There have been many discussions within the Lync community regarding how many conferences Lync Server 2013 can support or handle. Ironically, the number of conferences supported and the number of conferences that can be safely handled are two different topics, especially since the supported number of users per Front End Server has been adjusted to 6,660 users in Lync Server 2013. Let's explore the conferencing size limit touted by Microsoft as well as look at some additional numbers that can help you determine the number of conferencing users per Front End Server and the number of concurrent conferences per Front End Server.

Understanding the Conferencing Size Limit

Many Lync administrators have heard that Lync Server 2013 supports up to 250 on-premises users per conference, but not many Lync administrators know how this number came about. To begin, it's important to understand that this isn't a hard limit but rather a recommendation by the Microsoft Lync Server team. This recommendation is based on scalability testing, which took into account many factors, including how many users typically participate in conferences. Based on the data it collected, Microsoft found that the majority of conferences were collaborative meetings attended by four to six users. Interestingly, less than 1 percent of conferences had more than 250 users, so large conferences are the exception rather than the norm. The Microsoft Lync Server team recommends that if an organization has conferences that exceed 250 on-premises users, it should use a dedicated Lync Front-End Server pool for those conferences.

Although the 250 on-premises users per conference is a recommendation, there are some hard limits out there. The hard limits to keep in mind are:

  • A maximum of 12 Front-End Servers per pool
  • A maximum of 80,000 users per pool

Determining the Number of Conferencing Users and Concurrent Conferences

When planning or fine-tuning a Lync Server 2013 implementation, it's helpful to know the typical number of conferencing users per Front End Server and the number of concurrent conferences per Front End Server. The Lync Server 2013 user models state that in a typical organization with moderate conferencing needs, 5 percent of the organization's user base participate in a conference at any given time. There will be organizations with a higher percentage, but for the purposes here, let's stick with this statistic.

So, let's do some math. Suppose an organization has a user base of 7,500 users and three Front-End Servers in a pool. Given the 5 percent statistic, 375 concurrent users (7,500 × 0.05) are participating in a conference at any given time. Because there are three Front End Servers, the organization has 125 conferencing users per Front End Server (375/3).

Next, we need to factor in the average number of users in a conference. Recall that Microsoft found that the majority of conferences have four to six participants. So, for this example, let's say that each conference has an average of four users. This means the number of concurrent conferences that each Front End Server is handling is 31 (125 conferencing users per Front End Server/4 users per conference).

Let's compare this example's numbers with Microsoft's maximum limits for the number of conferencing users per Front End Server and the number of concurrent conferences per Front End Server.

Maximum number of conferencing users. If you take 5 percent of the maximum of 80,000 users per pool, you'll have 4,000 users (80,000 × 0.05) participating in a conference at any given time. If there are 12 Front End Servers in a pool, the maximum number of conferencing users per Front End Server is around 333 (4,000/12). In our example, the number of conferencing users per Front End Server is considerably lower than the maximum limit of what a Front End Server can safely handle (125 versus 333).

Maximum number of concurrent conferences. Because Microsoft found that the majority of conferences have four to six participants, let's say that each conference has four users, like we did with our example. With four users per conference, the maximum number of concurrent conferences that each Front End Server can safely handle is about 83 (333 users per Front End Server/4 users per conference). Once again, in our example, the number of concurrent conferences per Front End Server is considerably lower than the maximum number of concurrent conferences that Lync Server 2013 can safely handle (31 versus 83).

Conferencing Implementations Will Vary

Lync Server conferencing has come a long way in the past few versions, with the inclusion of features such as IM and audio, video, and application sharing. In addition, Lync Server 2013 can now host many different types of conferences and accommodate different types of users, such as remote, federated, and even anonymous users. The key concept to remember is that each organization's conferencing requirements will vary, depending on the number and types of conferencing users, the conferencing features they'll be using, and other factors. Those requirements will dictate the number of Front End Servers that need to be deployed for the Lync pool. You can find additional information on Front End Servers and sizing in my article "Determining How Many Front-End Servers to Deploy in Lync Server 2013."