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以太网无源光网络中的下游数据包调度节能方案随着网络规模的增长，无源光网络（Passive Optical Networks，PONs）的能量消耗显著增加。因此，设计有效的节能方案对于PONs来说非常重要。传统上，节能方案往往侧重于让负荷较低的光网络单元（Optical Network Units，ONUs）进入睡眠模式，这通常会导致较大的数据包延迟。而且，传统ONU睡眠模式不能独立让发送器和接收器睡眠，即使它们不需要传输或接收数据包，这种做法显然会浪费能源。因此，本文提出了一种基于以太网无源光网络（Ethernet Passive Optical Networks，EPONs）下游数据包调度的节能方案（Energy-Saving scheme based on downstream Packet Scheduling，ESPS）。我们设计了既适用于网络间（inter-ONU level）也适用于网络内（intra-ONU level）的下游数据包调度算法和规则，目的是减少下游数据包延迟。然后，我们提出了一种混合睡眠模式（hybrid sleep mode），该模式不仅包括ONU的深度睡眠模式，还包括独立的发送器和接收器睡眠模式。这确保了ONU的能量消耗是最小的。为了实现混合睡眠模式，我们设计了一种改良版的GATE控制消息，该消息涉及到10个睡眠过程中的时间点。在ESPS方案中，这10个时间点根据上行和下行分配的带宽进行计算。仿真结果显示，与传统以上传输核心调度（Upstream Centric Scheduling，UCS）方案相比，ESPS在能量消耗和实时及非实时数据包下游的平均延迟方面表现更优。仿真结果还显示，在大型网络中，每个ONU的平均能量消耗要比小型网络中的少。关键词包括：以太网无源光网络、数据包调度、混合睡眠模式、节能方案。知识要点如下： 1. 无源光网络（PONs）的能源消耗问题：随着网络规模的扩大，PONs的能量消耗持续增长，这促使研究者需要开发有效的节能方案。 2. 现有节能方案的局限性：传统节能方案主要集中在将低负载的ONUs置于睡眠模式以节省能源，但这种方式会造成较大的数据包延迟，并且传统睡眠模式不能独立关闭ONU的发送器和接收器，从而导致不必要的能源浪费。 3. EPONs中的下游数据包调度：为了减少下行数据包延迟，文章提出了专门针对下游数据包调度的算法和规则。 4. 混合睡眠模式（hybrid sleep mode）的提出：文章设计了一种新的混合睡眠模式，能够使ONU在不需要传输或接收数据包时进入深度睡眠状态，同时独立关闭发送器和接收器，从而最小化ONU的能耗。 5. 改良版GATE控制消息的设计：为了实现混合睡眠模式，设计了一种改良版的GATE控制消息，该消息包含了10个与睡眠过程相关的时间点，这些时间点根据上行和下行分配的带宽计算得出。 6. 节能方案（ESPS）的性能评估：通过仿真验证了ESPS方案在能量消耗和数据包延迟方面的优势，与传统的UCS方案相比，在实时和非实时数据包的传输效率和节能效果上都有明显提高。仿真结果还表明，ESPS方案在大型网络中的表现比小型网络中更好。这些知识点系统地阐述了以太网无源光网络的节能挑战和相应的解决方案，并且通过仿真实验验证了ESPS方案的有效性。这些内容能够帮助专业人士理解当前在EPONs中关于节能调度的最新研究进展及其潜在的应用价值。

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Energy-saving scheme based on downstream packet scheduling in ethernet

passive optical networks

Lincong Zhang, Yejun Liu

⇑

, Lei Guo, Xiaoxue Gong

College of Information Science and Engineering, Northeastern University, Shenyang 110819, PR China

article info

Article history:

Received 16 November 2012

Revised 14 December 2012

Available online 29 January 2013

Keywords:

Ethernet passive optical networks

Packets scheduling

Hybrid sleep mode

Energy-saving scheme

abstract

With increasing network sizes, the energy consumption of Passive Optical Networks (PONs) has grown

signiﬁcantly. Therefore, it is important to design effective energy-saving schemes in PONs. Generally,

energy-saving schemes have focused on sleeping the low-loaded Optical Network Units (ONUs), which

tends to bring large packet delays. Further, the traditional ONU sleep modes are not capable of sleeping

the transmitter and receiver independently, though they are not required to transmit or receive packets.

Clearly, this approach contributes to wasted energy. Thus, in this paper, we propose an Energy-Saving

scheme that is based on downstream Packet Scheduling (ESPS) in Ethernet PON (EPON). First, we design

both an algorithm and a rule for downstream packet scheduling at the inter- and intra-ONU levels,

respectively, to reduce the downstream packet delay. After that, we propose a hybrid sleep mode that

contains not only ONU deep sleep mode but also independent sleep modes for the transmitter and the

receiver. This ensures that the energy consumed by the ONUs is minimal. To realize the hybrid sleep

mode, a modiﬁed GATE control message is designed that involves 10 time points for sleep processes.

