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Cisco IP Telephony Solution Reference Network Design
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Chapter 1 IP Telephony Deployment Models
Multi-Site MPLS WAN Considerations
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Cisco highly recommends that you replicate key services (TFTP, DNS, DHCP, LDAP, and IP Phone
Services), all media resources (conference bridges and music on hold), and gateways at each site
with IP phones to provide the highest level of resiliency. You could also extend this practice to
include a voice mail system at each site. Under a WAN failure condition, only sites without access
to the publisher database might loose a small amount of functionality.
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Every 10,000 busy hour call attempts (BHCA) in the cluster requires 900 kbps of bandwidth for
Intra-Cluster Communication Signaling (ICCS). This is a minimum bandwidth requirement, and
bandwidth is allocated in multiples of 900 kbps.
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In addition to the real-time ICCS bandwidth, intra-cluster bandwidth is required for the SQL, CTI
Manager, and LDAP traffic. The amount of additional bandwidth is dependant on the use of the
system. For instance, the use of Extension Mobility increases the amount of SQL traffic between
servers.
·
Signaling or Control Plane traffic requires additional bandwidth when devices are registered across
the WAN with a remote Cisco CallManager server in the same cluster.
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A maximum Round Trip Time (RTT) of 40 ms is allowed between any two servers in the
Cisco CallManager cluster. This time equates to a 20 ms maximum one-way delay, or a transmission
distance of approximately 1860 miles (3000 km) under ideal conditions.
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The remote failover model requires Cisco CallManager Release 3.1 or later.
Call Admission Control for Clustering Over the IP WAN
If calls are allowed across the WAN between sites, then you must provide call admission control between
those sites by configuring Cisco CallManager locations for those sites in addition to the default location
for the other sites. Even if the bandwidth is over-provisioned for the number of devices, it is still best to
configure call admission control based on locations.
Multi-Site MPLS WAN Considerations
This section explains how to adapt the call admission control mechanisms, described previously in this
chapter, to multi-site deployments with Multiprotocol Label Switching (MPLS) WANs.
The main design difference between traditional Layer 2 WAN technologies and MPLS is that an MPLS
WAN does not conform to a hub-and-spoke topology but instead provides full-mesh connectivity
between all sites. This topology difference has implications on the call admission control mechanisms
that must be adopted.
Purely Centralized Deployments
In single-cluster centralized call processing deployments, the call admission control function is
performed by the locations construct within Cisco CallManager.
In a hub-and-spoke WAN topology (for example, Frame Relay or ATM), each link to and from a branch
site terminates at the central site. For example, in a Frame Relay network, all Permanent Virtual Circuits
(PVCs) from the branch routers are aggregated at the central site's head-end router. In such a scenario,
there is no need to apply call admission control to devices at the central site because the bandwidth
accounting occurs at the branch ends of the WAN links. Therefore, within the Cisco CallManager
Locations configuration, devices at the central site are left in the <None> location, while devices at each
branch are placed in their appropriate location to ensure proper call admission control.