Energy-efficiency of optical network units with vertical-cavity surface-emitting lasers

doi:10.1364/OE.20.014960

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Energy-efficiency of optical network units with vertical-cavity surface-emitting lasers

Elaine Wong, Michael Mueller, Maluge P. I. Dias, Chien Aun Chan, and Markus C. Amann »View Author Affiliations

Optics Express, Vol. 20, Issue 14, pp. 14960-14970 (2012)
http://dx.doi.org/10.1364/OE.20.014960

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▸ Article Outline
– Abstract
1. Introduction
2. Characterization of VCSEL transmitter block
3. Power consumption for Video-on-Demand (VoD) delivery over 10G-EPON
4. Energy savings of VCSEL-ONUs in doze and sleep mode operations
5. Summary
– References and links

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Abstract

The energy savings of 10 Gbps vertical-cavity surface-emitting lasers (VCSELs) for use in energy-efficient optical network units (ONUs) is critically examined in this work. We experimentally characterize and analytically show that the fast settling time and low power consumption during active and power-saving modes allow the VCSEL-ONU to achieve significant energy savings over the distributed feedback laser (DFB) based ONU. The power consumption per customer using VCSEL-ONUs and DFB-ONUs, is compared through an illustrative example of 10G-EPON for Video-on-Demand delivery. Using energy consumption models and numerical analyses in sleep and doze mode operations, we present an impact study of network and protocol parameters, e.g. polling cycle time, network load, and upstream access scheme used, on the achievable energy savings of VCSEL-ONUs over DFB-ONUs. Guidance on the specific power-saving mode to maximum energy savings throughout the day, is also presented.

1. Introduction

The exponential growth in global Internet traffic is currently being fuelled by the combination of increasing customer demand for bandwidth-intensive services, and the penetration of fiber networks into the access network segment. As the Internet expands, its energy consumption will not only exacerbate the global carbon footprint but will also significantly contribute to the operating expenditure of the network operator. Energy modeling studies of the Internet have highlighted that in the short-to-medium time frame, energy consumption is overwhelmingly dominated by the access network, particularly by the optical network unit (ONU) at each subscriber premises [1

J. Baliga, R. Ayre, K. Hinton, W. V. Sorin, and R. Tucker, “Energy Consumption in Optical IP Networks,” J. Lightwave Technol. 27(13), 2391–2403 (2009). [CrossRef]

]. To reduce energy consumption, efforts in developing optical transceivers and electronic circuits of low power consumption are underway. The IEEE 802.3az [2

IEEE. Std, 802.3az–2010, [Online]. Available: http://standards.ieee.org/getieee802/download/802.3az-2010.pdf

] and ITU-T G.sup 45 [3

“GPON power conservation,” ITU-T G-series Recommendations –Supplement 45 (G.sup-45), 05/2009.

] standards have specified idle mode operation for power savings. In an ONU, both the User Network Interface through power shedding operation, and the PON interface through doze and sleep mode operations, can be powered down.

Under doze mode operation, the ONU transmitter (TX) block which comprises the laser and driver, is powered down when no upstream traffic is to be transmitted, and under sleep mode operation, both TX and receiver (RX) blocks are powered down when no upstream or downstream traffic is observed. The RX block comprises the photodetector, front-end circuit, and back-end circuit. Out of the two modes, sleep mode is considered to be the more energy-efficient mode due to the powering down of both TX and RX blocks. However, sleep mode incurs an overhead T_rec of 2 ms for clock recovery and synchronization [4

J. Mandin, “EPON Powersaving via Sleep Mode,” IEEE P802.3av 10GEPON Task Force Meeting (2008).

]. This overhead is attributed to the time taken for the ONU to resynchronize to the central office (CO) clock after transitioning from sleep mode to active mode. Under heavy traffic conditions, a long T_rec prevents an ONU from transitioning into sleep mode. In contrast, doze mode operation eliminates the overhead T_rec through keeping the receiver and front-end circuitry continuously powered up. Instead, a small overhead of typically 760 ns [5

E. Igawa, M. Nogami, and J. Nakagawa, “Symmetric 10G-EPON ONU BMT Employing Dynamic Power Save Control Circuit,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, NTuD5, (2011).

] corresponding to the settling time T_sett of the TX block exists. Doze mode operation yields smaller energy savings as compared to sleep mode since only the TX block is powered down during dozing intervals.

In order to maximize energy-efficiency in an ONU, (a) the power consumed in active and power-saving modes should be minimized, and (b) the duration in which the ONU is in power-saving mode should be maximized. In this work, we address these critical points whilst maintaining the cost-efficiency of access networks through the use of 10 Gbps vertical-cavity surface-emitting lasers (VCSELs) as laser transmitters of next-generation energy-efficient ONUs. We critically investigate the energy savings arising from using 10 Gbps VCSEL-ONUs under active, doze, and sleep mode operations. To clarify, an ONU that operates in doze mode can transition between dozing (i.e. only RX is powered-up) and active intervals (i.e. both TX and RX are powered up). Likewise, an ONU that operates in sleep mode can transition between sleep (i.e. both TX and RX are powered down) and active intervals. In contrast, an active and always-on ONU has both its TX and RX continuously powered up.”

