A survey on Wireless Mesh Networks

Transcription

1 A survey on Wireless Mesh Networks IF Akyildiz, X Wang - Communications Magazine, IEEE, 2005 Youngbin Im ybim@mmlab.snu.ac.kr

2 Contents Introduction to WMNs Network architecture Critical design factors Advances and research challenges Physical layer MAC layer Routing layer Transport layer Cross-layer design WMN examples

3 Introduction to WMNs (1/4) Mesh networking defined in Wikipedia A way to route data, voice and instruction b/w nodes Allows for continuous connections and reconfiguration around broken or blocked paths by hopping from node to node Difference from other networks The component parts can all connect to each other via multiple hops Generally not mobile one type of ad hoc network

4 Introduction to WMNs (2/4) Two types of nodes Mesh routers Minimal mobility Form the mesh backbone compared with conventional wireless routers Additional routing functions for mesh networking Usually multiple wireless interfaces, but similar hardware Mesh clients Simpler hardware and software

5 Introduction to WMNs (3/4) Integration of WMNs with other networks Through the gateway/bridge functionalities of mesh routers Difference from typical ad hoc networks Diversify the capabilities of ad-hoc networks Application scenarios Broadband home networking Community networking Building automation High-speed metropolitan area networks Enterprise networking

6 Introduction to WMNs (4/4) Some products, some testbeds But, need considerable research effort Ex) scalability Researchers revisit the protocol design of existing wireless networks in WMNs view , ad hoc networks, wireless sensor networks Industrial standards groups are working on new spec for WMNs IEEE , IEEE , IEEE

7 Network architecture Infrastructure/Backbone WMNs Provides a backbone for conventional clients Enable integration of WMNs with existing wireless networks Client WMNs Provides peer-to-peer networks among clients No mesh router, same as a conventional ad hoc network Hybrid WMNs Combination of both clients access through mesh routers or other mesh clients directly

8 critical design factors (1/2) Radio techniques directional and smart antennas MIMO Multi-radio/multi-channel systems Scalability - all layers Mesh connectivity Broadband and QoS Most applications are broadband services Need more performance metrics besides delay, fairness delay jitter, aggregate & per-node throughput, packet loss ratio

9 critical design factors (2/2) Security Many schemes of wireless LANs or ad hoc are not fully applicable for WMNs Ease of use Autonomous network Network management tools Compatibility and Inter-operability Backward compatibility with conventional client nodes Integrating heterogeneous wireless networks

10 Physical layer adaptive error resilience through link adaptation modulation, coding rates For high-speed transmission Orthogonal frequency multiple access (OFDM) Ultra-wide band (UWB) For capacity increase, impairment mitigation Antenna diversity, Smart antenna, MIMO For better spectrum utilization and viable frequency planning Frequency-agile radios, Cognitive radios A software radio platform is promising technique for physical layer Open research issues Further improve the tx rate Interactive working with higher level protocols Cross-layer design, low-cost software radio

11 MAC layer (1/2) Single channel MAC approaches Modifying existing MAC protocols ex) Adjust parameters of CSMA/CA, but low throughput Cross-layer design Directional antenna-based MACs MACs with power control Innovative MAC protocols TDMA or CDMA MAC protocols But, complexity, cost, compatibility with existing MACs

12 MAC layer (2/2) Multi-channel MAC approaches Multi-channel single-transceiver MAC For cost, compatibility Multi-channel multi-transceiver MAC Only one MAC Multi-Radio MAC Each radio has its own MAC, PHY Multi-radio unification protocol(mup) coordinate communications in all channels Open research issues Scalable MAC - Need new distributed & collaborative schemes MAC/Physical cross-layer design Novel MAC protocols for advanced PHY techniques MIMO, CRs Network integration in the MAC layer Advanced bridging functions for different wireless radios to work seamlessly

