Signal Congestion Rejuvenates Interest in Cell Paging-Channel Protocol
by George Lawton
The unexpected signaling demand of smart phones has forced network operators to rethink network engineering and dust off an old protocol to keep networks flowing. Although attention has traditionally focused mostly on payload bandwidth, signaling bandwidth has been growing at a much faster rate. Many experts believe the 3G Partnership Projects (3GPP) cell paging-channel (Cell_PCH) protocol could help solve the problem.
The 3GPP included the protocol with the current mobile-phone specification three years ago, but until recently, carriers and vendors saw little benefit in implementing it. All of that changed with a new technique for conserving smart-phone battery power, which increased the network signaling load by up to five times, noted Phil Twist, head of marketing and communications for Nokia Siemens Networks.
The widespread adoption of Cell_PCH promises to extend battery life, while also keeping the signaling load manageable. In some cases, mobile operators could save 45 million euros by simply turning on Cell_PCH, rather than installing new switching infrastructure, said Twist.
Reports of trouble with the iPhone in New York and San Francisco have highlighted congestion problems. Although neither Apple nor ATT has publicly disclosed the source of these troubles, Michael Thelander, CEO of Signals Research Group, a wireless research consultancy, said the reports match a pattern of signal congestion. A signal congestion problems affects all the traffic in a larger region served by a radio network controller (RNC), whereas a payload-congestion problem would be isolated to data traffic in one small area.
"Part of the problem," Thelander said, "is that people designed networks to scale for data and not signaling."
Understanding Cell Signaling
There are four cell phone signaling states used in a 3G network: idle, forward-access channel (FACH), data channel (DCH), and paging channel (PCH).
In the idle state, which uses the least power, the network knows the phone is on and near a cell tower but doesn't know exactly where. The cell phone can use DCH for transmitting large quantities of data, but it consumes the most power. Cell_PCH can send many short messages efficiently, while keeping battery levels on par with the idle state. It's also far more efficient in terms of the number of signaling messages required to set up and tear down a short data session.
In terms of power usage, Twist said that DCH consumes 200–300 mA of power, compared with 150 mA for FACH, and 5 mA for both idle and PCH. For operators, the main issue is the number of signaling messages to go between different states. Up to 30 messages are required to go from idle to DCH compared to only seven messages between PCH and DCH. Cell_PCH also reduces the time required to set up a call from two seconds for fast dormancy to only a half second.
The Smart Phone "Fix"
When smart phones were first released, Cell_DCH was the only widely available protocol for sending short messages. First-generation iPhones burned through their batteries as they maintained a constant data connection throughout the day.
All this changed in June 2009, when Apple released the iPhone OS 3.0, which supported fast dormancy. The phone could quickly shut down its network connection to significantly increase battery life, but it had the side effect of increasing signaling traffic.
"Usually we had to worry about overall traffic growth," said Woojune Kim, vice president of advanced technology at Airvana. "But we found that the signaling component of traffic was growing the most significantly. As the smart phones became pervasive, they were generating more connections while sending less data."
Cell_PCH promises to help keep battery life down, while reducing signaling load. In one set of tests, Thelander found that it could reduce signaling load by 20 to 40 percent. "With Cell_PCH," he said, "you're reducing signaling rather than adding capacity. It's a more intelligent means of solving the signaling problem."
Challenges
PCH faces many challenges that will slow its adoption. Steve Kemp, Alcatel-Lucent director of wireless marketing, said many operators haven't adopted it. Reasons include a lack of load-balancing functionality in the Cell_PCH state, the latency to resume transmission, and incompatibility with user equipment. He also noted that many of the fast-dormancy features built into existing smart phones might defeat the benefits of Cell_PCH.
Thelander said the transition can be taxing to mobile operators. They need to test all of the equipment and ensure that it doesn't interfere with normal communications processing. "This could mean that what is 'simple,' actually ends up being quite complex and time consuming," he said.
Despite the challenges, Kemp expects most issues to work themselves out by 2011 with widespread adoption Cell_PCH to follow. In the long run, Cell_PCH will lose importance as 3GPP's Long-Term Evolution (LTE) 4G technology comes to market. LTE is much more efficient at managing signal congestion.
George Lawton is a freelance correspondent in Guerneville, California. You can contact him via his website http://www.glawton.com.
