Topic 6: Stigmergy, Swarm Intelligence and Ant Algorithms

1. Topic 6: Stigmergy, Swarm Intelligence and Ant Algorithms swarm intelligence stigmergy ant algorithms AntNet: routing AntSystem: TSP

2. Swarm Intelligence • “Swarm Intelligence (SI) is the property of a system whereby the collective behaviors of (unsophisticated) agents interacting locally with their environment cause coherent functional global patterns to emerge.” • characteristics of a swarm: • distributed, no central control or data source • no (explicit) model of the environment • perception of environment, i.e. sensing • ability to change environment • problem solving is emergent behaviour

3. Some natural Swarm Intelligence systems • Ants • find the shortest path to food • make cemeteries • sort their brood by size • etc. • Termites • build nests with complex features like • fortified chambers • spiral air vents • fungus gardens • etc. • Bees • gather pollen with high efficiency, exploiting the nearest richest food source first. • Geese • coordinate takeoff and landing • flight patterns • etc. • Fish / Birds /… • swarms

5. How Do Social Insects Coordinate Their Behaviour? • communication is necessary • two types of communication • direct • antennation, trophallaxis (food or liquid exchange),mandibular contact, visual contact, chemical contact, etc. • indirect • two individuals interact indirectly when one of them modifies the environment and the other responds to the new environment at a later time called stigmergy

6. Stigmergy • “La coordination des taches, la regulation des constructions ne dependent pas directement des oeuvriers, mais des constructions elles-memes. L’ouvrier ne dirige pas son travail, il est guidé par lui. C’est à cette stimulation d’un type particulier que nous donnons le nom du STIGMERGIE (stigma, piqure; ergon, travail, oeuvre = oeuvre stimulante).” Grassé P. P., 1959 • [“The coordination of tasks and the regulation of constructions does not depend directly on the workers, but on the constructions themselves. The worker does not direct his work, but is guided by it. It is to this special form of stimulation that we give the name STIGMERGY (stigma, sting; ergon, work, product of labour = stimulating product of labour).”]

7. Stigmergy Communication through marks in the environment … • Marks serve as a shared memory for the agents • Promotes loose coupling • Robust Stigmergy ~ action + sign: • Agents put marks in the environment (inform other agents about issues of interest) • other agents perceive these marks (influence their behavior) • manipulate marks • other agents perceive the marks • etc. Marks can be • static (environment does not manipulate marks over time) • e.g., flags • dynamic (environment manipulates marks over time) • e.g., pheromones, gradient fields

8. Example of Stigmergy:Ant cemeteries in nature • simple behaviour rules for ants • wander around • if you find a dead antpick it up with probability inversely proportional to the number of other dead ants nearby • if you are carrying a dead antput it down with probability directly proportional to the number of other dead ants nearby

9. Food Nest Example of Stigmergy: Trail Following and Ants Foraging Behaviour • while walking, ants and termites • may deposit a pheromone on the ground • follow with high probability pheromone trails they sense on the ground

10. Ants Foraging BehaviourExample: The Double Bridge Experiment 15 cm A Nest Food B Simple bridge % of ant passages on the two branches Goss et al., 1989, Deneubourg et al., 1990

11. Asymmetric Binary Bridge Experiment

12. Food foraging in nature

13. r = 1 Some Results r = 2 r is the length ratio among the two bridges r = 2 Short branch added later

14. Food Nest Evaporation and Refresh

15. “Artificial” Stigmergy Indirect communicationmediated by modifications of environmental states which are only locally accessible by the communicating agents Dorigo & Di Caro, 1999 • Characteristics of artificial stigmergy: • Indirect communication • Local accessibility

16. What Are Ant Algorithms? • Ant algorithms are multi-agent systems that exploit artificial stigmergy as a means for coordinating artificial ants for the solution of computational problems • examples • AntNet: network routing • AntSystem: TSP

17. AntNet: An Ant Algorithm for Routing in Packet-Switched Networks (e.g. Internet) • the routing problem • build routing table at each node • costs are dynamic • adaptive routing is hard

18. AntNet data structure • routing table • probabilities of choosing each neighbor nodes for each possible final destination • trips vector • contains statistics about ants’ trip times from current node k to each destination node d (means and variances) P(k,n,d)

19. Probabilistic rule to choose the path Artificial Ants in a Network Pheromone trail Memory depositing ? Source Destination

20. Simple AntNet Algorithm • ants are launched regularlyfrom each node to randomly chosen destination • ants build their paths probabilistically with probability function of • artificial pheromone values • heuristic values • ants memorize visited nodes and incurred costs • once destination is reached, ants deterministically retrace their path backwards, updating pheromone trails that is a function of the quality of the solution they generated • refresh + evaporation !!

21. AntNet Routing - Agents • Two kinds of Agents • Forward Ant • explores the network and collects information • when reaches the destination, changes into backward ant • Backward Ant • goes back in the same path as forward ant • update routing tables for all the nodes in the path

22. (N1,T1) (N2,T2) (N3,T3) (N4, T4) (N1,T1) (N2,T2) (N3,T3) (N2,T2) (N1,T1) (N1,T1) Forward Ant Forward Ant Forward Ant Forward Ant AntNet: F-Ants and B-Ants • F-ants • collect implicit and explicit information on paths and traffic load • implicit through arrival rate at destination • explicit by storing experienced trip times • share queues with data packets • B-ants • backpropagate fast • use higher priority queues 2 1 4 3 Backward Ant Backward Ant Backward Ant Backward Ant

23. Using Pheromone and Memoryto Choose the Next Node Memory of visited nodes tijd j i tikd tird k r

24. Ants’ Probabilistic Transition Rule • tijd is the amount of pheromone trail on edge (i,j,d) • Jik is the set of feasible nodes ant k positioned on node i can move to

