CDPD: Cellular Digital Packet Data



Chris Cole

Jerry deCesare

Joe Ford

Michelle Fruman

Ned Montag

Kevin Mulcrone


In today's fast paced, highly mobile and time poor society, few business people would argue with the notion that communications, in all its forms, is essential to being a productive, competitive, company of the nineties. Increasingly, customers are demanding untethered, wireless applications to conduct business. This has been demonstrated by the overwhelming success of cellular telephones during the past ten years which has, for all intents and purposes, made mobile voice communications commonplace. Wireless "data" communications will soon be just as commonplace as the currently popular office and desk top applications are replaced by a mobile professional with a portable and (various) hand held devices. To facilitate this evolution:

The following sections of this paper will focus on exploring the Cellular Digital Packet Data technology which is just one option for achieving mobility with respect to data transmissions.


What is CDPD?

Cellular Digital Packet Data (CDPD) is a technique used for transmitting small chunks of data, commonly referred to as packets, over the cellular network in a reliable manner. It allows users to send and receive data from anywhere in the cellular coverage area at any time, quickly and efficiently. CDPD technology provides extensive, high speed (data can be sent over the Airlink at a rate of 19.2 kilobits per second), high capacity, cost effective data services to mobile users. With this technology, both voice and data can be transmitted over existing cellular channels.

What defines the CDPD Network

By building CDPD as an overlay to the existing cellular infrastructure, and using the same frequencies as cellular voice, carriers are able to minimize the capital expenditures required to offer the service while offering the same coverage area (footprint) their customer base has grown accustomed to. In comparison, it costs approximately $1 million to build out a new cellular cellsite and only about $50,000 to build the CDPD overlay to an existing site.

The CDPD overlay network is made up of a combination of key components that operate together to provision the overall service. These components are described below:

The way these componenets interact with each other can be seen from the graphic depiction below:

How does CDPD work?

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.

How did CDPD originate?

In 1992, a CDPD development consortium was formed with key industry leadership from:

Ameritech Mobile Communications, Inc.

Bell Atlantic Mobile Systems

GTE Mobilnet, Inc.

McCaw Cellular Communications, Inc.

NYNEX Mobile Communications, Inc.

Pactel Cellular

Southwestern Bell Mobile Systems

with the goal of creating a uniform standard for sending data over existing cellular telephone channels. They wanted to find a cost-effective, reliable, flexible, two-way wireless data communications service for mobile professionals which could compete with all digital voice and data services. To this end, in 1993, they published, the Cellular Digital Packet Data System Specifications Release 1.0 guidelines which enabled modem and network providers to build compatible and competitive products which would function on / with the CDPD network.

Since then, the CDPD Forum has emerged as a trade association for wireless data services providers, equipment manufacturers, software developers and information providers who are shaping the technology and supporting the development of the CDPD commercial marketplace. It currently has in excess of 88 members representing all arenas of the wireless data communications industry.

Key Players

A sampling of the key players in this market today is:

Cellular Carriers

Ameritech Cellular

AT&T Wireless Services

Bell Atlantic NYNEX Mobile

GTE Mobilnet (PCSI)

Equipment Manufacturers

Hughes Network Systems

Motorola, Inc.

Pacific Communications Sciences, Inc.

Sierra Wireless, Inc.


CDPD is not better than Circuit Switch for transmitting data, but rather it is different. They both have their place in the cellular wireless solution, and it may take the combination of both services to provide the customer with the optimal solution.

As stated above, the two technologies are different. CDPD is connection-less. It sends each packet intermittently, when there is "space" available. Circuit Switching on the other hand, sends the data over a continuous connection. For this reason, CDPD would be the optimal solution for a customer who is sending information which is both "short" and "bursty", the circuit switch solution would be optimal when sending a large data transmission. Another important difference is that CDPD uses less power than circuit switching. Since the information is sent in short bursts, the device only has to be at high levels of power for short intervals. Additionally, CDPD has a "sleep" mode which allows the device to conserve power when not in use, without logging off of the network. Therefore, a mobile computing device will have longer battery life using the CDPD technology rather than circuit switched connections. Lastly, CDPD uses an encryption technique to prevent an outside source from receiving the transmitted data.

The following two tables demonstrate the effectiveness of CDPD technology as compared to Circuit Switched:

         CDPD               Circuit Switched      

Efficient for short    Efficient for very large   
to large burst         transmissions              

No call set-up or      Call set-up and take down  
take down delays       required                   

Broadcast              Point to point             
capabilities           connections                

One log-on at power    Log -on (call) for every   
up                     transmission               

Power ramping for      Power ramping for entire   
short bursts           connection                 

Airlink Security       Airlink unsecured          

       Application             CDPD         Circuit     

E-Mail                         Good           Fair      

Remote Terminal to Host        Poor           Good      

FAX                            Poor           Good      

Remote Equipment               Good           Fair      

Remote Control of              Good           Fair      

Small File Transfers           Good           Poor      
(1-1000 bytes)                                          

Large File Transfers           Poor           Good      
(5000+ bytes)                                           


How big is the market?

