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VoIP over 802.11 wireless

VoIP over 802.11 wireless Ask-the-Expert response

Let's consider how TeleSym's solution works in a public Internet access hot spot:

  • The PDA must first associate with the hot spot access point and satisfy access controls imposed by the hot spot operator. For example, many hot spots require users to log in with a browser before accessing the Internet with any other protocol. On a Wi-Fi enabled PDA, this is done with Pocket IE.

  • The PDA connects to a SymPhone call server – for TeleSym, that's outbound UDP on port 3344, but products vary. So, this service only works if the hotspot firewall allows outgoing UDP/3344. Not usually a problem, except in more tightly-firewalled hot spots.

  • The SymPhone call server now knows where the PDA is, and can route incoming calls to the PDA as long as this UDP pseudo-session remains connected. However, unsolicited inbound UDP is typically blocked by Internet-facing firewalls, so the client must stay in touch with the server (continue to run on active PDA and remain associated with AP).

  • Other SymPhone clients connected to the same call server can call each other peer-to-peer. With a PBX connector, incoming calls from the PSTN can reach the client, relayed through the SymPhone call server.
  • How well this actually works depends on many factors, ranging from the quality of the microphone on the PDA to utilization of the hot spot WLAN to congestion on the public Internet. If you want to try this yourself, TeleSym makes SymPhone client software available for free 30-day trial, routing calls through their own call server, PBX connector, and 3COM NBX in Seattle.

    Note that this usage model is different than simply carrying a cellphone. Hot spot coverage areas are small (hundreds of feet, not miles), interaction is required to connect to each hot spot, quality and usability will vary, and outside calls go through your own PBX, not directly to/from a local public carrier network.

    Cellphone manufacturers like Motorola and Qualcomm have announced plans to build Wi-Fi enabled phones and chipsets. Once that happens, carriers may offer seamless roaming between 802.11 and "3G" networks like GPRS. For example, a carrier like T-Mobile might offer a plan that lets dual-mode phones use 802.11 at hotspots and GPRS everywhere else. In that case, the carrier would track phone location, route incoming calls from the PSTN, allow VoIP protocols into the hotspot, and automate device authentication. In return for providing this additional infrastructure, the carrier will charge you for the call, of course.

    Dig Deeper on Wireless LAN (WLAN)

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