Performance Evaluation
4.2 Simulation Results
We first consider a 5 × 5 grid WMN where the gateway is located at the center of the network and each mesh point can communicate with up to 4 neighbors. Fig. 4.2 shows the simulation results. We can see that the single channel scheme has the worst performance because it does not benefit from multiple available channels. Our protocol outperforms the MJMM no matter which channel assignment strategy is used. The major reason is that our protocol reduces the channel switch overhead. We allow that several packets can be transmitted in a slot. Thus, when packets arrives at mesh points in V , mesh points in V can forward these packets immediately in the same slot. Thus, mesh points do not need to store many packets in their buffers.
This can relieve the buffer overflow problem. This can be verified by Fig. 4.3. We can see that our protocol has the best performance in terms of the packet drop rate.
(The packet drop rate is defined as the ratio of the number of packets dropped to the number of packet generated.) We can further observe that if there exists one quality of service (QoS) demand of 50% packet drop rate, our protocol can support
0
(a)Ripple Scheduling (b)Dynamic Scheduling
Figure 4.2: Comparison of throughput performance under a 5 × 5 grid network.
100 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(a)Ripple Scheduling (b)Dynamic Scheduling
Figure 4.3: Comparison of packet drop rates under a 5 × 5 grid network.
up to 12 traffic flows transmission, MJMM can support about 8 traffic flows and single channel can only support 4 traffic flows. Finally, we can see that the channel assignment strategies do not impact on performance deeply. In addition, we can observe that the performances of the ripple scheduling scheme and the dynamic scheduling scheme are similar. The reason is that although the dynamic scheme benefits from its flexibility, nodes may suffer more severe collision in the dynamic scheme.
The flow characteristic in WMNs may be different in various applications and therefore we conduct an experiment in which the ratios of down-link flows to up-link flows are varied. Fig. 4.4 and Fig. 4.5 show the results. We can see that our protocol has the best performance in all scenarios.
Next, we further investigate the performance of our protocol under random de-ployment. Three scenarios are conducted. In the 10-node scenario, 10 mesh points are deployed randomly with uniform distribution and the transmission range of mesh points is 45 units. In the 20-node scenario, 20 mesh points are deployed randomly
0
Down-Link flow ratio SingleChannel
Down-Link flow ratio SingleChannel
LowInterference LowDelay Middle MJMM
(a)Ripple Scheduling (b)Dynamic Scheduling
Figure 4.4: Comparison of throughput performance under a 5 × 5 grid network.
100 Drop rate (%) (Number of packets dropped /number of packets generated)
Different Down-link ratio SingleChannel Drop rate (%) (Number of packets dropped /number of packets generated)
Different Down-link ratio SingleChannel
LowInterference LowDelay Hybrid MJMM
(a)Ripple Scheduling (b)Dynamic Scheduling
Figure 4.5: Comparison of packet drop rates under a 5 × 5 grid network.
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(a) Ripple Scheduling (Throughput) (b) Ripple Scheduling (Packet Drop Rate)
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(c) Dynamic Scheduling (Throughput) (d) Dynamic Scheduling (Packet Drop Rate) Figure 4.6: Simulation results of the 10-node scenario.
with uniform distribution and the transmission range of mesh points is 40 units. In the 30-node scenario, 30 mesh points are deployed randomly with uniform distribu-tion and the transmission range of mesh points is 35 units. The results are shown in Fig. 4.6, Fig. 4.7, and Fig. 4.8. Our protocol is still the best one because our protocol can benefit from low channel switch overhead.
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(a) Ripple Scheduling (Throughput) (b) Ripple Scheduling (Packet Drop Rate)
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(c) Dynamic Scheduling (Throughput) (d) Dynamic Scheduling (Packet Drop Rate) Figure 4.7: Simulation results of the 20-node scenario.
