High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks

1. High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks Baruch Awerbuch, David Holmer, Herbert Rubens Szikszay Fábri Anna, ELTE IK Prog.terv.mat. 2009.05.05

2. Table of Contents • Basic terms • Network model • Traditional route selection techniques • General model of attainable throughput • MTM: Medium Time Metric • Advantages • Discussion • Summary

3. OAR • receiver based approach • allows high-rate multi-packet bursts • to take advantage of the coherence times of good channel condition • bursts also dramatically reduce the overhead at high rates

4. Basic terms • Ad hoc wireless network: • Decentralized • each node is willing to forward data for other nodes, and so the determination of which nodes forward data is made dynamically based on the network connectivity • Multi-rate network: • which allocates transmission capacity flexibly to connections • OAR: Opportunistic Auto Rate protocol

5. Network model • Network assumptions • the OSI/ISO layer is capable of operating using multiple rates • the ISO/OSI MAC layer is capable of selecting the rate used by the physical layer • the MAC layer is capable of providing information to the ISO/OSI network layer that indicates the selected rate • The network layer can then use this information to improve its routing decisions • Demonstration: information from lower layers can be utilized to enhance overall performance

6. Traditional route selection techniques 1. Minimum hop path • hop count: route selection criteria • Most ad hoc routing protocols • minimizes the total number of transmissions required to send a packet on the selected path • in single-rate wireless networks: OK • every transmission consumes the same amount of resources • multi-rate networks: • tendency: pick paths with both low reliability and low effective throughput

7. Throughput loss • Multi-rate wireless networks • the selection of minimum hop paths → paths where the links operate at low rates • Shortest path contains fewer nr of nodes • To cover the same distance → longer links →lower channel quality (lower rates) → low throughput • Shared medium: degrades the path of other flows in the network • Transmission of a packet at low link speed: takes TIME..

8. Reliability loss • Multi-rate wireless devices are designed to deal with connectivity changes (mobility & interference)‏ • In 802.11b prot.: 2 nodes move in opposite directions→ link speed drops • Tendency: lowest link speed path • No chance for auto rate protocol to deal with channel quality fluctuations

9. 2. Shortest Widest Path • selects the shortest path from the set of paths that have the fastest bottleneck link • commonly used routing criteria in wired networks • the total throughput of a path is directly related to the speed of the bottleneck link (each link in the path operates independently)‏ • often used when high throughput is required • Inappropriate for wireless networks

10. In wireless networks, individual links do not operate independently of one another • Individual transmissions affect a large area • compete for medium time • other transmissions along the same path • transmission in the same geographical area • does not consider the speed of links other than the bottleneck • even though these links my affect the bottleneck link!

11. Conclusion:the two paths equal (throughput) • each have equal bottleneck links → selection: which path contains the fewest hops

12. General Model of Attainable Throughput • Difficulty: in modeling the complex environment of wireless multi-hop networks • We ignore packet scheduling issues and consider a steady-state flow model • The model • each network edge may be fractionally shared by several flows • the sum of shares cannot exceed 100% • the transmission graph: G(V, E, ρ)‏ • transmission rate to each transmission edge ρ : E → R+

13. G can be directed • the transmission rate in the reverse direction of a bi-directional edge may be different than that in the forward direction • different node configurations and asymmetric channel effects • The interference graph: G(V˜ , E˜)‏ • the vertices of the interference graph to be the edges of the transmission graph: V˜ = E • ((a, b), (c, d)) ∈V˜ if (a, b), (c, d) ∈ E and if a transmission on (a, b) interferes with a transmission on (c, d)‏ • modeling the interference graph: difficult • interference neighborhood of any given edge (u, v) as follows. χ(u, v) = (u, v) ∪ ((x, y) : ((x, y), (u, v)) ∈ E)‏

14. a set of i flows: each φi originates from source si and is sinked by receiver ri . • we can represent each flow as a sum of path flows. Each path flow φij exists only on πij (path)‏ • each edge (u, v) in the transmission graph • the sum of the fractional shares used by all flows in the interference neighborhood of (u, v) must be less than or equal to 100%. • this is a more complicated version of the classic edge capacity flow constraint.

