Passive Optical Networks (PONs) are considered to be one of the energy efficient access technologies. Initially, when some of the PON systems (e.g. Time Division Multiplexing (TDM)-PON) were standardized (e.g. IEEE 802.3ah, IEEE 802.3av) it was recommended that an Optical Network Unit (ONU), which is installed at the user premises, should be always on. However, some statistics show that access network equipments (e.g. an ONU) have a utilization around 15\%. This indicates that there is a still room to improve energy efficiency in TDM-PONs (Ethernet PON (EPON), Gigabit PON (GPON)). One of the common and efficient approaches to maximize energy efficiency in a TDM-PON is to put an ONU in sleep mode turning off some of its power hungry components while it does not have any traffic to receive and/or transmit. However, the length of sleep interval plays a significantly important role on energy efficiency of an ONU and traffic delay. That is, there exists a very strong trade off relationship. If the length of sleep interval is increased, there would be less energy consumption at an ONU; however, the traffic will experience longer delay, and the other way around. The important fact is that improving energy efficiency in network equipments is increasingly important issue in today`s research as they are largely contributing on increasing the carbon footprint. However, as mentioned above that energy saving and traffic performance introduce a trade off relationship. Then, rationally, an PON operator cannot lose revenue just for the sake of saving energy. Therefore, a novel solution is required that will be able to save energy in ONUs and provide desired traffic performance at the same time. As the penetration rate of TDM-PONs are much higher than other PON solutions (e.g. wavelength division multiplexing PON (WDM-PON)), the research contributions presented in this dissertation consider TDM-PON environment. In this dissertation, a novel Adaptive Delay-Aware Energy Efficient (ADAEE) solution is proposed. The ADAEE solution not only reduces energy expenditure in ONUs but also ensures satisfactory downlink traffic delay requirement compared to the existing solutions. Performance of ADAEE is evaluated under constant bit rate, variable bit rate, and real network traffic traces.
To save energy, when an ONU switches into sleep mode, it requires to be available (listening optical channel for a predefined amount of time, which is termed as listening interval) periodically to the Optical Line Terminal (OLT) (once ONU is in sleep mode, the OLT cannot communicate with it). As it is mentioned above the longer the sleep interval (i.e. inter listening time) the higher energy saving, and vice versa. Then, if the sleep interval length is small, then there would be many listening intervals in which an ONU would get instruction from the OLT to switch back into sleep mode again in case the OLT does not have any traffic to transmit for that ONU. This phenomenon could be very common in the off-peak hours of a day (e.g. midnight). Consequently, an ONU would end up spending energy even it does have any traffic to receive during those listening intervals (during a listening interval an ONU needs to be totally powered on). Then, it would be an interesting solution in which an ONU wakes up from sleep mode only when the OLT has some traffic to transmit for it. Considering this, a novel solution is proposed in this dissertation using a Hybrid-ONU (H-ONU). This novel H-ONU is composed of an ONU module and a low-cost low-energy consuming sensor mote. The attached sensor mote (i.e. IEEE 802.15.4 in this case) overtakes the role of powering up whole H-ONU when it should leave sleep mode to receive traffic from the OLT. Numerical results presented here show that proposed solution can significantly reduce energy consumption while satisfying delay requirement of traffic compared to the existing sleep mode management approach.
Researchers from industries and academia have already came up with different solutions to maximize energy efficiency of TDM-PONs. Some solutions suggest to turn off an ONU`s components whenever possible based on its communication requirements. For example, if an ONU needs to transmit at a particular time instant, the receiver module could be turned off at that time if it does not have any role to perform. This would provide improvement in energy saving in ONUs. When an ONU turns on/off its components it winds up with multiple power levels. There are some analytical models have been introduced so far to measure energy efficiency of an ONU. However, these models only consider ONUs with two power levels. In some existing proposals, that center their effort to maximize energy efficiency of ONUs, propose more than two power levels for an ONU. In this dissertation, a comprehensive analytical model is presented to measure energy efficiency of an ONU having more than two power levels. Under two different sleep interval deciding algorithms, which are used in an OLT of a TDM-PON, the accuracy of this analytical model is evaluated by means of simulation.
슬립 구간의 길이는 ONU의 에너지 효율과 트래픽 지연에 큰 영향을 미치는 요소이다. 그러므로 이 논문은 ONU의 에너지를 절감함과 동시에 만족할만한 트래픽 성능 제공하는 방법을 제안하였다. TDM-PON의 실질 사용률은 WDM-PON 과 같은 다른 PON 방식에 비해 매우 크기 때문에 이 연구는 TDM-PON 환경을 고려하였다. 이 연구는 ADAEE (Adaptive Delay-Aware Energy Efficient)을 제안하였다. ADAEE는 ONU의 에너지 소비를 줄일 뿐만 아니라, 현존하는 솔루션과 대비하여 만족할만한 다운링크 트래픽 지연을 보장한다. 성능은 CBR, VBR, 실제 네트워크 트래픽을 적용하여 분석하였다.
에너지를 절감하기 위하여 ONU 는 슬립모드로 전화되며, ONU는 주기적으로 일어나 listening 구간 동안 OLT와 통신을 할 수 있어야 한다. 슬립구간의 길이가 길어질수록 더 많은 에너지를 절감할 수 있다. 슬립구간의 길이가 짧으면 ONU는 많은 listening 구간을 필요로 한다. 이 때, OLT가 전송할 데이터가 없다면, OLT는 ONU를 다시 슬립모드로 전환하는 명령을 보낸다. 이는 인터넷 사용량이 적은 밤 시간대에 매우 흔한 현상이다. 그러므로 ONU가 OLT로부터 수신할 데이터가 있을 때만 슬립모드에서 일어나는 방식이 이상적이다. 이와 같은 사실들을 고려하여 이 연구는 H-ONU (Hybrid-ONU)를 제안하였다. H-ONU는 ONU모듈과 에너지를 적게 소모하는 IEEE 802.15.4무선모듈 구성되었다. 무선모듈은 모든 슬립모드에 있는 H-ONU를 깨워 트래픽을 수신하게 하는데 사용된다.
ONU의 구성요소들을 겨고 끌 수 있기 때문에 ONU는 여러 전력 수준을 가질 수 있다. 지금까지 에너지 절감형 ONU를 위한 여러 분석적 모델이 제안되었지만, 대부분은 2가지 전력 수준만을 고려하였다. 몇몇 연구들은 ONU의 에너지효율을 최대화하기 위해 여러 단계의 전력 수준을 고려하였다. 이 연구에서는 2가지 이상의 전력 수준을 갖는 종합적인 분석모델이 제안하고 이의 에너지 효율이 측정하였다. 그리고 시뮬레이션을 통해, TDM-PON의 OLT에서 사용되는 두 가지 슬립 구간 결정 알고리즘에 대한 분석적 모델의 정확도가 계산되었다.
