Even though the cell tower often comes to mind during a cellular outage, subsections of a cell network can also experience temporary service outages. The GPRS network is actually an overlay network that operates on top of the GSM network. The former delivers IP data, while the latter delivers voice and SMS. But if the GSM network is down then everything is down: IP data, voice and SMS. This is rare because voice is the carrier's main source of revenue and, as such, is engineered for redundancy. In fact, even the U.S. Government tracks outages based only on voice minutes lost; it doesn't pay any attention to data outages. As a result, brief data outages are not a critical issue.
Because the GPRS network is a subset of the GSM network, IP traffic can suffer an outage while voice and SMS traffic still flow as usual. This is not a condition that many people would notice, but in the case of sole-path fire communications the line must be supervised every five minutes, which means even a brief outage would be detected. If this occurs, regulations stipulate that the site personnel and central station must be notified. For systems relying exclusively on GPRS, these brief but periodic outages will frustrate the customer and tarnish an integrator's reputation, which is why some equipment manufacturers always include SMS as either the primary or backup communications method. With SMS backup, even if the GPRS network fails the fire-communications protection remains intact and no one needs to be notified. This is another point to stress with an AHJ to demonstrate the resiliency of cellular as the sole-path.
Like many prior technologies, cellular must overcome resistance based on awareness, applicability and reliability. However, as with earlier code improvements, AHJs will gradually warm to the idea of cellular as the sole path for communications. With code knowledge and the proper equipment, savvy integrators can calm the AHJ's safety concerns and, for those efforts, watch as their RMR and customers' sense of appreciation grow.
Shawn Welsh is vice president of Marketing and Business Development, Telular Corporation, Atlanta.