Everyone agrees that in aviation today, we struggle with connectivity when comparing the Internet service we have at home to that available in flight. What isn’t agreed to is the reason why the experience on board an aircraft is so less satisfying than what we expect. Oftentimes, people point solely to bandwidth, or the “size of the pipe” to the aircraft available to move data from Point A to Point B. If connectivity was basic plumbing with one-way water flow, this may be true, but connectivity experience depends on two-way communication between Point A and Point B. This is when latency enters the equation.
Latency is the time it takes for a packet of data to travel from one point to another in a network. It has a large impact on user experience, even outweighing bandwidth unless the “pipe” is incredibly constrained. If the bandwidth— the absolute capacity of the connection— is big enough to handle data requests, then latency takes over as the real driver to user experience.
Think about all the things you want to do on your devices in flight: load a web page, Facetime a friend, play an online game. A low latency will let you do these activities with minimal wait time, while a high latency means that you will become frustrated as your game lags or a website struggles to load.
These applications rely on transferring data to and from your device by either UDP (User Datagram Protocol) or TCP (Transmission Control Protocol). An application using UDP will send data to the receiver in a streaming fashion, without regard to whether the receiver got the data or if the data was corrupted in transit. This provides the best possible experience for time-sensitive data, like live audio and video, in that delay is minimized and will be presented to the end user as quickly as possible. As the network latency increases, however, the delay in presentation increases.
When an Internet connection is established between two endpoints using TCP, data is sent from one point to the other in blocks. The TCP window size determines how much data can be sent at a time. This wait time between sending, receiving an acknowledgment, and sending the next block of data is increased as the network latency increases— and the longer we wait for data, the worse the user experience.
To illustrate the effect of latency on TCP bandwidth, let’s compare two different connections, where Connection 2 has a higher bandwidth than Connection 1, but also a higher latency. Latency is measured in RTT, or the Round Trip Time between two endpoints.
|Connection 1||Connection 2|
|Bandwidth||1 Mb/sec||10 Mb/sec|
|Latency||80 msec RTT||800 msec RTT|
|Maximum TCP Throughput* (assume TCP window size of 64kbytes)||6.6 Mbps||0.66 Mbps|
|Maximum TCP Throughput greater than bandwidth connection||Yes||No|
|Effective speed of the connection||1 Mbps||0.66 Mbps|
*TCP Window Size / Latency (s) = Maximum TCP Throughput
The example above clearly shows that although the bandwidth is larger on Connection 2, due to latency, the effective speed of the connection is inferior to Connection 1, which has a smaller bandwidth but much lower latency. This delay is a significant factor in degrading user experience for all Satellite-based technologies, which typically have Round Trip Time (RTT) latencies ranging between 500ms and 850ms. For ATG networks using 3G technology, which inherently have latencies in the 150ms to 300ms range, this also degrades user experience. Networks which are designed with latency below 100ms, such as SmartSky Networks, do not cause degradation in user experience due to this delay.
So how do high latency and effective lower throughput affect your Internet experience?
Let’s say you’re trying to hold a Facetime call with a friend or coworker. Conversations will be difficult to maintain as voice and video are delayed and will appear out of sync. Trying to load a web page to keep up with the financial markets? High latency will slow the website load time to a crawl and oftentimes the page will time out before all the required data is fully loaded. Playing an online game such as Fortnite? Hope your competitor is only a beginner, as your game actions will be based on a screen presentation that happened over 500msec ago. It’s tough to line up a shot on your moving opponent in these conditions.
The impact of network latency on user experience is clear. SmartSky has designed a network that has much lower latencies than those of any current aviation offering, providing a user experience on par with your home connection.
How do network coverage and beamforming also have an impact on your user experience and ability to connect?