There is an interesting phenomenon happening with ISP networks that I don’t see anybody discussing. During the last year, we saw a big change in the nature of our broadband usage in that many of us are connecting to remote work or school servers, or we are connecting to long Zoom calls.

We already can see that these changes have accelerated the average home usage of broadband. OpenSignal reports that the average broadband usage per home grew from 274 gigabytes per month just before the pandemic up to 462 gigabytes per month measured at the end of the first quarter of this year. Since much of the new usage came during the daytime, most ISPs reported that they were able to handle the extra usage. This makes sense because ISP networks in residential neighborhoods were relatively empty during the daytime before the pandemic—adding the additional usage at these non-busy times did not stress networks. Instead, the daytime hours started to become as busy as the evening hours, which have historically been the busiest time for residential networks.

But there is one impact of the way networks are now being used that is impacting ISPs. Before the pandemic, most of the use of the Internet in residential neighborhoods was bursty. People shopped or surfed the web, and each of these events resulted in short bursts to the Internet. Even video streaming is bursty—when you watch Netflix, you’re not downloading a video continuously. Instead, Netflix feeds you short, fast bursts of content that cache on your computer and keeps you ahead of what you are watching.

But our new network habits are very different. People are connecting to a school or work server with a VPN and keeping the connection for hours. Most Zoom video calls last 30 minutes to an hour. Suddenly, we’re using bandwidth resources for a long time.

In telephone networks, we used to refer to this phenomenon as holding times. Holding times were important because they helped determine how many trunks or external connections were needed to handle all of the demand. A longer holding time for a given kind of traffic meant that more external trunks were needed for that kind of calling. This is pure math—you can fit twice as many calls into an hour if the average holding time is five minutes instead of ten minutes. A telephone company would have multiple kinds of trunks leaving a central office—some trunks for local traffic between nearby exchanges and other trunks for different types of long-distance traffic. Traffic engineers measured average holding times to calculate the right number of trunks for each kind of traffic.

The fact that residents are maintaining Internet connections for hours is having the same kind of impact on broadband networks. The easiest place to understand this is in the neighborhood network. Consider a neighborhood served by DSL that has a DS3 backhaul provided by the telephone company—that’s 45 megabytes of capacity. Such a connection can support a lot of bursty traffic because requests to use the Internet come and go quickly. But the new, long-duration broadband holding times can quickly kill a DSL neighborhood connection, as we saw during the pandemic. If only 20 homes in the neighborhood (which might consist of 100 homes) connect to a school or work server using a 2 Mbps connection, then 40 of the 45 megabytes is fully occupied for that use and can’t be used for anything else. It’s possible for this local network to become totally locked with heavy VPN usage.

This kind of network stress doesn’t just affect DSL networks but every broadband technology. The connections inside the networks between homes and the hub have gotten far busier as people lock up Internet links for long periods of time. For technologies like DSL with small backhaul pipes, this phenomenon has been killing usage for whole neighborhoods. This is the phenomenon that killed the upload backhaul for cable companies. For technologies with larger backhaul bandwidth, this phenomenon means the backhaul paths are much fuller and will have to be upgraded a lot sooner than anticipated.

This phenomenon will ease somewhat if schools everywhere go live again. However, it appears that we’re going to continue to have people working from home. And video calling has moved into the mainstream. That means that backhaul connections inside ISP networks are a lot busier than any network engineer would have predicted just two years ago. While some of the extra traffic comes from increased broadband volumes, much of it is related to the much longer customer holding times—a term we’ve never used before with broadband networks.