Ad Hoc wireless Networks 是一種不需任何基礎建設架構支援的無線網路，是一相當方便的區域網路，這種特殊的網路型態，適合用於像是戰地、災難緊急救助等等臨時性需求而架設的網路，以便於在無任何基礎建設架構支援的環境下還能夠彼此收送data。在 Ad Hoc 無線網路的環境下，由於網路上的host會隨時移動，而造成網路topology隨時發生改變， Ad Hoc wireless Networks上的host必需要有能力處理並得知網路topology的改變，以便採取相對應的措施。因此host之間必須要藉由彼此之間交換訊息來了解當時網路的情況和一些重要的資訊，廣播 (Broadcast) 是最常被應用的一種方式。但是由於無線網路的高錯誤率 (High Bit Error Rate) ，常常造成broadcast packet損毀，使得週圍的host無從得知這些重要的資訊。在802.11中，由於廣播的方式缺少 Acknowledgement 去確認Receiver是否正確收到該封包，若發生collision，Sender也不知道，所以也不會再送一次封包，所以802.11的廣播方式是一種不可靠的傳輸方法。再者，現在許多已被提出的討論提高廣播可靠度的方法中，都是架構在兩個host間的傳送link是雙向的，這只是在一個理想的環境下的假設。事實上在無線網路的環境中，有許多link是屬於unidirectional link，是一種非對稱方式的傳輸，也就是說工作站A的傳送範圍可以包含到工作站B，但工作站B的傳送範圍卻不能包含到工作站A。
在這篇論文中，我們提出一種夠提高廣播可靠度的方式， HRBSDA (Highly Reliable Broadcast Scheme with Directional Antennas)，不僅能提高broadcast的可靠度，同時能降低因為unidirectional link所造成對reliable broadcast的影響。 HRBSDA 的作法是利用方向性天線和TDMA-like的概念，將DIFS (DCF Inter-Frame Space) 的訊框間隔切成數個minislot，Receiver使用這些minislot去確認是否要求Sender再送一次上一筆的data，如此一來不僅僅能夠提高廣播方式的可靠度，大大地降低Packet Loss Recovery Time，同時還能避免造成額外的overhead的問題，更因為方向性天線的特性，還可以降低broadcast packet及control message可能會對其他host傳輸造成hidden terminal的問題，提高整體的throughput和channel的utilization。

Ad hoc wireless networks are constructed by several mobile hosts and have a property that its topology is changed as mobile hosts moved. There is no stationary infrastructure or based station to coordinate packets transmissions and advertise the information of network topology or something important. The special networks are used in temporal wireless networks, such as battlefield, disease rescue place, and so on. So without any stationary infrastructure supported, mobile hosts can communicate with others immediately or indirectly. Because topology is often changed while mobile hosts moving, mobile hosts must exchange information to deal with the changed conditions. Mobile hosts often utilize broadcasting to exchange information with their neighbor hosts, but there is high bit error ratio in wireless networks, packet corruption occurs frequently, so that mobile host might lost some important information sent from its neighboring host. In 802.11 standard, lack of acknowledgement, broadcasting is an unreliable transmission, because sender host do not know whether all of it neighboring hosts received broadcasting packets correctly or no. Many proposed papers of reliable broadcast assumed that links between mobile hosts are bidirectional links, but bidirectional link is an ideal assumption. In real environment, links are unidirectional, so host A could send packets to host B immediately, but host B could not because of their transmission range are different.
In this paper, we propose a new reliable broadcast scheme, Highly Reliable Broadcast Scheme with Directional Antennas (HRBSDA). HRBSDA can reduce the influence of unidirectional links and reach for highly reliable broadcasting. HRBSDA uses directional antennas and concept of Time Division Multiple Access (TDMA)-like. HRBSDA divide DCF Inter-Frame Space (DIFS) into several minislots, and mobile hosts use these minislots to ask sender for retransmission of lost packets. By the way, HRBSDA can not only reach for highly reliable broadcasting, but also reduce Packet Loss Recovery Time, and avoid causing extra overhead. Using directional antennas HRBSDA can reduce collision, so that improving throughput and channel utilization.

