  The Session Initiation Protocol (SIP) is an application-layer control
protocol for creating, modifying and terminating multimedia sessions. An open
issue is the control of overload that occurs when a SIP server lacks sufficient
CPU and memory resources to process all messages. We prove that the problem of
overload control in SIP network with a set of n servers and limited resources
is in the form of NP-hard. This paper proposes a Load-Balanced Call Admission
Controller (LB-CAC), based on a heuristic mathematical model to determine an
optimal resource allocation in such a way that maximizes call admission rates
regarding the limited resources of the SIP servers. LB-CAC determines the
optimal "call admission rates" and "signaling paths" for admitted calls along
optimal allocation of CPU and memory resources of the SIP servers through a new
linear programming model. This happens by acquiring some critical information
of SIP servers. An assessment of the numerical and experimental results
demonstrates the efficiency of the proposed method.