13 Routing layer (1/2) Routing protocols with various metrics In [6], link quality source routing(lqsr) selects a routing path according to 4 metrics ETX, per-hop RTT, per-hop packet pair, hop count ETX : best for stationary nodes Hop count : best for mobile nodes Multi-radio routing A multi-radio LQSR(MR-LQSR) is proposed in [7] Weighted cumulative expected transmission time (WCETT) Consider link quality metric & the minimum hop count Multi-path routing Select multiple paths b/w source and dest Use another path when broken better load balancing & high fault tolerance But, depends on the availability of node-disjoint routes complexity

14 Routing layer (2/2) Hierarchical routing Form clusters. Select cluster head which communicate with other clusters Better performance in high node density complexity of maintaining the hierarchy Geographical routing Use the position of nodes in the vicinity & the destination Unlike early single-path greedy routing, planar-graphbased algorithm[9] guarantee delivery, but higher communication overhead

15 Transport layer (1/2) No proposed transport protocol for WMNs But, many protocols for ad hoc networks Reliable data transport TCP variants - Tackle these problems Non-congestion packet loss Unknown link failure Network asymmetry Different BW, loss rate, latency between forward & backward paths Impact ACKs Large RTT variations The change of routing path may be frequent The normal operation of TCP relies on a smooth measurement of RTT New transport protocols ATP(ad hoc transport protocol) Rate-based transmission, Quick-start for initial rate estimation Delay-based congestion detection, No retransmission timeout Better performance than TCP variants Not compatible with TCP

16 Transport layer (2/2) Real-time delivery A Rate control protocol(rcp) is needed to work with UDP An adaptive detection rate control(adtfrc) scheme Multi-metric joint detection approach for TCP-friendly rate control schemes Open research issues Cross-layer solution to network asymmetry Should avoid asymmetry b/w data and ACKs at routing layer MAC & error control need to treat TCP data & ACKs differently to reduce asymmetry Adaptive TCP Which can be used in heterogeneous networks

17 Cross-layer design Protocols should interactively work together with especially with the PHY Channels are variant in terms of capacity, bit error rate, etc Dynamic network topology impact multiple layers Two ways By considering parameters in other layers Ex) the packet loss rate in MAC is reported to transport layer differentiate congestion from loss Keep the transparency By merging several protocols Ex) MAC & routing Better performance Issues Loss of layer abstraction Incompatibility with existing protocols Unforeseen impact Difficulty in maintenance

18 RoofNet An experimental b/g mesh network in development at MIT CSAIL (computer science and artificial intelligence laboratory) open source software Join the existing RoofNet Start a new community network

19 TFA(Technology For All) Rice University's Networks Group has partnered with Technology For All (TFA) to deploy a multi-tier wireless network in an underresourced community in Houston's East End. Stared in ,000 users in summer of 2007 Fully programmable nodes a first-of-its-kind research testbed for large-scale urban wireless networks

20 Meraki A company in Mountain View, California Mission : bring affordable Internet access to the next billion people. 25 countries How it works Some offer APs The connections are extended by repeaters to neighbors Portland, Oregon example A hundred routers, $4, people, pay $1 per month By 5 DSL connections Same broadband quality with the normal connection

21 XO-1 known as the $100 Laptop or Children's Machine One Laptop per Child (OLPC) social welfare organization an inexpensive laptop computer intended to be distributed to children in developing countries to provide them with access to knowledge. Currently set to start at $188 Goal : $100 in IEEE b, set maximum to 2 Mb/s will use IEEE s to form the wireless mesh network. How to access to the Internet Whenever powered on, participate in a MANET with each node it can hear. If a computer has access to the Internet then all are able to share

22 Champaign-Urbana Community Wireless Network (CUWiN) A project started in 2000 Mission connect more people to Internet and broadband services, develop open-source hardware and software for use by wireless projects world-wide, and build and support community-owned, not-for-profit broadband networks in cities and towns around the globe. Network architecture intends to provide a meshed, ad-hoc, non-hierarchical network topology Routing protocol OSPF -> Hazy-Sighted Link State (HSLS) -> Adaptive Hazy- Sighted Link State (A-HSLS) scale to thousands or tens of thousands of nodes (they think) ETX

23 Conclusion Introduction to WMNs Network architecture Critical design factors Advances and research challenges Scalability Security WMN examples