In earlier chapters, we've talked about dial-plan design, PSTN trunks, and dial-tone services. We've covered some of the equipment used to link VoIP-based networks with legacy telephony systems: equipment like media gateways and ATAs. Signaling protocols like SIP, media protocols like RTP, and quality of service have been described. Each of these elements is dedicated to a specific, tactical duty in your telephony system.
But the "big picture" that contains and unifies all of these disparate technology objects together into a functional system is topology : the lay of the telephony land. At the base of this functional system is infrastructure. The transport, security, and directory services elements enabling your voice applications on the IP network are your VoIP infrastructure. Topology includes geographical issues, toothe physical locations of voice resources and connectivity maps of your wide area network.
There's quite a bit to take into account. In this chapter, we'll talk about methods for building a reliable VoIP infrastructure. We'll cover general IP WAN layouts, the use of private trunks to link PBX systems, disaster recovery and survivability , choosing a location for PSTN connect points, optimizing VoIP WAN links, and directory services for telephony.
You could think of VoIP servers and private trunks as Tinkertoy hubs and sticks. The old Tinkertoy hubs had six holes and could therefore host six sticks, which could connect to other hubs and so on, creating a network of Tinkertoys. This network could be straight and long, it could be shaped like a hexagon or star, or it could be a complex mesh, depending upon how the hubs and sticks were arranged. Such is the case with WAN topology, and particularly enterprise VoIP.
In the context of an IP WAN, the Tinkertoy hubs are routers (or switches), and the sticks are data links like frame-relay PVCs (permanent virtual circuits)or Ethernet switches. In the higher-layer context of a VoIP network, the Tinkertoy hubs are analogous to softPBX servers, and the sticks would be physical (T1 carrier) or logical (IP-based) voice trunks.