25. Using Pheromone and Heuristicto Choose the Next Node Memory of visited nodes a heuristic evaluation of link (i,j,d) which introduces problem specific information stored in pheromone table tijd;ijd j i tikd;ikd tird;ird k r

26. Using Pheromone and Heuristicto Choose the Next Node Memory of visited nodes a heuristic evaluation of link (i,j,d) which introduces problem specific information: for AntNet: proportional to the inverse of link (i,j) queue length stored in pheromone table tijd;ijd j i tikd;ikd tird;ird k r

27. B-ants update routing tables: • if B-ant has source node d and goes from node n to k • Increase the probability of the channel that backward ant comes from P(k,n,d) = P(k,n,d) + R.(1 – P(k,n,d)) • Decrease the probability of the other channels P(k,i,d) = P(k,i,d) - R.(1 – P(k,i,d)) for all i != n R (0 < R <= 1), function of - T: time experienced by F-ant - m: avg time for same destination memorized in trips table - sigma: std. deviation for same destination memorized in trips table P(k,n,d)

28. AntNet evaluation • extensive tests and experiments • American NSF net • Japanese NTT net • artificial networks • simple graphs • grid networks • in short: good results • similar throughput compared to other routing algorithms • remarkably smaller avg. packet delay

29. Ant Colony Optimization (ACO) • TSP has been a popular problem for the ACO models. • Key concepts: • Positive feedback • build a solution using local solutions, by keeping good solutions in memory • Negative feedback • want to avoid premature convergence, evaporate the pheromone • Time scale • number of runs are also critical

30. Traveling Salesman Problem (TSP)

31. Ant Optimization of TSP • seed all paths with initial pheromone • ants make a tour of all cities • ant probabilistically selects next city to visit considering distance and pheromone strength on each path • ants deposit pheromone on their path with an intensity inversely proportional to the length of their tour • pheromone decays with each time step • repeat steps 2 - 4 until some threshold is met

32. [] [] [] [] [] 1 2 3 4 5 A simple TSP example A B C D E dAB =100;dBC = 60…;dDE =150

33. [A] [E] [C] [B] [D] 1 5 3 2 4 Iteration 1 A B C D E

34. [A] [A] [A] [A] [A,D] 1 1 1 1 1 How to build next sub-solution? A B C D E

35. [C,B] [E,A] [A,D] [B,C] [D,E] 3 5 1 2 4 Iteration 2 A B C D E

36. Iteration 3 [D,E,A] [E,A,B] [C,B,E] [B,C,D] [A,D,C] 4 5 3 2 1 A B C D E

37. [D,E,A,B] [B,C,D,A] [E,A,B,C] [A,D,C,E] [C,B,E,D] 4 2 5 1 3 Iteration 4 A B C D E

38. [A,D,C,E,B] [C,B,E,D,A] [D,E,A,B,C] [B,C,D,A,E] [E,A,B,C,D] 1 3 4 2 5 Iteration 5 A B C D E

39. [A,D,C,E,B] [E,A,B,C,D] [B,C,D,A,E] [C,B,E,D,A] [D,E,A,B,C] 1 5 2 3 4 Path and Pheromone Evaluation L1 =300 L2 =450 L3 =260 L4 =280 L5 =420

40. End of First Run Save Best Tour (Sequence and length) All ants die New ants are born stopping criteria stagnation max.runs

41. Ant cycle for TSP • tabu list • random walk • priorities • backtracing and considering edge length 5 1 4 1 1 3 2

42. Tabu List n : number of cities = one city tourm : number of antsk : ant indexs : member in tabu list (current city) for each iteration in the ant algorithm cycle:insert town in „visited node list“ for (int k = 1 ; k <= m ; k++) { insert town of ant k in Tabuk(s) }

43. Random Walk & Priorities i,j : edge between nodes i,jnij : 1/distance(i,j)a : weight for markingb : weight for node „neighbourhoodness“ allowedk : unvisited cities for ant k from ant routing table: probability of ant k for going from city i to city j

44. Backtracing i,j : edge between nodes i,jtij (t): marking at time tr : evaporation rate (t,t+n) Q/Lk : const/tour-length of ant k With Tabu List, ant routing table: marking_delta = sum of markings of all ants k who passed (i,j) marking after tour = evaporation_rate * marking_old + marking_delta

45. Ant System (Ant Cycle) Dorigo [1] 1991 t = 0; NC = 0; τij(t)=c for ∆τij=0 Place the m ants on the n nodes Initialize Update tabuk(s) Tabu list management Choose the city j to move to. Use probability Move k-th ant to town j. Insert town j in tabuk(s) Compute the length Lk of every ant Update the shortest tour found = For every edge (i,j) Compute For k:=1 to m do Yes Yes NC<NCmax && not stagn. Set t = t + n; NC=NC+1; ∆τij=0 No End

46. 10-Cities-Problem CCA0 AntSystem marking distribution at beginning and after 100 cycles

47. Oliver30 Problem cycles: 342length : 420 a = 1b = 5r = 0.5

48. Some inherent advantages • positive feedback accounts for rapid discovery of good solutions • distributed computation avoids premature convergence • the greedy heuristic helps find acceptable solution in the early stages of the search process • the collective interaction of a population of agents

49. Disadvantages in Ant Systems • slower convergence than other heuristics • performed poorly for TSP problems larger than 75 cities

50. Improvements to AS: Ant Colony System (ACS) • ACS (pheromone update) • update pheromone trail while building the solution • ants eat pheromone on the trail • local search added before pheromone update