 |
|
|
A control channel is to a cellular system as a front desk is to a hotel. Anybody can walk into the lobby, but only those who register with the front desk and have their credit validated get a room. Control channels solve a fundamental problem in using radio, which is naturally a broadcast medium, for individual service. If voice channels could be directly accessed by mobiles, how could the system ensure that only one mobile uses a voice channel at a time? Moreover, since voice channels usually do not transmit when they are not being used, it would be impossible to know which frequencies were legitimate in any area.
Control channels solve this problem by forcing all mobiles to line up for service in each cell (or sector). Any mobile can request service, but until the control channel provides a room key (voice channel identifying number), the mobile has to cool its heels in the lobby. The most common requests for service by mobiles are registration (which announces the presence of a mobile, required for future call delivery), origination (making a call) and responding to a page (receiving a call). As systems and phones become more sophisticated, other operations, such as those related to authentication and short message service, are becoming more common on control channels. Once the mobile has tuned to the frequency of the voice channel, it has no need for the control channel for the rest of the call, and can use the voice channel for any further signaling that is required.
Obviously any system that makes customers line up has a potential for congestion. The question is how many clerks can work the front desk at once, and how fast can they work? Analog systems are limited in capacity and capability to one control channel with a raw capacity of about 1,000 bits/second for each cell (or sector). Digital systems (CDMA and TDMA) can have several control channels per cell, each with about 10 times the capacity of an analog control channel. This capacity improvement is not an inherent difference between analog and digital, but merely a reflection that digital is newer, and that its designers have learned from the past. It would be possible to design new analog mobiles around a new control channel, but this will probably not be done because it would require a fourth type of control channel – neither of the digital types are likely to be used because they both rely on the digital modulation scheme used for voice (CDMA or TDMA) – and thus th e analog mobile would have to be almost a digital mobile just to support the new control channel.
The easiest ways to increase the effective capacity of an analog control channel are to increase the density of cells, or to make more efficient use of it. Increasing the density of cells by shrinking and sectorizing them is often done to obtain greater voice capacity, and as a nice side effect, produces greater control channel capacity. The best way to greater efficiency is through configuring switch and base station algorithms that balance the load of paging and registrations. Originally, cellular systems paged in every cell to avoid tracking mobiles, but the load on all control channels increased with every new subscriber. Now, most cellular systems use registration to track a mobile to within a few cellsites, or even to within a single cellsite. This reduces the paging load to as little as one control channel transaction per system per call, as opposed to one per cell per call. However, it also increases the registration load. Many systems also perform time-based regis tration to limit the maximum time that mobiles can be ‘lost’. The load on the control channel can be reduced by triggering these registrations less often (at the unfortunate cost of more calls that cannot be terminated to a mobile).
Limitations of capacity and capability in analog systems do have one advantage. The analog control channel is considered by some experts to be more bulletproof than the slicker and sleeker digital control channels. It actually repeats every bit of transmitted data 5 times, with error detection, so that only one of the 5 transmissions has to be received correctly. Although this is a tremendous amount of redundancy, it should provide analog systems with greater immunity to long bursts of noise during signaling.
Control channels are not only a real-time bottleneck, but they can also stand in the way of progress, as most new features will require modifications to the control channel. Even though digital systems have their own control channels, several changes had to be made to the analog control channel to support them – changes which took advantage of the forward-compatibility built into the original analog specification (EIA/TIA-553, originally called IS-3). Unfortunately, not all analog phones implemented forward-compatibility correctly, so that implementation of TDMA and CDMA systems (and other capabilities, such as authentication, that affect the control channel) can degrade the operation of analog phones that used to work perfectly well. This problem is not talked about very openly, but it affects a significant percentage of older analog phones. It is serious enough for the TIA standards committee TR-45 (in the analog sub-committee TR-45.1) to initiate a project to defin e a single common definition of the analog control channel protocol.
Another compatibility problem is less technical, and more business oriented. As the digital standards committees take advantage of the greater digital control channel bandwidth, and add features to their standards, only new phones can support those new capabilities. This increases the cost of implementation of new services, as a new phone will have to be part of the package. Initially, adding sexy new features to digital was a way to promote digital, but too many layers of new features will start to add costs and confusion without increasing customer satisfaction. However, if consumers start to clamor for features like short message service, digital systems are much better positioned to satisfy that desire.
Control channels – it’s hard to live with ‘em, you certainly can’t live without ‘em. They make analog systems the old pickup truck of wireless systems – tough but unexciting. Digital systems have a new control channel under the hood, and lots of gadgets to keep their drivers happy.
© – Copyright