Sleep Mode Mechanism with Improved Upstream ... - IEEE Xplore

IEEE ICC 2014 - Selected Areas in Communications Symposium

Sleep Mode Mechanism with Improved Upstream Performance for Passive Optical Networks Jie Li1, N. Prasanth Anthapadmanabhan2, Chien Aun Chan1, Ka-Lun Lee1, Nga Dinh3, and Peter Vetter2 1

Centre for Energy-efficient Telecommunications (CEET), The University of Melbourne, Melbourne, Australia 2. Bell Labs, Alcatel-Lucent, New Jersey, USA, 3. Bell Labs Seoul, Alcatel-Lucent, Seoul, South Korea E-mail: [email protected]

Abstract – With the proliferation of advanced and interactive services together with the high demand for premium quality of experience, upstream traffic in passive optical networks is growing fast in recent times. Motivated by this, a new power saving mechanism called the sleep-transmit mode (STM) is proposed, which enables the optical network units (ONUs) at customer premises to transmit upstream data during sleep periods without turning on the receiver, thus conserving energy and improving upstream transmission delay. These advantages are achieved by pre-allocating bandwidth for upstream transmission to ONU before it enters the sleep state. Our analysis results show that using STM (1) the power consumption of an ONU is reduced by up to 29% compared to interrupting the sleep mode, and (2) the average additional upstream packet delay due to sleep mode is reduced by up to 10-fold compared to transmitting the upstream packets only after the sleep duration expires. Keywords - XGPON; cyclic-sleep mode; sleep-transmit mode; power saving mechanisms; energy efficiency; upstream packet delay

I.

INTRODUCTION

In conventional optical access networks, the optical network units (ONUs) at the customer premises are constantly powered on and consume a significant amount of power even during idle periods. Power saving mechanisms based on sleep modes are proposed in the International Telecommunication Union (ITU) standards [1, 2] to improve the energy efficiency of a passive optical network (PON) based on GPON/XGPON. Meanwhile, consumer traffic has changed greatly since triple play services emerged with the rapid growth in the upstream-hungry services, such as picture and video storage in the cloud, social media, and video chatting. The forthcoming era of Internet-of-things (IoT) will further boost the demand of upstream traffic from surveillance cameras, smart grids, wireless sensor networks, etc. Hence both power consumption and upstream quality of service need to be seriously considered when designing the sleep mode mechanism for next generation energy-efficient optical access networks. However, the sleep mode mechanisms proposed in recent literature [3-7] show an unavoidable trade-off between the power saving and the delay performance. The performance of the sleep mode can be improved by shortening the wake-up overhead [5, 8]. However, these approaches require new circuits and architectures that are not feasible for the already deployed PON system. In this work, we propose a new sleep mechanism called the sleep-transmit mode (STM), which

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allows ONU to transmit upstream packets during the sleep period by adding a Transmit state to the currently employed mechanism in PON standards. STM aims to optimize both power saving and transmission delay as applicable to upstream packets. The ONU state transition and timing diagram for cyclicsleep mode in XGPON (G.987.3) [1, 2] are shown in Fig. 1. In the Asleep state, both the receiver and transmitter of the ONU are turned off, and in all the remaining states, they are both turned on. Thus, sleep mode mechanisms generally require buffering of downstream packets at the OLT and of upstream packets at the ONU while the ONU is in the Asleep state. Also, upstream transmissions are not possible in the Asleep state. Suppose some upstream traffic arrives during Tsleep, there are two ways for transmitting upstream traffic when cyclic-sleep mode is employed: i) The ONU waits for Tsleep to expire after which it can send the upstream packet(s) during the ActiveHeld state; or ii) The ONU immediately activates a local wake-up indication (LWI) and transitions to the ActiveHeld state where it can send upstream packet(s).

Fig. 1: (a) State transition and (b) timing diagram of XGPON cyclic-sleep mode.

