Impact of SCTP-controlled Failovers for M3UA Users in a Dedicated SIGTRAN Network

Contrary to a datacommunications network, a telecommunications network logically comprises two networks: a transport and a signaling network. The transport network carries voice traffic, while the signaling network carries the control information that is needed for the administration and supervision of calls, and the administration of the telecommunications network itself. Traditionally, signaling traffic and voice traffic are both carried over TDM circuit-switched connections. However, currently this is about to change. With IP, telecom operators are seeing a way to improve the resource utilization and reduce the operational, maintenance, and network infrastructure costs. Still, the transition from TDM to IP will not happen overnight – maybe never. The traditional telecommunications network represents a huge capital investment and is still unsurpassed in terms of reliability an QoS. To address the situation with two different, co-existing networks, one TDM based and one IP based, the IETF SIGTRAN working group has developed an architecture for signaling traffic over IP. In particular, they have developed an architecture for running Signaling System No. 7 (SS7), the predominant signaling protocol, over IP. Together with the so-called SoftSwitch architecture, the SIGTRAN architecture [4] constitutes a complete solution for the integration of the two networks. The interoperability between the traditional TDM based telecommunications network and its IP counterpart requires that the signaling performance in the IP network is comparable to that of TDM. Although there has been some time since the SIGTRAN architecture was first published, it is still unclear if it will perform similar to the traditional telecommunications network, or if it will lead to unacceptable performance degradations. The SIGTRAN architecture specifies a common transport protocol for all SS7 signaling traffic, STCP [7], and a number of adaptation layers which runs on top of SCTP. Although several adaptation layers have been specified, it seems as if a majority of telecom companies have embraced the MTPL3 User Adaptation Layer [6](M3UA). This adaptation layer mimics the functionality of MTP-L3, the SS7 transport layer, and thus makes it possible to run all layers of the SS7 stack above MTP-L3 on IP unmodified. In order to provide network redundancy in a SIGTRAN network, SCTP supports so-called multi-homed associations. Multi-homed associations make it possible to manage several TCP-like connections, paths in SCTP, as one redundant logical connection. When one path goes down, SCTP performs a failover and switches all traffic to an alternative path. Since a similar failover mechanism is also available in M3UA, we hereafter denote failover management in the STCP layer SCTP-controlled failover. This paper presents an experiment in which the impact of SCTP-controlled failovers was studied. In particular, the experiment studied the impact these failovers have on the Message Signal Unit (MSU) transfer times, i.e., the signaling message transfer times, for an M3UA user in a dedicated SIGTRAN network. In addition, the experiment studied to what extent an increased link delay has a significant deteriorating effect on the MSU transfer times during an SCTP-controlled failover. A similar experiment as the one presented in this paper has been carried out by Jungmaier et al. [2]. However, their experiment considered the M2PA adaptation layer. Furthermore, Caro Jr. et al. at the University of Delaware have made extensive simulation studies on issues related to SCTP multihomed associations, and have suggested a two-level threshold mechanism [1] as an improvement to the existing SCTP failover mechanism. The remainder of the paper is organized as follows. Section II describes the experiment procedure and setup. The results of the experiment are presented in Section III. Finally, Section IV concludes the paper.