物聯網是一個將嵌有感測器、制動器或兩者皆有的實體物件相互連接的網路。
這些物件可以在沒有人為的操作下，感測環境並相互溝通、傳遞訊息，我們稱這些物件為物聯網裝置。
當物聯網裝置偵測到事件的發生，就會產生資料並傳送到相關的伺服器，透過這些收集來的資料，人們可以擷取有用的資訊並加以利用。
藉由對這些資訊的善加利用，人們生活的方方面面得到了空前未有的便利，這也是近年來物聯網得到各界關注與發展的原因。

由於大部分的物聯網裝置主要是透過電池供電，如何降低電力的消耗、提高對電的有效利用成為物聯網中非常重要的議題。
尤其對於某些應用來說，裝置一旦佈建、啟用，便很難為其更換電池，或者說更換電池的成本相當高，延長電池壽命則顯得至關重要。
另一方面，即使對於某一部分的應用而言，更換電池並不是難事，但基於對使用者友善，盡可能延長電池壽命也是有必要的，畢竟，一個不需要頻繁充電或更換電池的產品是更能得到消費者青睞的。
作為一個極具前瞻性、專為物聯網設計的網路通訊技術，窄帶物聯網在省電特性上，也已將目標設立在 5 瓦特的電能得可以持續使用十年。
這是一個極具挑戰的目標，需要各方面共同考量省電這一特性來達成。

在過去，第三代合作夥伴計劃提出了一個標準省電機制，稱為不連續接收。
此機制為一般行動用戶設計，使其可以週期性地進入睡眠狀態以達到省電的效果。
但由於一般行動用戶與物聯網裝置的特性相差甚遠，將不連續接收機制直接應用在物聯網裝置上並不適當。

本論文的第一項研究即針對物聯網裝置提出適合其應用之樂觀的不連續接收機制，此機制考量了無線資源控制連線的釋放與重建，因為裝置在釋放無限資源的情況下，電量的消耗會減少相當多。
更關鍵的是，我們提出以樂觀旗幟的使用來更進一步地協助物聯網裝置提早進入長睡眠周期以達到更高的省電效果。
在這一項研究中，我們發展出數學模型與模擬實驗來分析樂觀的不連續接收機制的表現，並將其與標準不連續接收、動態不連續接收兩個機制作實驗對比，結果顯示樂觀的不連續接收在省電指標方面大大的超越另外兩個機制的表現。
在最後，我們還提出了對於使用樂觀的不連續接收機制時，三項參數的設定建議。

對於一些物聯網的應用而言，可靠性也是一項重要特性，然而，為了減輕物聯網裝置的負擔，傳輸層多以用戶數據報協議為主，因此在應用層會有所謂的重傳機制來輔助。
訊息佇列遙測傳輸感測網路和受限制應用協議即是兩個支援發佈-訂閱模式的應用層協議，物聯網裝置可以透過這些協議發佈訊息給感興趣的訂閱者。
在這兩個協議當中，都提供了重傳機制，來補足傳輸資料的可靠性。
如果裝置在重送超時計時器到期後還沒收到認可訊號，它將重送資料，直到收到認可訊號或者已達重傳上限。
在這樣的方式當中，設定一個適當的重送超時是非常重要的，因為如果設定不當，有兩種情況可能會發生，一是資料的延遲相當高，二是重送次數會徒增。當重送次數增加，裝置的耗量也會相對應提高。

在第二項子研究中，我們提出閘道器協助之受限制應用協議，來動態調整重送超時計時器。
藉由閘道器的幫助，加上偷聽機制，物聯網裝置能更早地拿到適當的計時器數值，減少重傳次數，進而達到省電的效果。
模擬實驗將閘道器協助之受限制應用協議與其它四個方式作比較、分析，而結果顯示，閘道器協助之受限制應用協議比起其他方式更適合應用在物聯網裝置上。

The Internet of Things (IoT) is the network linking physical objects which have embedded sensors, actuators, or both to sense events and interact with the environment.
Those physical objects called IoT devices (or nodes) communicate with each other with limited human interaction.
When IoT devices detect some event happening, they generate data and send to associated receivers.
With those collected data, people are able to retrieve the information which is meaningful to them.

How to reduce the power consumption is always a critical issue, because a majority of IoT devices are battery-powered and some of them are deployed at inaccessible locations.
Even though the battery of some devices can be replaced or charged easily, prolonging the battery life in order to reduce the charging frequency is still important due to the user friendliness.
In terms of power efficiency, NB-IoT has targeted a ten-year battery life with the capacity of 5 Wh, which is a quite challenging goal.