Both of these options suffer from drawbacks. With option 1, the upstream traffic faces an unnecessary delay due to the wait for the expiration of Tsleep. In order to guarantee the QoS for upstream traffic, the value for Tsleep needs to be set quite conservatively which then reduces the power savings that is actually realized. With option 2, the unnecessary upstream delay in waiting for Tsleep to expire is eliminated. However, since the ONU interrupts the sleep mode, it spends more time

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in the high power ActiveHeld, ActiveFree and SleepAware(0) states while waiting for the re-enabling of the low power mode. Thus, this option results in reduced power savings. In contrast, STM is specifically designed to address the above issues. The power saving and upstream delay performance can be improved by enabling upstream packet transmission during the low power Asleep state as shown in Fig. 2. This is achieved by pre-allocating bandwidth for upstream transmission before an ONU enters the low power state. The rest of the paper is structured as follows: Section II describes our proposed STM mechanism and its operation. Section III provides mathematical models for performance analysis and Section IV evaluates the power saving and delay performance of the proposed mechanism. 6$2)) RU):,RU/:,65$ZDNH

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downstream packet is observed for a certain period of time (Thold), the OLT sends the Sleep_Allow (SA(ON)) message together with the PBW information (Tstm and Ttransmit) allowing the ONU to enter into the ActiveFree state. The ONU acknowledges this by sending a STM Sleep_Request (SR(STM)) message back to the OLT and immediately transitions to the SleepAware state. The ONU stays in the SleepAware state for a period of time (Taware). It enters into the Asleep state if there is no downstream or upstream traffic. Otherwise, the SleepAware period is interrupted by a forced wakeup indication (FWI) or local wakeup indication (LWI) and the ONU returns back to the ActiveHeld state. The main novelty of this proposed mechanism is to allow the ONU to initiate one or multiple upstream transmissions by transitioning to the Transmit state (i.e., Ttransmit) during the Tstm. By transitioning between the Transmit and Asleep state, this low power period Tstm is not interrupted. After a number of (Tsleep + Ttransmit) cycles, Tstm expires and the STM ONU returns to the SleepAware state just as in G.987.3.

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Fig. 2: (a) State transition, (b) timing diagram, and (c) example of the operation of sleep-transmit mode (STM).

II.

PROPOSED SLEEP-TRANSMIT MODE MECHANISM

Sleep-transmit mode (STM) adds a new state called the Transmit state (see Fig. 2(a)) to the sleep mode state transition diagram defined by G.987.3. In this Transmit state, the ONU has the transmitter turned on, but the receiver remains off. The timing diagram and the operational scheme of STM are shown in Fig. 2(b) and 2(c), respectively. The OLT operation includes pre-allocating an appropriate amount of upstream bandwidth to ONUs before the ONU transitions to the Asleep state. The pre-allocated bandwidth (PBW) information consists of one or multiple timeslots (Ttransmit) when the ONU can transmit an upstream burst of data during the STM period (Tstm) and the Asleep state interval (Tsleep) between two timeslots. On the ONU side, before transitioning to the Asleep state, it acquires the PBW information and it’s available for use by the ONU for any upstream transmission during the low power period (Tstm). The detailed operation is as follows. In the ActiveHeld state, if no

Fig. 3: Flowchart of sleep-transmit mode (STM)

If there are multiple ONUs operating in different states with different Tstm, the PBW information of each ONU is calculated by considering the status of other ONUs. Hence, the notion of a polling cycle (with period of Tp) is necessary. The OLT periodically checks and updates the status of ONUs during each Tp. Based upon the status of each ONU, the OLT updates each ONU with different PBW information. This is summarized in the flowchart shown in Fig. 3. The PBW is conveyed to the ONU in a common downstream frame transmitted by the OLT containing a pre-allocated bandwidth map field, which is referred as PBWmap following ITU-T Gigabit PONs. The allocation of Ttransmit to the ONUs during the Tstm is accomplished using a bandwidth allocation algorithm similar to traditional DBA [8], but the definition of

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such algorithm is beyond the scope of this paper. Another challenge is the synchronization of upstream transmissions during the Asleep state. At the instant the ONU transitions to the Asleep state during Tstm, it is in fact synchronized with the OLT. The ONU continues to maintain its clock and uses the same time reference to initiate an upstream transmission when transitioning from the Asleep state to the Transmit state. However, a problem occurs in which the ONU clock may have drifted with respect to the OLT clock during Tstm. A typical crystal has a frequency drift deviation of smaller than 100ppm [9] which translates to 1μs of clock drift in 10ms duration of Tstm. Since this drift is usually very small, the OLT can take it into account by adding a guard interval before or after Ttransmit in the PBW to avoid upstream traffic collision among ONUs.

