Smartphone customers in the U. S. must be wearing a "slap me" sign on their backs if the latest notification from Verizon is any indication. When Big Red joined the club and went to tiered data plans that allowed overage fees, those with existing unlimited plans were feeling good as they got grandfathered in. Verizon figured out how to coax those folks onto the tiered plans by promising to throttle the network speed for unlimited plan customers who are in the top 5 percent of data usage.
Verizon doesn't like calling the service curtailment "throttling", as they prefer the term "network optimazation".
The difference between our Network Optimization practices and throttling is network intelligence. With throttling, your wireless data speed is reduced for your entire cycle, 100% of the time, no matter where you are. Network Optimization is based on the theory that all customers should have the best network possible, and if you’re not causing congestion for others, even if you are using a high amount of data, your connection speed should be as good as possible. So, if you’re in the top 5% of data users, your speed is reduced only when you are connected to a congested cell site. Once you are no longer connected to a congested site, your speed will return to normal. This could mean a matter of seconds or hours, depending on your location and time of day.
In other words you can't predict when this will happen, what you can do to prevent it, nor how long you will be "optimized". Sounds like throttling to me.
The only customers subject to optimization are 3G (not 4G) customers with unlimited plans. Bandwidth curtailment will only happen when a top data user is connected to a congested cell tower. While not being specific about what sort of data usage will put you in the top 5 percent, the FAQ at Verizon states that currently that is "2 GB or more of data each month".
Full details about the optimization are on the Verizon web site.