| Abstract: | The Power Utility of Serbia Company intends to migrate portions of its time-division-multiplexing- based telephony network to IP. The high-level, capital project requirements include: ? Building a new VoIP infrastructure ? Exchanging approximately one-third of the existing TDM-based PBXs with new VoIPbased and hybrid PBXs ? Integrating these new PBXs with the remaining existing legacy PBXs to create a new hybrid network Telephony service availability should be five nines (99.999 percent), a requirement that created a derived requirement for network element redundancy. One of the most important elements is the softswitch. The softswitch is the equipment responsible for call control of IP phones and voice gateways at the level of the Power Utility of Serbia Company telephone network. Softswitch redundancy in a failover configuration proved to be most challenging because of a performance requirement to achieve a switchover time of six seconds or less, whereas commercially available softswitches typically support switchover times of a few minutes. Another significant challenge was integrating the legacy PBXs and new (VoIP) PBXs. Although industry marketing promotes SIP trunking as the preferred integration/interconnection method, we found that with the exception of bearer traffic (voice media) pathing, service interoperability between the equipment of different vendors was often problematic. Vendors typically suggest Q.Sig as an interconnection alternative when SIP trunking is infeasible, but some vendor interoperability problems persisted. This article presents technical solutions to these major issues that can benefit similar TDM-to-IP migration and hybridization projects. |
