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WIRELESS MESH NETWORKS Ian F. AKYILDIZ* and Xudong WANG** * Georgia Institute of Technology BWN (Broadband Wireless Networking) Lab & ** TeraNovi Technologies 4. ROUTING PROTOCOLS 2 Features for Optimal Routing in WMNs Scalability Fast route discovery and rediscovery (for reliability) Mobile user support (for seamless and efficient handover) 3 Features for Optimal Routing in WMNs Multiple Performance Metrics minimum hop-count ineffective!! e.g., link quality and round trip time (RTT) Robustness (link failures or congestions, fault-tolerant, load balancing) Adaptive Support of Both Mesh Routers and Mesh Clients (support mobility and power efficiency) 4 Features for Optimal Routing in WMNs Flexibility – Work with/without gateways, different topologies QoS Support – Consider routes satisfying specified criteria Multicast – Important for some applications (e.g., emergency response) 5 Prelim Routing Protocols for WMNs * Apply routing algorithms derived for Ad Hoc Networks 6 CLASSIFICATION OF ROUTING PROTOCOLS FOR AD HOC NETWORKS 7 OVERVIEW Flat – Reactive DSR – Dynamic Source Routing AODV – Ad hoc On demand Distance Vector – Proactive FSR – Fisheye State Routing FSLS – Fuzzy Sighted Link State OLSR – Optimized Link State Routing Protocol TBRPF – Topology Broadcast Based on Reverse Path Forwarding 8 OVERVIEW Hierarchical – CGSR – Clusterhead-Gateway Switch Routing – HSR – Hierarchical State Routing – LANMAR – Landmark Ad Hoc Routing – ZRP – Zone Routing Protocol 9 OVERVIEW Geographical Routing – DREAM – Distance Routing Effect Algorithm for Mobility – GeoCast – Geographic Addressing and Routing – GPSR – Greedy Perimeter Stateless Routing – LAR – Location-Aided Routing 10 Reminder: Ad Hoc Routing Protocols Proactive Protocols (Actively seeks for routes/paths) – Determine routes independent of traffic pattern – Traditional link-state and distance-vector routing protocols are proactive Reactive Protocols (Seek for routes/paths only when required) – Maintain routes only if needed Hierarchical Routing – Introduces hierarchy to the flat network Geographic Position Assisted Routing – Why send packets North when the destination is South? 11 Routing Protocols from Ad Hoc Networks Used for WMNs – Dynamic Source Routing (DSR) in Microsoft mesh networks D.B. Johnson, D.A. Maltz, and Y.-C. Hu, “The dynamic source routing protocol for mobile ad hoc networks (DSR),” IETF Internet-Draft, 2004. 12 Routing Protocols from Ad Hoc Networks Used for WMNs – AODV (ad-hoc on-demand distance vector) routing Used by many other companies Major building block for the routing framework of IEEE 802.11s C. E. Perkins, E. M. Belding-Royer, I. D. Chakeres, “Ad hoc On-Demand Distance Vector (AODV) Routing”, IETF Draft, Jan. 2004. IEEE 802.11s Task Group, “Joint SEE-Mesh/Wi-Mesh proposal to 802.11 TGs overview,” IEEE Doc: 802.11-05/0567r6, 2006. 13 Routing Protocols from Ad Hoc Networks Used for WMNs – Topology broadcast based on reverse path forwarding (TBRPF) protocol in Firetide Networks R. Ogier, F. Templin, and M. Lewis, “Topology dissemination based on reverse-path forwarding (TBRPF),” IETF RFC 3684, 2004. 14 Dynamic Source Routing (DSR) D. B. Johnson, D. A. Maltz, Y.-C. Hu, “Dynamic Source Routing Protocol for Mobile Ad Hoc Networks”, IETF Draft, April 2004. Based on Source Routing principle! On-demand Route computation performed on a per-connection basis 15 Dynamic Source Routing (DSR) Source, after route computation, appends each packet with a source-route information Intermediate hosts forward packet based on source route TWO PHASES: ROUTE DISCOVERY & ROUTE MAINTENANCE 16 Dynamic Source Routing (DSR): ROUTE DISCOVERY When node S wants to send a packet to node D, but does not know a route to D, node S initiates a Route Discovery Source node S floods (broadcasts) Route Request (RREQ) packet. RREQ packet contains * DESTINATION ADDRESS * SOURCE NODE ADDRESS and * A UNIQUE IDENTIFICATION NUMBER. 17 Dynamic Source Routing (DSR): ROUTE DISCOVERY If the node is the receiver (i.e., has the correct destination address) then returns the packet to the sender If the packet has already been received earlier (identified via ID) then discard the packet 18 Dynamic Source Routing (DSR): ROUTE DISCOVERY Each node receiving the packet checks whether it knows of a route to that destination. If it does not, it appends/adds its own identifier (address) to the route record and forwards the RREQ packet. 19 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Z S E F B C M J A G H K I L D N 20 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Broadcast transmission S [S] E F B C M J A L G H K I [X,Y] Z D N Represents transmission of RREQ Represents list of identifiers appended to RREQ 21 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Z S E [S,E] F B C A M J [S,C] H G K I L D N Node H receives packet RREQ from two neighbors: Potential for collision 22 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Z S E F B [S,E,F] C M J A L G H I [S,C,G] K D N Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once 23 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Z S E [S,E,F,J] F B C M J A L G H D K I [S,C,G,K] N • Nodes J and K both broadcast RREQ to node D • Since nodes J and K are hidden from each other, their transmissions may collide 24 Dynamic Source Routing (DSR): ROUTE DISCOVERY Y Z S E [S,E,F,J,M] F B C M J A G H K I L D N • Node D does not forward RREQ, because node D is the intended target of the route discovery 25 Route/Path Discovery in DSR Destination D on receiving the first RREQ, sends a Route Reply (RREP) RREP is sent on a route obtained by reversing the route appended to received RREQ RREP includes the route from S to D on which RREQ was received by node D 26 Route/Path Discovery in DSR Y S Z RREP [S,E,F,J,D] E F B C M J A G H K I L D N Represents RREP control message 27 DATA DELIVERY IN DSR Node S on receiving RREP, caches the route included in the RREP When node S sends a data packet to D, the entire route is included in the packet header – hence the name Source Routing Intermediate nodes use the Source Route included in a packet to determine to whom a packet should be forwarded 28 Data Delivery in DSR Y S DATA [S,E,F,J,D] Z E F B C M J A L G H K I D N Packet header size grows with route length 29 DSR Optimization: Route Caching Each node caches a new route it learns by any means When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F When node K receives Route Request [S,C,G] destined for K, it learns route [K,G,C,S] to node S 30 DSR Optimization: Route Caching When node F forwards RREP [S,E,F,J,D], node F learns route [F,J,D] to node D When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D A node may also learn a route when it overhears Data packets 31 Advantages of Use of Route Caching – can speed up route discovery – can reduce propagation of route requests 32 Use of Route Caching [S,E,F,J,D] [E,F,J,D] S B [F,J,D],[F,E,S] E F C M J [C,S] A [J,F,E,S] L G H [G,C,S] D K I N Z Represents cached route at a node (DSR maintains the cached routes in a tree format) [x,y,w] 33 Use of Route Caching: Can Speed up Route Discovery [S,E,F,J,D] [E,F,J,D] S [F,J,D],[F,E,S] E F B C [G,C,S] [C,S] A [J,F,E,S] M J L G H I [K,G,C,S] When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route D K RREP RREQ N RREQ Z 34 Use of Route Caching: Can Reduce Propagation of Route Requests Y [S,E,F,J,D] [E,F,J,D] S [F,J,D],[F,E,S] E F B C [G,C,S] [C,S] A [J,F,E,S] M J L G H I [K,G,C,S] D K RREP N RREQ Z Route Reply (RREP) from node K limits flooding of RREQ. In general, the reduction may be less dramatic. 