Datasheet

Cisco Preferred Architecture for
Midmarket Collaboration PAGE 16
Call Control
Note: SRST is not available for Cisco EX, MX, or SX Series endpoints. See the SRST data sheet for information about
endpoints that support SRST.
Dial Plan
A structured, well-designed dial plan is essential to successful deployment of any call control system. When designing a
dial plan, consider the following main factors:
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Dialing habits
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Endpoint addressing
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Routing
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Directory integration
•
Classes of service
Dialing Habits
Dialing habits describe what end users can dial to reach various types of destinations. Dialing habits can first be classified
as numeric dialing (for example, 914085550123) or alphanumeric dialing (for example, [e-mail]).
Typically, different types of destinations require support for different dialing habits. For example:
•
PSTN toll call: 91-<10 digits>
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PSTN international call: 9011-<country code + national significant number>
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Abbreviated intra-site dialing: for example, 4XXX
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Abbreviated inter-site dialing: for example, 8-<site code>-<intra-site number>
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+-dialing from directories: “+” followed by a fully qualified global PSTN number as described in ITU recommendation
E.164
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URI dialing: for example, [e-mail] for intra-company and inter-company dialing. Endpoints typically allow
omission of the right-hand side (host portion) of the URI and they automatically append the local host portion, so
that [e-mail] can also be abbreviated as bob.
Further dialing habits might have to be defined for services such as call pick-up, voicemail, and others. Also, future growth
should be considered so that more users and more sites can be added as needed without redesigning the dial plan.
Identifying dialing habits is most important when defining an enterprise dial plan in order to avoid overlaps between any
two dialing habits. For example, a trunk access code of 9 prohibits abbreviated intra-site dialing starting with 9. Avoiding
overlaps between dialing habits is crucial to avoid inter-digit timeouts, which lead to bad user experiences.
In migration scenarios, the dialing habits supported by the existing system can be used as a first estimate of the dialing
habits required in the new system. On the other hand, migration to a new communications system can also serve as a
reason to get rid of outdated customs and practices.
Endpoint Addressing
Each endpoint registered with the enterprise call control must have a unique numeric address. Endpoint addresses in
Cisco Unified CM are equivalent to the directory numbers provisioned on the lines of the endpoints. Use fully qualified
PSTN numbers (E.164 numbers) with a leading “+” as endpoint addresses. This format is typically referred to as +E.164
format. The benefits of using +E.164 endpoint addresses include:
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Wide use in voice networks
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No need to develop and maintain an enterprise numbering scheme
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Easy creation of correct caller ID presentation for all on-cluster and off-cluster call flows
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Easy implementation of directory lookups
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Simplified alternate routing to the PSTN in cases of WAN failure or bandwidth constraints