Cisco, which just plunked down $3 billion to buy Tandberg ASA, makers of videoconferencing gear, said Tuesday morning that it’s going to buy nine-year-old Starent Networks for $2.9 billion, a 21% premium.

Starent supplies mobile and converged carriers with IP-based mobile infrastructure widgetry and Cisco figures the mobile Internet is now at an inflection point as IP-enabled Smartphones and other connected mobile devices become ubiquitous.

It says global mobile data traffic is expected to more than double every year through 2013. Starent last year made $52 million on revenue up 74% to $254 million.

Cisco’s paying $35 a share cash. The deal should close sometime in the first half when Starent, under its CEO Ashraf Dahod, will become the new Mobile Internet Technology Group in Cisco’s Service Provider Business. Dahod will report to Service Provider general manager Pankaj Patel.

Cisco figures the acquisition will dilute its non-GAAP earnings in fiscal years 2010 and 2011 and become accretive in fiscal 2012.

Cisco will pair its skills in video and IP with Starent’s infrastructure solutions, promising rich, quality multimedia experiences on 3G and 4G networks.

Starent helps service providers scale and monetize their investments via differentiated experiences. It says it provides the multimedia intelligence, core network functions and services to manage access from any 2.5G, 3G, and 4G radio network to a mobile operator’s packet core network.

Its access-independent technology is deployed in CDMA2000 (1X, EV-DO), UMTS/HSPA and WiMax networks.

Starent is based in Massachusetts and has approximately 1,000 people worldwide.