35 Dynamic Source Routing: Advantages Routes maintained only between nodes who need to communicate – reduces overhead of route maintenance Route caching can further reduce route discovery overhead A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches 36 Dynamic Source Routing: Disadvantages Packet header size grows with route length Flood of route requests may potentially reach all nodes Care must be taken to avoid collisions between route requests propagated by neighboring nodes – insertion of random delays before forwarding RREQ 37 Ad Hoc On-Demand Distance Vector Routing (AODV) C. E. Perkins, E. M. Belding-Royer, I. D. Chakeres, “Ad hoc OnDemand Distance Vector (AODV) Routing”, IETF Draft, Jan. 2004. AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain paths AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate 38 AODV – Hop-by-hop routing as opposed to source routing – On-demand – When a source node wants to send a message to some destination node and does not already have a valid route to that destination, it initiates a Path Discovery Process to locate the destination (as in DSR case) – It broadcasts the RREQ packet to its neighbors 39 AODV RREQs are forwarded in a manner similar to DSR When a node re-broadcasts a RREQ, it sets up a reverse path pointing towards the source – AODV assumes symmetric (bi-directional) links When the intended destination receives a RREQ, it replies by sending a Route Reply (RR) RR travels along the reverse path set-up when RREQ was forwarded 40 AODV – When RREQ propagates, routing tables are updated at intermediate nodes (for route to source of RREQ) – When RREP is sent by destination, routing tables updated at intermediate nodes (for route to destination), and propagated back to source 41 AODV – Each node maintains its own sequence number and a broadcast ID. – The broadcast ID is incremented for every RREQ the node initiates 42 AODV – The node’s IP address and the broadcast ID uniquely identify a RREQ. – Along with its own sequence number and broadcast ID, the source node includes in the RREQ the most recent sequence number it has for the destination. 43 AODV – Intermediate nodes can reply to the RREQ only if they have a route to the destination whose corresponding Destination Sequence Number is greater than or equal to that contained in the RREQ. – During the process of forwarding the RREQ, intermediate nodes recording their route tables with the address of the neighbor from which the first copy of the broadcast packet is received establishing a reverse path. 44 AODV – If more same RREQs are received later, they are discarded. – RREP packet is sent back to the neighbors and the routing tables are accordingly updated. 45 Route Requests in AODV Y Z S E F B C M J A L G H K I D N 46 Route Requests in AODV Y Broadcast transmission Z S E F B C M J A L G H K I D N Represents transmission of RREQ 47 Route Requests in AODV Y Z S E F B C M J A L G H K I D N Represents links on Reverse Path 48 Reverse Path Setup in AODV Y Z S E F B C M J A L G H K I D N • Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once 49 Reverse Path Setup in AODV Y Z S E F B C M J A L G H K I D N 50 Reverse Path Setup in AODV Y Z S E F B C M J A L G H K D I N • Node D does not forward RREQ, because node D is the intended target of the RREQ 51 Route Reply in AODV Y Z S E F B C M J A L G H K I D N Represents links on path taken by RREP 52 Forward Path Setup in AODV Y Z S E F B C M J A L G H K I D N Forward links are setup when RREP travels along the reverse path Represents a link on the forward path 53 Data Delivery in AODV Y DATA Z S E F B C M J A L G H K I D N Routing table entries used to forward data packet. NOTE: Route is not included in packet header as in DSR. 54 Routing Protocols for WMNs * Special considerations - WMN routers differ from MANET routers * Power supply * Mobility * Separation of WMN routers and clients 55 Routing Challenges in WMNs Routing in WMNs is much more complicated than in Ad Hoc Networks REASONS: 1) Network topology is variable and inconsistent (same as ad hoc networks) 2) Depending on the performance goal in routing, it may not be possible to determine a routing path solely based on network topology 3) There may not be an optimal solution for a given routing problem 4) Routing traverses both mesh routers and mesh clients that have different networking capabilities 56 Design Principles 1) Maintaining a consistent and stable network topology 2) Performing dynamic and adaptive routing 3) Developing new routing metrics 4) Considering tradeoff between cross-layer design and single-layer solution 57 Design Principles 5) Deriving distributed algorithms for routing 6) Ensuring scalability in routing 7) Adaptively supporting both mesh routers and mesh clients 58 PERFORMANCE METRICS Per-Flow Parameters: (e.g., delay, packet loss ratio, and delay jitter and other parameters such as hop-count, per-flow throughput, and intra-flow interference) Per-Node Parameters: (computational complexity and power efficiency) Per-Link Parameters: (e.g., link quality, channel utilization, transmission rate, and congestion) Inter-Flow Parameters: (e.g., inter-flow interference and fairness) Network-Wide Parameters: (e..g., total throughput or total delay) 59 OVERVIEW OF ROUTING METRICS * Hop-Count * Per-Hop RTT * Per-Hop Packet Pair Delay * Expected Transmission Count (ETX) 60 OVERVIEW OF ROUTING METRICS * Expected Transmission on a Path (ETOP) * Expected Transmission Time (ETT) * Weighted Cumulative ETT (WCETT) * Bottleneck Link Capacity (BLC) * Expected Data Rate (EDR) * Airtime Cost Routing Metric 61 Hop Count * Minimum hop counting (the link quality is binary) * Simple and requires no measurements Disadvantages: * Can lead to poor throughput * Link quality all links do not have the same quality * Does not take packet loss or bandwidth into account * Route that minimizes hop count does not necessarily maximize the throughput 62 Per-Hop RTT Adya A, Bahl P, Padhye J, Wolman A and Zhou L, “A multi-radio unification protocol for IEEE 802.11 wireless networks”, Proc. IEEE BroadNets 2004 Measured by sending unicast probe packets between neighboring nodes Then calculating the time spent on the probe-ack procedure A weighted moving average is needed to get a smoother measurement, because one sample cannot really reflect the actual link status. 63 Per-Hop RTT Based on per-hop RTT, a routing protocol selects a routing path with the least sum of RTTs of all links on the path. Per-hop RTT is able to capture * the packet loss ratio in a link * the traffic load * queuing delay in two nodes on the link, and * contention status in all neighboring nodes. 64 Per-Hop RTT * Loss will cause RTT to increase due to ARQ * If ARQ fails, RTT is increased by some percentage * This metric is load dependent * Channel contention increases RTT 65 Per-Hop RTT Kim K-H and Shin KG, “On accurate measurement of link quality in multi-hop wireless mesh networks,” Proc. ACM MOBICOM 2006 Its effectiveness is constrained by two problems: PROBLEM 1: Per-hop RTT is too much dependent on the traffic load/queuing delay, which interferes with the accuracy of per-hop RTT and thus, can easily lead to route instability. If a separate queue is assigned to probe packets, then it can accurately measure the link quality but cannot reflect the traffic load. A solution to this problem is to adopt a link measurement scheme ! 