We show that in active mode, the power consumption of the VCSEL TX block is smaller than that of a DFB laser by an order of magnitude. Further, we demonstrate that the VCSEL-ONU considered in our work is able to transition between active and dozing with a fast T_sett of 330 ns, thereby maximizing the dozing duration. We also study the interdependency between network and protocol parameters, e.g. polling cycle time T_CYC, network load, and upstream access scheme, and the maximum achievable energy savings of VCSEL-ONUs. We show that the polling cycle time is not only an important protocol parameter that influences delay, jitter, and upstream utilization, but also the energy-efficiency of an ONU. When using an upstream scheme based on static time division multiple access (TDMA), we show through numerical analysis that energy savings is maximized under light network loading levels and long T_CYC. When using a dynamic bandwidth allocation (DBA) upstream access scheme, we show that energy savings is maximized under heavy network loading levels and long T_CYC. Through the observations of our work that is presented in this paper, we provide guidance on the specific power-saving mode to use in order to maximize energy savings. We highlight that the ability to switch between doze and sleep modes in a VCSEL-ONU provides the flexibility to maximize energy savings throughout the day irrespective of the network loading level, polling cycle and upstream access scheme.

2. Characterization of VCSEL transmitter block

Figure 1(a) shows the TX block of our proposed VCSEL-ONU, comprising a high-speed 1340 nm buried-tunnel-junction (BTJ) VCSEL and a 11.3 Gbps VCSEL driver. The design and characteristics of the 1340 nm BTJ VCSEL used in this work was previously reported in [6

W. Hofmann, M. Mueller, G. Bohm, M. Ortsiefer, and M. C. Amann, “1.55um VCSEL with enhanced modulation bandwidth and temperature range,” IEEE Photon. Technol. Lett. 21(13), 923–925 (2009). [CrossRef]

A. Gatto, A. Boletti, P. Boffi, C. Neumeyr, M. Ortsiefer, E. Rönneberg, and M. Martinelli, “1.3 µm VCSEL transmission performance up to 12.5 Gbps for metro access networks,” IEEE Photon. Technol. Lett. 21(12), 778–780 (2009). [CrossRef]

]. The threshold current of the BTJ-VCSEL used in our experiments was measured to be 2.5 mA. For a bias condition of 11.5 mA, the overall power consumption was measured to be 20 mW. Since typical DFB-laser thresholds at room-temperature are ~10 mA which coincide with typical bias-currents of VCSELs for which output-powers are in the range of 3 to 4 mW, the VCSEL is the laser of choice for upstream PON transmission when considering the power efficiency at these optical power levels.

Fig. 1 (a) 10 Gbps VCSEL transmitter (TX) block. (b) Measurement of T_sett of the VCSEL TX block: Oscilloscope traces of doze mode control from ON to OFF, and the resulting VCSEL output. (c) Oscilloscope traces of doze mode control from OFF to ON, and the resulting VCSEL output.

Another important point to note is that temperature stabilization is not required for the operation of our VCSEL. This is due to the use of an integrated heat sink which provides excellent heat management. A summary of the power consumption of the VCSEL-ONU under active, doze and sleep mode operations is listed in Table 1 . Note that the values listed include only the power consumption of the PON interface but not that of the User Network Interface [8

E. Wong, M. Mueller, P. I. Dias, C. A. Chan, and M. C. Amann, “Energy Saving Strategies for VCSEL ONUs”, Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, OTu1H5 (2012).

]. In active mode, the 10 Gbps VCSEL-ONU consumes a total of 3.984 W [9

Analog Devices ADN2530 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2530.pdf

,10

S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).

]. A comparable 10 Gbps DFB-ONU consumes 5.052 W [10

S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).

–12

Analog Devices ADN2531 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2531.pdf

]. The power consumed by the 10 Gbps VCSEL TX block was calculated to be 0.134 W [9

Analog Devices ADN2530 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2530.pdf

,10

S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).

]. In comparison, a 10 Gbps DFB TX consumes 1.202 W of power [10

S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).

,11

NEL Laser Diodes, [Online]. Available: http://www.nttelectronics.com/en/products/photonics/pdf/NLK5B5EBKA.pdf

]. The RX block for both types of ONUs was intentionally chosen to be identical to highlight the energy saving benefits of the VCSEL TX. The difference in power consumption between both types of ONU in active mode therefore lies in the power consumed by their respective TX block.

Table 1 Summary of Power Consumption and Transition Times of 10 Gbps VCSEL-ONU

Mode	TX block (W)	RX block (W)	P_ONU (W)	T_sett (µs)	T_rec (µs)
Active	0.134	3.85 [10 S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009). ]	3.984	0	0
Doze	0	3.85 [10 S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009). ]	P_doze = 3.85	0.33	0
Sleep	0	0.75 [10 S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009). ]	P_sleep = 0.75	0.33	2 [4 J. Mandin, “EPON Powersaving via Sleep Mode,” IEEE P802.3av 10GEPON Task Force Meeting (2008). ]

Also listed in Table 1 are the setting time, T_sett and recovery time, T_rec of the VCSEL-ONU considered in our work. The total T_sett of 330 ns of the VCSEL-ONU was measured experimentally by summing up the individual transition times of the TX block between active and doze modes and vice versa. This is shown in Figs. 1(b) and 1(c) which compares the oscilloscope traces of the doze mode control and the resulting VCSEL output. A ‘0’ to ‘1’ transition of the doze mode control powers down the TX block where as a ‘1’ to ‘0’ transition powers up the TX block. The VCSEL is inherently well-matched to 50 Ohm circuits as its differential series resistance is typically in that range [13

M. Mueller, C. Grasse, K. Saller, T. Gruendl, G. Boehm, and M. C. Amann, “1.3 μm High-Power Short-Cavity VCSELs for High-Speed Applications”, CLEO/QELS, San Jose, USA, CW3N.2, (2012).

