mode GATE
message to the ONUi
Update sleep period table
Figure 7. Flow chart of the Download Centric Scheduling (DCS)Scheme.
awake time, when the downstream traffic load changes. Inthe case of PIS, ONUs constantly stay in awake state andconsumes high energy (100% of the observation period).When the EMM is enabled in the EPON system, theawake time maintains at a low average percentage value ifthe Uplink Centric Scheduling (UCS) scheme is applied.As explained in Section IV-B, the period of being poweron is decided by allocated upstream transmission periodsin the UCS case. When each ONU is allocated with fixedamount of upstream bandwidth, the awake time is alsofixed. On the other hand, under the DCS case, ONUs wakeup and communicate with the OLT for either upstreamor downstream traffic. When arrival traffic is less thanthe optical output link rate, the awake time of EMM-DCis slightly increased due to the additional active periodfor downstream traffic. When the arrival traffic rate ishigh, the awake time increases and close to the level asin the PIS case. With respect with energy efficiency, theproposed energy management mechanism outperforms thetraditional power ignoring mechanism. Particularly, usingthe uplink centric scheduling scheme saves up to 90%power on time. The energy consumption in the DCS casevaries when the traffic load changes.
Fig. 9 shows the network throughput under the PIS andEMM cases. The downstream throughput is monitored,because the upstream throughput is not affected by theproposed power saving mechanism. When the EMMis deployed, both DCS and UCS schemes degrade thethroughput performance. In the DCS case, throughputis decreased due to the additional overhead by intro-ducing the sleep mode GATE control message. Becausethe wakeup time and sleep time are assigned to ONUswhen they receive every downstream packet, it causes aportion of control message overhead. In the UCS case,the network throughput is reduced greatly. The reasonis that the OLT can dispatch downstream traffic duringits allocated upstream transmission period. Assuming theOLT polls and grants all ONUs in a Round Robin (RR)order, each ONU can derive one over sixteen percentof total downstream link capacity. The link utilizationfactor, computed as the amount of traffic delivered to thedestination successfully over the total link capacity in thesimulation time. The higher the network throughput, thehigh link utilization. The proposed EMM receives similarresults compared to the PIS case, when the DCS schemeis applied. However, performance is highly downgradedin the UCS scheme.
The average queueing delay for analyzing performancedifferences under different power saving settings is shownin Fig. 10. The average delay of the EMM DC schemeis better than that of the UCS scheme since, with UCSscheme, downstream packets have to wait for their awakeperiod in the OLT buffer. We observe that when thetraffic intensity is low, average delay remains constantsince the transmission rate is high enough to serve undera range of values of traffic load. However, when theincoming traffic intensity reaches a certain point andcauses congestion, average delay increases rapidly. Asexpected, the average queueing delay is consistent withthe throughput performance, where the EMM DC schemeachieves better performance.
C. Impact of Upstream Bandwidth Allocation Schemes
Now we examine the impact of upstream bandwidthallocation schemes on the proposed energy managementmechanism with the uplink centric scheme. In this sce-nario, only upstream traffic from ONUs to the OLT isconsidered to highlight the results differed from differentupstream bandwidth allocation schemes. If the down-stream traffic is involved, then additional time is requiredfor transmitting downstream packets.
Fig. 11 illustrates a simple scenario that shows therelationship between the upstream bandwidth allocationalgorithm, TDM and DBA, with the ONU awake time.Because the downstream traffic is ignored, the sleep pe-riod is assigned to each ONU only based on the allocatedupstream bandwidth, as the EMM UCS performs. Inthe TDM case, each ONU is assigned with a fixed slottime. In the DBA scheme case, the OLT grants ONUsaccording to their bandwidth requests. Under low trafficload, buffered data at ONUs are small, and therefore,
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0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
0
10
20
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50
60
70
80
90
100
Ave
rag
y p
erc
en
tag
e o
f O
NU
aw
ake
tim
e (
%)
ONU Downstream Transmission Rate (Gbps)
EMM-UC (TDM)
EMM-DC (TDM)
PIS
Figure 8. Average ONU awake time in PIS, EMM-UC, and EMM-DCalgorithms.