In ESPS, the 10 time points are calculated according to the allocated bandwidths in both the upstream

and the downstream. The simulation results show that ESPS outperforms traditional Upstream Centric

Scheduling (UCS) scheme in terms of energy consumption and the average delay for both real-time

and non-real-time packets downstream. The simulation results also show that the average energy con-

sumption of each ONU in larger-sized networks is less than that in smaller-sized networks; hence, our

ESPS is better suited for larger-sized networks.

1. Introduction

Recently, developing ‘‘green networks’’ has been a critical focus,

as energy saving in networks has faced unprecedented challenges

[1–3]. In particular, the ‘‘last mile’’ broadband access network

has received signiﬁcant attention because energy consumption in

these networks accounts for approximately 75% of the entire net-

work [4]. As a predominant modern broadband access technology,

the Passive Optical Network (PON) has become the most feasible

and energy-efﬁcient solution. Due to the use of optical ﬁbers closer

to the end users in addition to the passive nature of the remote

nodes, PON consumes the smallest energy among the various ac-

cess technologies, which include Wireless Fidelity (WiFi) [5,6],

Worldwide Interoperability for Microwave Access (WiMax) [7], Fi-

ber-To-The-x (FTTx) [8], and Digital Subscriber Line (xDSL) [9].

Nevertheless, the energy consumption of PONs can be further re-

duced because the devices are not in use at all times.

In this paper, we focus on Ethernet PON (EPON), which has been

widely deployed in recent years. The devices used in the typical

Time Division Multiplex (TDM) EPON include an Optical Line Ter-

minal (OLT), a splitter and several Optical Network Units (ONUs),

which are constructed in a tree topology. In the upstream direc-

tion, the trafﬁc from the end users is ﬁrst aggregated at the ONUs

and is then sent to the OLT. All ONUs share the upstream channel

to the OLT in a time division manner. Conversely, in the down-

stream direction, the OLT broadcasts the downstream trafﬁc to

all ONUs. The ONUs only reserve the trafﬁc designated for them-

selves and discard the rest.

In EPON, the energy is mainly consumed by the OLT and ONUs,

where the power consumption of an OLT is 20 times of an ONU

[10]. Nevertheless, as the highest aggregate node, the OLT can

rarely be turned off because it connects the PON with the Internet.

As a result, current efforts to reduce the energy consumption in

EPON have primarily focused on ONUs. Several energy-saving mea-

sures are currently used for ONUs in EPON.

First, the power consumption of the ONUs can be reduced in

light of the hardware structure. Ref. [11] developed low-power

ONUs with integration technology, which decreased the power

consumption of the investigated ONUs from 10 W to 6 W.

http://dx.doi.org/10.1016/j.yofte.2012.12.007

⇑

Corresponding author. Address: College of Information Science and Engineering,

Northeastern University, P.O. Box 365, Shenyang 110819, PR China. Fax: +86 24

83684219.

E-mail address: haveball@yeah.net (Y. Liu).

Optical Fiber Technology 19 (2013) 169–178

Contents lists available at SciVerse ScienceDirect

Optical Fiber Technology

www.elsevier.com/locate/yofte

Second, the ONUs can be allowed to sleep when the trafﬁc is

low. In reality, devices in access networks, such as ONUs, tend to

be underused by approximately 15% [12]. Because the trafﬁc oc-

curs suddenly and dynamically, the ONUs do not keep busy all of

the time. Consequently, the ONUs can go into sleep modes under

low or zero trafﬁc conditions. Refs. [13,14] proposed algorithms

for ONU sleep when the trafﬁc load is lower than the low-level

threshold, at which point another sleeping ONU will be woken

up when the network trafﬁc load is higher than the high-level

threshold. In this case, the trafﬁc designated to the sleeping ONU

is rerouted to other active ONUs.

Third, energy-efﬁcient protocols can be designed in EPON. This

method designates the sleep start time and wakeup time for ONUs.

Refs. [15,16] proposed two ONU sleep schemes, named Upstream

Centric Scheduling (UCS) and Downstream Centric Scheduling

(DCS), which are based on the use of a Multipoint Control Protocol

(MPCP) in EPON. In UCS, the downstream trafﬁc load only can be

transmitted during the upstream transmission slot. If the down-

stream trafﬁc is signiﬁcantly more than the upstream trafﬁc, a

large delay is caused for the downstream trafﬁc due to queuing.

In DCS, if any trafﬁc exists either upstream or downstream, the

ONU should not go to sleep. Therefore, DCS reduces the down-

stream trafﬁc delay while consuming more energy than UCS.

In fact, 60–70% of the ONU power consumption is due to the

transceiver and back-end circuit [11]. Hence, allowing the trans-

mitter and receiver to sleep is another possible approach for

ONU energy-saving management. Ref. [17] proposed four power

levels for ONUs, achieved through the use of different transmitter

and receiver states. Two sleep mode scenarios for ONUs are pro-

posed, including multiple Dynamic Bandwidth Allocation (DBA)

cycles and one DBA cycle. In the multiple DBA cycles scenario,

the transmitter and receiver are allowed to sleep for more than

one cycle only if the sleep duration is no more than the maximum

tolerated time. In the one cycle scenario, the transmitter goes to

sleep after completing the transmission, and it is woken up via

the ONU MAC protocol. However, sleeping the receiver is signiﬁ-

cantly more complicated because the receiver does not know the

condition of the downstream trafﬁc. Hence, the transmission inter-

val of an ONU in the downstream is designed to be determined by

the sum of the downstream trafﬁc of all other ONUs. Then, the re-

ceiver can sleep for a period based on a function of the interval

time.

