WORKING OF CDPD

To effectively integrate voice and data traffic on the cellular system without degrading the level of service provided to the voice customer, the CDPD network implements a technique called channel hopping. The way this works is that when a CDPD mobile data unit desires to initiate data transmission, it will check for availability of a cellular channel. Once an available channel is located, the data link is established. As long as the assigned cellular channel is not needed for voice communications, the mobile data unit can continue to transmit data packet bursts on it. However, if a cellular voice customer initiates voice communication, it will take priority over the data transmission. At such time, the mobile data unit will be advised by the Mobile Data Base Station (which is the CDPD serving entity in the cell and constantly checks for potential voice communication on the channel) to "hop" to another available channel. In the event that there are no other available channels, then data transmission will be temporarily discontinued. It is important to note that these channel hops are completely transparent to the mobile data user. As far as the user can see, there is only one data stream being used to complete the entire transmission.

CDPD and current cellular voice network are essentially two separate networks that happen to share cellular airspace. During transmission across cellular telephone channels, there are moments when the channel is idle. In fact, industry research indicates that over 30 percent of the air time, even during heavy traffic times is unused. CDPD technology is able to detect and use these otherwise wasted moments, by packaging data in small packets and sending it in short bursts or chunks during the idle time. As a result, the cellular channel operates more efficiently while remaining transparent to the cellular voice network. Thus voice and data transmissions are unaffected. CDPD is based on the same communications protocol as the Internet; so mobile users have access to the broadest range of information.

CDPD accomplishes this amazing feat by transmitting the data in small chunks or packets. Then the chunks can be sent separately on whatever channel is available at the time of transmission. Here is an analogy to help visualize this:

Instead of channels or lines we now have pipes. The pipes can be open in the traditional way. Now no one can use the pipe while you have it open. You send your data down the pipe. Now you wait for a response. Nothing is going through the pipe but still no one else can use it. Majority of your data transmission using this method is idle time that some else could exploit. CDPD is the new way. You have little bundles of data. You find a pipe that is available and you open it. You send your data and shut the pipe again. If that pipe is available when you send more data, you use it. If not, you find a new pipe. You get a response through which ever pipe is unused when the host sends you the data. With CDPD you are assigned an IP address. This is better than a phone number. This allows you to change channels or �pipes� and still send and receive data as if you were directly connected to your host.