臺灣博碩士論文加值系統

有效率地利用網路資源，這對於無線網路上的行動裝置而言是一個重要的議題。這篇論文專注於如何有效地使用無線網路資源的兩個主題，分別是強化式標頭壓縮技術(RObust Header Compression，簡稱ROHC)，和以服務為基礎的換手方式(Service-Based Handoff)。
ROHC是一種設計用來降低在網路上傳輸的封包標頭大小的架構，它承襲了網際網路通訊協定封包標頭壓縮(IPHC)的精神進而設計了對應四類封包標頭所產生的不同壓縮方法，並且針對無線網路傳輸有較大資料錯誤機率的問題提出壓縮錯誤更正以及恢復的機制，因此比IPHC更有效地壓縮且更能抵抗傳輸時造成的壓縮解壓縮封包標頭的錯誤，然而，封包標頭的壓縮對於有效率地利用網路頻寬相當有幫助。
在無線區域網路環境(WLANs)下，如何選擇一個適當的存取點(Access Point，簡稱AP)對於行動裝置而言相當重要。在現行的作法上，網路實體層(physical layer)上的換手(handoff)大部分決定於測量偵測到的AP的訊號雜訊比(Signal to Noise Ratio 簡稱SNR)的值，且選擇一個有最高SNR值的AP當作換手的對象。一般來說，較好的SNR值代表著較高的訊號品質。然而，這不代表一個剛換手到新AP的行動裝置可以從這個AP得到足夠的網路資源。有鑒於此，我們提出一個新的換手機制，稱為以服務為基礎的換手方式(Service-Based Handoff)。這方法除了實體層的SNR值的測量外，更量測了第二層(MAC layer)的競爭程度，使得行動裝置可以選擇一個能提供更多網路資源的AP。

It is an important issue for mobile devices in wireless networks to utilize network resources effectively. This thesis focuses on two topics for efcient utilization of wireless network resources: RObust Header Compression (ROHC) and Service-Based Handoff. ROHC is designed
originally to reduce packet header size. Decreasing packet size is useful to utilize network resource effectively. In WLANs, how to select an appropriate access point for mobile
station is important. Currently, the handoff decision in physical layer is made mostly based on the measurement of the Signal to Noise Ratio (SNR). In general, better SNR indicates better radio quality. However it does not mean that the mobile station could get enough network resource in the new access point after handing off. A new handoff procedure called Service-Based Handoff is proposed. In addition to SNR, it also measures the contention level
to make a better selection of target access point.

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.ROHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.DesignandImplementation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1.States and Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.2. ROHC Decompression Flow . . . . . . . . . . . . . . . . . . . . . 8
2.2.3. Procedure upon CRC failure . . . . . . . . . . . . . . . . . . . . . 10
2.2.4. Correction of SN LSB wraparound . . . . . . . . . . . . . . . . . . 10
2.2.5. Repair of incorrect SN updates . . . . . . . . . . . . . . . . . . . . 12
2.2.6. ROHC Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.7. ROHC Feedback message Format . . . . . . . . . . . . . . . . . . 17
2.2.8. Performance Analysis . . . . . . . . . . . . . . . . . . . . . . . . 18
v
3. Service-based Handoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.1. IEEE 802.11 Handoff in physical layer . . . . . . . . . . . . . . . 21
3.2.2. MAC in IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3. Basic Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.3.1. Proposed Handoff Procedure Flow . . . . . . . . . . . . . . . . . . 24
3.3.2. PROBE-ACK Observation . . . . . . . . . . . . . . . . . . . . . . 26
3.3.3. CUSUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3.4. Rosenbaum’s test . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3.5. Simulation Model . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3.6. Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.3.7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

[1] C. Bormann,C. Burmeister,M. Degermark,H. Fukushima,H. Hannu,L-E. Jonsson,R.
Hakenberg,T. Koren,K. Le,Z. Liu,A. Martensson,A. Miyazaki,K. Svanbro, T.Wiebke,T.
Yoshimura,H. Zheng, “RObust Header Compression (ROHC): Framework and four pro-files: RTP, UDP, ESP, and uncompressed.” IETF RFC 3095, July 2001.

[2] “Packet Data Convergence Protocol (PDCP) Specification.” 3GPP technical specification
of release 4 - TS 25.323 V4.6.0, Sept. 2002.

[3] J.-C. Chen and T. Zhang, IP-Based Next-Generation Wireless Networks. Wiley, Dec.2003. To appear.

[4] IEEE, “Std 802.11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications,” Nov. 1997.

[5] Ghyslain Pelletier,Lars-Erik Jonsson,Mark A West,Carsten Bormann,Kristofer Sandlund,“RObust Header Compression (ROHC):A Profile for TCP/IP (ROHC-TCP).” IETF
draft-ietf-rohc-tcp-09.txt, Feb. 2005.

[6] H. Neave and P. Worthington, “Distribution Free Tests,” 1988