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Sending Data to a Mobile Unit

On the return path, when data is sent to the mobile unit, the CDPD network must be prepared to deal with mobile units that are actively mobile — moving in a car, for example. In this case, it is likely that the mobile unit would move from one serving backbone router to another during the session. The CDPD network accommodates the roaming mobile unit by always sending its data to its home backbone router. The home backbone router always advertises itself as the destination router for the mobile units it serves.

Redirect Procedure. The home backbone router knows the current location of the mobile unit because of the registration procedure. When sending information to a mobile unit, the home backbone router encapsulates it into frames using the CLNP protocol and sends them to the address of the current serving backbone router. Once the data arrives at the serving backbone router, it is de-encapsulated into its original form to be sent to the mobile unit. This method of handling data trans-missions at the home backbone router is called the redirect procedure (see Exhibit 5).

The redirect procedure takes advantage of the identification done during the registration procedure. The registration procedure serves two purposes:

  To authenticate the user’s access rights.
  To identify the current location of the user.

The redirect procedure uses this information to minimize network overhead. The alternative, in which all the backbone routers would update their global routing tables whenever a mobile unit moved, would saturate the network with overhead traffic. The CDPD network permits full mobility, but without imposing an undue burden on the network infrastructure.

IS THERE A MASS MARKET FOR CDPD?

Industry analysts estimate that the wireless data market could be worth $10 billion by the year 2000, providing service to about 13 million mobile data workers. Bell Atlantic Mobile, an early provider of CDPD-based services, predicts that as much as a fifth of its cellular revenues could come from data services by the end of the decade.

The eventual availability of low-cost CDPD modems does not guarantee a mass market for CDPD. For this to happen, commonly used applications must be adapted to the technology APIs. APIs are required to optimize new and existing applications for use over relatively low-bandwidth wireless links with their high overhead and delay. After overhead is taken into account, the wireless CDPD link will top out at 14.4K b/s. The average throughput falls between 9K b/s and 12K b/s, depending on the number of errors and retransmissions.


Exhibit 5.  The Redirect Procedure.

Although CDPD is ideal for vertical niche markets such as fleet dispatch and field service, the more popular applications include E-mail, facsimile, and RDB access. Several toolkits are available to give new and existing applications the capability to run over CDPD networks.

To improve application performance over low-bandwidth wireless links, middleware that uses intelligent agents is now available that allows laptop users to query a corporate database using a software agent at the corporate site. If the user does not want to wait for a response to a query, or the connection is lost, the agent collects the information and sends it over the wireless network when the user makes the next connection.

CONCLUSION

Mobile users who are already committed to wireless data services are among the early users of CDPD service. As the price of CDPD modems fall, coverage increases, and more applications become optimized for CDPD, the technology will have even wider appeal.

Cellular digital packet data networks are appealing because they offer seamless nationwide availability; work with the vast installed base of computers, applications, and data networks; and make use of existing private and public network infrastructures, encompassing all products and user equipment. The ultimate success of CDPD is, of course, closely tied to industry efforts to standardize its implementation. A universal standard for cellular packet data would facilitate terminal capability, allow users to roam between service areas, and simplify the introduction of wireless data services.


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