66 Per-Hop RTT PROBLEM 2: Accuracy of per-hop RTT measurement totally relies on the weighted moving average scheme. For large variations in measurements cause unreliable values for perhop RTT (no matter what weight is applied in the weighted moving average scheme.) Overhead of the probe-ack procedure REMARK: Per-hop RTT captures per-link performance parameters, although measurement is actually carried out at the network layer. 67 Disadvantages of Per-Hop RTT Disadvantages: * Does not take link data rate into account. * High overhead. * Load dependent metric may cause route flaps * Need to insert probe at head of interface queue to avoid queuing delay * Not scalable - every pair needs to probe each other 68 Per-Hop Packet Pair Delay Draves R, Padhye J and Zill B, “Routing in multi-radio, multi-hop wireless mesh networks,” Proc. ACM MOBICOM 2004. Measured by sending two back-to-back probe packets from a node to its neighbor First a small probe packet; Second large When the neighbor receives these two packets, it finds the delay in-between them and then sends such information back to the probing node Since relative delay is used to measure the per-hop delay, per-hop PPD measurement is less impacted by queueing delays or traffic load in a node 69 Per-Hop Packet Pair Delay * However, impact by traffic load still exists, because whether or not being able to send probe packets in a link of two nodes also depends on the queuing delays of other neighboring nodes. EXAMPLE: when Node A sends a probe packet to B, if A’s neighbor C is also sending a very high traffic load to A, then A has to delay its probe to B. Therefore, per-hop packet pair delay still has to capture the route instability issue. 70 Per-Hop Packet Pair Delay * Large overhead than per-hop RTT, due to the need of more probe packets Its performance is also dependent on the weighted moving average scheme, and assumes the variation of measurements are small. Similar to per-hop RTT, per-hop PPD only captures per-link performance parameters. 71 Expected Transmission Count (ETX) De Couto DSJ, Aguayo D, Bicket J, and Morris R, “A high-throughput path metric for multihop wireless routing,” in Proc. ACM MOBICOM 2003 ETX of a link is the expected number of transmissions before a packet is successfully delivered on a link For a route, the ETX is the sum of the ETXs on all links Captures the link quality and packet loss on both directions of a link The route ETX can detect interference among links of the same route the larger the route ETX, the more self-interference on the route. 72 Expected Transmission Count (ETX) Every period of t seconds a node sends a broadcast probe message to all its neighbors Each neighbor records the number of received probe messages (denoted by nw) during a period of w seconds, where w > t Thus, the delivery ratio of sending a packet from the probing node to its neighbor is: nw w /t 73 Expected Transmission Count (ETX) If a probing node embeds the information of nw from all its neighbors to the probe packet Then each of its neighbors can derive the packet delivery ratio from the neighbor to the probing node With the delivery ratio at both forward and reverse directions, denoted by df and dr, respectively, ETX is calculated as: 1 ETX d f dr 74 Advantages of Expected Transmission Count (ETX) Lower overhead because broadcast rather than unicast is applied to probe messages. Does not measure delays, so the measurement based on probe messages are not impacted by queueing delays in a node. 75 Disadvantages of Expected Transmission Count (ETX) – Probe messages experience different packet loss ratios than unicast messages because broadcast messages use more robust modulation and coding schemes, and thus have low transmission rates – ETX does not take into account the differences in packet size for different traffic flows and the different capacities for different links 76 Disadvantages of Expected Transmission Count (ETX) – Estimation method in ETX may not be accurate * It relies on the mean loss ratio; * Wireless links usually experience bursty losses. 77 SO FAR * ETX metric performs best in static scenarios * It is insensitive to load * RTT is most sensitive to load * Packet-Pair suffers from self-interference on multi-hop paths * Minimum hop count based routing seems to perform best in mobile scenarios * Schemes based on measurements of link quality does not converge quickly 78 Expected Transmission on a Path (ETOP) * When a routing path is selected in many routing protocols, the position of a link is not considered in the routing metric This is true if the link layer allows an infinite number of retransmissions, because a retransmitted packet has the same impact on upper layer no matter at which link retransmission happens However, if the link layer allows only a limited number of retransmissions, end-to-end retransmission has to be carried out. 79 Expected Transmission on a Path (ETOP) Jakllari G, Eidenbenz S, Hengartner N, Krishnamurthy S and Faloutsos M, “Link positions matter: a non-commutative routing metric for wireless mesh networks,” Proc. IEEE INFOCOM 2008 Comparing two links, even if their ETX is the same, the one closer to the destination can result in higher transport layer retransmissions, i.e., this link can lead to worse performance if it would be selected. ETOP solves the above problem by taking into account the relative position of a link on a routing path when the routing cost of the path is calculated. 80 Expected Transmission on a Path (ETOP) Consider a routing path with n links from node v0 to node vn its cost is denoted by Tn For a packet to be delivered end-to-end through this routing path, the needed number of end-to-end attempts is assumed to be Yn. In an end-to-end attempt j, the number of links that a packet has been traversed before it is dropped by the link layer is denoted as M The number of link layer transmissions at node j is assumed to be Hj . 81 Expected Transmission on a Path (ETOP) ETOP of a routing path is the expectation of Tn n 2 n1 E[Tn ] K (E[H j | H j K ] P[M j | M n]) E[Yn 1] E[H j | H j K ] j0 j0 Captures the total number of link layer transmissions of a given routing path under all possible end-to-end attempts Compared to ETX, ETOP can improve transport layer throughput, because a routing path is selected with a least number of overall link layer retransmissions. 82 Expected Transmission Time (ETT) and Weighted Cumulative ETT (WCETT) Draves R, Padhye J and Zill B, “Comparisons of routing metrics for static multi-hop wireless Networks,” in Proc. ACM SigComm, 2004 Expected Transmission Time (ETT) – An extended version of ETX. – Based on ETX, ETT considers the impact of both packet size and link quality S ETT ETX S: packet size, B: link bandwidth. B – ETT reflects the expected packet transmission time on a link. 83 Expected Transmission Time (ETT) and Weighted Cumulative ETT (WCETT) – For a routing path, the expected transmission time can be the sum of ETTs of all links on the path. – However, ETT does not take into account channel diversity in WMNs using multiple radios at some nodes – To resolve this issue, a routing metric called WCETT is proposed 84 Expected Transmission Time (ETT) and Weighted Cumulative ETT (WCETT) Weighted Cumulative ETT (WCETT) n: number of hops on a routing path, k: # of available channels for multi-radio operation. n WCETT (1 ) ETTi max X j i 1 X j 1 j k ETT so max 1 j k n i Xj finds the bottleneck channel of a HOP i on channel i given routing path. – First term considers the overall expected transmission time of the routing path. – Second term captures the transmission time on the bottleneck channels. – In this way, WCETT takes into account the tradeoff between overall routing delay and channel diversity utilization. 