Product Documentation

Technical overview

Jan 26, 2017

Citrix HDX RealTime Optimization Pack offers clear, crisp high-definition audio/video calls with Microsoft Skype for Business in an optimized architecture. Users can seamlessly participate in audio-video or audio-only calls to and from other Skype for Business users and other standards-based video desktop and conference room multipoint control unit (MCU) systems. All audio/video processing is offloaded from the Server to the end-user device or terminal, optimizing the quality of the call with minimal impact on server scalability.  

Key features

The Citrix HDX RealTime Optimization Pack provides XenApp and XenDesktop customers the following key features: 

  • Optimizes Skype for Business audio and video calling on Windows, Mac, and Linux devices. Our partner, Dell, supports Wyse ThinOS.
  • Compatible with Skype for Business Server 2015, Lync Server 2013, and Skype for Business Online (Office 365).
  • Enables call initiation from the Skype for Business dialpad, dial-in bar, contacts list, Conversation window, and Outlook or other Office application.
  • Supports both point-to-point and multi-party calling.
  • Supports Microsoft's proprietary RT-Audio codec. Also supports industry standards G.771, G.722, G.722.1, and G722.1c for audio calls from 3 kHz to 14 kHz (ultra wideband) and SILK, which delivers higher audio quality across a wide range of network environments, including the public Internet and mobile networks.
  • Supports RT-Video, H.264 Scalable Video Coding (SVC) and H.264 Advanced Video Coding (AVC). Video call rates range from 128kb/s to 2048kb/s. All video is encoded at up to 30fps (depending on the webcam used) and transmitted over RTP/UDP or TCP.
  • Includes 720p HD support and the following video resolutions:
    • 4CIF resolution (704 X 576 pixels)
    • VGA resolution (640 X 480 pixels)
    • CIF resolution (352 X 288 pixels)
    • SIF (352 X 240)
    • 360p (640x360)
    • QCIF resolution (176 X 144 pixels) or QSIF (176 X 120)
  • Supports Quality of Service (QoS) by observing the audio and video port ranges configured on the Skype for Business server (see and 
  • Supports Differentiated Services Code Point (DSCP) marking for media packets. For Windows push out QoS policies to the endpoints. For Linux and Mac OS X, there are RealTime Optimization Pack registry settings that must be applied in the user's profile on the server. 
  • Optimizes Quality-of-Experience (QoE) through adaptive jitter buffer, packet loss concealment, call rate adaptation, and other techniques.
  • Provides fallback to server-side media processing (Generic HDX RealTime) if there is no HDX RealTime Media Engine present on the user device.
  • Supports most webcams including built-in webcams on Windows devices.
  • Supports built-in webcams (Facetime cameras) on Mac devices.
  • Improves audio and video quality over lossy connections by enabling forward error correction (FEC).
  • Call Admission Control on the Skype for Business server, improves media quality in enterprise networks by tracking media bandwidth usage and denying calls that would overload the network by using too much bandwidth.
    • When enabled by administrators, all audio and video calls made with RealTime Optimization Pack inform the Skype for Business server infrastructure about bandwidth usage and follow all bandwidth policy constraints, including:
      • Limits audio and/or video bandwidth as required by the policies.
      • Downgrades video calls to audio only if bandwidth for video is not available. Note, a generic message displays.
      • Reroutes the call through the internet when bandwidth on the corporate network is not available. Note, a generic message displays.
      • Reroutes the call to voicemail when bandwidth is not available anywhere. Note, a generic message displays.
      • Reports Call Admission Control bandwidth constraints to the Quality-of-Experience monitoring database.
    • Call Admission Control works in all network configurations supported by Microsoft: that is, multiple regions, sites, links, routes, policies, and so forth. It works as designed for both on-premises and remote endpoints.  On remote endpoints, just as with native Skype For Business clients running remotely, only the internal portions of the media path are subject to Call Admission Control bandwidth policies. 


When you deliver the Skype for Business client in a vritualized environment, there are a small number of feature differences. For more information, see


Editing the registry incorrectly can cause serious problems that may require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

  • If Skype for Business is running locally on your device, right-click the Skype for Business icon in the system tray and Exit the application. Interaction issues are likely when running Skype for Business locally while it is also running in the data center as a hosted application.
  • Microsoft does not support the Lync and Skype for Business basic client with the HDX RealTime Optimization Pack. Workaround: Use the full version of Skype for Business.
  • RealTime Optimization Pack does not support direct media connections to and from public switched telephone network (PSTN) gateways (there is an optional feature of Skype For Business, also known as Media Bypass, see If Media Bypass is enabled by Skype For Business server administrators, PSTN calls involving RealTime Optimization Pack users automatically and transparently route media connections through the Mediation Server. There is no end user impact due to this feature limitation. Network administrators need to account for it when planning network capacity. 
  • When the Skype for Business client is delivered as a published application rather than as part of a full Windows desktop, desktop sharing is not supported and should not be used as the server desktop will be shared rather that the local desktop. Application sharing can be used to share other hosted applications during a Skype for Business call. 
  • Client-side recording is not supported. Citrix recommends evaluating third-party server-based recording solutions.
  • Gallery view is not supported in multiparty calls. Active speaker view is used in Skype for Business multiparty calls using Optimization Pack.
  • Panoramic webcams that deliver a 360-degree view of the meeting room are not supported.
  • Optimized delivery (redirection of media processing to the user device) in a double-hop XenApp/XenDesktop-Receiver scenario is not supported.
  • Web proxy limitations:
    • HTTP proxy authentication is not supported. Configure proxies with white lists to allow unauthenticated access to target Skype For Business servers (for example, Office 365 servers for cloud-based deployments).
    • Web Proxy Auto-Discovery Protocol (WPAD) and dynamic proxy detection are supported with Windows endpoints only. Configure Linux and Mac endpoints with a static HTTP proxy address.
  • On Linux terminals, the RealTime Media Engine installer disables multimedia redirection in Linux Receiver. This avoids Optimization Pack and Linux/Unix Receiver getting into a conflict when accessing video devices. However, this means that other Unified Communications applications cannot work with Generic USB redirection when accessed on a Linux terminal that has RealTime Media Engine installed.
  • The date and time strings on USB telephone devices with display capabilities are not properly localized.
  • The Plantronics Clarity P340 audio device is not supported. 
  • The Optimization Pack disables the use of hardware acceleration with the Logitech C920 camera on Windows, but support is provided for the C290 as a nonencoding camera. To enable hardware compression for the Logitech C920 on Windows, do the following:

1. Replace the Logitech driver with the stock Microsoft driver.

2. Create a registry setting that enables hardware acceleration with the C920.

On 32-bit and 64-bit Windows:

Name: EnableC920Compression
Type:  DWORD
Data:  1 (enables the hardware acceleration)
           0 or missing (disables hardware acceleration)

Note: Logitech does not recommend the C920 for business use cases. We recommend the more modern Logitech cameras (C930E, C925E), which work with standard Microsoft drivers.

Considerations and recommendations

  • With the inclusion of hardware acceleration for video, the amount of data being sent increases if you deploy devices that support hardware acceleration for video. Ensure you have sufficient bandwidth available between all endpoints or update your Skype for Business server media bandwidth policies accordingly.

  • In Fallback mode (when RealtTime Media Engine is not available on the endpoint and audio/video processing occurs on the server), video quality might degrade to the point of failure on virtual desktops with a single virtual CPU.

    We recommend you change the VDA configuration to have a minimum of two CPUs for users who might need Fallback mode. For more information, see and
  • Users might hear an echo or other artifacts when using any built-in microphone and speakers on endpoints.

    We recommend you use a headset on an endpoint or a speakerphone with hardware-based echo cancellation. 
  • When attempting to make high-definition video calls from a home office, take into account your users' network bandwidth and ISP routing policies. If you observe pixilation of the video or problems with lip sync, adjust the Maximum Packet Size (MTU) on the network interface card properties to a lower value such as 900 to avoid situations where ISPs perform traffic shaping based on packet size.
  • Various scenarios (for example, combining content sharing and audio/video conferencing) might not work properly when some conversation participants run 1.x versions of the RealTime Optimization Pack.

    We recommend participants using older versions of HDX RealTime Optimization Pack upgrade to this version of HDX RealTime Optimization Pack. 

  • Users might see an error when making a call or joining a session when they have multiple sessions running.

    We recommend running only one session.

  • Old versions of graphics card drivers might impact the stability of the RealTime Optimization Pack. H.264 hardware encoding and decoding on Intel and AMD chipsets works most reliably with the latest versions of graphics drivers available from the endpoint or chipset vendors. RealTime Optimization Pack might automatically disable these features if an unsupported driver version is detected.

Citrix Customer Experience Improvement Program (CEIP)

The Citrix CEIP usage and analytics program is a voluntary data collection program designed to improve your product experience. After installing this version of the RealTime Optimization Pack, you can participate in the programs anonymously.

You can change your participation in the program at any time. For more information, see

CEIP is voluntary. When you opt-in, the CEIP services running in Citrix products gather anonymous configuration and usage data from your deployment, and automatically sends the data to Citrix. CEIP collects these classes of data:

  • Configuration data
  • All system and account identifiers are anonymized before being uploaded.

How your privacy is protected:

  • Citrix does not collect any personally identifiable data.
  • Random identifier is created at install time, which tracks data transfers over time.
  • Citrix does not record information such as IP addresses, server names, or domain names.
  • All data is sent using HTTPS directly to Citrix servers - no third party data hosting services.
  • All data is secured on Citrix servers and is accessible only by authorized individuals.

CEIP Opt-out policies and the user interface (UI)

The RealTime Connector defines the following registry entries controlling CEIP metrics:

DWORD DisableCEIPMetrics

If absent or set to 0, the user controls the CEIP metrics collection. If present and set to nonzero, metrics collection is disabled and the opt-out UI is hidden.

On 64-bit Windows, the RealTime Connector checks both HKEY_LOCAL_MACHINE\Software\Citrix and HKEY_LOCAL_MACHINE\Software\Wow6432Node\Citrix for this value.

HKEY_CURRENT_USER\Software\Citrix\ \

If absent or set to 0, the user did not opt out of CEIP metrics collection. If present and set to nonzero, the user opted out of CEIP metrics collection.

In the Settings dialog screen, the RealTime Connector adds a checkbox.

[] Send anonymous usage metrics to Citrix

The checkbox is hidden if the administrator disables CEIP metrics collection by setting DisableCEIPMetrics. Otherwise, it appears. The checkbox is checked if the OptOutOfCEIPMetrics registry value is absent or set to zero, and it is unchecked if OptOutOfCEIPMetrics is present and set to nonzero. When the user changes the state of the checkbox, the RealTime Connector updates the registry setting and enables or disables CEIP metrics submission accordingly.