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client/agent/server model alleviates the impact of the limited bandwidth and
the poor reliability of the wireless link by continuously maintaining clients
presence on the fixed network via an agent. The agent splits the interaction
between mobile clients and fixed servers into two parts, one between the cli-
ent and the agent, and one between the agent and the server. This model
moves responsibilities from the client to the agent. Moreover, agents are used
in a variety of forms and roles. At one extreme, an agent acts as the complete
surrogate of a mobile host on the fixed network. At the other extreme, the
agent is attached to a specific service or application.
The client/intercept/server model [45] is used to address the shortcom-
ings of the client/agent/server model. In the client/agent/server model, the
mobile client cannot continue to operate uninterrupted when an event such
as a disconnection happens. That model requires changes to the client code,
and the agent can optimize only data transmission over the wireless link from
the fixed network to the mobile client but not vice versa. The client/inter-
cept/server model proposes the use of two agents, the client-side agent, which
is co-resident with the client, and the server-side agent, which resides on the
fixed networks. This model is transparent to both the client and the server,
offers flexibility in handling disconnections, and optimizes data transmis-
sions from the fixed network to the mobile client and vice versa.
10.7.2
Agents
Agent technology is not new in computer science [46]. It has been used in,
for example, artificial intelligence. In general, an agent is a computer pro-
gram that acts autonomously on behalf of a person or an organization. Each
agent has its own thread of execution so it can perform tasks on its own
initiative. An agent system is a platform that can create, interpret, execute,
transfer, and terminate agents. When an agent moves, it travels between exe-
cution environments called places. A place is a context within an agent system
where an agent can execute. The source place and the destination place can
reside in the same agent system or in different agent systems that support the
same agent profile [47].
The use of the agent technology in the implementation of mobile sys-
tems has the following advantages [48]:
Asynchronous communications. The elements involved in the com-
munication do not have to be connected all the time. That means
MUs may decide to disconnect while the agents that represent them
are working in other computers. This may be interesting, for
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