The third generation partnership project (3GPP) has proposed the standard power saving mechanism, called discontinuous reception (DRX).
DRX is designed for normal mobile users to periodically enter sleep mode in order to save power.
Since the characteristics of mobile users are different from that of IoT devices, it is inappropriate to apply DRX directly on IoT devices.

An optimistic DRX (ODRX) mechanism is proposed to be suitable for IoT devices.
ODRX considers the radio resource control connection release and re-establishment to save more power.
We introduce the optimistic flag to make IoT devices enter longer sleep periods earlier.
Analytical and simulation models are proposed to investigate the performance of ODRX, and ODRX is then compared with the standard DRX and dynamic DRX (DDRX) through simulation experiments.
The results show that ODRX outperforms standard DRX and DDRX by gaining significant extra power saving.
Finally, we also conclude some guidelines to configure ODRX parameters.

For some IoT applications, reliability is also an important feature.
Therefore, retransmission mechanism is provided in the application layer.
Message Queuing Telemetry Transport for Sensor Networks (MQTT-SN) and Constrained Application Protocol (CoAP) are two protocols in application layer supporting publish/subscribe model for IoT devices to publish message to interested subscribers.
Both MQTT-SN and CoAP run on User Datagram Protocol (UDP).
Retransmission mechanism is introduced in both MQTT-SN and CoAP to compensate the lack of data reliability.
If the device does not receive the acknowledgment (ACK) before retransmission timeout (RTO) expires, the device will retransmit the data.
Setting an appropriate RTO is crucial.
If RTO is too large, the delay will also be large.
If RTO is too small, unnecessary retransmission will be triggered frequently which causes more power consuming.

We propose a Gateway-assisted CoAP (GaCoAP) to dynamically compute RTO for IoT devices.
With the assistance of the gateway and overhearing mechanism, the IoT devices can obtain a suitable RTO early to reduce the number of retransmission and therefore, to save power.
Simulation are proposed to investigate the performance of GaCoAP compared with other four methods.
The experiment results show that GaCoAP is more suitable for IoT devices in terms of power saving.

