Information Technology Reference
In-Depth Information
Centralized Deployment PSTN Backup Call Flow
In the event the IP WAN fails, all phones in the branch lose connectivity to the CUCM at
the main location and no longer function. In this event, Survivable Remote Site Telephony
(SRST) is recommended to provide registration and call control signaling local to the
branch phones. SRST provides on-net IP-to-IP calling between branch phones, but if calls
from branch phones to other locations are placed (whether on-net or off-net), they will
fail. This is because of a lack of call-routing configuration on the branch gateway router.
Typically, the branch gateway is configured with a subset of CUCME capabilities to pro-
vide public switched telephone network (PSTN) access to branch phones. If the SRST
router dial-plan is configured properly, the branch users can still dial the same on-net ex-
tensions for calls to other locations, and the SRST router transparently modifies the dialed
digits for PSTN routing. Most users will not be aware of the WAN failure.
Key
To p i c
From the main location CUCM perspective, the branch phones have de-registered and are
considered unreachable. This is not entirely accurate (because they are reachable via the
PSTN) and is an undesirable failure of the system. To provide continuity of service, call-
routing options and settings are configured on the CUCM that provide an alternate path
to the branch via the PSTN:
1.
The call-routing table has a second option added to provide a PSTN gateway and ap-
propriate digit manipulation to provide the required PSTN dialed digits.
2.
The Call Forward UnRegistered (CFUR) option is configured on each branch phone
to specify the full PSTN number needed to reach the branch phone.
These two configurations, in combination with the appropriate SRST configuration, en-
sure that branch phones can both reach and be reached by the main location in the event
of WAN failure.
When the WAN recovers, the branch phones de-register from the SRST router, re-register
with the CUCM, and the normal WAN-based signaling and calling patterns resume.
Centralized Deployment Considerations and Limitations
Deploying CUCM in a centralized call-processing environment is one of the most popular
choices, providing full features and capabilities to branch offices while centralizing equip-
ment and management efforts at the headquarters. The following points should be consid-
ered during the design and planning of a Centralized deployment:
CUCM supports a maximum of 1000 locations and a maximum of 1100 H.323 or
Media Gateway Control Protocol (MGCP) devices per cluster.
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Although there is no limit to the number of phones at a branch, the number of phones
supported by the SRST router will be limited based on the router hardware model and
the configuration applied to it. The maximum number of phones supported in SRST
mode is 1500 on a 3945E branch router. The correct configuration to allocate re-
sources to support that many phones must be applied, and the appropriate amount of
RAM must be installed.
The WAN must be properly provisioned and QoS enabled to allocate the priority
queue bandwidth appropriate to the number of concurrent calls, and to limit delay for
both RTP voice and signaling traffic.
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