Cellular Digital Packet Data (CDPD)

 

Cellular Digital Packet Data (CDPD) systems offer one of the most advanced means of wireless data transmission technology. Generally used as a tool for business, CDPD holds promises for improving law enforcement communications and operations. As technologies improve, CDPD may represent a major step toward making our nation a wireless information society. While CDPD technology is more complex than most of us care to understand.

In this age of information, no one needs to be reminded of speed but also accuracy in the storage, retrieval and transmission of data. The CDPD network is a little one year old and already is proving to be a hot digital enhancement to the existing phone network. CDPD transmits digital packet data at 19.2 Kbps, using idle times between cellular voice calls on the cellular telephone network.

CDPD technology represents a way for law enforcement agencies to improve how they manage their communications and information systems. For over a decade, agencies around the world have been experimenting with placing Mobile Data Terminals(MDT) in their vehicles to enhance officer safety and efficiency.
Early MDT's transmits their information using radio modems. In this case data could be lost in transmission during bad weather or when mobile units are not properly located in relation to transmission towers. More recently MDT's have transmitted data using analog cellular telephone modems. This shift represented an improvement in mobile data communications, but systems still had flaw which limited their utility.
Since the mid-1990, computer manufacturers and the telecommunication industry have been experimenting with the use of digital cellular telecommunications as a wireless means to transmit data. The result of their effort is CDPD systems. These systems allow users to transmit data with a higher degree of accuracy, few service interruptions, and strong security. In addition CDPD technology represents a way for law enforcement agencies to improve how they manage their communications and information systems. This results in the capacity for mobile users to enjoy almost instantaneous access to information.

1.    WHAT IS CDPD?      

CDPD is a specification for supporting wireless access to the Internet and other public packet-switched networks. Data transmitted on the CDPD systems travel several times faster than data send using analog networks.

Cellular telephones and modem providers that offer CDPD support make it possible for mobile users to get access to the Internet at up to 19.2 Kbps. Because CDPD is an open specification that remains to the layered structure of the Open Systems Interconnection (OSI) model, it has the ability to be extended in the future. CDPD supports both the Internet's Connectionless Network Protocol (CLNP).
CDPD also supports IP multicast (one-to-many) service. With multicast, a company can periodically broadcast updates to sales and service people on the road or a news subscription service can transmit its issues as they are published. It will also support the next level of IP, IPV6. With CDPD we can assigned our very own address. With this address, we can virtually always connected to our host without having to keep a constant connection.

There are currently two methods for sending data over cellular networks: cellular digital packet data (CDPD) and cellular switched-circuit data (CSCD). Each has distinct advantages depending on the type of application, amount of data to send or receive and geographic coverage needs.

CDPD is currently available to roughly 50 percent of the population base. Two methods to transmit data are used, depending upon the service provider's network architecture. Some providers have radio channels dedicated to data transmission installed at existing voice cellular sites. Others use voice cellular channels and interleave data messages within the unused portion of voice radio signals. To use a CDPD data service, users require a laptop computer, a connector cable and a CDPD radio modem. Radio modems come in a PC-card format or connect to the user device with a serial cable.

Regardless of the method used, messages are broken up into discrete packets of data and transmitted continuously over the network. Messages are then "reassembled" into the original message at the receiving device. This technology supports roaming and is especially attractive for multicast (e.g., one-to-many) service, allowing updates to be periodically broadcast to all users. Users log on once per day to register on the network. Messages and transmissions automatically locate them. Major CDPD providers generally have roaming agreements to allow users to access the service when outside their home coverage area. For the mobile users, CDPD's support for packet switching means that a persistent link isn’t needed. The same broadcast channel can be shared among a number of users at the same time. The user's recognizes the packets intended for its user. As data such as e-mail arrives, it is forwarded immediately to the user without a circuit connection having to be established. There is circuit switched version, called CS CDPD that can be used where traffic is expected to be heavy enough to warrant a dedicated connection.  As a tool for transmitting data CDPD utilizes digital networks. Placing data, conversions, photographs, and multimedia into digital form and transmitting the information through a network with a large bandwidth permits more information to be sent more quickly with greater clarity. Thus, data send using CDPD is received in a quick, secure, and accurate fashion. Data send using CDPD systems is less likely to be lost between senders and receivers due to the position of mobile units, weather conditions,

or other anomalies.

CDPD is an overlay to the existing cellular network, which enables users to transmit packets of data over the cellular network using a portable computing device and a CDPD modem. CDPD offers a high-speed, high-capacity, low-cost system with the greatest possible coverage. Additionally data is encrypted for security. CDPD air link

Transmission has a 19,200 bps raw data rate.

The Raven is a rugged, full duplex Cellular Digital Packet Data (CDPD) modem that provides wireless transport capabilities for fixed and mobile applications. The Raven is an efficient and secures wireless packet data technology that is ideal for un-tethered applications. The CDPD Raven is ideally suited for all fixed and mobile Industrial and Commercial Applications, including Telemetry, SCADA, Public Safety, Dispatch, Field Service, Financial Transaction Processing, and Security. The Raven's embedded TCP/IP stack enables virtually any type of remote device to access

the CDPD network.