]. Therefore, its connection to well-designed, commercially available 50 Ohm driving circuits (such as that used in this work) results with low settling times.

Plots of the experimental bit-error-ratio (BER) measurements of a free-running and uncooled VCSEL-ONU are shown in Fig. 2 . The VCSEL-ONU is directly-modulated with 10 Gbps non-return-to-zero data (PRBS 2²³-1 pattern length) for back-to-back and 20 km single mode fiber (SMF) transmissions. The optical spectrum with a full width half maximum of 0.06 nm is shown inset. Considering that forward error correction (FEC) is mandatory as specified by the 10G-EPON standard [14

IEEE. Std, 802.3av – 2009, [Online]. Available: “http://standards.ieee.org/getieee802/download/802.3av-2009.pdf

] and in combination with the use of an avalanche photodiode at the OLT, results in Fig. 2 indicate that the VCSEL-ONU is able to support channel insertion losses specified in the standard, namely 20 dB (PR10) and 24 dB (PR20). FEC is necessary in high bit-rate links to compensate for the decrease in optical sensitivity of 10 Gbps receivers and to meet the standardized link budgets.

Fig. 2 Bit-error-ratio (BER) measurements of free-running and uncooled 10 Gbps VCSEL for back-to-back (B2B) and 20 km single mode fiber (SMF) transmission. Inset: Optical spectrum of laser output.

3. Power consumption for Video-on-Demand (VoD) delivery over 10G-EPON

The power consumption per customer for VoD downstream delivery over a 10G-EPON using VCSEL-ONUs and DFB-ONUs is compared. In our work, we consider distributed storage solutions for VoD delivery whereby video storage arrays are located at the CO, as shown in Fig. 3 . Studies of energy-efficient VoD and IPTV architectures have highlighted that distributed storage which are located close to customers minimizes transport energy requirements, while implementing centralized storage away from the customers minimizes storage energy requirements [15

J. Baliga, R. Ayres, K. Hinton, and R. Tucker, “Architectures for Energy-Efficient IPTV Networks,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf, San Diego, USA, ThQ5 (2008).

,16

C. Jayasundara, A. Nirmalathas, E. Wong, and N. Nadarajah, “Energy Efficient Content Distribution for VoD Services,” Proc. of the IEEE/OSA Opt. Fiber Commun. Conf, Los Angeles, USA, OWR3 (2011).

]. Since high downloads of popular content such as new release movies increase transport energy consumption, such content should be widely replicated throughout the network. Further, storing content closer to customers is now more than feasible than before given the ever-decreasing cost of data storage [17

M. Rabinovich and O. Spatscheck, Web Caching and Replication: Addison Wesley (2001).

]. The storage arrays at the CO would only store frequently requested popular content, thus leverage storage energy requirements with its transport energy requirements. Equation (1) describes the power consumption per customer for downstream VoD delivery over the 10G-EPON using data from major equipment vendors as listed in Table 2 .

Fig. 3 Video-on-Demand (VoD) delivery architecture over a 10G-EPON where distributed video storage is located at the central office (CO).

Table 2 Equipment Specifications

Equipment	Power/Capacity
Video server (Nexus 5010, 2 x UCS 6100 Fabric Interconnect, UCS 5108 Blade server chassis)	70 W/Gbps
Storage arrays (EMC vMax SE)	17 pW/bit
OLT line card (Alloptic edge10)	80 W

\begin{array}{l} P_{c u s t o m e r} (W) = \frac{P_{s t o r a g e}}{N} + \frac{P_{s e r v e r}}{N} + \frac{K P_{O L T}}{N} + P_{O N U} \\ = (\frac{V \times M \times 17 p W / b i t}{N}) + (\frac{B \times N_{a c t i v e} \times 70 W / G b p s}{N}) + \frac{K P_{O L T}}{N} + P_{O N U} \end{array}

(1)

In Eq. (1), parameter V = 28.8 Gbit is the HD video size (60 mins in length), M = 1000 is number of movies stored, N is the number of ONUs supported, K is the number of 10 Gbps OLT line cards required to support N ONUs, and B = 0.008 Gbps is the bit rate for a HD video stream, respectively. The terms on the right hand side represent the power contributions arising from the CO storage arrays, CO video server, OLT chassis, and ONU, respectively. Each line card serves a maximum of 32 ONUs and each ONU serves a single customer. The power consumption model described in (1) takes into account redundancy and a utilization of 60% to allow for some reserve capacity for storage and services. In (1), the value of P_ONU is dependent on whether the ONU is in DFB active (P_active = 5.052 W), VCSEL active (P_active = 3.984 W) or VCSEL doze (P_doze = 3.85 W) mode. Sleep mode operation is not considered in this section of the work since downstream delivery of VoD traffic requires that the RX block to be powered up.

