Architecture primer
-------------------

Overview 
..........

Understanding the dial plan model requires a basic understanding
of the architecture and the interaction between the elements. The
architecture consists of three discrete layers: 

* Aggregation layer
* Infrastructure layer
* Customer-premises layer

These layers comprise the majority of the call flow
interactions. 

The aggregation layer handles inbound/outbound call routing
between customer premises and PSTN, while all intra-customer interactions are
handled at the infrastructure layers.

The numbering plan facilitates this design, allowing for a simple
routing methodology, and is discussed in this section.

* Aggregation Layer
* Customer Premises Layer
* Voicemail - Cisco Unity Connection (CUC)



Aggregation layer
.................

The aggregation layer is responsible for the following:

* E.164 call routing
* Routing E.164 calls between two Automate customers
* Routing calls from the PSTN to IP Lines on Cisco Unified Communications Manager (UCM)
  clusters
* Routing calls from IP Lines (on UCM clusters)
  to centralized PSTN breakout
* Routing calls to other components within the aggregation layer
* Routing calls to Cisco Webex (can be a separate route)
* Routing calls to Cisco Contact Center Enterprise (can be a separate route)

The primary use of the aggregation is to aggregate central PSTN traffic.
Any call that is received at the aggregation layer must be treated as a call
either from or to the PSTN. The aggregation supports PSTN breakout like SS7 and SIP.

A service provider using an early version of Automate (VOSS-4-UC v10.x/11.5(x))
can only use the customer's own aggregation device (third party).

The aggregation layer is connected to the UC applications through the third-party
session border controller as shown in the following figure.
The SBC is used mainly as a media aggregation point. The SBC is used
for media anchoring and VRF translation functions. Additionally, it provides SIP
header normalization functions for inter-operating with various service provider
networks. The SBC is configured manually; it is not provisioned automatically
from Automate. For signaling, there is one-to-one
mapping of trunk from leaf clusters to signaling aggregation node, IMS or any other
third-party softswitch. In general, there is no dial plan requirement for the SBC.

Figure 1. Logical diagram showing SBC adjacency for each customer

|dialplan-Logical-Diagram-Showing-SBC-Adjacency-for-Each-Customer|

                                                                           
.. note::

   The Forced On-Net feature in UCM can be used
   between users of a customer; that is, a user dialing the PSTN number of another
   user of the same customer.                          
                                                                      

Calls between various clusters and the aggregation traverse SIP trunks.
SIP is the only protocol that must be configured for these types of trunks.

Customer premises layer
.......................

This layer connects customer endpoints: phones, mobile devices, and local gateways
to the SP network; and provides end user interfaces to network management software.
This layer may handle PSTN routing and Survivable Remote Site Telephony (SRST)
if a local breakout design is used. This layer can also include C-series servers
for UC applications as part of the Extender
deployment model.

Voicemail - CUC
..................

Cisco Unity Connection (CUC) is connected to the leaf clusters as a SIP connection
(telephony integration). For each Cisco UCM IP phone line that exists on the
leaf cluster that requires voicemail, there is also a voice mailbox definition
on the CUC. On CUC, the voice mailbox
definition is associated to the telephony integration and leaf cluster that
contains the line definition. The leaf clusters have a voicemail pilot number
defined that helps to route calls to the CUC. With this
scenario, each leaf cluster can use the same number for the voicemail pilot number.


.. note::

   For Shared Architecture Dial Plan (G3) dial plan, the voicemail pilot number
   can be shared within a customer (that is, across locations) but it has to be
   unique across customers that belong to the shared architecture of UCM and CUC.


.. |dialplan-Logical-Diagram-Showing-SBC-Adjacency-for-Each-Customer| image:: /src/images/dialplan-Logical-Diagram-Showing-SBC-Adjacency-for-Each-Customer.png