Energy-saving problems also have been studied in various

PONs. Refs. [18–20] noted that a 10G-EPON consumes more energy

than a 1G-EPON due to the signiﬁcantly higher line rate. Based on

this idea, they developed a single-threshold policy [18,19] and a

dual-threshold policy to prevent frequent switching of the link

rates [20]. In addition to adding the adaptive link rate, the cyclic

sleep mode is also used as one of the energy-saving modes that

are speciﬁed in the ITU-T Recommendation G. sup45, while the

other is doze mode [21]. Further, ITU-T G.984 for Gigabit PON

(GPON) speciﬁes the ONU sleep modes for four types, including

the ONU power shedding mode, the ONU dozing mode, the ONU

fast/cyclic sleep mode and the ONU deep sleep mode [22]. In the

ﬁrst mode, beyond the optical link, some functions and services

can be turned off. The power consumption in this mode is the high-

est among the four modes. In the ONU dozing mode, the transmit-

ter can be turned off without the presence of upstream data. This

mode consumes the second highest power after the ONU power

shedding mode. The ONU fast/cyclic sleep is deﬁned by allowing

the ONUs to stay in a sequence of sleep cycles. Its power consump-

tion is less than that of the ONU dozing mode. The ONU deep sleep

mode consumes the least power because both the transmitter and

the receiver can be turned off. Meanwhile, the network perfor-

mances are also most seriously degraded in this mode. Recently,

a method for combining the four modes has appeared. An ONU

deep sleep mode combined with a dozing mode was proposed to

achieve the maximum possible energy saving in Ref. [23].

Although Refs. [13–16,18–23] studied various energy-saving

methods, they did not focus on energy consumption in the inde-

pendent components of the ONUs. In Ref. [17], although the energy

consumption of the transmitter and receiver was considered, the

performances including the packet delay were not addressed.

Therefore, an energy-saving scheme that considers both the com-

ponents of the ONU and the packet delay remains a challenge,

and this challenge has motivated our work.

In this paper, we consider the energy-saving components of the

ONUs, which include the transmitter, the receiver and the ONU in

its entirety. We ﬁrst propose an algorithm and a rule for down-

stream packet scheduling under inter- and intra-ONU conditions

to ensure that the real-time packet that arrives earliest also can

be sent ﬁrst and that all real-time packets can be sent before the

non-real-time packets. Then, we develop a hybrid sleep mode for

ONUs that considers the combination of ONU deep sleep and inde-

pendent sleep for transmitters and receivers. To realize this combi-

nation, a modiﬁed GATE control message is designed with 10 time

points for the sleep processes of the transmitter, the receiver and

the entire ONU. Based on the packet scheduling algorithm and rule

in addition to the hybrid sleep mode, we propose an ONU sleep

scheme named the Energy-Saving scheme based on downstream

Packet Scheduling (ESPS), which minimizes the energy consump-

tion of ONUs and reduces the packet delay as much as possible.

To the best of our knowledge, the work in this paper is the ﬁrst

study to combine the independent sleep mode for transmitters and

receivers with the ONU deep sleep mode, and this work also is the

ﬁrst study to design the ONU sleep scheme by scheduling both in-

ter- and intra-ONUs. The rest of the paper is organized as follows:

Section 2 gives the problem statement; Section 3 presents the

scheduling of downstream packets; Section 4 proposes the hybrid

sleep mode; Section 5 describes the proposed ESPS scheme in de-

tail; Section 6 details the simulation and its analysis; and Section 7

concludes this paper.

2. Problem statement

2.1. Assumptions and notations

In the ONU deep sleep mode, both the transmitter and receiver

are turned off, while the other components (e.g., the digital cir-

cuitry) remain active at all times. Therefore, the power of the

ONU in a deep sleep state is produced by the always-active compo-

nents. When the transmitter/receiver remains active, the always-

active components are also working. We assume there are K ONUs

in a given EPON, and enable tx and rx to denote the transmitter and

the receiver of ONU, respectively. We let acc denote the always-ac-

tive components. Because the energy-saving components that we

consider in this paper include tx, rx and acc, we give the relative

parameters for those components in the following list.

 i: The index of the ONU, 1 6 i 6 K.

 N: The maximum polling cycle that OLT polling ONUs in the

simulation time.

 n: The index of the polling cycle, 1 6 n 6 N.

 E

wakeup

: The energy consumption of the wakeup process for an

ONU.

 P

, P

: The power consumed by the transmitter and the recei-

ver while active, respectively.

 P

sleep

: The power consumed when an ONU is in a state of deep

sleep.

 T

tx;i

: The active duration of the transmitter of an ONU

in the nth

cycle, 1 6 i 6 K.

170 L. Zhang et al. / Optical Fiber Technology 19 (2013) 169–178

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