Pheromone based Mobile Agent Migration Mechanisms
4.1 Introduction and Related Work
Chapter 4
Pheromone based Mobile Agent
as physical entities or paths in the environment or as synthetic messages.
Kitamura et al. [130] have used pheromone communication for understanding swarm intelligence in terms of complex and adaptive population behaviours for robots. They describe robots that can lay pheromones on a desktop using coloured pens mounted on them. Susnea et al. [131] describe virtual pheromone based control of mobile robots where robots relay their position information to a remote program which in turn uses a pheromone based algorithm to compute the alignment to be effected by the robot. Parunak et al. [132] discuss pheromone based computations by a group of small sensor bots thrown onto a hostile area to check out the maneuverability of vehicles in the region.
Research on various techniques for coordinating a large number of robots has been conducted by Payton et al. [133]. Anies and Russell [134] have discussed pheromone communication between robots using chemical signals.
They use a physical chemical for the task of assessing the quorum size of a group of robots equipped with gas sensors. The communication is bi-directional and their investigations have revealed the potential advantages and drawbacks of implementing physical pheromone signaling between robots.
Pheromone based algorithms have also been used for routing in MANETS. Roth and Wicker [135] describe TERMITE, a distributed routing algorithm for mobile wireless ad-hoc networks inspired by the hill-building behaviour of termites. It is designed using a swarm intelligence framework [136]
for achieving better adaptivity, low control overhead and low per-node computation. Contributions of both Gunes and Spaniol [137] and Roth and Wicker [135], [138] have shown enhanced performance over traditional approaches used for MANET routing. Gunes and Spaniol use an ant-colony based algorithm for routing in MANETs while Roth and Wicker use stigmergy to reduce the amount of control traffic.
In the succeeding sections a pheromone based bi-directional search strategy that can drastically reduce the time taken by the mobile agents in reaching robots that require a service, is described.
4.2 PherCon: A Mechanism for Search and Service Using Conscientious Mobile Agents and Pheromone diffusing Robots
Mobile agents normally use the conscientious method for migration within the network. In this approach they keep track of the nodes already visited and avoid migration onto such nodes. This section presents an enhanced migration strategy nicknamed PherCon, wherein both the mobile agent and the robotic node requiring the former’s services, play a pro-active role in discovering one other. The mobile agent uses the conscientious migration strategy while a robotic node requiring its service, referred to as the Robot Requiring Service (RRS), diffuses pheromones to attract the former. An illustration of AB which uses PherCon, is depicted in Figure 4-1. The mobile agents populating the network carry the information similar to that of the Y-shaped antibodies mentioned in Chapter 2, as their payloads. This includes the name of the service and the program to effect the service. The name of the service resembles the information in the constant region of the Y-shaped antibody while its associated program is analogous to the rule-set. Whenever an robotic node (RRS) within the network requires a program or a service, it diffuses pheromones with the highest concentration possible, along the paths connecting its neighbouring nodes. The neighbours in turn lay the same beyond, with lesser concentrations forming a network of pheromones in the vicinity of the RRS. Thus, if the mobile agent is anywhere in the vicinity or within the neighbours of this RRS it will be guided, from thereon by the pheromone concentration gradient, along the shortest path towards the RRS. If pheromones are diffused many hops away from an RRS, it will proportionately decrease the time required for convergence of the agent to the RRS. As can be seen from Figure 4-1, the RRS diffuses pheromones (red dotted lines) onto its immediate one-hop neighbours R1, R2, R3 and R4 which in turn re-diffuse them to their respective neighbours viz. R5, R6, R7 and R8 at lesser pheromone concentrations. An agent resident at R10 continues its migration using the conscientious approach to eventually reach R7 via R9 to find a pheromone
trail. If the information within this pheromone matches that carried by the agent, then it starts to climb the pheromone concentration gradient to finally reach the RRS via the shortest path henceforth. This matching of the information within the pheromone with that contained by the agent is akin to matching the type of the robot and the type stored within the constant region of the Y- shaped antibody as discussed in section 2.4. The pheromones diffused by the RRS and the conscientious migration on part of the agents carrying the requested services constitute a bi-directional search. However it is unlike the normal bi-directional search which has no guidance mechanism when it nears the destination. The RRS seems to extend and lay a path for the mobile agent while the latter searches concurrently and conscientiously for such paths thereby facilitating faster service of the former. While the mobile agent uses the conscientious approach till it senses the pheromone, the RRS spreads out a pheromone network around it to trap and guide the agent towards it.
Fig. 4-1 Illustration of the Mobile Agent based Networked Robot System
with Pheromoning Robots and Mobile Agents [78]
4.2.1 Pheromones in PherCon
When a robot requires a service, it diffuses pheromones onto its immediate neighbours. These pheromones inherently carry the following information.
1) RRS ID: Pheromones carry the robot id of the RRS that originally initiated its diffusion within a field called RRS ID. This RRS ID stored within the pheromone ensures that the mobile agent does not perform a redundant service, described later.
2) Service Information: This information allows a mobile agent to