15. Linear Programming (LP) methods are required to achieve an optimal throughput solution • Theorem 1: • In the case of a complete interference graph in the stated multi-rate ad hoc wireless network model, a routing protocol that chooses a path that minimizes the sum of the transmission times minimizes network resource consumption, and maximizes total flow capacity.

16. Medium Time Metric • It is designed to allow any shortest path routing protocol to find throughput optimal routes assuming full interference • assigns a weight to each link • proportional to the amount of medium time used by sending a packet • Weight of a path • Sum: proportional to the total medium time consumed • packet traverses the whole path • → shortest path protocols that use the medium time metric find paths that minimizes the total transmission time

17. Assumption: full interference • A general optimal algorithm must • monitor the medium time utilization at every node in the network, • disseminate that information (to aid routing decisions)‏ • unnecessary to use multiple paths simultaneously • Multiple paths available: random choice & exclusive usage • Using additional paths: no advantage

18. Computing link weights • MTM: paths that minimize the total consumed medium time should be selected • Uses existing shortest paths protocols • Assign WEIGHT to LINKS • Medium time consumed ~ package sending • Possible solution • Inverse rate scheme (Cisco)‏ • In wired networks: usage of MTM no advantage • There is no transmission interference in wired nws • Wires are isolated :)‏ • BUT: prediction about medium time consumed is sometimes wrong (because of MAC overhead)‏ • MAC overhead

19. The solution: • To induct a package size dependency into the protocol • Transmission of a small packet is dominated by the MAC overhead & is almost the same (regardless to link rate)‏ • MTM would use different light weights for different package size • easy to implement in link state protocols • topology information (to compute alternate routes using different sets of weights)‏ • More difficult for distance vector protocols • additional communication overhead for each additional set of weights

20. An implementation of the MTM for a distance vector protocol • It is tuned for the dominant packet size • using link weights ~ to the medium time • used by packets of the tuned size • Larger packets: longer path with even higher rate links • Smaller packets: paths that are shorter but with lower rate links • the tuned packet size was chosen (1500 byte IP packet)‏ • OAR protocol significantly changes the MAC layer packet exchange • the expected medium time consumed by a packet at a given rate changes significantly • MTM weights must be calculated to match the change in consumed medium time

21. Advantages • Simplicity • shortest path metric • it can be incorporated into existing distance vector or link-state protocols • the majority of existing wireless ad hoc routing protocols fall into these categories • MTM protocols only need to track changes in link rates • MTM paths naturally avoid low-rate links • nodes connected by a high-rate link consider distance before the link breaks • nodes move apart, the auto rate protocol reduces the link speed • proactive routing protocols: update their paths → avoid path failures • by continuously switching to higher rate links.

22. Discussion • link rates by definition change faster than link connectivity • some routing protocols may consume more overhead when using MTM when compared with min hop • distance is the dominant factor that determines the link rate • even in the worst case, the MTM metric should only change a constant amount more than connectivity • better than traffic sensitive routing, • traffic loads change much faster than either link rates or link connectivity

23. The MTM selects paths that have a greater number of hops than the minimum • higher rate hops: less total medium time than the minimum number of hops • BUT: increased number of senders could cause other detrimental effects: packet drop • When the density of the network is low: topology sparsely connected • few choices for routing protocols to select from • MTM and min hop will tend to pick the same path

24. Node density ~ increased throughput

25. Summary • general theoretical model of the attainable throughput in multi-rate ad hoc wireless networks • MTM is derived from a detailed analysis of the physical and medium access control layers • Selects optimal throughput paths and tends to avoid long unreliable links • Minimizes the total medium time consumed sending packets from a source to a destination • This results in an increase in total network throughput (20% to 60%)‏