Cloud identity models are purely managed and maintained on the Cloud, while Hybrid models are managed on-premises and then synchronized to the Cloud. But what does this mean for organizations running an on-premises Skype

for business setup who want to transition to Microsoft Teams? Well, this depends really on whether you're planning to move to a Cloud-Only solution or you're planning to maintain your on-premises infrastructure. In either scenario, there are options and settings that can fit your organization's needs. Since Skype and Teams have an overlap in some capabilities, it is likely that organizations will eventually look to move into Teams. However, this roller isn't an instantaneous process and in that case it is important to understand what a hybrid environment looks like. If a user is using both Skype and Microsoft Teams, they cannot communicate with Skype users from their Teams client.

This functionality requires users to be moved from their on-premises Skype for business to the Cloud and the coexistence mode to be set to Teams Only. Organizations can also have a split domain, which utilizes direct routing to allow use of an organization's on-premises voice infrastructure. In this scenario, users are homed and split between the on-premises Skype server and Skype for business online. This configuration requires Azure Active Directory Connect to synchronize users from the on-premises domain to Azure AD and allow users to communicate and collaborate with users homed in either location. But what does this all mean for Microsoft Teams? Well, organizations can enable something known as Coexistence to continue using both Skype for business alongside Microsoft Teams. 

Coexistence is effectively a term used to run both Microsoft Teams and Skype for Business in the same environment. There are several coexistence modes organizations can employ to fit any situation. These modes determine how and where calls and chats get routed and the application that a user utilizes for chats, calls, and meetings. These modes can be assigned on a per-user basis, allowing for more granular control over a Microsoft Teams deployment. There are five coexistence modes organizations can assign users, and they are: Teams Only, Skype for Business Only, Islands, Skype for Business with Teams collaboration, and Skype for Business with Teams collaboration and meetings. Each of these modes operate in a different way, so let's go over them each one by one.

When a user is assigned as Teams Only, they will gain all of the capabilities of Microsoft Teams alongside the ability to join Skype meetings from users not set to Teams Only mode. All functionality and communication is done through Teams, and the user no longer has access to schedule or initiate a Skype for business chat or meeting. The best way to think of Teams Only mode is that the assigned user utilizes Teams for effectively everything. And the only time they encounter Skype for business is when another user invites them to a Skype meeting at which point the user will be able to join as if there was no change. Skype for Business only, on the other hand, can be thought of as the same thing I just said, only reverse it. So, Skype for business is the main software they utilize. Generally, this option is not preferred as users will not get access to the functionality and features provided in Teams.

Island mode is unique in the sense that it allows users to access both Skype for business and Teams. However, both tools are kept on separate Islands. Skype for business users talk to Skype for business users, and Microsoft Teams users talk to Microsoft Teams users. This mode effectively disables the connection between Skype for business and Teams so that users from one tool cannot communicate with users on the other tool. Both of the other options, Skype for business with Teams collaboration and Skype for business with Teams collaboration and meetings, have some sort of blend of both tools. They are both meant for organizations that want interoperability between Skype for business and Teams, and therefore Islands mode would not be ideal. They can also be thought of as the ideal coexistence modes while planning a complete Microsoft Teams deployment and upgrade as they will help users get used to Microsoft Teams throughout the process of a deployment.

Specifically, Skype for business with Teams collaboration is meant for users to continue using Skype for business to chat, call, and host meetings, but pushes users to collaborate using Microsoft Teams. Skype for business with Teams collaboration and meetings takes us a step further with the only difference being that users schedule meetings in Microsoft Teams as well as a collaboration feature. This empowers users not only with the additional functionality of Teams collaboration but with Teams and channels and also superior meeting capabilities built right into Microsoft Teams. With the exception of Islands mode, each of these options have some form of interoperability. It is important to note that if messages are sent in Island mode, then those chats will only be received in the same client.

Specifically, Skype messages will go to Skype and Teams messages will go to Teams. In any other mode, the receiving user will determine where the chat is received, whether in Skype for business or Microsoft Teams. For example, if they're in Teams Only, they'll receive a message in Teams, or in the other modes they'll receive messages in Skype. It is important that users are made aware of how they should be communicating as chat will fluctuate depending on the coexistence mode set.