Wireless Networks Lecture 32 Wireless Mesh Networks Part

Wireless Networks Ü Lecture 32 Ü Wireless Mesh Networks Part II Ü Dr. Ghalib A. Shah 1

Outlines Ü MAC Layer ► ► ► Scalability Single Channel Multi-Channel Some Ideas Research Issues Ü Network Layer ► ► ► Routing Wish List Route Optimization Criteria Routing fairness Routing – Cross-layer design Ü Qo. S Support at each layer Ü WMN Standards 2

Last Lecture Ü Ü Ü Introduction to WMN Characteristics WMN vs MANET Architecture Applications Critical factors influencing performance ► Radio techniques, scalability, Qo. S, security, Ease of Use, Mesh connectivity 3

MAC Layer Ü MAC for WMNs is concerned with more than one hop communication. Ü MAC is distributed and cooperative and works for multipoint-to-multipoint communication. Ü Network self-organization is needed for the MAC. Ü Mobility affects the performance of MAC 4

Ü The scalability of MAC can be addressed in two ways. ► Enhance the existing or propose new for single channel to increase E 2 E throughput ► Allow transmission on multiple channels of each network node 5

Basic Techniques Ü Scheduled ► Fix scheduled TDMA ► Polling ► Impractical due to lack of: • • Central coordination point Reasonable time synchronization Ü Random Access ► CSMA – simple and popular ► RTS/CTS – protects the receiver 6

Ü Channels can be implemented by: ► FDMA ► CDMA (code assignment is an issue) ► SDMA (with directional antennas) ► Combinations of the above 7

Single channel MAC Ü Improving existing MAC protocol ► 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. Ü Cross-layer design with advanced physical layer. ► MAC based on directional antenna and power control Ü 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 8

MAC – Multichannel Why? Ü Increases network capacity 2 Ch-1 2 3 Ch 1 -1 Ch 1 3 4 Ch-1 User bandwidth = B/2 4 3 2 2 1 Ch-2 User bandwidth = B Chain bandwidth = B B = bandwidth of a channel 9

MAC – Multichannel MCCL 802. 11 PHY Ch-1 2 3 -2 Ch 4 IP -1 Ch Ü Perhaps, if a new Multi. Channel Coordination Layer (MCCL) is introduced between MAC and Network Ü Must work within the constraints of 802. 11 Ü May increase the capacity of the network 1 2 Ch-2 3 1 10

Multi-channel MAC (MMAC) Ü Multi-channel single transceiver ► One channel active at a time ► Different nodes may operate on different times. ► Hence, coordination is required Ü Multi-channel multi-transceiver ► A radio includes multiple parallel RF chips and baseband processing modules to support several simultaneous channels. ► On top of multiple channel in physical layer, only single MAC to coordinate operations. Ü Multi-radios MAC ► A node has multiple radios each with its own MAC and physical. ► A virtual MAC protocol such as a multi-radio unified MAC protocol is required to coordinate communication among all. 11

MMAC Functions Ü Maintaining data structure of all channels in each node. ► classified into three types depending on its status of allocation. Ü Negotiating channels during ad hoc traffic indication message (ATIM) window. ► Negotiations are done through a pre-defined channel known to all nodes. Ü Selecting a channel. ► The criterion is to use a channel with the lowest count of source –destination pairs that have selected the channel. 12

Multi-radio Unification Protocol Ü Discovering neighbours. ► After the discovering procedures, neighbors are classified into MUP enabled and legacy nodes. Ü Selecting a NIC ► based on one-hop round trip time (RTT) measurements. MUP selects the NIC with the shortest RTT between a node and its neighbors. Ü Utilizing the selected NIC for a long period. ► This period is determined by a random process and in the order of 10– 20 s. Ü Switching channels. ► After the random time period, all NICs are measured again through one-hop probe messages. If an NIC has a certain amount of quality improvement than the existing NIC, then it is selected for sending packets. 13

Open research issues Ü Ü Ü Ü Scalability issue in multi-hop ad hoc network has not been solved yet. CSMA/CA based MAC protocols solve partial problems A distributed TDMA or CDMA MAC can be the solution. Mesh routers and clients hold different characteristics like mobility, power consumption etc. A single solution may not be applicable for both. Some Mesh routers integrate various wireless networking technologies and require advance bridging functions Existing research focuses on capacity, throughput and fairness. But many applications may require broadband multimedia communication in WMNs. MAC developed with Qo. S metrics 14

Network Layer Ü WMN will be tightly coupled with internet and IP has been widely accepted in different wireless networks. Ü However routing differs from IP and cellular. 15

Routing Ü Finds and maintains routes for data flows Ü The entire performance of the WMN depends on the routing protocol Ü May be the main product of a mesh company Ü May be missing 16

Routing – Wish List Ü Scalability Ü ► Overhead is an issue in mobile WMNs. ► Work with/without gateways, different topologies Ü Ü Fast route discovery and rediscovery ► Essential for reliability. Ü Mobile user support ► Seamless and efficient handover Flexibility Qo. S Support ► Consider routes satisfying specified criteria Ü Multicast ► Important for some applications (e. g. , emergency response) 17

Existing Routing Protocols Ü Internet routing protocols Ü Ad-hoc routing protocols (e. g. , OSPF, BGP, (e. g. , DSR, AODV, RIPv 2) 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 Ü Ü Ü Ü Minimum Hops Minimum Delays Maximum Data Rates Minimum Error Rates Maximum Route Stability Power Consumption Combinations of the above Ü Use of multiple routes to the same gateway Ü Use of multiple gateways 19

Routing – Cross-Layer Design Ü 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. Ü 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) Ü 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. Ü Routing – Application ► Especially with respect of satisfying Qo. S constraints 21

Network Layer - Fairness Ü Fairness ► Equal share of resources to all participants. ► Special case of priority based Qo. S. Ü Horizontal – nodes 1, 2 GW 2 1 ► The MAC layer’s fairness ensures horizontal fairness. Ü Vertical – nodes 3, 4 ► MAC layer is no longer sufficient GW 3 4 22

Fairness Problem G 2 G S 2 Ü Unfair Ü Inefficient 1 S 1 Ideal GW Real 23

Qo. S Support required at every layer Ü Physical Layer ► Robust modulation ► Link adaptation Ü MAC Layer ► Offer priorities ► Offer guarantees (bandwidth, delay) Ü Ü Transport ► Attempt end-to-end recovery when possible Ü Application ► Negotiate end-to-end and with lower layers ► Adapt to changes in Qo. S Network Layer ► Select “good” routes ► Offer priorities ► Reserve resources (for guarantees) 24

WMNs Standards Ü WPAN: Bluetooth, Zigbee Ü Wi. Fi: 802. 11 a, b, g, n Ü Wi. MAX: 802. 16 Range Wi. MAX 50 Km 100 m WPAN 100 kb 1 Mb Wi-Fi 10 Mb 100 Mb Data Rate 25

WMNs Standards Ü IEEE 802. 16 a 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 Ü 802. 11 s WLAN Mesh ► MAC layer needs to be extended to a wireless DS to support broadcast/multicast ► Multi-hop capability added to 802. 11 g/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 Ü 802. 11 s MCF Sublayer 28

Summary Ü MAC Layer ► ► ► Scalability Single Channel Multi-Channel Some Ideaa Research Issues Ü Network Layer ► ► ► Routing Wish List Route Optimization Criteria Routing fairness Routing – Cross-layer design Ü Qo. S Support at each layer Ü WMN Standards 29