85 Expected Transmission Time (ETT) and Weighted Cumulative ETT (WCETT) ETT enhances the performance of ETX by mapping packet size and link BW into the transmission time. However, it uses a similar estimation scheme as that of ETX, so it has similar problems of ETX, i.e., inaccurate estimation, bottleneck routes, etc. 86 Expected Transmission Time (ETT) and Weighted Cumulative ETT (WCETT) WCETT is not applicable to WMNs based on single-radio multi-channel operation for two reasons: 1) broadcast probe messages cannot be sent on different channels of the same radio simultaneously; 2) the channel switching time can be comparable to ETT of a link. 87 Bottleneck Link Capacity (BLC) Liu T and Liao W, “Capacity-aware routing in multi-channel multi-rate wireless mesh networks,” in Proc. IEEE ICC, 2006 BLC is derived based on the expected busy time (EBT) of transmitting a packet on a link. EBT can be estimated by considering the packet loss rate (PLR) and transmission mechanism in the MAC layer. – If RTS-CTS-Data-Ack handshake is used for packet transmission as in an IEEE 802.11 MAC, Thandshake EBT 1 ep Thandshake: Total transmission time of one RTS-CTS-Data-Ack Ep: PLR. 88 Bottleneck Link Capacity (BLC) Based on EBT, a residual capacity of a link considered as defined as the ratio between the idle time and EBT. – Considering a path P, if the residual capacity of link i is LCi, then BLC is given by BLC min LCi iP K K: Length of the routing path P μ: fine-tuning parameter. 89 Bottleneck Link Capacity (BLC) Residual capacity of the bottleneck link of a routing path Dividing the minimum residual capacity by a certain number is for penalizing a long routing path Because busy time is considered in BLC, load-balancing in links has been taken into account 90 Bottleneck Link Capacity (BLC) However, the self-interference of a routing path is not considered, as the minimum residual capacity is considered in BLC – If two routing paths have different self-interferences, then the bottleneck link can have the same residual capacity. – The same problem applies to interference from other routing paths. 91 Expected Data Rate (EDR) EDR integrates the expected transmission count and expected transmission contention degree (TCD) into the same routing metric – TCD of a link is the time that is spent on retransmitting non-acknowledged packets over a given period. – Considering link k on a routing path, if the sum of TCDs of links that interfere link k is Ik, then the the EDR of link k is EDRk I k ETX k Γ: maximum transmission rate of link k. For EDR of a routing path, it is defined as the EDR of the bottleneck link. 92 Problems of Expected Data Rate (EDR) – EDR integrates two closely related parameters: ETX ad TCD. – In fact, given the same packet length, if ETX is large, TCD is large too. why ETX and TCD have to be combined like an EDR equation remains a question. 93 Problems of Expected Data Rate (EDR) – Even if the link rate is considered in the metric, it does not consider the fact that multiple rates instead of only the maximum rate are available in each link. – Interference range of a given link k is difficult to determine, so Ik is hard to derive. – EDR of a routing path cannot take into account the selfinterference, as the EDR of a bottleneck link is used as the EDR of the entire routing path. 94 Airtime Cost Routing Metric IEEE 802.11s Task Group, Draft, Amendment to Standard for Information Technology – Telecommunications and Information Exchange Between Systems LAN/MAN Specific Requirements – Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Amendment: ESS Mesh Networking, IEEE P802.11s/D1.00-2006.). – To identify an efficient radio-aware path among all the candidate paths, – A default routing metric in IEEE 802.11s draft – Reflects the amount of channel resources consumed for transmitting a frame over a particular link. 95 Airtime Cost Routing Metric – The path which has the smallest sum of airtime cost is the best path. – The airtime cost Ca for each link is calculated as: Oca, Op, and Bt depend on the used transmission technology. Ca [Oca OP Bt 1 ] r 1 e pt Oca: channel access overhead, Op: protocol overhead, Bt: number of bits in a test frame. r and ept : bit rate in Mbit/s and frame error rate for the test frame size Bt, respectively. 96 Comparison of Different Routing Metrics Many routing metrics try to capture link-layer performance parameters by using a procedure in the network layer In fact, these schemes can be enhanced by performing link-quality measurements directly in the link layer and then use such measurements in the network layer This method implies that the routing metrics should involve cross-layer interactions 97 Comparison of Different Routing Metrics Routing Metrics Layers Hop-count Network Per-hop RTT Per-hop PPD ETX Network Network Network Captured Performance Parameters Advantages Shortcomings Number of hops Simple and low Minimum hop-count is usually not the performance goal Packet loss, traffic load, queuing delay, contention Multiple link metrics captured high overhead in sending probes, estimation accuracy depends on traffic load Packet loss, transmission Delay Multiple link metrics captured, less impacted by traffic load High overhead in measuring delay, performance dependent on the measurement accuracy, no load balancing Captured multiple link metrics, relative lower overhead by using broadcast Measurement is not accurate due to differences between broadcast and unicast, no load balancing, cannot capture packet loss variations, can have bottleneck link Packet loss, retransmission, contention 98 Comparison of Different Routing Metrics Routing Metrics ETOP ETT & WCETT Layers Captured Performance Parameters Advantages Shortcomings Network, Link End-to-end attempts, link retransmission Link position considered in routing Difficulty in deriving the metric Network Same link metrics of ETX and also link bandwidth and packet size Improve ETX by considering link bandwidth and packet size, channel diversity Same problems of ETX, Not applicable to single-radio multichannel operation 99 Comparison of Different Routing Metrics Routing Metrics BLC EDR Airtime Cost Layers Captured Performance Parameters Advantages Shortcomings Network, Link MAC handshake, time, packet loss rate, hop count Residual capacity of a link is considered, so load-balancing is performed indirectly Bottleneck link of a route does not consider self-interference Network link metrics as that in ETX, contention time Use contention time of all interfering links to consider interference Have the same problems as those in ETX, hard to find interfering links Resource consumed by a packet on a link Captures the impact of the dynamic environment to a link Overhead in probing, Airtime cost captured by probe message may be different from a packet Link 100 Remaining Issues – The measurement or estimation method for a routing metric may not be accurate – It may also cause large overhead, especially for a large scale network. – Performance comparisons between different routing metrics need further research, although some work has been done for a few routing metrics. 