A. Non-interrupted sleep mode As shown in Fig. 1, the OLT buffers the downstream traffic for each ONU and only transmits the buffered-packets during a pre-determined activity slot [3]. The upstream packets are buffered and are sent during a specific timeslot after the ONU is out of the sleep duration. As shown in Fig. 4(a), in a conventional fixed cyclic-sleep mode, the ONU will not be interrupted during the sleep duration (Tsleep_a). During the SleepAware state, the ONU wakes up to check the traffic condition and control messages from the OLT. The ONU enters the ActiveHeld state by either receiving the FWI from the OLT for any downstream packet or triggering the LWI locally for any upstream packet. The ONU goes back to the Asleep state if there is no downstream or upstream packet. Otherwise, it returns to the ActiveHeld state and starts to receive/transmit the packets. Since the time used to send/receive the wake-up indication is almost negligible, the time spent in the Taware is mainly constrained by the wake up overhead (Twake), i.e., synchronizing the downstream data. Twake ranges from 0.6 ms to 14 ms [10]. In our analytical model, we assume Taware=Twake and the first SleepAware period Taware(0) together with Thold is the triggering time (Ttrigger= Thold +Taware(0)). We assume that the packet arrivals in both downstream and upstream are independent Poisson processes with rates Ȝds and Ȝus respectively, resulting in an overall packet arrival rate (Ȝ) of Ȝds+Ȝus. If only upstream traffic is considered, the probability of no packet arriving is given as exp(-ȜusTtrigger) during Ttrigger and exp(-Ȝus(Tsleep_a+Taware)) during a sleep cycle. In other words, the probability (pro(n)) of having at least one packet arrived at the n-th sleep cycle while no packet arrived during the previous (n-1) sleep cycle(s) is given as [11] ­1− e−λusTtrigger n=0 ° , (1) pronism (n) = ® −λus (Tsleep _ a +Taware ) º ª n ≥1 ° pron−1 ⋅ «1− e ¬ ¼» ¯ where pron-1 is the probability of no packet arrived in previous −λ T

Fig. 4: Timing diagrams for the upstream transmission in (a) non-interrupted sleep mode, which waits for Tsleep expires before transmission, (b) interrupted sleep mode, which wakes up immediately by LWI, and (c) sleep-transmit mode (STM).

III.

PERFORMANCE ANALYSIS

In order to evaluate the impact of power consumption and upstream packet delay in STM, we compare three different sleep mode mechanisms, as depicted in Fig. 4, for upstream transmission. We compare the proposed STM mechanism against the two options mentioned in Section I. Fig. 4 conceptually illustrates the advantages of the proposed STM in both upstream packet delay and power saving performance.

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sleep _ a . cycles, which is given as e us trigger ⋅ e The average power consumption of an ONU under noninterrupted sleep mode mechanism (Pnism) is listed in Table I, where Pactive is the power consumption of transmit/receive packets during the ActiveHeld state, Pidle denotes the idle power consumption during Ttrigger in the ActiveHeld state and the first SleepAware(0) state, Psleep is the power consumption of the Asleep state, and Pcycle is the total power consumption during one sleep cycle, which is given as Tsleep·Psleep+ Taware·Pidle. T(0) represents the expected arrival time of the first packet that arrives during the idle period Ttrigger. Based on [12], T(0) is given as

T (0) =

1

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(2) ⋅ Ttrigger . −λ T 1 − e us trigger TT denotes the packet transmission time, i.e., the time for an ONU to finish transmitting backlogged packets. For n=0, as long as one packet arrived, the ONU is interrupted from Ttrigger

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IEEE ICC 2014 - Selected Areas in Communications Symposium

and transitions to (or remains in) the ActiveHeld state. During the packet transmitting time (TT(0)), the new packets arriving during this period should also be considered. If the ONU is able to enter into the Asleep state (n•1), TT becomes a function of the upstream buffered backlog of the ONUs and its link access rate. Only those packets arrived during the last sleep cycle (Ȝus(Tsleep_a+Taware)) should be considered. If the packet forwarding rate of the ONU, denoted as ȝ, is assumed to be the same as its link access rate divided by the packet size, TT for non-interrupted sleep mode (TT_nism) is given as 1 ­ n=0 °(1 + λusTrrt ) μ − λ us us ° . (3) TT _ nism (n) = ® ª λus (Tsleep _ a + Taware ) º 1 °« n ≥1 + λusTrrt » ⋅ − λus (Tsleep _ a +Taware ) °« − λus μ » us 1 − e ¼ ¯¬ The average additional upstream packet delay is the average delay occurs when an upstream packet arrives at the ONU during the sleep period before a transmission opportunity becomes available. The average additional upstream packet delay shown in Fig. 4(a) for non-interrupted sleep mode (Dnism) is depicted in Table I. It is noted that the delay due to the queuing and the propagation delay during the active state are not accounted in the delay calculation. Hence Dnism is