The Raven is currently installed with many different types of Remote Terminal Units (RTUs), which perform remote metering, and monitoring functions in the oil, gas, and water industries. The CDPD Raven can be a low-cost replacement for existing landline, private radio and circuit-switched cellular installations.

2.    KEY CDPD CHARACTERISTICS

2.1                         AVAILABILITY

 CDPD is available in more than half of the geographic United States and in 30 international markets. CDPD carriers use “roaming” agreements to extend their regional services to other areas outside their region. Therefore, as long as CDPD services are available, CDPD subscribers can use CDPD as they cross service areas or if they are on travel to different parts of the US.

2.2                         COVERAGE

Carriers will typically deploy networks to provide services in areas with high population density, such as metropolitan areas and along roadways. Consequently, carriers may not provide full coverage in rural areas or beyond these major roadways. This is a key consideration for users that expect and need contiguous service off the beaten path. Whether in the carrier’s region or when roaming, CDPD users will often experience coverage gaps similar to cellular voice services. This often occurs in less populated areas or away from major roads. Coverage gaps can be caused by environment or buildings that interfere with the signal. They are also due to “dead spots” within the region, where the carrier’s signal is too weak or non-existent. Users should match operational requirements to CDPD coverage considerations to ensure the service is available when and where they need it.

2.3                         RELIABILITY

Users of commercial systems share the airwaves and compete for capacity with one another. Therefore, users may experience jamming if there is more demand than network capacity. Overcrowding within CDPD networks will cause delays in setting up a connection and transmitting information. The possibility and effect of overcrowding depends, in part, on the type of CDPD network implemented. The two network types are channel hopping networks and dedicated channel networks.CDPD channel hopping takes advantage of capacity unused by cellular voice subscribers to transmit information. This means that the capacity available for CDPD is directly associated with the level of use by cellular voice calls. During peak periods or emergencies when voice traffic on cellular networks tends to increase significantly, the possibility of CDPD users experiencing overcrowding may increase significantly causing call set-up and transmission delays. The other configuration uses dedicated channel networks. This technique dedicates capacity on a portion of the cellular network that is available for CDPD use only. Therefore, CDPD capacity does not vary as cellular voice calls increase or decrease. CDPD transmissions do not compete with cellular voice calls. To the CDPD user, this is an important improvement because CDPD performance is not affected when the cellular voice system is congested. Users must still compete with other CDPD users for call setup and channel capacity. Cellular carriers will often use channel hopping when they first introduce CDPD service, in smaller cities, or where CDPD usage is low. They will implement dedicated channels in larger cities where the service has been offered for a number of years and demand is high. These network implementation strategies have significant implications for reliability. Users should exercise caution to ensure that service reliability will not affect critical operational requirements.

2.4                         TRANSMISSION SPEED

CDPD can provide a maximum link data rate of 19.2 kilobits per second (kbps). The actual user transmission speeds are less typically from 10 to 12 kbps when the application overhead is Included . The exact transmission speed varies among vendors and is affected by the level of traffic on the network. This can mean a 10 kilobyte file (which contains approximately 1500 words, or 3 text pages) will take up to 15 seconds to transmit. Times to send files of different sizes are provided in the fig 3.4

2.5                         CALL SET-UP TIMES

Call set-up time is the time it takes to begin transmitting information after the user pushes the transmit button. CDPD call set-up times range from less than 1 second to 3 or 4 seconds. Differences in call set-up times may depend on the user terminal processing speed and the users’ ability to access the network from the user’s point of view, the overall time to transmit data is equal to the call set-up time plus the transmission duration. Using the example from above, the overall time needed to transmit a 10 kilobyte file is 18 seconds (3 seconds for set up and 15 seconds for transmission).

2.6                         PRIVACY AND SECURITY

CDPD privacy features include automatic identification and authentication, to limit unauthorized, fake access and data interception. Channel-hopping transmissions are more difficult to intercept than dedicated channel transmissions, since they often change channels between transmissions of packets. However, intercepting channel-hopped communications is possible. CDPD does use a standard form of encryption to protect information sent over the air. For users with more robust security requirements, end-to-end encryption schemes should be considered. Users should also consider the security practices of the service provider, with the respect to physical, operational, and information security.

3.    FEATURES

• Data network overlay on analog cellular telephone system
• Uses same 30 KHz channels @ 800 MHz; potentially same coverage as cellular system
• Schedule data packets to unused voice channels

·   CDPD uses strong encryption between the modem and its service provider, so you can feel reasonably confident that no one will be able to intercept your employees' signals and read confidential data.

·   CDPD supports the full range of TCP/IP applications, including telnet, HTTP, FTP.                                                                                                                                    

• Provides IP packet service

1)      19.2 kbps raw data transfer rate.

2)      Full duplex transreceiver.

3)      600 mw transmit power.

4)      Integrated TCP/IP protocol stack.

5)      Compact size modem.

6)      strong aluminum case.

7)      LEDs show status of CDPD operation.