101 Remaining Issues – The design of many existing routing metrics is still “ad-hoc”, * i.e., why the proposed routing metric can improve the network performance is not really justified; * usually only simulation results are used to prove the effectiveness of a routing metric. * the side-effect of such a design is that the effectiveness of a routing metric may be limited to a certain type of WMNs. – A routing metric may not be able to capture enough network parameters for a routing protocol to optimize the network performance. 102 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 103 SET 1: Hop-Count based Routing Algorithms Light Client Management Routing (LCMR) Protocol Orthogonal Rendezvous Routing (ORR) Protocol HEAT Protocol 104 Light Client Management Routing (LCMR) Protocol Wehbi B, Mallouli W and Cavalli A, Light client management protocol for wireless mesh networks. Proc. 7th Int. Conf. on Mobile Data Management (MDM), 2006 End-to-end routing path from a source to a destination client consists of * proactive route among mesh routers and * reactive routes between clients and mesh routers To find the best route from one client to another, hop-count is used as the routing metric. LCMR does not require routing functionality in clients Mesh routers supporting clients take care of routing 105 Light Client Management Routing (LCMR) Protocol Mesh routers need to maintain two tables: 1. MAC and IP addresses of local clients 2. IP addresses of remote clients as well as the IP addresses of remote mesh routers associated with remote clients 106 Light Client Management Routing (LCMR) Protocol Based on these two tables, • When a local client needs to set up a routing path to a remote client, • its associated mesh router can find out which remote mesh router is responsible for forwarding traffic to the remote client • Based on such information, mesh routers can then set up a routing path between them using proactive routing and hop-count metric. 107 Disadvantages of Light Client Management Routing (LCMR) Protocol LCMR has a high overhead of maintaining the two tables on each mesh routers, as all clients’ IP addresses need to be collected and stored at each mesh router. 108 SET 1: Hop-Count based Routing Protocols Light Client Management Routing (LCMR) Protocol Orthogonal Rendezvous Routing (ORR) Protocol HEAT Protocol 109 Orthogonal Rendezvous Routing (ORR) Protocol Cheng B, Yuksel M and Kalyanaraman S, Orthogonal rendezvous routing protocol for wireless mesh networks. Proc. IEEE Int. Conf. on Network Protocols (ICNP), 2006. Each node can define its neighbors’ directions relative to its local North. Relying on such information, ORR can reduce the state information for routing, and it does not need flooding for route construction. 110 Orthogonal Rendezvous Routing (ORR) Protocol Compared to geographic routing, ORR does not need exact location of nodes. Idea In 2-D Euclidean space two orthogonal lines can have at least two intersect points with another group of two orthogonal lines, if these two groups of orthogonal lines have different centers. 111 Orthogonal Rendezvous Routing (ORR) Protocol To construct routing paths, a source node sends route discovery in orthogonal directions, while a destination node sends route dissemination in orthogonal directions. Thus, there is at least one intersect point, called rendezvous point where both route discovery and route dissemination messages are received. 112 Orthogonal Rendezvous Routing (ORR) Protocol In this way a routing path is established between the source and the destination Also, routing path from the source to the rendezvous point is a reactive route and the remaining path to the destination is a proactive route. 113 Shortcomings of Orthogonal Rendezvous Routing (ORR) Protocol 1. Direction of a node needs to be configured freely 2. Network is not really a 2-D space. (If a 3-D space is considered, the theory for ORR may not be valid) 3. ORR may not work if the node density is high or topology change frequently happens 4. Routing path selection procedure is based on hop-count. (However, other metrics such as link quality can be adopted to enhance the ORR). 114 SET 1: Hop-Count based Routing Protocols Light Client Management Routing (LCMR) Protocol Orthogonal Rendezvous Routing (ORR) Protocol HEAT Protocol 115 HEAT Protocol Baumann R, Lenders V, Heimlicher S and May M, HEAT: scalable routing in wireless mesh networks using temperature fields. Proc. IEEE Int. Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM), 2007 Anycast routing protocol HEAT considers all nodes in a WMN as a temperature field – Gateways have the highest temperature – Temperature of a non-gateway node will be determined by hop-count to the gateways and the robustness of a routing path from this node to gateways. 116 HEAT Protocol Once temperatures of all nodes are determined according to this procedure, the packets from any node to gateways can simply follow the following method: The node forwards the packets to its neighbor with the highest temperature, and this neighbor will repeat the same process until reaching gateways. 117 Problems of HEAT Protocol Totally depends on the assumption that the traffic of WMNs only needs to be routed between gateways and nongateways For other scenarios, anycast routing is not supported. Moreover, how to consider other routing metrics in HEAT remains an open problem 118 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 119 Set 2: Link-level QoS based Routing Algorithms – Link quality source routing (LQSR) protocol – Multi-radio LQSR (MR-LQSR) routing protocol - ExOR Routing Protocol – AODV-spanning tree (AODV-ST) protocol 120 Link Quality Source Routing (LQSR) Protocol Draves R, Padhye J and Zill B, Comparisons of routing metrics for static multi-hop wireless networks. Proc. ACM SIGCOMM, 2004. Based on Dynamic Source Routing (DSR) Contains all basic DSR functionalities, such as * Route Discovery (Route Request and Route Reply messages) and * Route Maintenance (Route Error messages). 121 Link Quality Source Routing (LQSR) Protocol However, LQSR holds two major differences compared to DSR. – LQSR is implemented as layer 2.5 protocol instead of as a network layer protocol – LQSR supports link quality metrics. 122 Link Quality Source Routing (LQSR) Protocol Layer 2.5 architecture brings two significant advantages. – On the one hand, no modification is needed for the higher layer software, i.e., LQSR routing protocol is transparent to higher layer software. – Also no modification is required for link layer software. 123 Link Quality Source Routing (LQSR) Protocol Performance of LQSR varies based on different routing metrics and network mobility: – For stationary nodes in WMNs, the routing metric ETX, achieves the best performance 124 Link Quality Source Routing (LQSR) Protocol For mobile nodes: Minimum hop count method outperforms the three link quality metrics, i.e., per-hop RTT, per-hop packet pair delay, and ETX REASON: As the sender moves, the ETX metric cannot quickly track the changes in the link quality. 