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
0
100
200
300
400
500
600
Th
rou
gh
pu
t T
x to
ON
U.m
ea
n (
Mb
ps)
ONU Downstream Transmission Rate (Gbps)
EMM-UC (TDM)
EMM-DC (TDM)
PIS
Figure 9. Network throughput in PIS, EMM-UC, and EMM-DCalgorithms.
small amount of requested time slots. The percentageof awake time is increased along with the increasedtraffic rate. Because the maximum upstream slot time isconstrained as 2 ms, the DBA based approach has similarresults under the heavy traffic load.
Fig. 12 and Fig. 13 show the network throughput andqueuing delay for the downstream traffic, respectively.The volume of downstream traffic is fixed at 1 Gbpsand destination is uniformly distributed among 16 ONUs.Consistent with the curve of awake time, the downstreamthroughput increases when the ONU receives high inputtraffic and requests for large timeslots. On the other hand,when the required upstream transmission period is small,ONUs are turned off most of the observation time anddownstream data are queued at the OLT. Thus the queuingdelay is high under light traffic load in the dynamicbandwidth allocation case.
From previous studies [18], dynamic bandwidth alloca-tion schemes improve EPON channel utilization and pro-vide QoS guarantee. Our simulations over the upstreamchannel have demonstrated that the dynamic upstreambandwidth allocation scheme gains system power effi-ciency. However, the delay performance is downgradedwhen the upstream traffic load is low.
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0
0
20
40
60
80
100
120
140
160
180
200
220
Avg
Qu
eu
e D
ela
y in
ON
U.m
ea
n (
ms)
ONU Downstream Transmission Rate (Gbps)
EMM-UC (TDM)
EMM-DC (TDM)
PIS
Figure 10. Average queueing delay in PIS, EMM-UC, and EMM-DCalgorithms.
0 10 20 30 40 50 60 70 80 90 100
0
10
20
30
40
50
60
70
80
Ave
rag
y p
erc
en
tag
e o
f O
NU
aw
ake
tim
e (
%)
ONU Upstream Transmission Rate (Mbps)
EMM-UC (TDM)
EMM-UC (DBA)
Figure 11. Awake time compared in different upstream bandwidthallocation schemes.
D. Impact of the sleep mode GATE message overhead
In this study, simulations show impacts of the sleepmode GATE control message. The MPCP control mes-sages, GATE and REPORT, are formatted according tothe IEEE 802.3ah specification. The sleep mode GATEmessage, Gs, is an extension of the standard GATEmessage. The additional two 4 bytes fields are embeddedas assigned sleep time and wakeup time. The total sizeof Gs message is set to 41 bytes. In the UCS case,the sleep mode GATE message is used to poll ONUs,grant upstream bandwidth, and inform assigned sleepperiod. The Gs is generated and sent to ONUs in thesame means as the original MPCP case. However, inthe DCS case, Gs message is transmitted together witheach downstream packet, which results in amount ofcontrol message overhead. Therefore, in this test case,we simulate a network scenario with 16 ONUs and focuson EMM DCS scheme.
Fig. 14 shows the number of transmitted Gs as afunction of incoming packet sizes. In all cases, the totalnumber of Gs is increased at the downstream data rateincreases. The size of data packets are varied between400 bits and 9400 bits. Under a certain input data rate,the larger the data size, the less transmitted Gs controlmessage. With respect to bandwidth utilization, increasednumber of Gs message introduces high value of control
JOURNAL OF COMMUNICATIONS, VOL. 6, NO. 3, MAY 2011 259
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0 10 20 30 40 50 60 70 80 90 100
0
10
20
30
40
50
60
70T
hro
ug
hp
ut T
x to
ON
U.m
ea
n (
Mb
ps)
ONU Upstream Transmission Rate (Mbps)
EMM-UC (TDM)
EMM-UC (DBA)
Figure 12. Network throughput compared in different upstream band-width allocation schemes.
