IEEE802.16標準為一種無線都會型的網路(Wireless Metropolitan Access Network; WMAN)，可以提供較大的傳輸距離。為了保障各種多媒體的服務品質(Quality of Service; QoS) ，服務流被分為五種不同的類別。而在IEEE 802.16標準中並沒有定義排程演算法。因此，如何設計一個排程演算法來滿足所有服務類別的需求，是一個值得探討的議題。輪詢是一種上傳頻道的傳送機制，在傳輸資料前，當基地台(Base Station; BS)進行輪流詢問後，被詢問的用戶端工作站(Subscribe Station; SS)才可以傳送頻寬要求訊息。

在可附掛的頻寬需求輪詢法(Polling according to the piggyback capable bandwidth request for WiMAX network; PAPCBR)[1]中，基地台會根據工作站的延伸之即時輪詢服務流(ertPS)和即時輪詢服務流(rtPS)是否有頻寬要求，來調整輪詢間隔(Polling Interval)的大小，若延伸之即時輪詢服務流(ertPS)有頻寬要求，輪詢間隔直接降至10ms，若無要求，輪詢間隔往上加10ms，至40ms而不再增加；即時輪詢服務流(rtPS)若有頻寬要求，輪詢間隔減10ms，至40ms而不再減少，若無要求，輪詢間隔往上加10ms，至150ms而不再增加。此外，即時輪詢服務流(rtPS)可以將頻寬要求附掛在延伸之即時輪詢服務流(ertPS)的頻寬要求封包上面，也就是說，當基地台對該工作站的延伸之即時輪詢服務流提出輪詢時，此時工作站的延伸之即時輪詢服務流(ertPS)與即時輪詢服務流(rtPS)皆可提出頻寬要求，如此一來，當延伸之即時輪詢服務(ertPS)的資料分配完畢後，讓即時輪詢服務(rtPS)的資料也可以得到上傳的機會，讓訊框的資源能更有效的分配。

而在本論文中，我們根據PAPCBR提出了一個根據封包遺失率附掛的頻寬需求輪詢法(PALDPBR)，其目的是為了進一步改善即時性輪詢服務流(rtPS)的封包遺失率。當負載過重且封包遺失率過大時，附掛於延伸之即時輪詢服務流(ertPS)的即時輪詢服務流(rtPS)頻寬需求，由於優先權的問題，無法得到排程。因此，此演算法中，即時性輪詢服務流(rtPS)除了可以附掛在延伸之即時輪詢服務流(ertPS)上面，亦可根據封包遺失率的大小，將頻寬要求附掛在非即時性服務流(nrtPS)和盡力服務(BE)流上。而在本論文，我們將即時性輪詢服務流(rtPS)的封包遺失率的門檻值，設定為0.1。一旦即時性輪詢服務流(rtPS)的封包遺失率大於0.1，就可以將要求附掛於非即時性服務流(nrtPS)和盡力服務流(BE)上，如此一來，隨著負載增加，即時性輪詢服務(rtPS)的排程機會也會跟著增加，使得頻寬需求會比PAPCBR演算法，更有機會得到得到滿足，封包遺失率的表現會比PAPCBR還好。這是因為PAPCBR演算法只可以附掛在延伸之即時輪詢服務流(ertPS)上面。模擬的結果顯示出，即時輪詢服務流(rtPS)隨著負載增加，吞吐量和封包遺失率，皆比PAPCBR演算法有所改善。

IEEE 802.16 is a standard for Wireless Metropolitan Area Network (WMAN). It can provide longer distance communication. To ensure the Quality of Service (QoS) for different multimedia applications, the service flows are classified into five classes. Nevertheless, IEEE802.16 does not provide standard for bandwidth scheduling of this network. Therefore, it is a good issue for exploring to design a scheduling algorithm to meet the QoS requirement of all classes. Polling is a mechanism which the base station (BS) polls subscribe stations (SS) in turn, and then subscribe station can only send bandwidth request in the event of being polled by the base station.

In “Polling according to the piggyback capable bandwidth request for WiMAX network”(PAPCBR)[1], base station changes polling interval of extended real-time Polling Service (ertPS) and real-time Polling Service (rtPS) according to bandwidth request from subscribe stations. If ertPS has bandwidth request, polling interval will be decreased to 10ms directly; if not, polling interval will be increased by 10ms until it is 40ms. And if rtPS has bandwidth request, polling interval will be decreased 10ms until it is 40ms; if not, polling interval will be increased 10ms until it is 150ms. Moreover, rtPS can piggyback its bandwidth request on the request of ertPS. In other words, once polling interval of ertPS arrives, ertPS and rtPS can send bandwidth request simultaneously. Therefore, rtPS can also get opportunity to transmit data in uplink.