The power-saving effectiveness of using VCSEL rather than DFB transmitters in ONUs can be observed in Fig. 4 . Plots of the power consumption per customer for downstream VoD delivery over a 10G-EPON evaluated using (1) for VCSEL and DFB ONUs are compared. The general trend indicates that as the network scales, the power contributions from CO storage arrays, CO video server, and OLT chassis become less significant since these are shared between an increasing number of supported ONUs, N. As validated by the results in Fig. 4, the ONU becomes the dominant contributor of power consumption as the network scales up. At N = 2048, the power consumption per customer in a network that uses active DFB-ONUs is 8.6 W. By comparison, that of network that uses active VCSEL-ONUs is 7.52 W. In doze mode, the power consumed per customer is 7.38 W. The percentage of power savings arising from implementing VCSEL-ONUs over active DFB-ONUs is superimposed in Fig. 4. For N = 2048, a 12.8% of power savings can be achieved through using active VCSEL-ONUs. A further improvement of up to 14% can be achieved when these ONUs transition into doze mode.

Fig. 4 Power consumption per customer and percentage of power savings as a function of supported ONUs.

4. Energy savings of VCSEL-ONUs in doze and sleep mode operations

An impact study of the interdependency between network and protocol parameters such as polling cycle time T_CYC, network load, and upstream access scheme, and the energy savings of VCSEL-ONUs, is presented in this section. In our work, we consider a 10G-EPON that supports up to 32 VCSEL-ONUs. The arrival rate of the Ethernet packets for upstream transmission at each VCSEL-ONU follows the Poisson distribution with exponential arrival times. The Ethernet packet length is chosen to be uniformly distributed between 64 and 1518 bytes, and with an average packet length of 791 bytes. The guard time between consecutive VCSEL-ONU transmissions is 1 µs [18

B. Skubic and D. Hood, “A comparison of DBA for EPON, GPON, and NG TDM PON,” IEEE Commun. Mag. 47, 540–548 (2009).

]. In order for 32 VCSEL-ONUs to efficiently share the upstream bandwidth, we consider two distinct upstream access schemes, namely the static TDMA and DBA upstream access schemes. For the static TDMA case, each VCSEL-ONU is allocated a designated transmission slot within each polling cycle. The polling cycle, T_CYC, defined as the time between consecutive transmissions from the same VCSEL-ONU, has a fixed duration and is a crucial design parameter that impacts the packet delay, jitter, and bandwidth utilization [18

B. Skubic and D. Hood, “A comparison of DBA for EPON, GPON, and NG TDM PON,” IEEE Commun. Mag. 47, 540–548 (2009).

]. The optimum choice of T_CYC is a tradeoff between satisfying the quality-of-service requirement of these different performance parameters.

For the DBA scheme, standardized REPORT and GATE messages are used to dynamically request and allocate bandwidth between the VCSEL-ONUs and the OLT. The REPORT control message is a fixed 64 byte overhead that is transmitted by the ONU once per polling cycle. Upon receiving the REPORT messages from all ONUs, the OLT sends a GATE message to each ONU allocating its bandwidth. In order to avoid upstream collisions between multiple transmitting VCSEL-ONUs, the transmission start-time of each ONU is also provided in its GATE message. Hence for DBA upstream access, depending on the requested bandwidth which in turn reflects the upstream network load, the actual value of T_CYC differs from cycle to cycle. Typically, the length of a polling cycle is proportional to the total amount of demanded upstream bandwidth from all ONUs or equivalently total network load. Some DBA access schemes specify a maximum polling cycle time T_CYCMAX in order to limit the allocated bandwidth per polling cycle to a maximum value. Specifying T_CYCMAX also bounds packet delay to a maximum value. A complete list of network and protocol parameters used in our numerical analyses presented in this section can be found in Table 3 .

Table 3 Network and Protocol Parameters

Parameter	Value
Downstream, R_D, and upstream line-rate, R_U	10 Gbps
Normalized network load, L	0.1 to 1
No of ONUs, N	32
Polling cycle time, T_CYC (static TDMA) Maximum polling cycle time, T_CYCMAX (DBA)	10 μs to 1s
REPORT message transmission time, T_REPORT	0.0512 μs (64 bytes)
Average Ethernet frame size, T_PKT	0.6328 μs (791 bytes)
Guard time between transmission slots, T_G [17 M. Rabinovich and O. Spatscheck, Web Caching and Replication: Addison Wesley (2001). ]	1 μs (1250 bytes)

As discussed in Section 1, the energy-efficiency of a VCSEL-ONU increases with the time spent dozing or sleeping, and the amount of power saved in these modes. The general formula for evaluating the percentage of energy savings η is given by:

η = 100 % \times [1 - (\frac{T_{a c t i v e} P_{a c t i v e} + T_{d o z e ​ / s l e e p} P_{d o z e / s l e e p}}{T_{C Y C} P_{a c t i v e}})]

(2)

In (2), T_active is the time interval within a polling cycle when the VCSEL-ONU is on (i.e. both TX and RX blocks are powered up), T_doze/sleep is the time interval within a polling cycle when the VCSEL-ONU is dozing or sleeping, and P_doze/sleep is the power consumed during the dozing or sleeping. The parameter T_doze/sleep is reduced by the overheads incurred during the polling cycle. These include T_sett in doze mode, T_rec in sleep mode, and T_REPORT in doze and sleep modes.

