Skype for Business Cloud Connector is an option for those organizations who have existing telephony infrastructure in-place but not using Skype for Business on-premises or Hybrid infrastructure. With the help of cloud connector customer can opt for Skype for Business Online while they still can leverage their telephony infrastructure for PSTN connectivity.

Let me clarify one thing: Organizations can use either Cloud PBX with PSTN calling service or Skype for Business Hybrid infrastructure to get most/all of the features and functionalities.

Cloud Connector only comes in picture where organizations are not using Lync Server / Skype for Business. When it comes to topology, it again only applicable to Sites which have PSTN infrastructure which needs to be connected with Skype for Business online. Cloud connector edition provides scalability as well as high availability.

Scalability: Multiple instance of cloud connector can be deployed with one or more PSTN sites. Maximum 200 sites can be deployed associated to one Skype for Business online tenant.

High Availability: Up to 4 cloud connector edition can be deployed in 3+1 configuration to provide high availability. In 3+1 configuration, you may get up to 99.8% availability while with 2+2 configuration you may get 99.9% availability.

Cloud Connector comes in two versions:

1. Small Version (Supports up to 50 simultaneous calls)
2. Large Version (Supports up to 500 simultaneous calls)

Let me describe multiple supported topologies through illustration:

Single instance of cloud connector with single PSTN site

Multiple instance of cloud connector with single PSTN site

Multiple instance of cloud connector with multiple PSTN sites

As cloud connector is an option for PSTN calls then number of PSTN calls will become a key selective area. Single PSTN site can be configure for up to 150 simultaneous call with small version while large version can support up to 1500 simultaneous calls. If you need more than 1500 simultaneous call in a single site then you can deploy multiple PSTN site in the same location to scale it up.