T-Mobile has been ramping up its plans for a nationwide 4G LTE network since early 2012, and now it seems that plan is almost complete.

On Friday it emerged that OpenSignal, a third-party firm that collects and monitors network data, noticed activity on T-Mobile 4G LTE networks in New York, San Francisco, Kansas City, Las Vegas, Seattle, Denver, New Orleans and San Diego.

There have been plenty of hints lately that T-Mobile's LTE will soon roll out, but perhaps none are more telling than this.

That network activity is likely coming from inside T-Mobile, and if the carrier is testing its own networks then it seems T-Mobile 4G LTE really is upon us.

Don't stop me now

T-Mobile promised in January that it would unleash a 4G LTE network this year.

That network would cover more than 200 million customers by the end 2013, the carrier said.

More recently, the fourth-largest carrier in the U.S. teased a 4G LTE light-up before the end of the month, likely heralded by an event next week on March 26.

In preparation, T-Mobile released an over-the-air update to Samsung Galaxy Note 2s on its network that switched on LTE support for those devices.

The carrier's release of the BlackBerry Z10 is expected to coincide alongside its LTE rollout, while it's also highly likely the T-Mobile iPhone 5 will make an appearance before the year runs out.

Crunching numbers

OpenSignal claimed that several devices, including the Galaxy Note 2, Galaxy S3 and newly announced Galaxy S4, were all detected on T-Mobile's 4G LTE network.

Those devices hit speeds of up to 25MB per second downloading and 8MB per second uploading, with 40ms latency, according to the network firm.

As CNET pointed out today, those numbers come from an extremely small sample on a network likely still in testing, and should not be considered representative of T-Mobile's 4G LTE network before it actually goes public.

T-Mobile will be the fourth and final major U.S. carrier to join the 4G LTE fray. As of this month, Verizon has the widest 4G LTE coverage, while AT&T had the fastest 4G LTE speeds.

Via CNET