considered as 0 when n=0. Although Fig. 4(a) appears to have a low average power consumption since it does not interrupt the Asleep state (Tsleep_a), the upstream traffic experience a large additional packet delay in waiting for the Asleep state (Twait_a) and the SleepAware state (Taware) to expire, and then transitions to the ActiveHeld state to initiate the upstream bandwidth allocation. This allocation includes at least one round-trip-time (Trtt). In the model, it is considered as 200 μs for network reach of 20 km [2]. B. Interrrupted sleep mode In Fig. 4(b), any upstream arrival immediately activates the LWI, which transitions the ONU to the ActiveHeld state with only a small overhead. The overhead includes the wakeup overhead for the transceiver (Twake) to recovery from the Asleep state and a small amount of time for the ONU waiting to send the Sleep_Request (SR(Awake)) back to OLT. After transitioning to the ActiveHeld state, the ONU spends Trtt waiting for the upstream bandwidth allocation. This allows the upstream delay to be reduced as compared to the case of noninterrupted sleep mode as shown in Fig. 4(a). However, since the low power mode is interrupted, this option leads to a higher opportunity of entering higher power consumption states (ActiveHeld, ActiveFree, and SleepAware

Table I: The power and delay analysis of the power saving options shown in Fig. 4 ONU power consumption Non-interrupted sleep mode (fixed cyclic-sleep mode) ª Ttrigger Pidle + nPcycle + Trtt Pidle + TT _ nism (n) Pactive º ª (T (0) + Trtt ) Pidle + TT _ nism (0) Pactive º ∞ » Pnism = pronism (0) « » + ¦ pronism (n) « T (0) + Trtt + TT _ nism (0) «¬ Ttrigger + n (Tsleep _ a + Taware ) + Trtt + TT _ nism (n) »¼ «¬ »¼ n=1 Interrupted sleep mode ªT ª (T (0) + Trtt ) Pidle + TT _ ism (0) ⋅ Pactive º ∞ P + (n − 1) Pcycle + T (n)′Psleep + (Twake + Trtt ) Pidle + TT′_ ism (n) Pactive º ′ (n) « trigger idle » Pism = proism (0) « » + ¦ proism T (0) + Trtt + TT _ ism (0) Ttrigger + (n − 1) (Tsleep _ a + Taware ) + T (n)′ + Twake + Trtt + TT′_ ism (n) «¬ »¼ «¬ »¼ n =1 ∞ ªT P + nPcycle + Trtt Pidle + TT′′_ ism (n) ⋅ Pactive º ′′ (n) « trigger idle » + ¦ proism «¬ Ttrigger + n (Tsleep _ a + Taware ) + Trtt + TT′′_ ism (n) »¼ n =1 Sleep-transmit mode ª Ttrigger Pidle + (n − 1) Pc + (Tstm − TT _ stm (n) ) Psleep + TT _ stm (n) Ptransmit + Taware Pidle º ª (T (0) + Trtt ) Pidle + TT _ stm (0) Pactive º ∞ » Pstm = prostm (0) « » + ¦ prostm (n) « T (0) + Trtt + TT _ stm (0) Ttrigger + n (Tstm + Taware ) «¬ »¼ ¬« ¼» n=1 Average additional upstream packet delay Non-interrupted sleep mode (fixed cyclic-sleep mode) ∞ § Tsleep _ a + Taware · Dnism = ¦ pronism (n) ⋅ ¨ + Trtt ¸ 2 n =1 © ¹ Interrupted sleep mode ∞ ∞ §T · ′ (n) ⋅ (Twake + Trtt ) + ¦ proism ′′ (n) ⋅ ¨ wake + Trtt ¸ Dism = ¦ proism 2 © ¹ n =1 n =1 Sleep-transmit mode ∞ § Tsleep _ c · Dstm = ¦ prostm (n) ⋅ ¨ ¸ n=1 © 2 ¹

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.(5)

n ≥ 1 during Taware

It is noted that Twake = Taware in the last term of our model. Since Twake cannot be interrupted by an upstream arrival, a complete Pcycle has to be accounted if the upstream packets arrived during Twake. The expected arrival time of the first packet that arrived during these three states (T) are given as T (n) = T ( n )′ =

