Perspectives: VoIP Pros and Cons

Much has been written and spoken about implementing Voice over Internet Protocol (VoIP) for aeronautical applications. Leading this charge are aeronautical communications service providers and product manufacturers, hyping and promoting VoIP for the aircraft cabin. The technology is readily available, requires little or no development effort and can provide a quick and inexpensive solution. There appears to be significant interest by customers to install VoIP in their aircraft cabins.

However, "all that glitters is not gold." It is our experience at ICG, based on discussions with prospective customers, that VoIP service often is misunderstood. There seem to be misconceptions as to the advantages and performance VoIP would offer in limited bandwidth applications.

VoIP allows users to make telephone calls using a broadband Internet connection instead of a conventional phone line. Typically, users can call other users within the same service or virtually anyone who has a telephone number, including local, long distance, mobile and international numbers. Calls initiated via a VoIP corded or cordless phone are delivered via the Internet, in a data format, either to users within the same network or to the Public Switched Telephone Network (PSTN) for delivery to a conventional telephone or mobile subscriber. For delivery to the PSTN, the caller must select a Service Providers gateway that will receive the call via the Internet and then convert the VoIP data packets back to a standard telephony format for delivery to the PSTN. Most broadband service providers offering Aero VoIP services will provide PSTN access at a ground-side server or have arrangements with a Service Provider offering these gateway services.

In a typical VoIP aircraft system, standard 802.11 wireless devices can be used to access a wireless router to route VoIP traffic, thereby eliminating cabin wiring for corded handsets. A single system can manage voice and data communications via a broadband satellite link, and standard telecom features such as conferencing, transfer, etc., can be managed by the server.

However, a critical requirement for a successful aircraft VoIP system is the ability of the onboard server, or CTU, to convert between VoIP formats and traditional telephony formats to permit connections via other aircraft systems not requiring a broadband connection, such as Iridium or Inmarsat Aero CEPT-E1 channels.

Unfortunately, the major advantage of VoIP — free service — achieved in a terrestrial application, is not available on an aircraft. And there are many other factors to consider when deciding whether to adopt VoIP as a cabin solution.

VoIP requires significant bandwidth on a satellite or other air-to-ground system. The bandwidth required depends on the Codec algorithms selected by the server and mobile device. A "toll quality" VoIP call using the G.711 codec algorithm requires 80Kbps, for a single call. Because this bandwidth requirement is more than a single 64Kbps Aero HSD channel, most providers are going with the G.726 algorithm, which requires 38K per call and is "near toll quality." Other codec algorithms are available to reduce the bandwidth requirements further, but the audio begins to acquire an electronic sound and fidelity is dramatically reduced. We can see that multiple channels of high-quality VoIP will require significant bandwidth and is a very inefficient method of utilizing a broadband connection. Essentially, it is impractical and expensive to use an HSD channel for a single VoIP telephone call. Alternate facilities such as an Aero I channel or Iridium are far less expensive.

The single largest drawback is the user device. Though the intention is for passengers to use their private PDAs and other voice-capable devices, there is no guarantee these devices will support the codec algorithm required by the server. Many new handsets are available on the market, both desktop devices requiring an Ethernet connection or wireless devices operating at 802.11 B/G. Typically, they are not aircraft approved and designated as carry on devices. Many do not have interactive noise reduction or echo cancellation.

Often, it is the intention of customers to use personal user devices such as PDAs and WiFi-capable cell phones on an aircraft equipped with a VoIP Session Initiation Protocol (SIP) server. Not all user devices are compatible with a SIP server and the audio quality varies dramatically between devices, making some models virtually unusable or frustrating to use. The operating system most commonly found in PDAs is "Palm OS," which will not operate with SIP VoIP services.

It is undetermined at this time what VoIP format and protocols will be supported by 802.11 capable cell phones. Once these phones become available in North America, an evaluation will be required to determine compatibility with SIP server devices.