CUCM: Single-Site/Campus Deployment
As illustrated in Figure 2-3, the single-site model for CUCM consists of a CUCM cluster located at a single site or campus with no telephony services provided over a WAN.
Figure 2-3 Single-Site Deployment
All CUCM servers, applications, and digital signal processor (DSP) resources are located in the same physical location or at multiple physical buildings with local-area networks (LAN) or metropolitan-area network (MAN)–based connectivity. LANs are normally defined as having connectivity speeds of 1000 Mbps (1 Gbps) and above, while MANs are typically in the multi-megabit range. In this model, calls beyond the LAN or MAN use the public switched telephone network (PSTN). Besides the voice gateway, Cisco Unified Border Element (CUBE) can also be used to connect all PSTN traffic via IT Service Provider (ITSP) cloud.
Each cluster supports a maximum of 40,000 IP phones. If there is a need to deploy more than 40,000 IP phones in a single-site configuration, multiple clusters can be implemented inside a LAN or within a MAN and connected through intercluster trunks. Gateway trunks that connect directly to the PSTN manage external calls. If an IP WAN exists between sites, it is used to carry data traffic only; no telephony services are provided over the WAN.
Design Guidelines for Single Site/Campus Model
To accommodate future scalability, Cisco recommends that best practices specific to the distributed and centralized call-processing models be used in a single-site deployment.
Current calling patterns within the enterprise must be understood. How and where are users making calls? If calling patterns indicate that most calls are intrasite, using the single-site model will simplify dial plans and avoid having to provision additional dedicated bandwidth for voice across the IP WAN.
Because Voice over Internet Protocol (VoIP) calls are within the LAN or campus network, it is assumed that bandwidth is not a concern. Using G.722 or G.711 codecs for all endpoints will eliminate the need for DSP resources for transcoding, and those resources can be allocated to other functions, such as conferencing and Media Transfer Protocols (MTPs).
All off-net calls will be diverted to the PSTN (via voice gateway or CUBE) or sent to the legacy private branch exchange (PBX) for call routing if the PSTN resources are being shared during migratory deployments.
To ensure successful operations, a network infrastructure designed for high-availability, fault-tolerant connectivity options should be utilized. In addition, reliable Power over Ethernet (PoE), quality of service (QoS) mechanisms, and monitoring services are recommended. When designing a single campus deployment, do not oversubscribe CUCM to scale larger installations. A single-site deployment does not always equate to a single cluster. If the site has more than 40,000 IP phones, install multiple clusters and configure ICTs between the clusters (or provision mega-cluster).
Benefits of Centralized Call Processing Model
A single infrastructure for a converged network solution provides significant cost benefits and enables CUCM to take advantage of the many IP-based applications in the enterprise.
Single-site deployment allows each site to be completely self-contained. Calls between sites will be routed over the PSTN. Extra provisioning of WAN bandwidth is not needed. Dial plans are also easier to provision. There are no service issues in the event of an IP WAN failure or insufficient bandwidth, and there is no loss of call-processing service or functionality.
In summary, the main benefits of the single-site model are as follows:
Ease of deployment
A common infrastructure for a converged solution
Simplified dial plan
No transcoding resources are required, due to the use of a single codec