1

λus 1

λus

−

e

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−

e

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C. Sleep-Transmit Mode (STM) In comparison, STM (as shown in Fig. 4(c)) enables a reduction of upstream delay. The average additional upstream packet delay for the STM (Dstm) is listed in Table I. The upstream bandwidth pre-allocation eliminates the need for downstream data recovery during Tstm. Hence the wakeup overhead for the transmitter to transition from the Asleep state to Transmit state is with a value of less than a microsecond, which mainly depends on the laser ON/OFF time [8, 13]. Thus, the transmitter of the ONU can potentially be turned ON/OFF as required during any of the states. Twait_c shown in the Fig. 4(c) is the time that the ONU waits to enter the Transmit state while it is still in the Asleep state. On average, for a fixed Tsleep_c, Twait_c is equal to Tsleep_c/2 and the maximum length of each Tsleep_c during the STM period (Tstm) can be approximately given as T −T ( n) , (7) Tsleep _ c = stm T _ stm N transmit + 1 where Ntransmit is the number of Ttransmit state within a STM duration (Tstm). In order to compare with the interrupted sleep mode as shown in Fig. 4(b), a reasonable value of Ntransmit is chosen to provide a similar upstream delay Dstm with respect to Dism, where Twait_c is approximately equal to Twake+Trtt. TT_stm

NUMERICAL RESULTS

Fig. 5 illustrates the power consumption (in %) relative to the full power consumption and the average additional upstream packet delay of an ONU with the non-interrupted sleep mode and with the proposed STM. The parameters used in analytical models are listed in Table II.

n=0

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during each state is given as 1 ­ n=0 °(1 + λusTrtt ) μ − λ us us ° . (8) TT _ stm (n) = ® 1 °ª λus (Tstm + Taware ) º ⋅ n ≥1 °«¬1 − e−λus (Tstm +Taware ) »¼ μus − λus ¯ Since the low power period (Tstm) is not interrupted, the power saving is much higher compared to the interrupted sleep mode. The power consumption of an ONU with STM (Pstm) is listed in Table I, where Pc is the total power consumption of a Tstm cycle without any upstream transmission, which is given as Tstm·Psleep+Taware·Pidle. Ptransmit is the power consumption in the Transmit state. During this state, only the transmitter is turned ON at the ONU.

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states) for a certain upstream traffic load. The analytical models for the average power consumption (Pism) and the average additional upstream packet delay (Dism) of an ONU under interrupted sleep mode mechanism are listed in Table I, respectively. The probability of an upstream packet arrived during Ttrigger, Tsleep_a, and Taware, (proism) are given as

Fig. 5: Power savings and average additional upstream packet delay performances of the ONU with non-interrupted sleep mode and with sleeptransmit mode (STM).

Table II: Key parameters for performance analysis Parameter Value Parameter Value Ttrigger 2ms Trtt 0.2ms Taware 1ms Nstm 10 Tsleep_a 20ms Psleep 23%×Pactive Tstm 20ms Pidle 74%×Pactive Twake 1ms Ptransmit 60%×Pactive Packet size ȝus 2.5Gbps 1518B The ONU with the proposed STM demonstrates slightly lower power consumption compared to the non-interrupted sleep mode. For the presence of any upstream packet, the ONU with the non-interrupted sleep mode is required to transition to the ActiveHeld state before it re-enters into the low power state. Transitioning into the ActiveHeld state also incurs additional power consumption consumed during Trtt and Ttrigger. In contrast, once the ONU with STM transitions into the low power period (Tstm), it does not require to return to the ActiveHeld state to transmit upstream packets. Hence, at the low traffic condition, the ONU can always remain in Tstm to conserve power.

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In terms of the additional upstream delay performance, the non-interrupted sleep mode suffers from a large delay at low traffic due to the fact that the ONU needs to wait for the expiry of the sleep duration. As shown in Fig. 5, with the same sleep duration, the ONU with STM can attain up to 10 fold of upstream packet delay reduction (from 10.6 ms to 1 ms) compared to non-interrupted sleep mode at low upstream traffic. In order to guarantee the QoS for upstream delay, the sleep duration (Tsleep_a) of the non-interrupted sleep mode would need to be set conservatively. As shown in Fig. 5, shortening Tsleep_a improves the delay performance. However, it also results in a significant power consumption increase (15%) at low traffic load. Fig. 6 shows the power saving and the average additional upstream packet delay of an ONU with the interrupted sleep mode and an ONU with the proposed STM. The ONU with the interrupted sleep mode shows relatively poorer power saving performance as expected. The proposed STM also demonstrates its advantage in providing almost the same upstream packet delay compared to the interrupted sleep mode.