[1] ‘’MQ Telemetry Transport,’’ MQTT.org, accessed: 2017-08-20.
[2] ‘’OMNet++ Simulator," https://omnetpp.org/, accessed: 2017-08-24.
[3] ‘’Request-response vs. publish-subscribe, part 1: What's the diff?’’
https://blog.opto22.com/optoblog/request-response-vs-pub-sub-part-1, accessed: 2019-06-02.
[4] 3GPP, ‘’Study on Alternatives to E.164 for Machine-Type Communications (MTC),’’ Third Generation Partnership Project, TR 22.988, Sep. 2012.
[5] 3GPP, ‘’System improvements for Machine-Type Communications (MTC),’’ 3rd Generation Partnership Project (3GPP), TR 23.888, 2012.
[6] 3GPP, ‘’Cellular system support for ultra-low complexity and low throughput cellular Internet of Things,’’ Third Generation Partnership Project, TR 45.820, 2015.
[7] 3GPP, ‘’Service requirements for Machine-Type Communications (MTC); Stage 1,’’ 3rd Generation Partnership Project (3GPP), TS 22.368, 2015.
[8] 3GPP, ‘’Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode,’’ 3rd Generation Partnership Project (3GPP), TS 36.304, Nov. 2016.
[9] 3GPP, ‘’Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol speci_cation,’’ 3rd Generation Partnership Project (3GPP), TS 36.321, Mar. 2017.
[10] Z. Abbas and W. Yoon, ‘’A survey on energy conserving mechanisms for the internet of things: Wireless networking aspects,’’ Sensors, vol. 15, no. 10, pp. 24818-24847, 2015.
[11] A. Al-Fuqaha, m. guizani, M. Mohammadi, M. Aledhari, and M. Ayyash, ‘’Internet of Things: A Survey on Enabling Technologies, Protocols and Applications,’’ IEEE Communications Surveys and Tutorials, vol. 17, p. Fourthquarter 2015, 11 2015.
[12] I. Ali and Z. Ullah, ‘’Internet of Things Security, Device Authentication and Access Control: A Review,’’ International journal of computer science and information security, 08 2016.
[13] I. Analytics, ‘’State of the IoT 2018: Number of IoT devices now at 7B - Market
accelerating,’’ IoT Analytics, Tech. Rep., Aug. 2018.
[14] I. Andrea, C. Chrysostomou, and G. Hadjichristofi, ‘’Internet of Things: Security vulnerabilities and challenges,’’ in 2015 IEEE Symposium on Computers and Communication (ISCC), July 2015, pp. 180-187.
[15] L. Atzori, A. Iera, G. Morabito, and M. Nitti, ‘’The Social Internet of Things (SIoT) - When social networks meet the Internet of Things: Concept, architecture and network characterization,’’ Computer Networks, vol. 56, no. 16, pp. 3594 - 3608, 2012. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S1389128612002654
[16] E. Baccelli, C. Mehlis, O. Hahm, T. C. Schmidt, and M. Wahlisch, ‘’Information centric networking in the IoT: Experiments with NDN in the wild,’’ in Proceedings of the 1st ACM Conference on Information-Centric Networking. ACM, 2014, pp. 77-86.
[17] E. Balandina, Y. Koucheryavy, and A. Gurtov, ‘’Computing the retransmission timeout in CoAP,’’ in Internet of Things, smart spaces, and next generation networking. Springer, 2013, pp. 352-362.
[18] E. Borgia, ‘’The Internet of Things vision: Key features, applications and open issues,’’ Computer Communications, vol. 54, pp. 1 - 31, 2014. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0140366414003168
[19] C. Bormann, A. Betzler, C. Gomez, and I. Demirkol, ‘’CoAP Simple Congestion Control/Advanced (Work in Progress), July 2014,’’ URL http://tools.ietf.org/id/draft-bormann-core-cocoa-02.txt, 2015.
[20] A. Bujari, M. Furini, F. Mandreoli, R. Martoglia, M. Montangero, and D. Ronzani, ‘’Standards, security and business models: Key challenges for the IoT scenarios,’’ Mobile Networks and Applications, vol. 23, no. 1, pp. 147-154, Feb. 2018.
[21] H. Chang, S. Lu, T. Chuang, C. Lin, M. Tsai, and S. Sou, ‘’Optimistic DRX for machine-type communications,’’ in 2016 IEEE International Conference on Communications (ICC), May 2016, pp. 1-6.
[22] M. Chen, Y. Miao, Y. Hao, and K. Hwang, ‘’Narrow Band Internet of Things,’’ IEEE Access, vol. 5, pp. 20557-20577, 2017.
[23] Cisco, ‘’Cisco visual networking index: forecast and trends, 2017-2022 white paper,’’ Cisco, Tech. Rep., Feb. 2019.
[24] G.-H. Dai and J.-H. Yu, ‘’Research on NB-IoT background, standard development, characteristics and the service,’’ Mobile Communications, vol. 40, no. 7, pp. 31-36, 2016.