For static TDMA upstream access, the percentage of energy savings η arising from implementing a VCSEL-ONU in doze or sleep mode is given by:

η = 100 % \times [1 - \frac{(T_{C Y C} . \frac{L}{N} + T_{s e t t l i n g / r e c}) P_{a c t i v e} + (T_{C Y C} - T_{a c t i v e}) P_{d o z e / s l e e p}}{T_{C Y C} P_{a c t i v e}}]

(3)

where L is the aggregate network load and N is the number of supported ONUs in the 10G-EPON. In our work, a normalized network load of 1 represents 10 Gbps upstream traffic that is aggregated from N = 32 ONUs. In (3), note that the overheads T_sett and T_rec result in additional time spent within T_active, and thus reducing T_doze/sleep. For DBA upstream access, the percentage of energy savings η from implementing doze/sleep mode is given by:

η = 100 % \times [\begin{array}{l} 1 - \frac{(T_{C Y C M A X} . \frac{L}{N} + T_{R E P O R T} + T_{s e t t / r e c}) P_{a c t i v e}}{N (T_{C Y C M A X} . \frac{L}{N} + T_{R E P O R T}) P_{a c t i v e}} \\ - \frac{((N - 1) (T_{C Y C M A X} \frac{L}{N} + T_{R E P O R T}) - T_{s e t t / r e c}) P_{d o z e / s l e e p}}{N (T_{C Y C M A X} . \frac{L}{N} + T_{R E P O R T}) P_{a c t i v e}} \end{array}]

(4)

The energy savings from implementing a VCSEL-ONU in doze and sleep modes is summarized in Fig. 5 . Using (3), the energy savings for the case of static TDMA upstream access is evaluated. Results in Fig. 5(a) indicate that a VCSEL-ONU in doze mode is minimally dependent on the network load and value of T_CYC. The variation in energy savings is small ranging from (a) a minimum of 22.9% at maximum network load and minimum T_CYC, i.e. T_doze is minimize, to (b) a maximum of 23.7% at minimum network load and maximum T_CYC, i.e. T_doze is maximized. The small T_sett incurred in doze mode ensures that tangible energy savings can still be achieved at very low values of T_CYC. As for sleep mode operation, results in Fig. 5(b) show zero energy savings for T_CYC < T_rec (~2 ms). For T_CYC > T_rec, energy savings of up to 84.7% at minimum network load and maximum fixed polling cycle can be achieved. Such high energy savings is attributed to a maximized T_sleep and to the large reduction in VCSEL-ONU power consumption when in sleep mode.

Fig. 5 Percentage of energy savings η vs. network load and polling cycle for doze and sleep modes with static TDMA and DBA upstream access.

Using (4), the energy savings for the case of DBA upstream access is evaluated. Results for doze mode operation shown in Fig. 5(c) show energy savings ranging from (a) a minimum of 22.7% when network load and T_CYCMAX are minimum, to (b) a maximum of 23.05% when network load and T_CYCMAX are maximum. These results can be explained as follows. For the case of DBA upstream access, the length of the polling cycle is proportional to the network load. Therefore, a lightly-loaded network would result in a shorter polling cycle. If this polling cycle is less than the specified maximum of T_CYCMAX, then T_doze and hence the percentage of energy savings are minimized. As the network load is increased, the polling cycle is extended, resulting in an increase in T_doze and the percentage of energy savings. Results obtained for sleep mode operation shown in Fig. 5(d) show a similar behavior to that of the doze mode. No energy savings is observed at least until T_CYCMAX > 2 ms. Beyond that, the energy savings of up to 82.3% can be achieved when network load and T_CYCMAX are maximum. Such high energy savings is attributed to a maximized T_sleep and to the large reduction in VCSEL-ONU power consumption when in sleep mode.

Comparing doze mode and sleep mode operations, the latter has always been considered as the more energy-efficient mode due to the powering down of both TX and RX blocks. However, results in Fig. 5 clearly show that depending on the network load, chosen polling cycle time, and implemented upstream access scheme, the energy savings from sleep mode can be lower than that of doze mode. We illustrate this fact by evaluating the energy savings of a 10G-EPON which network load varies throughout the day. The results in Fig. 6 highlight that rather than implementing just sleep mode in VCSEL-ONUs, the flexibility to switch between doze and sleep modes in the device will maximize energy savings throughout the day. Figure 6(a) shows a normalized upstream network load of the PON as a function of time of day (TOD). This daily profile was simulated based on upstream video requests as outlined in [19

Y. Hongliang, Z. Dongdong, Y. Z. Ben, and Z. Weimin, “Understanding user behavior in large-scale video-on-demand systems,” Proc. 1st ACM SIGOPS/EuroSys European Conference on Computer Systems 2006, Leuven, Belgium, 1–12 (2006).