V.

CONCLUSIONS

With the rapid growth of upstream traffic from user devices in passive optical networks, it is important to emphasize the upstream performance (i.e., power consumption and delay) when sleep mode mechanisms are employed at the ONUs.We propose a new sleep mechanism called sleep-transmit mode (STM) that provides a solution addressing this concern by adding a Transmit state to the currently employed mechanism in PON standards. This idea enables the ONU to transmit its upstream packets without interrupting the low power state. Compared to non-interrupted fixed-cycle sleep mode, the STM demonstrates a similar power saving performance while at the same time provides up to a 10-fold upstream packet delay reduction. When compared to interrupted sleep mode, which provides the lowest upstream packet delay, STM achieves up to 29% power reduction while also providing comparable delay performance for upstream packets. Moreover, STM is compatible with and can be integrated into current PON standards such as XGPON. ACKNOWLEDGEMENTS This work is supported by Alcatel-Lucent Bell Labs, Victoria State Government of Australia, and the Seoul Metropolitan Government R&BD Program WR080951. REFERENCES [1]

[2] [3]

Fig. 6: Power savings and average additional upstream packet delay performances of the ONU with interrupted sleep mode and with sleeptransmit mode (STM).

[4] [5]

[6] [7]

[8] Fig. 7: Power saving performance of an ONU with sleep-transmit mode (STM) with shortened Ttrigger.

[9]

One possible way to enhance the power saving performance of the ONU with STM is to shorten the triggering period (Ttrigger=Thold +Taware(0)). Since the ONU with STM is able to transmit upstream packet during the STM period, Ttrigger could be set as short as possible without affecting the upstream delay performance. Fig. 6 and Fig. 7 show the power saving comparison of the ONU with STM and with interrupted sleep mode for Ttrigger of 2ms and 0.1ms, respectively. The power saving is improved from 11% to 29% by shortening Ttrigger.

[10] [11]

[12]

[13]

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ITU-T, "GPON power conservation," in Series G Supplement 45: Transmission systems and media, digital systems and networks, ed, 2009. ITU-T, "Transmission convergence (TC) specifications," in G.987.3: 10-Gigabit-capable passive optical networks (XG-PON), ed, 2010. A. Dixit, et al., "ONU power saving modes in next generation optical access networks: progress, efficiency and challenges," Opt. Express, vol. 20, pp. B52-B63, 2012. S. Lei, et al., "Energy-efficient PON with sleep-mode ONU: progress, challenges, and solutions," IEEE Network, vol. 26, pp. 36-41, 2012. W. Shing-Wa, et al., "Sleep Mode for Energy Saving PONs: Advantages and Drawbacks," in GLOBECOM Workshops, 2009 IEEE, 2009, pp. 1-6. A. R. Dhaini, et al., "Toward green next-generation passive optical networks," IEEE Communications Magazine, vol. 49, pp. 94-101, 2011. J. Zhang and N. Ansari, "Toward energy-efficient 1G-EPON and 10GEPON with sleep-aware MAC control and scheduling," IEEE Communications Magazine, vol. 49, pp. s33-s38, 2011. B. Skubic, et al., "A comparison of dynamic bandwidth allocation for EPON, GPON, and next-generation TDM PON," IEEE Communications Magazine, vol. 47, pp. S40-S48, 2009. Improving the Accuracy of a Crystal Oscillator [Online]. Available: www.semtech.com L. Valcarenghi, et al., "Energy efficiency in passive optical networks: where, when, and how?," IEEE Network, vol. 26, pp. 61-68, 2012. N. Dinh and A. Walid, "Power saving protocol for 10G- EPON systems: A proposal and performance evaluations," in Proc. IEEE Global Communications Conference (GLOBECOM) 2012, pp. 3135-3140, 3-7 Dec. 2012. N. P. Anthapadmanabhan et al., "Analysis of a probing-based cyclic sleep mechanism for passive optical networks," in Proc. IEEE Global Communications Conference (GLOBECOM) 2013, 9-13 Dec. 2013. A. G. Weber, et al., "Measurement and simulation of the turn-on delay time jitter in gain-switched semiconductor lasers," IEEE Journal of Quantum Electronics, vol. 28, pp. 441 - 446, 1992.