Cellular Telemetry Overview
Evolution of Transmission of Data over the Cellular Infrastructure
Similar to the phone system, the cellular infrastructure was originally designed for voice communication. Also similar to the phone system, procedures were developed to use the cellular system for data transmission in addition to voice. Circuit switched data was first to break into the cellular data market. Circuit switched cellular is based on the same concept as a PSTN telephone modem, but uses the wireless cellular network. A connection is established between the two parties and the data is passed. Since this type of service requires a connection to be established and held for the duration of the data transmission, the data costs were based on time and not the amount of data sent.
Next to evolve was the Cellular Digital Packet Data (CDPD) that still uses the voice channel, but sends data packets addressed to the receiver. CDPD technology sends and receives data over the existing cellular infrastructure. Cellular RF voice channels are not in use 100 percent of the time, and CDPD makes use of the unused channel time to send packets of data. The user becomes an active node in the network once the mobile user (in a cellular system the base station is in a fixed location but phones are not in a fixed location and are mobile) is registered on the system. This means that a registered user’s data sent to or from the mobile will be delivered immediately. Since CDPD is a packet-switched network sharing the radio channel, there is no connection time charge. CDPD usage is measured in terms of the amount of data transferred, rather than how long the user is registered on the network. This contrasts with Circuit Switched Data where the customer is billed for connect time whether or not data is being transferred.
Like the telephone system, the CDPD user only needs to buy a modem and establish an account with the provider to use the existing system. The system operator handles maintaining and upgrading the cellular infrastructure while the customer is responsible for the modem. The transmitter must be an active node (connected) to the network to send the data packet, but the receiver is not required to be active to receive the packet. If the receiver is an active node then the data packet is delivered immediately, if not, the system stores the data until the user is connected and the data sent. Today, the cellular infrastructure uses the control channel to send packets of data.
Advantages to Cellular Telemetry
- Low profile, non directional antenna
- Easy to set up and low maintenance costs
- Two-way communications
- Event notification by pager, Internet, other cell phone, etc.
Disadvantages to Cellular Telemetry
- Requires cell phone coverage area
- Monthly service fee (may vary depending on local area cell phone service provider)
- Cell phone service provides may change cell towers or communication protocol, thereby effecting communications to your remote location
- Connection may be dropped during peck cellular transmissions activities
The Cellular Infrastructure
All analog and digital mobiles use a network of base stations and antennas to cover a large area. The area a base station covers is called a cell; the spot where the base station and antennas are located is called a cell site. Cell sizes range from sixth tenths of a mile to thirty miles in radius for (1km to 50km). GSM use much smaller cells, no more than 6 miles (10km) across. A large carrier may use hundreds of cells.
Each cell site's radio base station uses a computerized 800 or 1900 megahertz transceiver with an antenna to provide coverage. Each base station uses carefully chosen frequencies to reduce interference with neighboring cells. Narrowly directed sites cover tunnels, subways and specific roadways. The area served depends on topography, population, and traffic. In some GSM systems, a base station hierarchy exists, with pico cells covering building interiors, microcells covering selected outdoor areas, and macrocells providing more extensive coverage to wider areas.
AMPS: Advanced Mobile Phone service. Conventional cellular service. Mostly analog, with some digital signals providing call setup and management. A first generation service, now only installed in remote regions.
IS-136: D-AMPS 1900. Feature rich cellular. Mostly digital, although backward compatible with analog based AMPS. AT&T uses it for their nationwide cellular network. Uses time division multiple access or TDMA. Incorporates the old standard IS-54, an early second generation system at the time. IS-136 operates at either 800 Mhz or 1900 Mhz. AT&T is moving to a transitional technology whereby three standards, in some form, will work together: IS-136, GSM, and the newer General Packet Radio Service or GPRS. Eventually AT&T will stop using IS-136, replace it with GSM, and eventually replace that with a wideband CDMA system.
IS-95: All digital cellular using CDMA, a spread spectrum technique. Sprint PCS uses this technology. Sometimes called by its trade name of PCS 1900. A second generation or early digital service.
GSM: European cellular come to North America at 1900 Mhz. Fully digital with advanced features. Each mobile has intelligence within the phone, using a smart card. Uses TDMA. Among others, Pacific Bell uses GSM. Will migrate in a few years to a wideband CDMA technology.
iDEN: Proprietary cellular scheme devised by Motorola and used nationwide by NEXTEL. Combines a cell phone with a business radio. TDMA based.
Two primary cellular modems being used in data collection are GSM and CDMA.
GSM is the world’s dominate cellular standard. It was the first European digital standard, developed to establish cellular compatibility throughout Europe. Its success has spread to all parts of the world and over 80 GSM networks are now operational. GSM originally stood for the French name “Groupe Spéciale Mobile”, but was later changed to Global System for Mobile Communications. However the original GSM acronym stuck. GSM operates at 900 and 1800 MHz in many parts of Europe and in England. Works at 1900 MHz in some parts of the United States, which is Time Division Multiple Access (TDMA) based.
GSM telemetry uses a circuit switched technology that allows data to be transmitted over a GSM link at speeds of 9.6 kbps (9600 baud). GSM is a form of multiplexing, which divides the available bandwidth among the different channels.
CDMA (Code Division Multiple Access) is a form of multiplexing (access to the same resource will be given to more than one user), which allows the use of a particular frequency for a number of signals, optimizing the use of available bandwidth. It is a cellular technology that uses spread-spectrum techniques, meaning it spreads the information contained in a particular signal over a much greater bandwidth than the original signal. Therefore, in CDMA technology every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized (ADC) into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.