After you design the
data network, design the Cisco Unified Communications infrastructure. Before
you can deploy any telephony applications, you need the Unified CM cluster and CTI Manager in place to dial and receive calls.
Several services that are important to your solution run on each Unified CM server:
For details on the architecture of all these services, see the Cisco Collaboration System
Solution Reference Network Designs at http://www.cisco.com/en/US/docs/voice_ip_comm/uc_system/design/guides/UCgoList.html.
High availability design for a cluster
requires that you understand how the Unified CM, CTI Manager, and CallManager services
interact. Unified CM uses the CTI Manager service to handle its CTI resources. CTI
Manager acts as an application broker that abstracts the physical binding of
applications to a particular Unified CM server. The CallManager service registers and
monitors all the Cisco Unified Communications devices.
The CTI Manager
accepts messages from the Agent PG, a CTI application, and sends them to the
appropriate resource in the cluster. The CTI Manager acts like a JTAPI
messaging router using the Cisco JTAPI link to communicate with Agent PGs. The JTAPI client library in Unified CM
connects to the CTI Manager instead of connecting directly to the CallManager
The CallManager service
acts as a switch for all the Cisco Unified Communications resources and devices
in the system. The CallManagers on each Unified CM server
link themselves across the public network with the Signal Distribution Layer
(SDL). This link keeps the cluster in sync. Each CTI Manager connects with the
Unified CM and CallManager services on its server. CTI
Managers do not connect directly with other CTI Managers in the cluster.
Agent PGs use a CTI-enabled user account in
Unified CM, typically called the "JTAPI user" or "PG user". The Agent PGs sign in to the
CTI Manager to connect to the devices for that user. If the appropriate device is
resident on the local CallManager, the CTI Manager handles the request for that device.
If the device is not resident on its local subscriber, then the CallManager service
forwards the request to the appropriate subscriber through the private link to the other
The following figure
shows the connections in a cluster.
Figure 18. Connections in
Unified Communications Manager Cluster
availability, distribute device registrations across all the subscribers in the
cluster. If you concentrate the registrations on a single subscriber, the
traffic puts a high load on that subscriber. The memory objects that the Agent PGs use to monitor registered devices also add to the device
weights on the subscribers.
If the PG that is
connected to a subscriber fails, the redundant PG that takes over and sends all
the requests to another subscriber. Then, the local CallManager
service must route the CTI Manager messaging for those requests across the
cluster to the original subscriber. The additional messaging in this failover condition creates greater
load on the cluster.