Wireless networks - Lecture 32: Wireless mesh networks (Part 2). The main topics covered in this chapter include: MAC layer; network layer; QoS support at each layer; WMN standards; network self-organization; increases network capacity;...
Wireless Networks Lecture 32 Wireless Mesh Networks Part II Dr Ghalib A Shah Outlines MAC Layer ► ► ► ► ► Scalability Single Channel Multi-Channel Some Ideas Research Issues Network Layer ► ► ► ► ► Routing Wish List Route Optimization Criteria Routing fairness Routing – Cross-layer design QoS Support at each layer WMN Standards Last Lecture Introduction to WMN Characteristics WMN vs MANET Architecture Applications Critical factors influencing performance ► Radio techniques, scalability, QoS, security, Ease of Use, Mesh connectivity MAC Layer MAC for WMNs is concerned with more than one hop communication MAC is distributed and cooperative and works for multipoint-to-multipoint communication Network self-organization is needed for the MAC Mobility affects the performance of MAC The scalability of MAC can be addressed in two ways ► Enhance the existing or propose new for single channel to increase E2E throughput ► Allow transmission on multiple channels of each network node Basic Techniques Scheduled ► Fix scheduled TDMA ► Polling ► Impractical due to lack of: • Central coordination point • Reasonable time synchronization Random Access ► CSMA – simple and popular ► RTS/CTS – protects the receiver c f Channels can be implemented by: ► ► ► ► c t f FDMA CDMA (code assignment is an issue) SDMA (with directional antennas) Combinations of the above t c c t f c t s1 f s2 f c t t s3 c f t f Single channel MAC Improving existing MAC protocol ► ► Cross-layer design with advanced physical layer ► By changing parameters of CSMA/CA based MACs like contention window size and backoff procedures for multi-hop Contention based approaches are not scalable and throughput degrades with increase in contention MAC based on directional antenna and power control Innovative solutions ► ► ► Need new ideas to overcome low end-end throughput for multi-hop ad hoc environment TDMA or CDMA based MAC needs to be explored Compatibility, cost and complexity are the important factors in designing new protocols MAC – Multichannel Why? Increases network capacity Ch-1 Ch-1 -1 Ch -2 Ch Ch-1 User bandwidth = B/2 Ch-2 User bandwidth = B Chain bandwidth = B B = bandwidth of a channel MAC – Multichannel MCCL 802.11 PHY Ch-1 -2 Ch IP -1 Ch Perhaps, if a new MultiChannel Coordination Layer (MCCL) is introduced between MAC and Network Must work within the constraints of 802.11 May increase the capacity of the network Ch-2 10 Network Layer WMN will be tightly coupled with internet and IP has been widely accepted in different wireless networks However routing differs from IP and cellular 15 Routing Finds and maintains routes for data flows The entire performance of the WMN depends on the routing protocol May be the main product of a mesh company May be missing 16 Routing – Wish List Scalability ► ► Overhead is an issue in mobile WMNs Fast route discovery and rediscovery ► Essential for reliability Flexibility QoS Support ► Mobile user support ► Seamless and efficient handover Consider routes satisfying specified criteria Multicast ► Work with/without gateways, different topologies Important for some applications (e.g., emergency response) 17 Existing Routing Protocols Internet routing protocols Ad-hoc routing protocols (e.g., OSPF, BGP, (e.g., DSR, AODV, RIPv2) OLSR, CBR, TORA) ► Well known and trusted ► Designed on the assumption of seldom link changes ► Without significant modifications are unsuitable for WMNs in particular or for ad hoc networks in general Ad Hoc Networks ► Newcomers by comparison with the Internet protocols ► Designed for high rates of link changes; hence perform well on WMNs ► May be further optimized to account for WMNs’ particularities Wireless Mesh Networks 18 Routing - Optimization Criteria Minimum Hops Minimum Delays Maximum Data Rates Minimum Error Rates Maximum Route Stability Power Consumption Combinations of the above Use of multiple routes to the same gateway Use of multiple gateways 19 Routing – Cross-Layer Design Routing – Physical ► ► ► Link quality feedback is shown often to help in selecting stable, high bandwidth, low error rate routes Fading signal strength can signal a link about to fail → preemptive route requests Cross-layer design essential for systems with smart antennas Routing – MAC ► ► ► Feedback on link loads can avoid congested links → enables load balancing Channel assignment and routing depend on each other MAC detection of new neighbors and failed routes may significantly improve performance at routing layer 20 Routing – Cross-Layer Design (cont) Routing – Transport ► Choosing routes with low error rates may improve TCP’s throughput ► Especially important when multiple routes are used ► Freezing TCP when a route fails Routing – Application ► Especially with respect of satisfying QoS constraints 21 Network Layer - Fairness Fairness ► ► Horizontal – nodes 1, ► Equal share of resources to all participants Special case of priority based QoS GW The MAC layer’s fairness ensures horizontal fairness Vertical – nodes 3, ► MAC layer is no longer sufficient GW 22 Fairness Problem G G S2 Unfair Inefficient S1 Ideal GW Real 23 QoS Support required at every layer Physical Layer ► ► MAC Layer ► ► Robust modulation Link adaptation Offer priorities Offer guarantees (bandwidth, delay) Transport ► Attempt end-to-end recovery when possible Application ► ► Negotiate end-to-end and with lower layers Adapt to changes in QoS Network Layer ► ► ► Select “good” routes Offer priorities Reserve resources (for guarantees) 24 WMNs Standards WPAN: Bluetooth, Zigbee WiFi: 802.11a, b, g, n WiMAX: 802.16 Range 50Km 100m WiMAX WPAN 100kb 1Mb WiFi 10Mb 100Mb 25 Data Rate WMNs Standards IEEE 802.16a WMAN Mesh ► ► ► “mesh mode” in addition to the point-to-multipoint(PMP) mode defined in IEEE 802.16 Operating in the licensed and unlicensed lower frequencies of 2– 11 GHz, allowing non-line-of-sight (NLO) communications, spanning up to a 50 km range Supporting multihop communications 26 WMNs Standards 802.11s WLAN Mesh ► ► ► ► ► ► ► MAC layer needs to be extended to a wireless DS to support broadcast/multicast Multi-hop capability added to 802.11g/a/b Auto configure on power up Multi-channel multi-radio operation Topology discovery MAC Path selection protocol Modified forwarding for QOS and mesh control 27 WMNs Standards 802.11s MCF Sublayer 28 Summary MAC Layer ► ► ► ► ► Scalability Single Channel Multi-Channel Some Ideaa Research Issues Network Layer ► ► ► ► ► Routing Wish List Route Optimization Criteria Routing fairness Routing – Cross-layer design QoS Support at each layer WMN Standards 29 ... network capacity Ch-1 Ch-1 -1 Ch -2 Ch Ch-1 User bandwidth = B/2 Ch-2 User bandwidth = B Chain bandwidth = B B = bandwidth of a channel MAC – Multichannel MCCL 802.11 PHY Ch-1 -2 Ch IP -1 Ch Perhaps,... WMAN Mesh ► ► ► ? ?mesh mode” in addition to the point-to-multipoint(PMP) mode defined in IEEE 802.16 Operating in the licensed and unlicensed lower frequencies of 2– 11 GHz, allowing non-line-of-sight... WLAN Mesh ► ► ► ► ► ► ► MAC layer needs to be extended to a wireless DS to support broadcast/multicast Multi-hop capability added to 802.11g/a/b Auto configure on power up Multi-channel multi-radio