How Does Desktop Sharing Work?

In Lync Server 2013 the Application Sharing Conferencing Server service is responsible for desktop and application sharing in conferences. This role is installed on all Lync Front End servers, and is one of the many MCUs in Lync Server 2013. Because each Lync Server handles all conferencing modalities, the boundaries for each MCU are often mixed up. The following list describes the MCUs in Lync Server 2013, and what they are used for:

• Web Conferencing—Manages conferencing data collaboration (see the section “Collaboration Content” for more information).

• A/V Conferencing—Manages audio and video media streams in conferences.

• IM Conferencing—Manages Instant Messaging conferencing in conferences.

• Application Sharing Conferencing—Manages desktop and application sharing media streams in conferences.

The App Sharing MCU was introduced in Lync Server 2010, and in Lync Server 2013 the functionality remains the same. The key functionality that this MCU allows for is the capability to present and share control of specific applications. This MCU uses the following protocols:

• C3P/HTTP—The C3P protocol is used to communicate conferencing control commands. C3P over HTTP is used by the conferencing servers to communicate with each other.

• C3P/SIP—Lync clients also communicate with the conferencing server using C3P to create conferences and to communicate changes to the conference. For Lync clients this is done securely over SIP.

• SIP/SDP—Application and desktop sharing traffic is delivered over RTP, and because of this, Session Description Protocol (SDP) is used to establish the RDP over RTP media stream. Lync clients communicate with the App Sharing MCU over SIP to perform this negotiation and establish a media stream.

• RDP/RTP—Lastly, the desktop and application sharing media stream uses the RDP codec, and it is delivered over an RTP media stream.

Application and desktop sharing is also available in peer-to-peer sessions between two users. These sessions do not interact with the App Sharing Conferencing Server, because they are only between two participants. These sessions do use the same protocols and the functionality is very much the same as in conferences.

It is important not to overlook the bandwidth requirements for desktop sharing. The Microsoft RDP codec can provide very high-quality desktop sharing capabilities; however, this can come at a huge cost in bandwidth utilization. Bandwidth utilization for desktop sharing is dynamic, and can fluctuate depending on many variables such as these:

• Resolution of the shared desktop

• Application that is shared

• Amount of movement in shared desktop

• Number of monitors shared

These factors can cause Lync Server 2013 desktop sharing to use up to 10Mbps of bandwidth. On average, desktop sharing in peer-to-peer and conferencing scenarios use between 70Kbps and 2Mbps. Administrators should consider controlling this functionality, while still considering the end-user experience. Limiting the bandwidth available for desktop sharing will limit the quality of the sharing experience. Lync Server 2013 allows administrators to control the bandwidth used by application sharing in the Lync conferencing policy. See the “Configuring Data Conferencing Options” section for more information on how to configure these polices. When you are controlling the bandwidth used by Lync desktop sharing, it is important to understand that reducing the bandwidth used will only degrade the sharing frame rate. At this time, it is not possible to adjust color depth, resolution, or other quality factors in Lync Server 2013. If bandwidth is limited for Lync desktop sharing, users will have a negative experience, but this might be required to maintain network capacity in certain scenarios.

Although Lync 2013 application and desktop sharing can use up to 10Mbps of bandwidth, more realistic numbers are provided in Table 19.7. These numbers are provided by Microsoft as guidance for network scaling, assuming common usage scenarios.

Table 19.7. Lync Server 2013 Desktop Sharing Bandwidth Requirements

It is best practice for organizations to use the bandwidth estimates referenced in Table 19.6 when planning to implement Lync Server 2013 desktop sharing.