]. The upstream network load is low during early morning but peaks around 1400 hrs and 2100 hrs. To provide a fair comparison with identical maximum bounded transmission delay, T_CYC of the static TDMA upstream access scheme and T_CYCMAX of the DBA upstream access scheme were intentionally fixed to 2.8 ms. For the TDMA upstream access, the polling cycle is a constant 2.8 ms. For DBA upstream access, the polling cycle varies according to the network load, reaching a maximum of 2.8 ms at 2100 hrs as shown in Fig. 6(b).The energy savings from implementing doze, sleep, and active modes in a VCSEL-ONU over an active DFB ONU for the case of static TDMA and DBA upstream access scheme is illustrated in Fig. 6(c) and 6(d) respectively. The blue line in both figures represents the percentage of energy savings from implementing an active VCSEL-ONU over an active DFB, i.e. η = 100% (1- (P_active,VCSEL/P_active,DFB)). Results in Fig. 6(c) show that in order to maximize energy savings throughout the day, sleep mode is favored during the first half of the day whereas doze mode is preferred during the second half. Both sleep and doze modes provide improved energy-efficiency as compared to an always on, active VCSEL-ONU. The energy savings from using a VCSEL-ONU in doze, sleep, and active modes for the DBA upstream access scheme, is illustrated in Fig. 6(d). From an energy conservation viewpoint, doze mode operation provides the highest energy savings with the least variance throughout the day. This is due to the fact that for DBA upstream access, the polling cycle is constantly minimized for all network loading levels. Consequently, at low network loading levels, the polling cycle cycle may be less than T_rec of 2 ms. In this case, the ONU cannot transition into sleep mode to conserve energy and therefore, energy savings is zero. Regardless of the choice of upstream access, our results indicate that using a VCSEL-ONU and more importantly one that can switch between doze and sleep modes will ensure that energy-savings of the network is always maximized for any given network load.

Fig. 6 (a) Normalized network load vs time of day (TOD), (b) Polling cycle vs TOD, (c) Percentage of energy savings from sleep and doze mode operations with static upstream access, and (d) Percentage of energy savings from sleep and doze mode operations with dynamic upstream access.

5. Summary

In this work, we addressed the efforts in reducing the power consumption of next-generation optical transceivers by critically investigating energy-savings arising from using 10 Gbps VCSEL-ONU transmitters. In terms of energy-efficiency, we showed that there are two main benefits in using VCSEL-ONUs, namely a fast setting time and low power consumption during active and power-saving modes. These benefits allow the VCSEL-ONU to maximize its duration in doze and sleep modes and to minimize its power consumption during active, doze, and sleep modes. The percentage of energy savings from using VCSEL-ONUs in doze and sleep modes over active DFB-ONUs was numerically investigated. Our calculations show that in the case of static TDMA upstream access, the energy savings from doze mode is minimally affected by network load and the fixed polling cycle. As for sleep mode, tangible energy savings is observed for polling cycles only beyond the recovery and synchronization overhead time. For DBA upstream access, energy-efficiency is maximized for heavy loads and long maximum polling cycle times. Since the polling cycle for DBA upstream access is dynamic and proportional to the network load, a heavily-loaded network and one with a long maximum polling cycle time will maximize the duration in power-saving mode.

Regardless of static or dynamic upstream access, for the VCSEL-ONU considered in our work, results show that the energy savings from doze mode operation is least affected by both network load and polling cycle. Results also show that sleep mode is not necessarily the most energy-efficient mode. The ability to switch between doze and sleep modes in a VCSEL-ONU thus provides the flexibility to maximize energy savings throughout the day irrespective of the network loading level, polling cycle and upstream access scheme. Finally, it is imperative note that the implementation of power saving modes along with the bandwidth allocation scheme in a network must not compromise its quality-of-service (QoS) performance. Our preliminary results obtained so far via packet level simulations and which we endeavor to report in a future journal publication, show that implementing sleep and doze mode VCSEL-ONUs does not affect the QoS performance of the network.

Acknowledgments

The authors would like to thank VERTILAS GmbH for providing the VCSELs used in our work, and also the Australian Research Council and Go8-DAAD Germany Joint Research Co-Operation Scheme for funding the research.

