A Motorola developed network, used only by Nextel (RIP) and a few other companies.
Didn't consider SMS initially as SMS is part of the GSM spec - and iDen is, well, very proprietary. Very popular for the Push-To-Talk (PTT) feature popular with the trades and construction.
Integrated Digital Enhanced Network - Wikipedia, the free encyclopedia
Nextel was ultimately bought by Sprint in perhaps the worst business acquisition of the decade.