Check SRND
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/uc6_1.html
WAN Considerations
For clustering over the WAN to be successful, you must carefully plan, design, and implement various
characteristics of the WAN itself. The Intra-Cluster Communication Signaling (ICCS) between
Unified CM servers consists of many traffic types. The ICCS traffic types are classified as either priority
or best-effort. Priority ICCS traffic is marked with IP Precedence 3 (DSCP 24 or PHB CS3). Best-effort
ICCS traffic is marked with IP Precedence 0 (DSCP 0 or PHB BE). The various types of ICCS traffic
are described in Intra-Cluster Communications, page 2-20, which also provides further guidelines for
provisioning. The following design guidelines apply to the indicated WAN characteristics:
• Delay
The maximum one-way delay between any two Unified CM 6.0 servers should not exceed 20 msec,
or 40 msec round-trip time (RTT). Beginning with Cisco Unified CM Release 6.1, the maximum
one-way delay between two Unified CM servers can be up to 40 msec, or 80 msec round-trip time.
Measuring the delay is covered in Delay Testing, page 2-21.
• Jitter
Jitter is the varying delay that packets incur through the network due to processing, queue, buffer,
congestion, or path variation delay. Jitter for the IP Precedence 3 ICCS traffic must be minimized
using Quality of Service (QoS) features.
• Packet loss and errors
The network should be engineered to provide sufficient prioritized bandwidth for all ICCS traffic,
especially the priority ICCS traffic. Standard QoS mechanisms must be implemented to avoid
congestion and packet loss. If packets are lost due to line errors or other “real world” conditions, the
ICCS packet will be retransmitted because it uses the TCP protocol for reliable transmission. The
retransmission might result in a call being delayed during setup, disconnect (teardown), or other
supplementary services during the call. Some packet loss conditions could result in a lost call, but
this scenario should be no more likely than errors occurring on a T1 or E1, which affect calls via a
trunk to the PSTN/ISDN.
• Bandwidth
Provision the correct amount of bandwidth between each server for the expected call volume, type
of devices, and number of devices. This bandwidth is in addition to any other bandwidth for other
applications sharing the network, including voice and video traffic between the sites. The bandwidth
provisioned must have QoS enabled to provide the prioritization and scheduling for the different
classes of traffic. The general rule of thumb for bandwidth is to over-provision and under-subscribe.
• Quality of Service
The network infrastructure relies on QoS engineering to provide consistent and predictable
end-to-end levels of service for traffic. Neither QoS nor bandwidth alone is the solution; rather,
QoS-enabled bandwidth must be engineered into the network infrastructure.