125 Set 2: Link-level QoS based Routing Protocols – Link quality source routing (LQSR) protocol – Multi-radio LQSR (MR-LQSR) routing protocol – ExOR Routing Protocol – AODV-spanning tree (AODV-ST) protocol 126 Multi-Radio LQSR (MR-LQSR) Routing Protocol Draves R, Padhye J and Zill B, “Routing in multi-radio, multi-hop wireless mesh networks”, Proc. ACM MOBICOM, 2004. Based on LQSR, and thus, also based on DSR Major difference from LQSR is WCETT To make LQSR perform well in a mesh network with multiple radios per node, WCETT is used as the routing metric in the routing protocol 127 Multi-Radio LQSR (MR-LQSR) Routing Protocol WCETT takes into account both link quality metric and the minimum hop-count Achieves good tradeoff between delay and throughput because it considers channels with good quality and channel diversity in the same routing protocol 128 Multi-Radio LQSR (MR-LQSR) Routing Protocol MR-LQSR assumes nodes are stationary This is true for mesh routers, but obviously not applicable to mesh clients Performance of MR-LQSR can also be degraded by the mobility of nodes, i.e., mesh clients. 129 Multi-Radio LQSR (MR-LQSR) Routing Protocol In WMNs, multi-channel operation over a single radio is another alternative to increase the network capacity. But MR-LQSR is not applicable because WCETT is limited to multi-radio mode. 130 Set 2: Link-level QoS based Routing Protocols – Link quality source routing (LQSR) protocol – Multi-radio LQSR (MR-LQSR) routing protocol –ExOR Routing Protocol – AODV-spanning tree (AODV-ST) protocol 131 ExOR Routing Protocol Biswas S and Morris R, “ExOR: opportunistic multihop routing for multi-Hop wireless networks,” in Proc. ACM SIGCOMM, 2005. * Proposed to improve throughput based on cooperative broadcasting packets from source to destination without explicitly setting up a routing path 132 Source’s behavior Collects enough packets of the same destination to form a batch – ExOR operates on batches of packets for efficiency Selects a set of nodes to be candidate forwarders, and includes the prioritized list in the overhead of every packet 133 ExOR Routing Protocol * This priority of a forwarding node is determined by the cost to the destination, which is evaluated by the accumulative ETX to the destination node. 134 ExOR Routing Protocol Set of nodes that are selected for forwarding packets are determined based on the packet loss ratio between the source and these nodes. Although many nodes can receive packets from the source node, only a subset of nodes are selected as forwarding nodes in order to reduce the overhead. 135 Forwarders’ Behavior How can a node know whether it is one of the forwarders or not? Check the forwarder list in the overhead of the received packet – If the node finds itself in the list, buffer the packet and keep state of this batch – If no, discard the packet 136 Forwarder’s Behavior Highest priority forwarding node sends its own batch of packets following the same procedure as done by the source node. This process is repeated until 90% of packets in each batch are received by the destination node. The remaining 10% of nodes will rely on traditional minimum hop-count routing to deliver. 137 Forwarders’ Behavior How can a node know whether the packet it receives has also been received by a node with higher priority or not? ExOR designs a “batch map” to record, for every packet in the batch, the highest-priority node known to have received that packet. 138 Advantages of ExOR Routing Protocol Most of packets are delivered without setting up routing paths, which is similar to anycast routing. Moreover, ExOR can improve throughput for two reasons. – It tries to use the best link to deliver packets through cooperation of forwarding nodes. – Progress of packet forwarding can be continued even if some nodes on a traditional path experiences bad link quality or out of order. 139 Set 2: Link-level QoS based Routing Protocols – Link quality source routing (LQSR) protocol – Multi-radio LQSR (MR-LQSR) routing protocol – ExOR Routing Protocol – AODV-spanning tree (AODV-ST) protocol 140 AODV-Spanning Tree (AODV-ST) Protocol Ramachandran K, Buddhikot MM, Chandranmenon G, Miller S, Almeroth K and Belding-Royer E, On the design and implementation of infrastructure mesh networks. Proc. IEEE WIMESH, 2005. AODV ST is designed for multi-radio WMNs ADOV-ST performs hybrid routing, i.e., for traffic inside the mesh network AODV is used where the spanning-tree based routing is used for traffic to/from gateways. ETT is used as the routing metric 141 Drawbacks of AODV-Spanning Tree (AODV-ST) Protocol AODV may not be efficient for intra-mesh traffic The WCETT proposed for multi-radio WMNs is not applicable, because AODV is a distance vector routing protocol 142 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 143 Set 3: Interference Based Routing: IRMA Wu Z, Ganu S and Raychaudhuri D, IRMA: integrated routing and MAC scheduling in multihop wireless mesh networks. Proc. IEEE WiMesh, 2006. TDMA instead of CSMA/CA as MAC Based on the TDMA, an integrated routing and MAC scheduling algorithm (IRMA) is derived to find a routing path for each traffic flow then determine slot allocation on each link considering * BW allocation information * Link status, and * Topology information in the network. 144 Interference based Routing: IRMA A centralized scheme Relies on an existing solution to collect the node, link, and topology related information of the entire network and get traffic specifications of traffic flows. Such signaling can be done in a global control plane (GCP) that can be implemented in a separate dedicated channel or a dedicated time slot 145 Interference based Routing: IRMA Two routing mechanisms defined in IRMA: – link scheduling with minimum-hop routing – link scheduling with bandwidth-aware routing (preferred) 146 Interference Based Routing: IRMA Link Scheduling with Minimum-Hop Routing A routing path is selected by shortest path using minimum hop-count. Then time slots along this path are determined by the centralized algorithm by considering the latest flow information in the network. may result in congested paths or links as the minimum hopcount routing does not consider the available BW. 147 Interference Based Routing: IRMA Link Scheduling with bandwidth-aware Routing Available BW on each link is factored when a routing path is selected. Based on the selection, time slots are then determined for each link on the path. Thus, such a scheme can not only avoid contentions in traffic flows but can also avoid bottleneck or congested links. 148 Shortcomings of Interference Based Routing: IRMA 1. Not scalable with the network size (a centralized scheme) 2. Assumes an efficient scheme to collect all control information, which is a challenging issue for all routing protocols. 