Queuing Delay and Achievable Throughput in Random Access Wireless Ad Hoc Networks

Transcription

1 Queuing Delay and Achievable Throughput in Random Access Wireless Ad Hoc Networks Nabhendra Bisnik and Alhussein Abouzeid Rensselaer Polytechnic Institute Troy, NY

2 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 2

3 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 3

4 Important Questions q Important questions: q How throughput scales with network size? q How delay scales with network size? q Relation between delay and throughput? q What are the tradeoffs? q We developed queuing network models to analyze delay and throughput of multihop wireless ad hoc networks 4

5 Delay in Multihop Wireless Networks q End-to-end delay is sum of queuing and transmission delays at intermediate nodes q Queuing delay depends on q Packet arrival process how much traffic is handled by network? q Node density how many interferers are there? q MAC protocol how the channel is shared? q Traffic pattern how many times a packet is transmitted before it reaches destination q Modeling all the factors is quite challenging 5

6 Throughput in Multihop Wireless Networks q Maximum achievable per node throughput of a network is the maximum rate at which the nodes of a network may generate traffic while keeping delay finite q Maximum achievable throughput is inversely proportional to q Average time a node takes to serve a packet q Average number of flows served by a node 6

7 Related Work q Gupta and Kumar Capacity of Wireless Networks q Under optimal scheduling, per node throughput scales as q E.Gamal et al Throughput Delay Trade-off in Wireless Networks q D(n) = (n T(n)) q Assuming that: q Packet size scales with throughput q Infinite backlog at source q Centralized and deterministic scheduling q Delay is proportional to number of hops traversed 7

8 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 8

9 Network and Interference Model q Network consists of n nodes that are distributed uniformly and independently distributed over a unit torus q Transmission rate of each node = W bits/sec q Interference Model: node i can successfully forward a packet to node j only if q r ij r(n) q r jk > r(n) nodes k transmitting simultaneously with i 9

10 A q : Neighbors of A All nodes within distance r(n) of A q + : Interfering neighbors of A All nodes within distance 2r(n) of A Transmission of A is guaranteed to be successful if none of the interfering neighbors of A transmit simultaneously 10

11 MAC Model q Before transmitting a packet each node counts down a random timer q The duration of the time is exponentially distributed with mean 1/ q Once the timer of a node expires it starts transmitting and at the same instant the timers of all interfering nodes is frozen The MAC model captures the collision avoidance mechanism of IEEE and is still mathematically tractable 11

12 Traffic Model q Each node is source, destination and relay of traffic q Size of each packet is fixed and equals L bits q Each node generates packets at rate packets/sec q When a node receives a packet from its neighbor: q The packet is absorbed by the node with probability p(n) (absorption probability) q The packet is forwarded to a randomly chosen neighbor with probability 1-p(n) q In other words, the fraction of packets received by a node that are destined to it equals p(n) p(n) characterizes the degree of locality of traffic Low p(n) average hops between a source destination pair is large 12

13 Queuing Network Model q In order to characterize delay, ad hoc network modeled as G/G/1 queuing network q Each node of the network is a station of queuing network q Incorporate queuing delays at source and relay in delay analysis q Diffusion approximation used to analyze the resulting queuing network 13

14 The queuing network 14

15 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 15

16 Main Results q Mean service time ( ) Average time it takes for a node to serve a packet Service time in absence of interference Where, Term introduced by interfering neighbors 16

17 Interpreting the Service Time Result q Transmitter and receiver, in absence of interference q Service time = Wait for timer to expire + transmission time = 17

18 Interpreting the Service Time Result q Now suppose there are k interferes, each with packet arrival rate q Fraction of time for which the channel is occupied by the interferers = q The fraction of time the channel is available to the transmitter = q In our model = i and k = 4nA(n), therefore 18

19 Main Results q Average end-to-end delay ( ) Average time in which packet reaches the destination after being generated at source Where, The value of end-to-end delay is governed by and SCVs of service and inter-arrival times. 19

20 Main Results q Maximum achievable throughput ( ) or, Where, As expected, MAT varies inversely with mean path length, node density and communication radius of nodes 20

21 Comparison with Kumar-Gupta Results q When parameters of our model are comparable to that of Kumar-Gupta model i.e. and or The bound is similar to Gupta-Kumar bound but is not achievable. This is expected as channel capacity is wasted due to random access. 21

22 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 22

23 Deviation from Real Networks q The MAC model does not take into account the packet collisions an essential feature of random access MAC q We assume all interfering nodes freeze their transmission timer as soon as a packet transmission begins. q In reality a node freezes its timer only when it hears a transmission q Thus, a transmission is successful if all interfering neighbors hear the transmission before their timers expire 23

24 Probability of Success q If a node has I interfering neighbors then q The expected probability of success, P s is given by 24

25 Improved Performance Bounds q Taking packet collision into account, for a more practical MAC the average service time is bounded by q Maximum achievable throughput is bounded by 25

26 Plots of the Deviation 26

27 Optimal Timer Rate q The optimal timer rate,, for which maximized is solution of this equation is q If, 27

28 Outline q Introduction q Queuing Network Model q Main Results q Deviation from Real Networks q Simulation Results 28

29 Simulation Results Comparison of theoretical and simulation results Diffusion Approximation yields pretty good results. 29

30 Conclusion and Future Work q Developed queuing network models for multihop wireless ad hoc networks q Used diffusion approximation to evaluate average delay and maximum achievable per-node throughput q Investigated the deviation of results from real life networks q Future work: extend analysis to many to one cases, taking deterministic routing into account 30

31 Thanks! 31