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%)‏