[25] E. Davis, A. Calveras, and I. Demirkol, ‘’Improving packet delivery performance of publish/subscribe protocols in wireless sensor networks,’’ Sensors, vol. 13, no. 1, pp. 648-680, 2013.
[26] N. De Caro, W. Colitti, K. Steenhaut, G. Mangino, and G. Reali, ‘’Comparison of two lightweight protocols for smartphone-based sensing,’’ in 2013 IEEE 20th Symposium on Communications and Vehicular Technology in the Benelux (SCVT). IEEE, 2013, pp. 1-6.
[27] P. T. Eugster, P. A. Felber, R. Guerraoui, and A.-M. Kermarrec, ‘’The Many Faces of Publish/Subscribe,’’ ACM Comput. Surv., vol. 35, no. 2, pp. 114-131, Jun. 2003. [Online]. Available: http://doi.acm.org/10.1145/857076.857078
[28] E. Felemban, C.-G. Lee, and E. Ekici, ‘’MMSPEED: multipath Multi-SPEED protocol for QoS guarantee of reliability and. Timeliness in wireless sensor networks,’’ IEEE transactions on mobile computing, vol. 5, no. 6, pp. 738-754, 2006.
[29] J. Francois, T. Cholez, and T. Engel, ‘’CCN traffic optimization for IoT,’’ in 2013 Fourth international conference on the network of the future (NOF). IEEE, 2013, pp. 1-5.
[30] GSA, ‘’Global progress to 5G - trials, deployments and launches,’’ Global mobile Suppliers Association, Tech. Rep., Jul. 2018.
[31] P.-J. Hsieh, G.-Y. Lin, C.-Y. Chen, and H.-Y. Wei, ‘’Accurate Modeling of the DRX Mechanism with Predetermined DRX Cycles Based on the 3GPP LTE Standard,’’ Mobile Networks and Applications, vol. 21, no. 2, pp. 259-271, 2016. [Online]. Available: http://dx.doi.org/10.1007/s11036-015-0662-8
[32] U. Hunkeler, H. L. Truong, and A. Stanford-Clark, ‘’MQTT-S|A publish/subscribe protocol for Wireless Sensor Networks,’’ in 2008 3rd International Conference on Communication Systems Software and Middleware and Workshops (COMSWARE’08). IEEE, 2008, pp. 791-798.
[33] S. Jain, M. Braathen, and T. Instruments, ‘’Measuring the power consumption on cc2530znp using cc2530 znp mini kit,’’ Application Note AN108, 2011.
[34] S. C. Jha, A. T. Koc, R. Vannithamby, and M. Torlak, ‘’Adaptive DRX configuration to optimize device power saving and latency of mobile applications over LTE advanced network,’’ in 2013 IEEE International Conference on Communications (ICC), June 2013, pp. 6210-6214.
[35] P. Karn and C. Partridge, ‘’Improving round-trip time estimates in reliable transport protocols,’’ in ACM SIGCOMM Computer Communication Review, vol. 17, no. 5. ACM, 1987, pp. 2-7.
[36] R. M. Karthik and A. Chakrapani, ‘’Practical algorithm for power efficient DRX configuration in next generation mobiles,’’ in INFOCOM, 2013 Proceedings IEEE, April 2013, pp. 1106-1114.
[37] F. Kelly, Reversibility and Stochastic Networks. John Wiley & Sons, 1979.
[38] S. D. T. Kelly, N. K. Suryadevara, and S. C. Mukhopadhyay, ‘’Towards the Implementation of IoT for Environmental Condition Monitoring in Homes,’’ IEEE Sensors Journal, vol. 13, no. 10, pp. 3846-3853, Oct 2013.
[39] L. Kleinrock, Queueing Systems: Volume I - Theory. New York: Wiley, 1976.
[40] A. T. Koc, S. C. Jha, R. Vannithamby, and M. Torlak, ‘’Optimizing DRX configuration to improve battery power saving and latency of active mobile applications over LTE-A network,’’ in 2013 IEEE Wireless Communications and Networking Conference (WCNC), April 2013, pp. 568-573.
[41] A. T. Koc, S. C. Jha, R. Vannithamby, and M. Torlak, ‘’Device Power Saving and Latency Optimization in LTE-A Networks Through DRX Configuration,’’ IEEE Transactions on Wireless Communications, vol. 13, no. 5, pp. 2614-2625, May 2014.
[42] M. Koster, A. Keranen, and J. Jimenez, ‘’Publish-Subscribe Broker for the Constrained Application Protocol (CoAP),’’ Internet Engineering Task Force, Internet-Draft draft-ietf-core-coap-pubsub-08, Mar. 2019, work in Progress. [Online]. Available: https://datatracker.ietf.org/doc/html/draft-ietf-core-coap-pubsub-08
[43] A. Kunz, H. Kim, L. Kim, and S. S. Husain, ‘’Machine type communications in 3GPP: From release 10 to release 12,’’ in 2012 IEEE Globecom Workshops, Dec 2012, pp. 1747-1752.
[44] M. Leo, F. Battisti, M. Carli, and A. Neri, ‘’A federated architecture approach for Internet of Things security,’’ in 2014 Euro Med Telco Conference (EMTC), Nov 2014, pp. 1-5.
