Verizon Wireless 's "network optimization plan" quietly went into effect last week, meaning the carrier began throttling its heaviest 3G data users. It has been plotting this move since February, but enacted it now to encourage these heavy users to upgrade to its fast-growing Long Term Evolution (LTE) network. (See Verizon Sells Out iPhone Pre-Orders, Throttles 3G.)

Throttling will only affect those 3G customers on unlimited data plans that fall in the top 5 percent of data usage and are on a congested cell site, stipulations that Verizon thinks makes it more desirable than its competitors. Users that fall into this category will experience reduced speeds for their current billing cycle and the one following.

Verizon is using this strategy to reduce traffic on its network, but also as a way to encourage its customers to upgrade to LTE smartphones on a more lucrative tiered data plan. Verizon engineer Harrison Duong told Light Reading Mobile at last week's developer conference that it welcomes data traffic on its LTE network, and that includes traffic from chatty apps and from consumers. (See Verizon Rallies Developers for LTE and Photos: Verizon's Developer Conference .)

Why this matters
Verizon already covers more than half of the urban population with LTE, and it's on track to blanket its entire 3G footprint with the faster network by 2013. This is an advantage it will market to consumers and hold over AT&T Inc. (NYSE: T)'s head for as long as it can. (See AT&T LTE Launch Set for Sunday, Verizon Wireless Covers 160M+ With LTE and Verizon Says LTE Will Match 3G Footprint in 2013.)

As part of its bid to make 4G more accessible, the carrier Monday unveiled a new LTE Pantech Co. Ltd. smartphone for $99, significantly less than all its 4G phones to date.

As of now, Verizon does not throttle its LTE users, although it also doesn't offer them unlimited data. Verizon says that it reserves the right to include 4G LTE users at a later date, but right now throttling only applies to its 3G network.

For more
Read up on Verizon's LTE migration below.

— Sarah Reedy, Senior Reporter, Light Reading Mobile