In Section 5.3, we discussed the sharing of the precious radio channel resource among CDPD mobile devices. In this section we discuss the sharing of the radio resources by the CDPD network and the cellular telephone network.
The CDPD network is designed to operate as an overlay on the existing cellular voice system. Figure 5.29 illustrates the AMPS cellular system as using a frequency based demand assignment reservation scheme. As a call request is made, it is assigned to an available channel. The channel is freed on completion of the call.
In order to ensure that a cellular call request can be serviced, cellular service providers deploy a large number of radio channels at each cell site. Using queueing analysis and typical telephony traffic models, cellular carriers attempt to engineer the radio channel layout to achieve a low blocking factor. The blocking factor is the probability that no channels are available to service a new call request. Using the typical target value of 2% blocking,5.24 theory shows that there would be significant excess channel capacity even during the typical busy hour.
The CDPD system design exploits this characteristic and attempts to make use of this excess capacity. Channel hopping is the concept of using the unused radio resource between voice calls. In order to eliminate the need to alter the voice system's operation, the data channel is managed by moving or hopping among the unused cellular channels. Figure 5.30 illustrates this operation. In effect, this is an attempt to "create" a RF data channel out of otherwise unused and unusable radio resource.
The CDPD system was designed as a transparent overlay on the cellular telephone network. As such, the CDPD system must not rely on the cooperative programming of the voice equipment. In other words, it must not require interaction with the cellular telephone infrastructure equipment to determine which radio channels are appropriate for use. The CDPD infrastructure equipment must devise methods to create the data channel.
The basic approach involves the MDBS to move the data traffic to different channels over time in order to avoid impacting voice traffic. This could be achieved through the MDBS "learning" the channel assignment algorithm of the cellular system, and continually moving to the channel least likely to be assigned next. This is known as a planned channel hop. Unfortunately, this involves much intelligence, since different cellular manufacturers use vastly different algorithms for selecting the next (voice) channel assignment. Futhermore, manufacturers may evolve their channel assignment algorithms and thus require even more intelligent5.25 MDBSs to evolve along with them.
The unlikelihood of creating the perfect planned hop algorithm means there will be instances where a voice call is assigned to a channel already occupied by CDPD data traffic. The CDPD specification addressed this problem by requiring that MDBSs use a device to sense the initiation of a voice call. This device, called the Sniffer5.26 , continually monitors the RF channel for non-CDPD transmissions. When it detects a non-CDPD transmission, it immediately5.27 terminates the CDPD transmission and moves the CDPD traffic onto a different unoccupied channel. This is an unplanned hop.
The two channel hopping mechanisms are not without problems. The inefficiencies introduced by a need to change frequencies and cause all the mobile devices to reacquire the new channel is obvious. Unfortunately, there are other effects.
First, the CDPD forward channel is a continuously transmitting channel. As such, it adds energy to the RF environment. Some of the cellular telephone infrastructure equipment continually monitors the RF energy of the available channels. If it senses significant energy on any of its allocated channels for a set minimum period of time, it declares that channel as noisy and prevents if from being used for voice calls.5.28 To ensure that the CDPD traffic channel doesn't cause this type of "channel sealing," the CDPD channel must hop very frequently, sometimes often enough to seriously impact the efficiency of the channel.
Second, the channel hopping concept was created with the promise of creating RF spectrum. However, some researchers are skeptical about this claim. They have argued that in many cellular systems today, the limiting factor is interference. In their claims, the cellular system's capacity will be reduced by the addition of RF energy into the system, regardless of whether it is stealing time between calls or not. In their presentations, they claim that a hopping channel will cause the same capacity reduction impact as dedicating a single channel for use by CDPD.
Finally, it should be noted that the ability to operate in a "hopping mode" relies on the assumption of low voice channel blocking rates. With the rapid growth of cellular adoption in the early 1990's, this assumed condition is often not met, especially in environments that are attractive to cellular voice usage, such as busy highways and airports. When the voice channel blocking rates rise significantly above about 5% or so, the hopping algorithms are unable to find a vacant channel to run on and the CDPD channel goes into a temporarily quiescent state. When this occurs, mobiles are forced to either find another channel or drop their virtual connection (and any running applications).
The specification team felt that these considerations placed important questions on the validity of the channel hopping approach. However, the field trial did provide some level of proof that the concept is usable. Furthermore, it was an important mechanism that was conceptually well accepted by the cellular carriers in terms of limiting the impact on their voice operations.
As of this writing, some CDPD service providers have trial channel hopping sites. These have operated well in periods of light voice traffic. However, current usage has been low and the true effectiveness of channel hopping will not be tested until CDPD traffic increases. It is worthwhile to note that many of the CDPD service providers have decided to operate with dedicated CDPD channels.