Network Characteristics: The Importance of Return Link Capacity

Posted by Brian Staik on Sep 6, 2018 11:58:54 AM
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You might not give much thought to your Internet connection's overall speed, but you probably do pay attention to your overall user experience. Most people think speed tests are the key to putting a measurable quantity on user experience – and most only pay attention to the speed to the user, not the speed from the user to the provider. You might be surprised to learn that in addition to latency, upload speed, or speed from the user to the provider, is key to a great Internet experience.

If you need to email or access large data files or presentations, use VoIP and video conferencing applications, or use a cloud-based office suite or online file storage like Google Drive or Dropbox, a sufficient return link is critical to having a good user experience. The return link is the path from the user device to the service provider network. Most inflight connectivity systems have been designed, built and deployed with the forward link having a much higher capacity or speed than the return link (typically a 90/10 split). How does this asymmetric connection compare to a network where the return link capacity is almost equal to the forward link?

Asymmetric Connection

The type of asymmetric connection seen with other inflight and some ground providers is weighted heavily in the forward link direction and only works when being used primarily for pulling data from the servers on the Internet.  With this type of usage, the return link is simply being used to carry the TCP acknowledgments to support the data passing to the user over the forward link.

This sizeable forward-link biased, asymmetric connection will not perform well when the return link is small (constrained) to begin with and then saturated with mixed traffic like web browsing, video streaming, and applications that push data over the return link, such as emailing larger files, cloud storage, and voice service. In the constrained and saturated scenario, the return link delays the return of TCP acknowledgments. This additional delay and latency results in:

  • Slower web page load times and, in the worst cases, failure to load web pages
  • Poor quality of streaming video: degraded video quality or video buffering
  • Slower file downloads from cloud storage or even the inability to upload files to the cloud service

Have you ever tried to send an important and time-critical presentation while en-route to your destination, only to find it won’t send until you connect on the ground? Even worse, it ties up other email exchange while waiting for a pipe that won’t ever be wide enough to send the data. Voice over IP service users will likely experience dropped voice packets to the called party, resulting in choppy-sounding audio or possibly no audio heard by the ground party.

Clearly, the heavily biased forward link connection is not well-suited to the wide-ranging productivity and personal entertainment applications users are generating nowadays.

Greater Return Link Capacity

When the return link approaches the capacity of the forward link— as in the case of SmartSky Networks, with approximately a 60% forward/40% return link bias— it allows for the best possible user experience in the cockpit and the cabin, whether you are accessing the cloud, sending large files and presentations, streaming videos, or conference calling.

In addition, this return link capacity enables a cost-effective means of streaming data off-aircraft to support a wide range of applications for both passengers and flight crews/operations, including:

  • Streaming aircraft maintenance information to allow a fast reaction to aircraft issues
  • Live in-flight turbulence data for shared situational awareness
  • Support for heavy data streaming for aircraft systems such as engine monitoring

The capability to reliably move data off the aircraft en masse creates the potential to be an alternative or supplement to the traditional VDL (very high frequency data link) communications system. These applications and more are all possible with this greater return link capacity.

How does latency impact user experience? Learn more by clicking below.

Latency Matters

Topics: Knowledge Base


  1. the distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave. 
  2. a person's ideas and way of thinking, especially as it affects their ability to communicate with others.

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