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(a) Ripple Scheduling (Throughput) (b) Ripple Scheduling (Packet Drop Rate)
0 Drop rate (%) (Number of packets dropped /number of packets generated)
Number of flows
(c) Dynamic Scheduling (Throughput) (d) Dynamic Scheduling (Packet Drop Rate) Figure 4.8: Simulation results of the 30-node scenario.
Chapter 5
Implementation
We have implemented our channel management protocol on the Realtek RTL8186 [1] platform. The RTL8186 is a chipset solution for IEEE 802.11s draft 1.0 (Note that IEEE 802.11 Working Group has set up the Mesh Network Task Grop(TGs) to define the standard of 802.11-based wireless mesh network). The 802.11s-supporting module is developed by Realtek-NCTU Joint Research Center[3] and this mesh network product will be commercialized in the future. We directly implemented our channel management protocol as a extension of RTL8186 wireless driver. Our protocol has software timer, channel packet queue and management frames logic and all mesh points support all of these functions. In the rest of this Chapter we describe the details of our implementation.
• Software Timer :With the design of frame structure mentioned in Chap. 3.1, each mesh point has to know whether it is at the broadcast slot or not and the mesh points in set V0 have to know the exact slot among k − 1 general slots (from slot 2 to slot k in one frame) for their channel switching sched-ule. We register a software timer in the RTL8186 operating system for the above requirements. The register operations are completed at the initial steps of RTL8186 and these steps are shown at Fig. 5.3. Based on the fact that the measured channel switching delay is about 8∼12ms, to register a 400ms software timer for one slot becomes reasonable because the channel switching delay may affect the network throughput seriously if the slot interval is too short.
• Channel Packet Queue:In single-radio and multi-channel environment, each mesh point in set V0 executes one channel switching operation for ev-ery 400ms. Among this 400ms slot interval, the relayed packets in these set
Figure 5.1: Realtek RTL8186 platform.
User space
Kernel
sw_timer
Channel queues Multi-channel
Management Frame functions
Wireless Driver
Physical Layer
Figure 5.2: System architecture.
Assign wlan_device.base_addr rtk8185_init_one()
Initialize DRAM for S/W TKIP calculation
Opmode
Get_chip_version (set RF BB/MAC chip type) rtk8185_init_sw
Request IRQ rtk8185_init_hw_pci Driver_state=opened Initialize priv->timer
Register a SW_Timer
ΞΞΞΞ ΞΞΞΞ
Figure 5.3: Initial steps of RTL8186.
V0 mesh points are buffered at their channel packet queues if the receivers of these packets are at different channels. Similarly, the mesh points in set V also buffer the packets destinated to one mesh point in set V0 that is at different channel. In short, packets can be delivered smoothly only when the transmitter and receiver are at the same channel.
• Management Frames:We have designed and used some management frames to meet many requirements (such as one mesh point wants to get its 2-hop neighbors’ permission) in our channel management protocol. Functions for processing these management frames are also directly implemented in RTL8186 wireless driver. To follow the frame design mentioned in 802.11s draft 1.0, we use the Category and Action fields to help wireless driver to identify our channel management frame. The Category field is set to 5 (rep-resenting mesh management) and the Action field is set to 255 (rep(rep-resenting vendor specific mesh management). Beside the above two fields, we add a new ID field to recognize our CHL GRANT, CLH REQ and broadcast frame (for switching schedule). The frame format of these three management frame is shown at Table 5.1.
• Performance Test: With above implementation, we simply evaluate the performance of our channel management protocol in a chain topology. The environment of our test is shown at Fig. 5.4(a). Four RTL8186 boards (A, B, C,
Table 5.1: Management frame formats used in our channel management protocol.
