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In a centralized SIP trunk deployment model, only the headquarters (centralized hub) of the organization has direct SIP connectivity to the IP PSTN as shown in the figure 1 below . All external calls to or from remote sites of the business go through the headquarters.

Caption
0 Figure 1 Centralized SIP Trunk Model

In the centralized SIP Trunk model, the centralized 

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 deployed in the enterprise headquarters controls the IP trunk border that connects the enterprise to the service provider or the outside world, and the private network border interconnecting all IP endpoints and IPT/UC servers that reside at headquarters and branch offices. Following advantages are realized:

• Easy to manage SIP trunks
• Excellent QoS and CAC capabilities

SBC in an Enterprise-hosted IP-PBX (Registration mode)

Enterprise hosts an IP-PBX locally in the corporate network. There maybe multiple or the same IP-PBX maybe shared by multiple sites.
The SIPConnect standard is an important initiative in this space, and is published through the SIPForum. SIPConnect defines two modes of operation:

• Static mode – The Service Provider network appears as a peer network to the enterprise network
• Registration mode – The SIP-PBX plays a role similar to multiple enterprise end points

Caption
0 Figure 1 Enterprise-hosted IP-PBX

Configuration Steps

1. Enterprise hosts an IP-PBX locally in the corporate network. There may be multiple IP-PBXs, or the same IP-PBX may be shared by multiple sites.
2. Registration is communicated through the SBC.
3. The carrier side SBC acts similar to an A-SBC in a regular IAD scenario, but supports additional routing and registration features (such as group registration of a set of IADs).
   1. The carrier may offer additional features and functionality via an AS or a feature server in the network.
   2. Peering SBC (or Media gateway) is used to break calls out to other networks.

SBC in an Enterprise-hosted IP-PBX (Static mode)

Enterprise hosts an IP-PBX locally in the corporate network. There may be multiple IP-PBXs, or the same IP-PBX may be shared by multiple sites. In this model the end points are not registered to a point in the carrier network.
The SIPConnect standard is an important initiative in this space and is authored/published through the SIPForum. SIPConnect defines the two modes of operation:

• Static mode—The Enterprise network appears as a peer network to the Service Provider network
• Registration mode—The SIP-PBX plays a role similar to a subscriber endpoint device supporting multiple end-users 

Caption
0 Figure 1 Enterprise-hosted IP-PBX

Configuration Steps

1. Enterprise hosts an IP-PBX locally in the corporate network.
2. The carrier may offer additional features and functionality via an AS or a feature server in the network. However, the calls may be routed directly to the final destination and not via the AS.
3. Peering

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   (or Media gateway) is used to break calls out to other networks.

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