In this dissertation, we propose “Polling According to Loss-rate Dependent Piggyback Bandwidth Request for WiMAX Networks”(PALDPBR) based on PAPCBR. Our goal is to improve the packet loss-rate of rtPS. When the traffic load is heavy and packet loss-rate is large, bandwidth can not be allocated to rtPS due to priority problem. Therefore, in this algorithm, rtPS can not only piggyback on the request of ertPS, it can also piggyback on the request of non-real-time Polling Service (nrtPS) and Best-Effort (BE) according to packet loss-rate. In this thesis, we set 10% as the threshold of packet loss-rate for rtPS. As soon as the packet loss-rate of real-time Polling Service is greater than 10%, rtPS can piggyback its bandwidth request on the request of nrtPS and BE. Therefore, rtPS has more opportunities than that in PAPCBR algorithm to get the bandwidth allocation. The simulation shows the result is better than that in PAPCBR algorithm.

[1]黃俊璟，「Polling according to the piggyback capable bandwidth request」，碩士論文，國立台灣科技大學，台北市(2015)。
[2]TELECOM CLOUD(n.d.). Connected world: a changing wireless paradigm. Retrieved March 2, 2015, from http://www.telecom-cloud.net/connected-world-a-changing-wireless-paradigm
[3]黃偉承，「A loss-rate ensured scheduling algorithm for WiMAX network」，碩士論文，國立台灣科技大學，台北市(2014)。
[4]IEEE 802.16 std., “IEEE standard for local and metropolitan networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” Oct 2004.
[5]廖文嘉，「Emergency first MCS algorithm for WiMAX network」，碩士論文，國立台灣科技大學，台北市(2016)。
[6]OHNO, Y., SHIMIZU T., HIRAGURI, T., and NAKATSUGAWA, M., “Novel Frame Structures to Improve System Capacity and Latency Performance of a Time-Division Duplex Multihop Relay Wireless Access System,” IEEE Wireless Communications and Networking Conference, pp. 1-6 (2009).
[7]Introduction to WiMAX (IEEE 802.16) and 3GPP LTE-A (2017) 取自: http://ocw.nctu.edu.tw/upload/classbfs1210030031115251.pdf
[8]楊育振，「Finite ratio MCS algorithm for WiMAX network」，碩士論文，國立台灣科技大學，台北市(2015)。
[9]IEEE Std 802.16-2012, “IEEE Standard for Air Interface for Broadband Wireless Access Systems” , August 2012.
[10]E. Lee and H. K. Park, “Packet Scheduling Scheme for Multiple Services in Mobile WiMAX System”, Second International Conference on Computer and Network Technology, pp. 60-63, 2010
[11]Fei Yin, Pujolle J. and Chen k. “Performance Improvement by Efficient Polling in WiMAX Network,” IEEE International Symposium on Wireless Pervasive Computing, pp. 1-5 , 2009.
[12]Teixeira, M.A. and Guardieiro, P.R. “A Predictive Scheduling Algorithm for the Uplink Traffic in IEEE 802.16 Networks,” Advanced Communication Technology (ICACT) Vol. 1, pp. 651- 656, 2010.

[13]Gupta S. “Comparison of Various Scheduling Algorithms in WiMAX: A Brief Review,” International Conference on Advances in Management and Technology , pp. 34-36,2013。
[14]陳政隅，「Bandwidth Allocation according to Queue Length and packet loss rate-a scheduling algorithm for WiMAX network」，碩士論文，國立台灣科技大學，台北市(2013)。
[15]H.C. Jang and K. C. Yang, “A QoS Aware Multi-Modulation CAC for WiMAX,” in International Symposium on Computer Science and Society, pp. 365-368, 2011.
[16]J. Chen, C. C. Wang, C. D. Tsai, C. W. Chang, S. S. Liu, J. Guo, W. J. Lien, J. H. Sum, and C. H. Huang, “The Design and Implementation of WiMAX Module for ns-2 Simulator”, Proc. of the ACM/ISCT/VALUETOOLS, October 2006.