INDEX …………………………………………………………………………………… i
Index of Figure ……………………………………………………………………… iii

Chapter 1. Introduction ..………………………………………………………… 1
Chapter 2. Background & Related Works ………………………………………… 3
2.1 IEEE 802.11 Ad Hoc wireless Networks………………………………… 3
2.2 ACK-based Scheme…………………………………………………………… 7
2.3 NACK-based Scheme ………………………………………………………… 8
2.4 RTS/CTS Handshake-like Scheme ………………………………………… 10
2.5 TDMA-like Scheme…………………………………………………………… 11
2.6 Unidirectional Link Problem…………………………………………… 12
Chapter 3. Highly Reliable Broadcast Scheme with Directional Antennas… 13
3.1 Assumptions ………………………………………………………………… 14
3.2 Idea and Overview of HRBSDA …………………………………………… 15
3.3 HRBSDA ………………………………………………………………………… 16
3.4 Advantage of HRBSDA ……………………………………………………… 25
Chapter 4. Simulations and Simulation Results………………………………… 26
4.1 Simulation Environment…………………………………………………… 26
4.2 Simulation Results ………………………………………………………… 28
4.2.1 Packet Loss Recovery Time ..……….…………………………………… 28
4.2.2 Rebroadcast Ratio ………………….……………………………………… 32
4.2.3 Goodput ……………………………………………………………………… 34
4.2.4 Extra Overhead ……………………………………………………………… 36
4.2.5 Probability Experiment …………………………………………………… 38

Chapter 5. Conclusion ……………………………………………………………… 41
Reference ………………………………………………………………………………… 42

[1] IEEE Standards Department, “Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, IEEE standard 802.11-1997” 1997.

[2] Institution of Electrical and Electronic Engineers. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Higher Speed Physical Layer Extension in the 2.4 GHz Band, 1999.

[3] Nasipuri, A. and Ye, S. and You, J. and Hiromoto, R.E., “A MAC protocol for mobile ad hoc networks using directional antennas,” Proceedings of IEEE WCNC 2000, Chicago, IL, USA, Sept. 2000.

[4] Mineo Takai and Jay Martin and Rajive Bagrodia and Aifeng Ren, “Directional virtual carrier sensing for directional antennas in mobile ad hoc networks,“Proceedings of the third ACM international symposium on Mobile ad hoc networking & computing, New York, NY, USA, June 2002.

[5] Romit Roy Choudhury and Xue Yang and Nitin H. Vaidya and Ram Ramanathan, “Using Directional Antennas for Medium Access Control in Ad Hoc Networks,” Proceedings of the eighth annual international conference on Mobile computing and networking, New York, NY, USA, Sept. 2002.

[6] Gopalsamy, T. and Singhal, M. and Panda, D. and Sadayappan, P., “A Reliable Multicast Algorithm for Mobile Ad hoc Networks,” Proceeding of Distributed Computing Systems, Altera Corp., CA., 2002.

[7] Impett, M. and Corson, M.S. and Park, V., “A Receiver-Oriented Approach to Reliable Broadcast in Ad Hoc Networks,” Proceedings of IEEE WCNC 2000, Chicago, IL , USA, Sept. 2000.

[8] Tang, K. and Gerla, M. “MAC reliable broadcast in ad hoc networks,” Proceedings of IEEE MILCOM 2001, Los Angeles, CA, USA, 2000.

[9] Shiann-Tsong Sheu and Yihjia Tsai and Jenhui Chen, “A Highly Reliable Broadcast Scheme for IEEE 802.11 Multi-hop Ad Hoc Networks,” Proceedings of IEEE ICC 2002, New York, NY, USA, April, 2002.

[10] Pearlman, M.R. and Haas, Z.J. and Manvell, B.P., “Using Multi-Hop Acknowledgement to Discover and Reliable Communicate over Unidirectional Link in Ad Hoc Networks,” Proceedings of IEEE WCNC 2000, Chicago, IL, USA, Sept. 2000.