Chapter 2, "Building a Cisco IPC Express Network," covers the choice between centralized architectures (those based solely on Cisco CallManager) and distributed or autonomous site architectures (those based on Cisco CME) for implementing VoIP networks. In many real-world cases, this isn't a simple either/or decision, because many networks include both types of structures, discussed in Chapter 2 as the hybrid network architecture.
Real enterprise VoIP networks that have been designed consistently from the ground up and that adhere to a single uniform architectural approach are rare. The technologies available to network designers have evolved rapidly over the past decade or two. This rapid evolution is likely to continue for some time. It requires organizations to continually rethink their network architectures to take advantage of the latest available enhancements. Not only do the technologies change, but so do the companies trying to make best use of them. Companies split and merge and reinvent themselves in a continuous effort to stay profitable and competitive. This leads to real-world networks made up of a mixture of architectures formed by the ad hoc fusion of components contributed by multiple network designs.
Looking at VoIP networks that incorporate Cisco components, you commonly see both central-site Cisco CallManager networks using Survivable Remote Site Telephony (SRST) at some remote branch offices coupled with Cisco CME systems used at other remote offices. Being able to interconnect these systems is a fairly important consideration. In fact, some businesses deliberately design their networks using both central and distributed models to take into account issues with geographic variation in the availability of WAN services. For example, in the banking industry, central Cisco CallManager designs have been widely used in city branches located in metropolitan areas where adequate bandwidth and quality of service (QoS)-enabled WAN links are fairly readily available. On the other hand, Cisco CME systems have been used in small-town bank branches located in more rural areas where WAN services might be less sophisticated and unable to support voice.
Both Cisco CallManager and Cisco CME support H.323, which you can use to create Cisco CallManager-to-CME links. Cisco CME also supports SIP for VoIP interconnect. SIP is also being introduced as a WAN trunking interface on Cisco CallManager. This chapter focuses only on the H.323 interconnect option, because the SIP interconnect option is still a work in progress as SIP support on successive Cisco CallManager releases evolves. However, you can expect that most of the architectural issues raised in this chapter are also applicable in the SIP context.
The descriptions contained in this chapter apply to the Cisco CME 3.1 and 3.2 releases and the Cisco CallManager 3.3(3) and 4.0 releases. Newer releases may have different behaviors and options than those described here.
Basic Calls Between Cisco CallManager and Cisco CME