3. MAC layer is assumed to have TDMA operation, which is not the case for many WMNs. 149 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Scalable Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 150 Set 4: Routing with Load Balancing Song W and Fang X , ”Routing with congestion control and load balancing in wireless mesh networks,’ Proc. Int. Conference on ITS Telecommunications, 2006 Routing is directly determined by considering network congestion Given a source and its destination, a routing path is determined by using the route with the least congestion If more than one paths have the same number of congested nodes, the route with minimum hop-count is selected Congestion state of a link is determined by the number of retransmissions of RTS and ACK packets. If the congestion exceeds a threshold, the congestion weight on this link increases 151 Routing with Load Balancing: CAR Liu T and Liao W, Capacity-aware routing in multi-channel multi-rate wireless mesh networks. Proc. IEEE ICC, 2006. A capacity-aware routing (CAR) protocol is proposed to balance load among links and channels in a multi-radio WMN CAR assumes channel assignment on radios lasts long time and can be static. With static channels on each radio in the network, CAR determines the BLC in a reactive manner for each traffic flow. 152 Routing with Load Balancing: CAR Transmissions start on a routing path that is determined according to the best BLC metric. However, this path can be switched to a new one if the source finds the new path has a higher BLC value. 153 Routing with Load Balancing: CAR Due to the bottlenecked link capacity in routing, CAR improves throughput and delay performance. CAR may not achieve optimal performance because the path selection on different flows affect each other but is not coordinated among such flows. 154 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Scalable Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 155 Set 5: Routing Based on Residual Link Capacity Raniwala A, Gopalan K and Chiueh T ,”Centralized channel assignment and routing algorithms formulti-channel wireless mesh networks,” ACM Mobile Computing and Communications Review, 2005. An alternative scheme to consider link capacity in routing is to get the information of residual link capacity A protocol called Hyacinth is developed to perform routing and channel assignment for a multi-channel WMN. 156 Routing Based on Residual Link Capacity Hyacinth considers traffic from/to gateways in WMNs and thus uses tree-based routing for such traffic. Each node advertises its costs of its path from/to the gateway Based on such information, a neighbor that finds a lower value in the cost will leave its old parent node and selects the new node as the new parent node. 157 Routing Based on Residual Link Capacity With such a procedure, all nodes in the network build up routing paths to the gateway like a spanning tree To reflect the cost of a path, residual capacity of a link the routing metric. Links are selected which have the largest available capacity 158 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Scalable Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 159 Set 6: End-to-End QoS Routing Quality Aware Routing Protocol Ring Mesh Routing Protocol 160 Quality Aware Routing Protocol Koksal CE and Balakrishnan H, ”Quality-aware routing metrics for time-varying wireless mesh networks,” IEEE Journal on Selected Areas in Communications, 2006. End-to-end QoS can be considered in a routing protocol by ensuring end-to-end packet loss rate, delay, or bandwidth. End-to-end packet loss rate is ensured where both ETX and ENT are used as routing metrics ENT is used to determine what routing paths can be used 161 Quality Aware Routing Protocol − Allowed packet loss rate in a link is given − Based on this packet loss threshold and the measurement of the link, a positive number δ used by ENT is derived. − With δ and an existing probe scheme, ENT of each link is determined. 162 Quality Aware Routing Protocol − ENT is then compared with the maximum number of transmissions of a packet before it is discarded at the link layer − If ENT is larger than the link-layer value, the routing cost of the link will become ∞ − Otherwise, ETX is used for the routing cost of the link 163 Quality Aware Routing Protocol − As a result, all links that do not satisfy packet loss requirement will be excluded from routing paths − After this step, routing path selection is performed by just choosing a path with smallest routing cost One critical issue: How to determine the allowed packet loss rate of a link given the threshold of end-to-end packet loss requirement? 164 Set 6: End-to-End QoS Routing Quality Aware Routing Protocol Ring Mesh Routing Protocol 165 RingMesh Routing Protocol Lin D, Moh T and Moh M ,”A delay-bounded multi-channel routing protocol for wireless mesh networks using multiple token rings: extended summary,” Proc. 31st IEEE Conference on Local Computer Networks (LCN), 2006. End-to-end delay RingMesh is developed based on a token ring protocol proposed for wireless LANs 166 RingMesh Routing Protocol Multiple token rings are created and organized from the gateway to all other nodes like a spanning tree scheme Different channels are used in neighboring rings First ring containing the gateway is called a root ring Next ring connected to the root ring is a child ring. 167 RingMesh Routing Protocol These two rings share a common node which is called a pseudo gateway Following this process, other child rings are connected together all the way to the root ring For a node in the network, which ring it can join depends on the delay from it to the gateway: the node joins a ring that can satisfy the end-to-end delay requirement. 168 Shortcomings of RingMesh Routing Protocol − How to form multiple rings to support multiple gateways to improve the delay performance is not addressed? − It is also unknown what happens if no ring can be joined by a node − No mechanism is also available to determine the delay from a node to the gateway, which is not a trivial task − Thus, end-to-end delay aware routing is still a challenging research topic 169 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Scalable Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 170 Set 7: Resilient Opportunistic Mesh Routing (ROMER) Protocol Yuan Y, Yang H,Wong SHY, Lu S and Arbaugh W,”ROMER: resilient opportunistic mesh routing for wireless mesh networks,” Proc. IEEE WIMESH, 2005 ROMER creates forwarding mesh on the fly for each packet ROMER assumes that there is an existing scheme that can find the minimum cost from each mesh router to the gateway Then the credit is determined while a packet is forwarded on the fly 171 Resilient Opportunistic Mesh Routing (ROMER) protocol When a packet is to be delivered from a mesh router, e.g., Node S, to the gateway, the source mesh router needs to set a credit cost. If the minimum cost from S to the gateway is Cmin,S and the credit cost is Ccredit,S, then S has a budget cost of Cmin,S + Ccredit,S to the gateway. When the packet is sent to the next mesh router, e.g., node A, the budget is reduced by the cost of the traversed link clinkSA 172 Resilient Opportunistic Mesh Routing (ROMER) protocol At mesh router A, the needed credit is computed according to the requirement of Ccredit,A + Cmin,A + ClinkSA <= Cmin,S + Ccredit,S, i.e., the remaining credit at mesh router A is Ccredit,A = Cmin,S + Ccredit,S − Cmin,A − ClinkSA. If the ratio of the remaining credit over the initial credit Ccredit,S is less than a threshold, e.g., (Cmin,A / Cmin,S)2 , then the packet at mesh router A shall be discarded; 173 Resilient Opportunistic Mesh Routing (ROMER) protocol Otherwise, mesh router A forwards the packet according to a randomized opportunistic forwarding scheme. The above process is repeated until the packet is delivered to the gateway 174 Resilient Opportunistic Mesh Routing (ROMER) protocol Finally, when multiple intermediate routers receive the same packet from a mesh router and all have enough credit to forward the packet, they need to follow a randomized opportunistic forwarding scheme to forward packets. 