NAVIGATION, COMMUNICATIONS & POSITIONING Cellular digital packet data introduced to US Gulf

Consortium product enhances speed and reliability of transmission

Edgar Daniels
PetroCom

Cellular digital packet data (CDPD) wireless communications transmission has been introduced to the US Gulf of Mexico. The first successful test of the system was conducted by PetroCom in January of this year.

The CDPD network was announced in 1992 by a consortium of cellular carriers that included Ameritech, Bell Atlantic, GTE Mobilnet, Contel Cellular, McCaw Cellular, NYMEX Mobile Communications, PacTel Cellular, and Southwestern Bell. The purpose of the consortium was to oversee the development of the CDPD specification.

The CDPD network is a fast and efficient digital system that will overlay the existing analog cellular network. It outdates most systems used for data transmission currently in place for speed, dependability, and cost effectiveness.

The system a versatile alternative to existing microwave and radio technologies currently serving supervisory control and data acquisition (SCADA) requirements in the Gulf of Mexico.

CDPD is accessible for SCADA systems and provides wireless data connection for personal computers and local area networks (LAN).

Differences

Four major differences exist between CDPD and wireline or circuit switched data over cellular transmission:

CDPD is a packet switched network. Data is transmitted in discrete chunks or packets of data, rather than as a continuous stream of data. Therefore, CDPD does not require a continuously open "pipe." The modem attached to the sending device (assuming a mobile device) needs only to break the data into packets before sending to the network.
The system is based upon the popular interact protocol (IP). Therefore, both the sending and the receiving device has IP addresses, not telephone numbers.
Since CDPD is an IP-based network, a modem is not required at the receiving device if the receiving device is connected, via telephone line, to the CDPD network.
There are no connect time charges associated with CDPD.

Instead of paying for the connect time used, one pays for only the amount of data transmitted, regardless of the time it takes to transmit that data.

CDPD is an open architecture based upon proven data networking technology. The open architecture:

Employs common, off-the-shelf -working technology;
Is data communications, rather than voice-oriented;
Supports security through sophisticated encryption techniques;
Supports multiple, connectionless protocols such as Internet;
Provides standard interfaces to user applications.

CDPD was designed to work with current data networks. By supporting a wide range of the data network services offered by conventional networks, the system provides seamless connections to host computers.

The system does not require changes to higher network protocols, allowing the user to use the system with few or no changes to current applications. Frequency hopping techniques minimize the impact on cellular voice traffic.

The system provides seamless service while roaming. It can be used as a wireless extension of any existing TCP/IIP network.

Components

Mobile end system (MES): This is typically a SCADA, RTU, laptop, PC, or LAN terminal with a CDPD transceiver/modem attached. In many cases, the MES can be a non-personal device, such as a telemetry system. The MES can be fixed (alarms) or mobile (dispatch).
Mobile data base station (MDBS): This station is located at each cell site and is primarily responsible for radio frequency management. For example, if data is transmitting while the MES is traveling, the MES may travel through several cells. The MDBS handles the channel hand-offs and informs the MES of frequency changes. From the network protocol transport perspective, the MDBS acts as a bridge.
Mobile data intermediate system (MDIS): This component is located in the mobile telephone switching office of the cellular carrier and is responsible for managing multiple MDBS units. The MDIS validates the MES on the network. From the network transport protocol perspective, the MDIS is similar to a router with additional address mobility management.
Intermediate system (IS): This component sends packets to the outside world. Hosts connected to the IS think that they are receiving data from an IP network.
Fixed end system (FES): This component is the customer's host or server computer and is usually the final destination of a message sent from an MES. However, some applications will involve two mobile computers communicating with one another.

Each unit of user data (a packet) is a self-contained entity. The network routes and handles each packet independently

of every other packet in the network. In the event that the network loses a packet, the end-to-end connections are able to recover the packet, just as they do in current wired networks. The maximum packet size is 2,000 characters (2 Kbytes).