In this paper we investigate traffic load control mechanisms for controlling congestion in signaling networks based on three types of thresholds. The goal of the paper is to analyze congestion controlling mechanisms and develop corresponding queuing models of SIP servers. The study is based on hysteretic congestion control, which has been developed for Signaling System 7 (SS7). Models for describing the hysteretic control are developed. The current state and problems of basic overload control mechanism proposed by Internet Engineering Task Force (IETF) for SIP signaling networks are investigated. Approaches to building mathematical models of SIP servers in the form of a queuing system with hysteretic control are proposed. © 2012 Springer-Verlag.