175 Resilient Opportunistic Mesh Routing (ROMER) protocol Probability that each intermediate router can forward a packet depends on the quality of the link to the parent router 176 Resilient Opportunistic Mesh Routing (ROMER) protocol Intermediate router with the best link quality forwards the packet with probability 1, while other intermediate routers forward the packet with a probability of (Rl / Rmax), where Rl is the current rate of the considered link and Rmax is the current rate at the best link. 177 Drawbacks of Resilient Opportunistic Mesh Routing (ROMER) Protocol ROMER has to rely on an existing scheme to find out the minimum cost from each mesh router to the gateway What type of cost is the best for ROMER and how to dynamically update the cost to best serve ROMER remain open questions ! 178 OVERVIEW OF ROUTING ALGORITHMS Hop-Count based Routing Link Level QoS Based Routing Interference Based Routing (IRMA) Routing with Load Balancing Routing Based on Residual Link Capacity End-to-End QoS Routing Reliability Aware Routing Joint Channel Assignment and Routing (Layer 2.5 Routing) 179 Multichannel Protocols Multi-channel operation is widely adopted in WMNs to improve network capacity Single-channel routing protocols may be run in each of the channels of the WMN – Easy design but not optimal, and does not guarantee availability of spectrum in the routes Multi-channel routing protocols are better suited 180 Multichannel Protocols Two types of multi-channel routing protocols –Type 1: Consider the impact of multi-channel operation such as link quality, interference, packet loss, bandwidth –Type 2: Conduct close routing/MAC cross-layer design such as joint channel allocation and routing 181 Multichannel Protocols Most existing protocols are of type 1 – As an example, in MQ-LSR ignores the close relationship between traffic distribution and channel allocation by assuming different radios are assigned non-overlapping channels – It also incorrectly assumes that the channel assignment changes relatively infrequent, leading to the necessity of joint channel-route assignment 182 Joint Channel Assignment and Routing Alicherry M, Bhatia R and Li L, “Joint channel assignment and routing for throughput optimization in multi-radio wireless mesh networks,” in Proc. ACM MobiCom, 2005 For infrastructure WMNs (IWMNs) Assumption: Aggregated traffic load at mesh routers and channels assigned to each router is not changing frequently Channels assigned to radios on a node are determined together with routing paths with an objective to obtain interference-free link schedule and achieve maximum throughput. 183 Joint Channel Assignment and Routing Tang J, Xue G and Zhang W, “Interference-aware topology control and QoS routing in multichannel wireless mesh networks,” ACM MobiHoc, 2005. Assumes the channel assignment can be static in WMNs Goal: mathematical formulation of the joint design between channel assignment and routing However, no actual protocol is proposed 184 Distributed Joint Channel and Routing Protocol Avallone S and Akyildiz, IF and Ventre G, “A channel and rate assignment algorithm and a layer-2.5 forwarding paradigm for multi-radio wireless mesh neetworks,” IEEE/ACM Transactions on Networking, 2009 It takes into account the number of flows that are possible to route on each link Amount of flows is obtained from a solution to the joint channel assignment and routing problem 185 Distributed Joint Channel and Routing Protocol Objective Enable every router to utilize each of its links in proportion to their assigned flow rates Routing protocol only requires a partial knowledge of the network topology and does not make use of a destinationbased routing table Hence, the name Layer-2.5 (L2.5) given to the routing protocol. 186 Layer-2.5 Routing Algorithm Each mesh router is configured with the set of precomputed flow rates associated with its links Packets are forwarded using such information (rather than routed using routing tables) – Layer-2 information are used, hence the name Each mesh router attempts to keep the utilization of the outgoing links proportional to their pre-computed flow rates 187 Channel Assignment & Routing * An approximate solution: Determine pre-computed flow rates A pre-computed flow rate is determined for every link based on the given optimization objective Determine the channel assignment Channels are assigned to radios in the attempt to make such pre-computed flow rates schedulable Adjust the pre-computed flow rates The pre-computed flow rates may be adjusted in order to obtain a set of schedulable flow rates given the computed channel assignment 188 Open Research Issues Performance Benchmark – Large number of routing metrics and routing protocols available for WMNs. – Considering routing protocols for other multi-hop wireless networks, the number is much bigger – Confusion about which routing metric and what type of routing protocols can provide the best performance => benchmark to investigate and compare different routing metrics and protocols. 189 Open Research Issues – It is expected to include theoretical analysis of performance bound, practical consideration of protocol design, and performance evaluation either through simulations or testbeds. In [1], a comparative study is carried out for different routing strategies for WMNs. Some design guidelines are provided in [2] for multihop wireless networks. However, such work is still far from providing a benchmark of selecting routing metrics and protocols. [1] Wellons J, Dai L, Xue Y and Cui Y, “Predictive or oblivious: a comparative study of Routing strategies for wireless mesh networks under uncertain demand," Proc. IEEE SECON, 2008 [2] Yang Y and Wang J, “Design guidelines for routing metrics in multihop wireless networks,” Proc. IEEE INFOCOM, 2008. 190 Open Research Issues New routing metrics – How to integrate multiple routing metrics into the same routing protocol is another challenging issue. 191 Open Research Issues Scalable routing – This is a critical requirement by WMNs, achieved by few routing protocols so far. – Hierarchical routing protocols can only partially solve this problem due to their complexity and difficulty of management. – Geographic routing needs GPS or similar cost, complexity. Additional traffic by inquiry of destination position. – Scalability is also related to MAC protocols. Thus, an eventual scalable routing protocol must be closely integrated with the MAC protocol. 192 Open Research Issues Network coding and routing – Can potentially improve the performance of WMNs E.g., research to apply network coding to WMNs [1], [2], Benefits of network coding to a multichannel WMN in [2] – However, as network coding is still in an early phase of being applicable to a practical networking protocol, integrating network coding with routing is still a new and challenging research direction [1] Omiwade O, Zheng R and Hua C, “Practical localized network coding in wireless mesh networks,” Proc. of IEEE SECON, 2008 [2] Zhang X and Li B, “On the benefits of network coding in multi-channel wireless networks,” Proc. of IEEE SECON, 2008 193