Category Action ID
CHL GRANT 5 255 1
CHL REQ 5 255 2
Broadcast Switching Schedule 5 255 3
D) form a chain mesh network and board A is the mesh gateway. When finish-ing our channel management protocol, board A, B and D own a fixed channel since they are belong to set V and board C arranges its channel switching schedule between channel 1 and 11. We attach each RTL8186 board with one notebook (through wired network) and these 4 notebook (NB1∼NB4) play the roles of traffic generators or destinations. More specifically, NB2, NB3 and NB4 generate data traffic with constant bit rate and these packets are all destinated to NB1. Fig. 5.4(b) presents this network throughput experiment.
In this experiment, the used traffic type is UDP (User Datagram Protocol), the experiment time is set to 300 seconds and the link capacity is 2 Mbps.
The network throughput evaluation result is shown at Fig. 5.5 and this ex-periment proves that our protocol outperforms 1.5 times than single channel mesh network when the gateway only equips one radio in both of the schemes.
Figure 5.4: Performance test in a realistic 802.11s mesh network.
0 200 400 600 800 1000
300 250
200 150
100 50
throughputs(Kbps)
Time (secs) SingleChannel
OurProtocol
Figure 5.5: Network throughput in our experiment.
Chapter 6 Conclusions
This paper is a research combined with theory and implementation. We propose a new channel management protocol to mitigate channel switching overhead and in-crease network throughput. Our protocol is a distributed and hierarchical approach to deploy a multi-channel environment in tree-based wireless mesh networks. Under simulation, it proves that the network throughput in our protocol can outperform 1∼2 times than the modified JMM and 2∼4 times than single channel schemes.
Beside verification by simulation, we also implement our protocol in IEEE 802.11s wireless mesh networks with real world platform (Realtek RTL8186). We conduct an experiment to evaluate the performance of our protocol and this experiment also proves that our protocol can be realized and perform well.
Bibliography
[1] Realtek Semiconductor Corp., http://www.realtek.com.tw/.
[2] Cisco Mesh networks. http://www.region43.org/docs/800mhz/Cisco200602.pdf.
[3] Realtek-NCTU Joint Research Center. http://realtek.cs.nctu.edu.tw/.
[4] Tropos Mesh networks. http://www.connect802.com/tropos.htm.
[5] A. Adya, P. Bahl, J. Padhye, A. Wolman, and L. Zhou. A Multi-Radio Unifica-tion Protocol for IEEE 802.11 Wireless Networks. In InternaUnifica-tional Conference on Broadband Networks (BROADNETS), pages 344–354, October 2004.
[6] I. F. Akyildiz, X. Wang, and W. Wang. Wireless mesh networks: a survey.
Computer Networks, 47(4):445–487, March 2005.
[7] P. Bahl, R. Chandra, and J. Dunagan. SSCH: Slotted Seeded Channel Hop-ping for Capacity Improvement in IEEE 802.11 Ad-hoc Wireless Networks. In Proceedings of the ACM International Conference on Mobile Computing and Networking(MobiCom), September 2004.
[8] J. Chen and Y.-D. Chen. AMNP: Ad Hoc Multichannel Negotiation Protocol for Multihop Mobile Wireless Networks. In Proceedings of the IEEE Interneational Conference on Communications (ICC), June 2004.
[9] R.-G. Cheng, C.-Y. Wang, and L.-H. Liao. Ripple: A distributed medium access protocol for multi-hop wireless mesh networks. In IEEE VTC2006-Spring, May 2006.
[10] R. Draves, J. Padhye, and B. Zill. Routing in multi-radio, multi-hop wireless mesh networks. In Proceedings of the ACM International Conference on Mobile Computing and Networking(MobiCom), September 2004.
[11] M. Gerla, R. Bagrodia, L. Zhang, K. Tang, and L. Wang. TCP over Wireless Multihop Protocols: Simulation and Experiments. In Proceedings of the IEEE International Conference on Communications (ICC), June 1999.
[12] M. X. Gong and S. F. Midkiff. Distributed Channel Assignment Protocols: A Cross-Layer Approach. In Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), March 2005.
[13] IEEE. Draft amendment: ESS mesh networking. IEEE P802.11s/D1.07, September 2007.
