Sprint is finally ready for a nationwide launch of push-to-talk over its CDMA network, announcing this week plans to turn on Qualcomm’s QChat technology in 40 markets, including most of the major metro areas. But the delays that have plagued the next-generation push-to-talk (PTT) offering may have eaten up all of Sprint’s time-to-market advantage, as Verizon Wireless plans to launch practically the same service this summer.

Though Verizon hasn’t given any details about how extensive its initial service offering will be, if it launches the service in a large footprint, it would leave Sprint with only a few months advantage in the market. The competition between the two may result in the resurgence of PTT as a hot-ticket service over operators networks, and it will likely bring pressure on all operators to enable PTT interoperability between their networks.

By virtue of its acquisition of Nextel, Sprint is the clear market leader in PTT, Nextel’s iDEN network having set a standard for the two-way walkie-talkie service to which all other platforms have aspired. Sprint, like Verizon, tried to replicate PTT over the CDMA 1X network using Motorola’s Winphoria technology, but it proved to be a poor substitute for Moto’s iDEN technology, which incorporated PTT directly into the network architecture.

In 2006, Sprint revealed it was shifting its PTT strategy. Instead of trying to use the CDMA voice channel, it would use the new data capabilities of its 3G network and VoIP. Adopting QChat was supposed to highlight two new network enhancements, Sprint’s new CDMA EV-DO Revision A upgrade, which supported the uplink speeds and low latencies necessary for VoIP, and its new IP Multimedia Subsystem (IMS) architecture, which would theoretically allow Sprint to meld the next-generation PTT service with other applications as well as mediate sessions between the CDMA network and legacy iDEN network.

The implementation, however, was easier said than done. While the network upgrades went up last year, Sprint had trouble getting QChat to work seamlessly with its new IMS platform. The Session Initiation Protocol (SIP) signaling it used to initiate and manage PTT sessions was too slow to simulate the sub-second call initiation of the iDEN network, and Sprint was forced to write a faster, yet proprietary protocol to manage the QChat sessions across both networks. In addition, Qualcomm’s ongoing court and regulatory battles over intellectual property cast a cloud over the future availability of 3G handsets embedded with Qualcomm chips, which contain both the Rev. A and QChat technology necessary to run the service.

Sprint finally unveiled the service at its Sprint Ahead technology showcase last summer, but it would be another nine months before the service went live in its initial trial markets, and almost a year before it goes nationwide this weekend. Meanwhile, Verizon confirmed this week it would have its own PTT service ready to go this summer, though it revealed few details. Sprint has said QChat is available exclusively to Sprint, though Verizon may be using a version of QChat that integrates with its BREW application deck.

Two major PTT launches this summer may build momentum for interoperability of push-to-talk services across different carriers’ networks. Many two-way mobile services have started out confined to an operator’s network, only to take off when inter-carrier communications was enabled. SMS is probably the best example, having become operators’ single-largest data revenue driver after the development of SMS exchanges.

So far PTT has been confined to the carriers that run the service, but Sprint’s linking Direct Connect across the Sprint and Nextel brands shows that PTT can traverse disparate networks owned by the same entity. Sprint, however, has made the investment on ensuring that interoperability between iDEN and CDMA networks, but Sprint is unlikely to extend those capabilities to its competition. Nextel has already built up an extremely loyal following of blue-collar workers, emergency personnel and professionals, all drawn by the PTT service. By bringing Direct Connect to the CDMA network, Sprint hopes to build upon that loyal base by offering a greater variety of handsets and advanced data services that the iDEN network simply couldn’t support.

Interoperability also presents a technology problem. While Sprint and Verizon are both using Qualcomm technology for their CDMA implementations, the other major push-to-talk platform, supplied by Kodiak Networks, is incompatible. Kodiak supplies its PTT server to AT&T, Alltel, MetroPCS and half a dozen other regional providers in the US, none of which interoperate with one another, much less with operators using different technology. In Europe, operators and Sonim Technologies have made efforts to develop an interoperable PTT standard, called PTT over Cellular (PoC), but the platform has gained little traction in the US.