The Dublin-born transaction management software Openet has today announced that it completed a Series D round of funding totaling $21 million.

The round is $16 million in equity funds and $5 million in long term debt. Openet is used by 80 network operators in 28 countries — including AT&T, BT, Orange, Verizon and Vodafone — factoring different ways to charge users based on the amount of data they use.

This round was led by NS Solutions with participation from Balderton Capital, Cross Atlantic and Kreos Capital. 

Balderton Capital, which has been an investor in Openet since 1999, has participated in each of the company's funding rounds and Barry Maloney, a partner at Balderton, has served on the Openet board since 2001.

From this round, Openet added Margaret Rice-Jones, the ex-CEO of Irish carrier Aircom, to its board. 

Openet said that it will be using the new influx of funds to build out its distribution channels, product development, product management and engineering.

"This funding round gives us additional flexibility to capitalize on our market opportunities," said Niall Norton, chief executive officer of Openet, in a statement. "The high level of interest from outside investors reflects the important role Openet's real-time transaction management solutions play for our customers as they compete in an increasingly dynamic and uncertain market."

Openet focuses on real-time network control that helps major carriers manage lots of data.

Focus was on Openet a year ago when the company was discussing how different data could be charged differently depending on the amount of strain it puts on the network -- such as charging more to view a YouTube video than reading a news website.

Users, obviously, were shocked at the idea that the type of content they were accessing being charged differently and it also raised a lot of questions around the privacy they have on their mobile devices. 

And back in early 2011, IBM partnered up with Openet to create a new telecom center of excellence in Austin, Texas. The center's focus was to help mobile operators with their R&D to cope with billing and bandwidth demand driven by growing use of smartphones and tablets.

The company has been profitable for the last two years, and in 2011 had a turnover of approximately $120 million.