[45] J.-M. Liang, J.-J. Chen, H.-H. Cheng, and Y.-C. Tseng, ‘’An Energy-Efficient Sleep Scheduling With QoS Consideration in 3GPP LTE-Advanced Networks for Internet of Things,’’ IEEE Journal on Emerging and Selected Topics in Circuits and Systems, vol. 3, no. 1, pp. 13-22, Mar. 2013.
[46] J.-M. Liang, P.-C. Hsieh, J.-J. Chen, and Y.-C. Tseng, ‘’Energy-efficient DRX scheduling for multicast transmissions in 3GPP LTE-Advanced wireless networks,’’ in 2013 IEEE Wireless Communications and Networking Conference (WCNC), April 2013, pp. 551-556.
[47] J. Lin, W. Yu, N. Zhang, X. Yang, H. Zhang, and W. Zhao, ‘’A Survey on Internet of Things: Architecture, Enabling Technologies, Security and Privacy, and Applications,’’ IEEE Internet of Things Journal, vol. 4, no. 5, pp. 1125-1142, Oct 2017.
[48] R. Ludwig and K. Sklower, ‘’The Eifel retransmission timer,’’ ACM SIGCOMM Computer Communication Review, vol. 30, no. 3, pp. 17-27, 2000.
[49] R. Mahmoud, T. Yousuf, F. A. Aloul, and I. A. Zualkernan, ‘’Internet of things (IoT) security: Current status, challenges and prospective measures,’’ 2015 10th International Conference for Internet Technology and Secured Transactions (ICITST), pp. 336-341, 2015.
[50] Y. Miao, W. Li, D. Tian, M. S. Hossain, and M. F. Alhamid, ‘’Narrowband Internet of Things: Simulation and Modeling,’’ IEEE Internet of Things Journal, vol. 5, no. 4, pp. 2304-2314, Aug 2018.
[51] S. Mohan, R. Kapoor, and B. Mohanty, ‘’Latency in HSPA Data Networks,’’ Qualcomm, Tech. Rep., 2011.
[52] A. Osseiran, F. Boccardi, V. Braun, K. Kusume, P. Marsch, M. Maternia, O. Queseth, M. Schellmann, H. Schotten, H. Taoka, H. Tullberg, M. A. Uusitalo, B. Timus, and M. Fallgren, ‘’Scenarios for 5G mobile and wireless communications: the vision of the METIS project,’’ IEEE Communications Magazine, vol. 52, no. 5, pp. 26-35, May 2014.
[53] C. Paxson, M. Allman, J. Chu, and M. Sargent, ‘’Computing TCP's Retransmission Timer (RFC 6298),’’ Internet Engineering Task Force, 2000.
[54] H. Shariatmadari, R. Ratasuk, S. Iraji, A. Laya, T. Taleb, R. Jantti, and A. Ghosh,
‘’Machine-type communications: current status and future perspectives toward 5G systems,’’ IEEE Communications Magazine, vol. 53, no. 9, pp. 10-17, September 2015.
[55] Z. Shelby, K. Hartke, and C. Bormann, ‘’The constrained application protocol (coap)(rfc 7252),’’ IETF, Tech. Rep., 2014.
[56] A. Stanford-Clark and H. L. Truong, ‘’MQTT for sensor networks (MQTT-SN) protocol specification,’’ 2013.
[57] H. Suo, J. Wan, C. Zou, and J. Liu, ‘’Security in the Internet of Things: A Review,’’ in 2012 International Conference on Computer Science and Electronics Engineering, vol. 3, March 2012, pp. 648-651.
[58] K. Wang, X. Li, and H. Ji, ‘’Modeling 3GPP LTE Advanced DRX Mechanism Under Multimedia Traffic,’’ IEEE Communications Letters, vol. 18, no. 7, pp. 1238-1241, July 2014.
[59] K. Wang, X. Li, H. Ji, and X. Du, ‘’Modeling and Optimizing the LTE Discontinuous Reception Mechanism Under Self-Similar Traffic,’’ IEEE Transactions on Vehicular Technology, vol. 65, no. 7, pp. 5595-5610, July 2016.
[60] R. Want, B. N. Schilit, and S. Jenson, ‘’Enabling the Internet of Things,’’ Computer, vol. 48, no. 1, pp. 28-35, Jan 2015.
[61] J. Wu, T. Zhang, and Z. Zeng, ‘’Performance analysis of discontinuous reception mechanism with web traffic in LTE networks,’’ in 2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), Sept 2013, pp. 1676-1681.
[62] J. Wu, T. Zhang, Z. Zeng, and H. Chen, ‘’Study on discontinuous reception modeling for M2M traffic in LTE-A networks,’’ in Proc. of the 15th IEEE International Conference on Communication Technology (ICCT), Nov. 2013.
[63] W. Xiang, K. Zheng, and X. S. Shen, 5G Mobile Communications. Springer Publishing Company, Incorporated, 2016.
[64] S.-R. Yang, ‘’Dynamic Power Saving Mechanism for 3G UMTS System,’’ Mobile Networks and Applications, vol. 12, no. 1, pp. 5-14, 2007.
[65] K. Zhao and L. Ge, ‘’A Survey on the Internet of Things Security,’’ in 2013 Ninth International Conference on Computational Intelligence and Security, Dec 2013, pp. 663-667.
[66] K. Zhou, N. Nikaein, and T. Spyropoulos, ‘’LTE/LTE-A Discontinuous Reception Modeling for Machine Type Communications,’’ IEEE Wireless Communications Letters, vol. 2, no. 1, pp. 102-105, Feb. 2013.