[14] IEEE 802.11 Working Group. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, 1997.
[15] IEEE 802.11a Working Group. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications - Amendment 1: High-speed Physical Layer in the 5 GHz band, 1999.
[16] D. B. J. Hauser and S. Conner. Draft PAR for IEEE 802.11 ESS Mesh (R3).
Technical Report IEEE 802.11-03/759r22, IEEE, Novermber 2003.
[17] N. Jain, S. Das., and A. Nasipuri. A Multichannel CSMA MAC Protocol with Receiver-Based Channel Selection for Multihop Wireless Networks. In IEEE International Conference on Computer Communications and Networks (IC3N), October 2001.
[18] P. Kyasanur and N. H. Vaidya. Routing and interface assignment in multi-channel multi-interface wireless networks. In Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), March 2005.
[19] S.-C. Lo and C.-W. Tseng. A Novel Multi-Channel MAC Protocol for Wire-less Ad Hoc Networks. In Vehicular Technology Conference, 2007. VTC2007-Spring., pages 46–50, April 2007.
[20] A. Nasipuri and S. R. Das. Multichannel CSMA with Signal Power-Based Chan-nel Selection for Multihop Wireless Networks. In IEEE Vehicular Technology Conference (VTC), September 2000.
[21] A. Nasipuri, J. Zhuang, and S. Das. A Multichannel CSMA MAC Protocol for Multihop Wireless Networks. In Proc. of IEEE Wireless Communications and Networking Conference (WCNC), September 1999.
[22] J. S. Pathmasuntharam, A. Das, and A. K. Gupta. Primary channel assignment based MAC (PCAM) - a multi-channel MAC protocol for multi-hop wireless networks. In Wireless Communications and Networking Conference (WCNC), March 2004.
[23] A. Raniwala and T. cker Chiueh. Architecture and Algorithms for an IEEE 802.11-Based Multi-Channel Wireless Mesh Network. In Conference on Com-puter Communications (INFOCOM), March 2005.
[24] J. So and N. H. Vaidya. Multi-channel MAC for Ad Hoc Networks: Handling Multi-channel Hidden Terminals Using a Single Transceiver. In Proceedings of the ACM International Symposium on Mobile Ad Hoc Networking and Com-puting (MobiHoc), pages 222–233, May 2004.
[25] W.-H. Tam and Y.-C. Tseng. Joint Channel Link Layer and Multi-Path Routing Design for Wireless Mesh Networks. In Conference on Computer Communications (INFOCOM), May 2007.
[26] A. Tzamaloukas and J. Garcia-Luna-Aceves. A Receiver-Initiated Collision-Avoidance Protocol for Multi-Channel Networks. In Proceedings of IEEE IN-FOCOM, 2001.
[27] R. Vedantham, S. Kakumanu, S. Lakshmanan, and R. Sivakumar. Component Based Channel Assignment in Single Radio, Multi-channel Ad Hoc Networks.
In ACM International Conference on Mobile Computing and Networking (MO-BICOM), pages 378–389, May 2006.
[28] S.-L. Wu, C.-Y. Lin, Y.-C. Tseng, and J.-P. Sheu. A New Multi-Channel MAC Protocol with On-Demand Channel Assignment for Multi-Hop Mobile Ad Hoc Networks. In International Symposium on Parallel Architectures, Algorithms, and Networks (I-SPAN), pages 232–237, December 2000.
[29] Q. Xue and A. Ganz. Temporal Topologies in Multi-channel Multihop Wire-less Access Networks. In International Conference on Broadband Networks (BROADNETS), October 2005.
[30] B. Zhang, M. N. Shirazi, and K. Hattori. A Tree-Based Channel Assignment and Sibling-Aware Routing Protocol for Multi-Channel Wireless Mesh Networks.
In IEICE Transactions on Fundamentals of Electronics, Communications and Computer Science, July 2007.