CDPD technology is now available in 39 of the top 50 Metropolitan Serving Areas (MSAs) within the United States and is following internationally as well, with introductions in four cities in Ecuador.

In a continuing effort to provide seamless nationwide CDPD coverage, five interoperability agreements have been signed to date between the leading cellular carriers for CDPD services and more are in the works. According to Jay Sheth, director, business market planning, GTE Wireless Products and Services, "The critical components of a wireless data solution - CDPD coverage, interoperability agreements, the increasing variety of devices and applications - are all moving in the right direction. This is momentum." In second quarter alone, six new CDPD modems and five new CDPD applications have been unveiled.

The following map depicts the CDPD coverage areas which incorporate over 3,000 communities nationwide.

Wireless data is not a future technology. Niche applications have been transmitting data using specialized technology (such as Specialized Mobile Radio - SMR) and are well established. Further, progressive cellular telephone users have been using wireless modems with portable computers for several years. In 1994, there were over 500,000 mobile data users (as estimated by BIS Strategic Decisions), and the projection is to exceed ten times that number by the year 2000.

As can be seen from the pie charts above, initial adopters of CDPD services are expected to be vertical market segments. As the price for the service decreases the product will be more widely used by business workers and consumers, as is demonstrated by the growth in the mobile professional category of users.

Currently, field service applications account for almost one fourth of the mobile data applications in use. The primary feature is automated dispatch and tracking. The primary benefit delivered is decreased time between field technician service calls, resulting in more calls per day. This produces real productivity gains. Finally, giving field technicians the ability to access databases instantly from anywhere means faster, more accurate trouble shooting and problem resolution and that translates to increased customer satisfaction.

Field sales is a cross industry application similar to Field Service. Instant on-site queries for inventory, pricing and pending orders increases field sales revenue because better customer service results in increased purchases. Immediate order entry and confirmation speeds product delivery, increasing cash flow. Overall, less paperwork and faster answers increases sales productivity and again, customer satisfaction.

Transportation and Package Delivery is a proven mobile data application, accounting for about one third of current users. Real time routing reduces costs through better asset utilization.

Retail point of sale facilitates more spontaneous purchases, and by reducing long lines at registers and putting sales people on the floor to help customers.

Telemetry includes remote meter reading for utility companies or for gas and oil production to monitor usage, performance and inventory. Telemetry also covers remote security monitoring and alarm backup.

What applications are being developed for the technology?

Since CDPD is an integrated technology, it can be used in a variety of applications, and is designed to maximize utilization of the existing cellular infrastructure. In broad terms, the applications could fall into the categories of: transaction, batch or broadcast applications and be developed on the basis of specific business solutions for specific industries such as: Transportation / Package Delivery, Security, Public Safety, Retailing, etc.

Transaction Applications

Transaction applications would be designed to provide communication between two network users, and oftentimes involves two way messaging in the form of an inquiry and response.

Some typical transaction applications might include:

Some typical batch applications might include:

Some typical broadcast applications might include:

How is the technology priced?

Pricing is on a per packet basis which means the user pays only for the data that is sent, whereas cellular voice and circuit switched data subscribers are charged for usage of the channel, regardless of whether there is (voice) communications present or not.


Florida Blood Services, located in Tampa, Florida is a customer which uses the technology to access host computer database information from laptops aboard deployed bloodmobiles. Through the use of CDPD technology, they can enter donor information remotely, whereas before they would take all of the donor information while in the field and then have to re-enter all of the information when they returned to the office at the end of the day. As a result, the staff has become more efficient and can therefore stay longer in the field. Additionally, it gives coordinators the ability to remotely "observe" how the blood drive is going.

Yellow Cab, located in San Francisco, California, is a customer who has started outfitting cabs with small terminals which allow the customers to automatically pay by credit card. The terminals use the CDPD technology to validate credit card information and receive payment verification. It takes less than five seconds, which is a significant savings to the traditional approach they had previously employed of validating customer credit card information through radio dispatch.

Lakeland Police Department, located in Lakeland, Florida has deployed CDPD transmission systems coupled with a PacketCluster Patrol software application from Cerulean Technology in patrol cars thereby establishing instant wireless access to federal, state and local motor vehicle and criminal records. Additionally, the technology allows the officers to communicate car to car, or car to dispatch silently through e-mail messages. This information exchange is more secure than the traditional voice communications as the data is encrypted and cannot be picked up by traditional police scanners.

Columbia Gas, has deployed CDPD technology to collect information from remote terminal units which monitor gas flow points and collect data at measurement and regulation stations. As this is a more cost effective way of collecting the data than traditional methods, they are able to do more frequent polling during heating season, and more effectively perform diagnostics for transmission problems.


Mobile data services are here today and provide solid business benefits that are quantifiable. Users of traditional data networks should be aware of the capabilities wireless data communications applications offer, and must consider how these applications can be used effectively as business tools.

For more information on CDPD, please refer to:

The CDPD Report Card at,

GTE Mobilnet at

PCSI's home page at

Unwired Planet at