0 10 20 30 40 50 60 70 80 90 100
0,0
0,5
1,0
1,5
2,0
Avg
Qu
eu
e D
ela
y in
ON
U.m
ea
n (
s)
ONU Upstream Transmission Rate (Mbps)
EMM-UC (TDM)
EMM-UC (DBA)
Figure 13. Average queuing delay compared in different upstreambandwidth allocation schemes.
overhead, therefore, results in low bandwidth utilization.The delay performance is illustrated by Fig. 15. The
delay difference is hardly to tell when the downstreamtraffic load is low. When traffic load becomes high, thelarge packet size case produces better delay performance.This is for the reason that higher channel utilization canbe achieved by reducing the ratio of the overhead of Gs
message and the size of payload.
E. Performance comparison with multiple ONUs
We now consider a scenario where 32 ONUs areconnected to the OLT. With this network structure, anONU is allocated less upstream transmission periods.Fig. 16 shows the awake periods are reduced in both UCSand DCS schemes compared to the 16 ONUs structure.Consequently, the queuing delay is increased (shown inFig. 17). When the traffic load becomes high enough,the downstream traffic takes all transmission time andconsumes the same power as in the PIS case.
VI. CONCLUSION
In this paper, we focus on the energy consumptionin the EPON system. We proposed and investigated ananalytical model for the energy expenditure in EPON.We developed an energy management mechanism foroptimal assignments of sleep periods to ONUs. The delayconstraints that need to be satisfied is analyzed in the
0,0 0,2 0,4 0,6 0,8 1,0 1,2
0
400000
800000
1200000
To
tal n
um
be
r o
f tr
an
sm
itte
d s
lee
p m
od
e G
AT
E m
essa
ge
s
ONU Downstream Transmission Rate (Gbps)
400 bits
1400 bits
2400 bits
3400 bits
4400 bits
5400 bits
6400 bits
7400 bits
8400 bits
9400 bits
Figure 14. the number of transmitted Gs as a function of incomingpacket sizes.
0,0 0,2 0,4 0,6 0,8 1,0 1,2
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Avg
Qu
eu
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ela
y in
ON
U.m
ea
n (
s)
ONU Downstream Transmission Rate (Gbps)
400 bits
1400 bits
2400 bits
3400 bits
4400 bits
5400 bits
6400 bits
7400 bits
8400 bits
9400 bits
Figure 15. Average queuing delay performance as an impact of thesleep mode GATE message overhead.
scheduling problem. We solve the problem by proposingtwo heuristic scheduling algorithms: the upstream centricscheduling algorithm and the downstream centric schedul-ing algorithm. An EPON system with energy efficiencybased scheduling schemes are simulated in the OPNETsimulator. We verified the solution quality with respect toa set of chosen metrics such as awake time and queuingdelay. Specifically, we compared the energy performanceand the network performance under different traffic loadand different upstream bandwidth allocation schemes. Wefound that the energy management mechanism reduces thepower consumption, and the DCS algorithm outperformsthe UCS algorithm in the delay performance.
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Ying Yan received the B.Eng. degree in electrical engineeringfrom the Beijing University of Technology, China, in 2002and the M.S. degree in electronics engineering in 2004 fromthe Technical University of Denmark. After completing theMaster degree, Ying Yan began to pursuit her Ph.D. studies ontelecommunication engineering in 2005. During 2006-2007, sheworked as a research scientist at the department of communi-cation platforms in the Technical Research Centre of Finland(VTT), Finland. While pursuing the Ph.D. study, Ying Yanparticipated in European projects (the IST-MUPBED projectand the ICT-ALPHA project) and a Danish national project (theHIPT project). Currently she is an Assistant Professor at theTechnical University of Denmark. Her research interests includescheduling algorithms, resource management mechanism, qual-ity of service, and energy efficiency in fixed wireless networksand Carrier Ethernet transport network.
Lars Dittmann was born in 1962 and received the M.Sc.EE and Ph.D. from the Technical University of Denmark in1988 and 1994, and is currently professor at the universitywithin the area of integrated networks. He has since January1999 been heading the network competence area (coveringboth optical and electrical networks) within DTU Fotonik atthe Technical University of Denmark and was prior to thatresponsible for electronic switching and ATM networks at theCenter for Broadband Telecommunication. Main research areais network architectures and performance.
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