Two Way Radio Technology
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Push To Talk

How does the “walkie talkie” feature on a Nextel phone work?

  • Nextel phones offer a service called Direct Connect that allows you to push a single button and connect with another Nextel customer, assuming you’re both in a local coverage area.
  • Unlike traditional walkie-talkies that cover a limited distance, push-to-talk services would work wherever a carrier’s network reaches.
  • Direct Connect uses Push to Talk (PTT) technology, requires the person speaking to press a button while talking and then release when done.
  • Users can specify a group (of up to 100 people) to send a broadcast message to at the same time, much like a dispatcher’s radio.
  • Nextel is unique among service providers because it has an entirely separate cellular network with its own frequencies and equipment in addition to the normal cell network shared with other providers. This network is based on Motorola’s Integrated Digital Enhanced Network (iDEN) and makes direct connect possible.
  • The network uses the 80 MHz portion of the radio spectrum assigned to specialized mobile radio (SMR) service.
  • Nextel has purchased a large segment of these frequencies in a significant number of the national and international cellular service markets
  • iDEN uses TDMA technology to split a 25 KHz frequency into six separate time slots. Using a combination of half-duplex and full-duplex signals iDEN is able to provide:
    • Normal cell phone voice communications
    • Messaging (pager, e-mail)
    • Digital two-way radio (one to one/ one to group)
    • Data services
  • The Two-way radio service uses a half-duplex signal, while normal cell phone calls uses two separate frequencies, one to send and one to receive for each call while a direct connect call uses only a single frequency.
  • A Direct Connect call:
    • You hit the Direct Connect button, which is configured with the number(s) of person or group you are calling.
    • Your phone establishes a session with the Nextel iDEN-based network.
    • The network determines that this is a dispatch call (Direct Connect) rather than an interconnect call (a normal cell phone call)
    • The network then determines if it is one to one or a group call. If it is a group call, the network duplicates the digital voice packets for each phone in the group.
    • The network routes the packets to the phone (or phones) of the person (or group) you are calling.
    • Their phone alerts them that they have a Direct Connect call.
    • They answer by pressing the talk button
    • The call is completed and everyone disconnects.

Ethics of Push To Talk

Article in Chicago Tribune, "Push Afoot for Walkie Talkies"

  • "License to be rude. With a phone call you start by asking how someone is and some pleasantries. With push-to-talk, you jump straight in with what you want."
  • Annoying in indoor settings where others around you can hear both your half of the conversation but also the other end.
  • Urging new carriers to market push-to-talk for new uses, such as instant communications among teens and gamers. Sports enthusiasts watching games at different locations might use the function to razz one another when their team flubs a play.

[Read full article]