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Feature Article
March 2003

The Protocol Challenge


As discussed in our last column, there are a number of standard and proprietary call control protocols being used for VoIP solutions. This month, we are going to detail some of the additional development challenges related to supporting these popular call control protocols over WiFi networks.

Regardless of whether a vendor chooses to implement a standard or a proprietary call control protocol, they all typically suffer the same problems when it comes to implementing them in a wireless environment -- as these architectures were designed to assume a constant Ethernet network connection, and certain protocol features rely on Ethernet implicit characteristics.

WiFi, because of its transport vulnerability to interference and a wireless phone�s dependence upon battery power, imposes some additional, unique wireless challenges. There are two specific protocol-associated challenges to be addressed with a WiFi VoIP solution:

� Out-of-range detection (i.e., no WLAN coverage);

� Out-of-band signaling (i.e., reliable call control and process signaling).

Challenge 1 -- a WiFi telephone has the peculiar behavior of being able to �disappear� from the network while on a call. This �disappearance� can be due to a dead battery or to the mobile user walking �out-of-coverage.� This condition is typically not accounted for in many of the aforementioned call control architectures. In such cases, the �system� must detect the fact that the device is no longer connected to the network and quickly reconcile resources allocated to the call control in a prompt manner. If left to TCP/IP socket time outs, this can be a matter of minutes, which means that that phone cannot receive or make calls because the associated call resource is still allocated.

Challenge 2 -- Certain control signaling often must be accomplished during the progress of a phone call that has an implied reliability. An example of this would be the act of picking up voice mail. Interfacing with most voice mail systems is accomplished via DTMF (Dual Tone Multiple Frequency) signaling. These signals may be sent �in-band,� that is, embedded as part of the active audio stream, or �out-of-band.� In a wireless implementation, any �in-band� signaling has no guaranteed reliability. What this means is that user-initiated keystrokes may not be properly transmitted to the voice mail system, thereby blocking access the system. A robust wireless implementation must accommodate an out-of-band alerting mechanism to ensure reliable service for such operations.

The question that must be answered is: �do any of the existing call control protocols directly address these wireless challenges?� The answer is: NO. So, what options are open for potential users of WiFi VoIP solutions?

With so many wireless VoIP solutions beginning to hit the market, how do these products address the specific WiFi VoIP protocol challenges? The products in the current market tend to fall into two categories with respect to this issue:

� They ignore the problem -- That is, there is no provision in their protocol architecture that addresses the two challenges previously described.

� They have a proprietary solution -- Solutions that control both sides of the RF domain (mobile unit and gateway) can implement proprietary solutions.

Many of the PDA-based soft-phones that are being introduced into the market fall into the first category. These are products from software vendors that see a market opportunity that seems easily filled with an application level (Layer 7) solution. While they may work fine when connected to a hard-wired network, they are exposed to the WiFi weaknesses described earlier.

Embedded solutions where a vendor supplies the wireless handset and the network gateway will fall into the second category. Vendors will often work together to produce proprietary solutions to address these problems. This approach can offer a robust solution� and are currently the most robust solutions for today�s market.

There are a number of solutions on the market that address the WiFi challenges and that have been successfully deployed. These solutions typically come from vendors that have experience in both telephony and wireless LAN technologies, where their product literature will typically clearly identify features where the wireless �challenges� are addressed.

As the WiFi and VoIP markets mature and grow, there will be a �brighter� tomorrow because of evolving VoIP standards and increased sophistication of the WiFi VoIP vendor community. This will result in a broader spectrum of robust WiFi VoIP offerings with greater interoperability between vendors. All of this will benefit you -- the consumer.

So, are VoIP protocols �wireless friendly?� Not by design, but market demand continues to spur on the creativity of the vendor community and standards bodies to support a wireless perspective and evolve solutions that are �wireless friendly.�

Richard Watson is director of telephony product marketing for Symbol Technologies� Wireless Systems Division in San Jose, CA. Prior to taking on the marketing role for Symbol�s NetVision family of WiFi Telephony products, he managed the software engineering team for three years and was responsible for developing Symbol�s WiFi Telephony products.

[ Return To The March 2003 Table Of Contents ]

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