iDEN is a wireless communication architecture developed by Motorola in the early 1990's. It was designed as a high-concentration replacement for the then standard DAMPS system. iDEN operates mostly in the 800MHz range, though new deployments are using the 1.5GHz range.

The core of the iDEN system is the TDMA transmission standard which Motorola also helped develop. The voice signals are coded using VSELP, or Vector Sum Excited Linear Prediction which serves for compressing the voice data before transmission. TDMA also works in concert with QAM, Quad Amplitude Modulation which allows 64kbps over the 25mHz TDMA channel.

Among other features intrinsic with the iDEN system is the capability of direct handset communications where two iDEN handsets can form communications loop between them (commonly refereed to as "radio mode" or "walkie-talkie mode"). iDEN also offers rudimentary triangularion and GPS locating facilities because the TDMA protocol works in conjunction with GPS satellite timecodes.

iDEN, an acronym for Integrated Digital Enhanced Network, is a cellular phone system created by Motorola.

It first entered actual development in 1992, known at the time as MiRS (Motorola Integrated Radio System). However, due to some system performance issues, mainly due to the signal, the system did not make a good impression. Motorola spent some time reworking this problem, improving signal quality, and reintroduced the system as iDEN. (The system continues to be known as MiRS in Israel, and there are plenty of internal references to system as MiRS.)

iDEN has the features of any other cellular phone service - voice mail, caller ID, a short message service, a variety of phones (known as radios or subscriber units by those working on the system) and such. It also contains a packet data service which allows enabled phones to get information from the internet, or services provided by the carrier.

Carriers using the iDEN system inclue Nextel (United States, and spreading into other countries), SoLink (SE United States), Mike (Canada).

It is easy to identify a phone designed for an iDEN system. There's a pair of buttons with white dots in the center, one located under the small screen on the left, and one on the right, used to navigate the menus, and also a large, long button along the left side of the radio (looking at the front for orientation). Currently the only company making phones compatible with the system is Motorola, though there are a couple companies that have licensed the specs and may make their own phones.

The biggest features to differentiate it are the dispatch-style group calls, and the person to person walkie-talkie style private call, billed by one carrier, Nextel, as "Direct Connect" service. The group call allows a person to talk to an entire group of people at once, simply by selecting the group (a person can belong to multiple groups), pressing the "push-to-talk" button, and speaking. The private call works similarly - select the private ID (the equivalent of a phone number) of the person you wish to talk to, and press the "push-to-talk" button, and you can start talking directly to the other radio. No waiting for someone to answer.

You also have the option of sending a "Call Alert" - it will start the other radio beeping, with your private ID displayed, and will continue beeping until they cancel the alert.

The system continues to grow in capabilities. Restrictions about who you can call using the private call and group call are continually disappearing as Motorola's iDEN Division adds new features. At the time I'm writing this, you can private call anyone else in the same "urban area" as you (a urban area is the entire area serviced by one system, they tend to be centered around large cities). Being able to private call anyone in the same network (for example, a person in Chicago calling a person in Los Angeles), regardless of urban area, is in testing at this point, due to reach the field sometime in late 2002/early 2003.

As required by the CALEA act passed by the United States government, the iDEN system allows conversations to be surveilled by law enforcement. Interconnect, private calls, and group calls can all be monitored either for who is calling whom, as long as one person is targetted for surveillance, or for voice. Law Enforcement must furnish part of the equipment to do so, as only part of the surveillance system is implemented on the system. Surveillance cannot be detected by the subscriber, whether by listening to voice quality, delay when talking, or even if they had access to the data to control the phone. The surveillance "feature" is disabled for all non-US systems installed - it is not possible for a conversation to be surveilled on those systems without the intervention of Motorola personnel. (We were none to happy about adding the "feature", but it wasn't a choice)

The iDEN system is uses TDMA (Time Division Multiple Access) for the radio channels. The channel is divided into 15-millisecond time slots, and allows either 6 radios total to share the slot for dispatch calls (a 6:1 scheme), or 3 radios for interconnect (telephone) calls (a 3:1 scheme). This will be changing shortly, as new technology is in testing that compresses the voice data into a smaller space, yet has slightly better sound than the current technology, allowing them to use 6:1 for interconnect, and 12:1 for dispatch, doubling the spectrum capacity.

iDEN is a very software heavy system. All of the call setup, teardown, switching, and such is done entirely through software, millions of lines of it. It is almost entirely message-based - different tasks on each functional box are running at the same time, and they communicate with each other through internal and external messages, based either on inputs, or due to timers (such as with "heartbeat" messages). Many tasks even send messages internally. To those of you who have been through computer science classes dealing with automata and finite state machines - it's not just theory. The call processing for the dispatch and group calls alone uses 11 of them. (9 before development of inter-urban area calls)

(I would love it if someone else who's interested in such things could go a little more detail into the workings of the system based on the publically available information. Being involved in the development, I known quite a bit more about it's workings, specifically the workings of the private/group calls, but I really don't want to take the chance of stepping into proprietary information territory, with all the bad consequences that could come from that.)

Interesting easter egg for iDEN phones - enter the sequence "#", "*", "Menu", "right" on the newer phones to access the test menu. Be careful, you can chance some important settings here.

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