References and links

1.	J. Baliga, R. Ayre, K. Hinton, W. V. Sorin, and R. Tucker, “Energy Consumption in Optical IP Networks,” J. Lightwave Technol. 27(13), 2391–2403 (2009). [CrossRef]
2.	IEEE. Std, 802.3az–2010, [Online]. Available: http://standards.ieee.org/getieee802/download/802.3az-2010.pdf
3.	“GPON power conservation,” ITU-T G-series Recommendations –Supplement 45 (G.sup-45), 05/2009.
4.	J. Mandin, “EPON Powersaving via Sleep Mode,” IEEE P802.3av 10GEPON Task Force Meeting (2008).
5.	E. Igawa, M. Nogami, and J. Nakagawa, “Symmetric 10G-EPON ONU BMT Employing Dynamic Power Save Control Circuit,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, NTuD5, (2011).
6.	W. Hofmann, M. Mueller, G. Bohm, M. Ortsiefer, and M. C. Amann, “1.55um VCSEL with enhanced modulation bandwidth and temperature range,” IEEE Photon. Technol. Lett. 21(13), 923–925 (2009). [CrossRef]
7.	A. Gatto, A. Boletti, P. Boffi, C. Neumeyr, M. Ortsiefer, E. Rönneberg, and M. Martinelli, “1.3 µm VCSEL transmission performance up to 12.5 Gbps for metro access networks,” IEEE Photon. Technol. Lett. 21(12), 778–780 (2009). [CrossRef]
8.	E. Wong, M. Mueller, P. I. Dias, C. A. Chan, and M. C. Amann, “Energy Saving Strategies for VCSEL ONUs”, Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, OTu1H5 (2012).
9.	Analog Devices ADN2530 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2530.pdf
10.	S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).
11.	NEL Laser Diodes, [Online]. Available: http://www.nttelectronics.com/en/products/photonics/pdf/NLK5B5EBKA.pdf
12.	Analog Devices ADN2531 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2531.pdf
13.	M. Mueller, C. Grasse, K. Saller, T. Gruendl, G. Boehm, and M. C. Amann, “1.3 μm High-Power Short-Cavity VCSELs for High-Speed Applications”, CLEO/QELS, San Jose, USA, CW3N.2, (2012).
14.	IEEE. Std, 802.3av – 2009, [Online]. Available: “http://standards.ieee.org/getieee802/download/802.3av-2009.pdf
15.	J. Baliga, R. Ayres, K. Hinton, and R. Tucker, “Architectures for Energy-Efficient IPTV Networks,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf, San Diego, USA, ThQ5 (2008).
16.	C. Jayasundara, A. Nirmalathas, E. Wong, and N. Nadarajah, “Energy Efficient Content Distribution for VoD Services,” Proc. of the IEEE/OSA Opt. Fiber Commun. Conf, Los Angeles, USA, OWR3 (2011).
17.	M. Rabinovich and O. Spatscheck, Web Caching and Replication: Addison Wesley (2001).
18.	B. Skubic and D. Hood, “A comparison of DBA for EPON, GPON, and NG TDM PON,” IEEE Commun. Mag. 47, 540–548 (2009).
19.	Y. Hongliang, Z. Dongdong, Y. Z. Ben, and Z. Weimin, “Understanding user behavior in large-scale video-on-demand systems,” Proc. 1st ACM SIGOPS/EuroSys European Conference on Computer Systems 2006, Leuven, Belgium, 1–12 (2006).

OCIS Codes
(060.4250) Fiber optics and optical communications : Networks
(060.4510) Fiber optics and optical communications : Optical communications
(250.7260) Optoelectronics : Vertical cavity surface emitting lasers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 9, 2012
Revised Manuscript: June 1, 2012
Manuscript Accepted: June 4, 2012
Published: June 19, 2012

Citation
Elaine Wong, Michael Mueller, Maluge P. I. Dias, Chien Aun Chan, and Markus C. Amann, "Energy-efficiency of optical network units with vertical-cavity surface-emitting lasers," Opt. Express 20, 14960-14970 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-14960

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References

J. Baliga, R. Ayre, K. Hinton, W. V. Sorin, and R. Tucker, “Energy Consumption in Optical IP Networks,” J. Lightwave Technol.27(13), 2391–2403 (2009). [CrossRef]
IEEE. Std, 802.3az–2010, [Online]. Available: http://standards.ieee.org/getieee802/download/802.3az-2010.pdf
“GPON power conservation,” ITU-T G-series Recommendations –Supplement 45 (G.sup-45), 05/2009.
J. Mandin, “EPON Powersaving via Sleep Mode,” IEEE P802.3av 10GEPON Task Force Meeting (2008).
E. Igawa, M. Nogami, and J. Nakagawa, “Symmetric 10G-EPON ONU BMT Employing Dynamic Power Save Control Circuit,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, NTuD5, (2011).
W. Hofmann, M. Mueller, G. Bohm, M. Ortsiefer, and M. C. Amann, “1.55um VCSEL with enhanced modulation bandwidth and temperature range,” IEEE Photon. Technol. Lett.21(13), 923–925 (2009). [CrossRef]
A. Gatto, A. Boletti, P. Boffi, C. Neumeyr, M. Ortsiefer, E. Rönneberg, and M. Martinelli, “1.3 µm VCSEL transmission performance up to 12.5 Gbps for metro access networks,” IEEE Photon. Technol. Lett.21(12), 778–780 (2009). [CrossRef]
E. Wong, M. Mueller, P. I. Dias, C. A. Chan, and M. C. Amann, “Energy Saving Strategies for VCSEL ONUs”, Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, OTu1H5 (2012).
Analog Devices ADN2530 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2530.pdf
S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).
NEL Laser Diodes, [Online]. Available: http://www.nttelectronics.com/en/products/photonics/pdf/ NLK5B5EBKA.pdf
Analog Devices ADN2531 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2531.pdf
M. Mueller, C. Grasse, K. Saller, T. Gruendl, G. Boehm, and M. C. Amann, “1.3 μm High-Power Short-Cavity VCSELs for High-Speed Applications”, CLEO/QELS, San Jose, USA, CW3N.2, (2012).
IEEE. Std, 802.3av – 2009, [Online]. Available: “ http://standards.ieee.org/getieee802/download/802.3av-2009.pdf
J. Baliga, R. Ayres, K. Hinton, and R. Tucker, “Architectures for Energy-Efficient IPTV Networks,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf, San Diego, USA, ThQ5 (2008).
C. Jayasundara, A. Nirmalathas, E. Wong, and N. Nadarajah, “Energy Efficient Content Distribution for VoD Services,” Proc. of the IEEE/OSA Opt. Fiber Commun. Conf, Los Angeles, USA, OWR3 (2011).
M. Rabinovich and O. Spatscheck, Web Caching and Replication: Addison Wesley (2001).
B. Skubic and D. Hood, “A comparison of DBA for EPON, GPON, and NG TDM PON,” IEEE Commun. Mag.47, 540–548 (2009).
Y. Hongliang, Z. Dongdong, Y. Z. Ben, and Z. Weimin, “Understanding user behavior in large-scale video-on-demand systems,” Proc. 1st ACM SIGOPS/EuroSys European Conference on Computer Systems 2006, Leuven, Belgium, 1–12 (2006).

IEEE Commun. Mag.

B. Skubic and D. Hood, “A comparison of DBA for EPON, GPON, and NG TDM PON,” IEEE Commun. Mag.47, 540–548 (2009).

IEEE Photon. Technol. Lett.

W. Hofmann, M. Mueller, G. Bohm, M. Ortsiefer, and M. C. Amann, “1.55um VCSEL with enhanced modulation bandwidth and temperature range,” IEEE Photon. Technol. Lett.21(13), 923–925 (2009). [CrossRef]
A. Gatto, A. Boletti, P. Boffi, C. Neumeyr, M. Ortsiefer, E. Rönneberg, and M. Martinelli, “1.3 µm VCSEL transmission performance up to 12.5 Gbps for metro access networks,” IEEE Photon. Technol. Lett.21(12), 778–780 (2009). [CrossRef]

J. Lightwave Technol.

J. Baliga, R. Ayre, K. Hinton, W. V. Sorin, and R. Tucker, “Energy Consumption in Optical IP Networks,” J. Lightwave Technol.27(13), 2391–2403 (2009). [CrossRef]

Other

IEEE. Std, 802.3az–2010, [Online]. Available: http://standards.ieee.org/getieee802/download/802.3az-2010.pdf
“GPON power conservation,” ITU-T G-series Recommendations –Supplement 45 (G.sup-45), 05/2009.
J. Mandin, “EPON Powersaving via Sleep Mode,” IEEE P802.3av 10GEPON Task Force Meeting (2008).
E. Igawa, M. Nogami, and J. Nakagawa, “Symmetric 10G-EPON ONU BMT Employing Dynamic Power Save Control Circuit,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, NTuD5, (2011).
Y. Hongliang, Z. Dongdong, Y. Z. Ben, and Z. Weimin, “Understanding user behavior in large-scale video-on-demand systems,” Proc. 1st ACM SIGOPS/EuroSys European Conference on Computer Systems 2006, Leuven, Belgium, 1–12 (2006).
E. Wong, M. Mueller, P. I. Dias, C. A. Chan, and M. C. Amann, “Energy Saving Strategies for VCSEL ONUs”, Proc. of IEEE/OSA Opt. Fiber Commun. Conf., Los Angeles, USA, OTu1H5 (2012).
Analog Devices ADN2530 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2530.pdf
S. W. Wong, L. Valcarenghi, S.-H. Yen, D. R. Campelo, S. Yamashit, and L. Kazovsky, “Sleep mode for energy saving PONs: Advantages and Drawbacks,” Proc. of IEEE GLOBECOM Workshops, (2009).
NEL Laser Diodes, [Online]. Available: http://www.nttelectronics.com/en/products/photonics/pdf/ NLK5B5EBKA.pdf
Analog Devices ADN2531 data sheet, [Online]. Available: http://www.analog.com/static/imported-files/data_sheets/ADN2531.pdf
M. Mueller, C. Grasse, K. Saller, T. Gruendl, G. Boehm, and M. C. Amann, “1.3 μm High-Power Short-Cavity VCSELs for High-Speed Applications”, CLEO/QELS, San Jose, USA, CW3N.2, (2012).
IEEE. Std, 802.3av – 2009, [Online]. Available: “ http://standards.ieee.org/getieee802/download/802.3av-2009.pdf
J. Baliga, R. Ayres, K. Hinton, and R. Tucker, “Architectures for Energy-Efficient IPTV Networks,” Proc. of IEEE/OSA Opt. Fiber Commun. Conf, San Diego, USA, ThQ5 (2008).
C. Jayasundara, A. Nirmalathas, E. Wong, and N. Nadarajah, “Energy Efficient Content Distribution for VoD Services,” Proc. of the IEEE/OSA Opt. Fiber Commun. Conf, Los Angeles, USA, OWR3 (2011).
M. Rabinovich and O. Spatscheck, Web Caching and Replication: Addison Wesley (2001).

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