System Architecture

Though "open system" standards are in the works, vendors of RFDC systems within the AIDC marketplace currently offer proprietary networks using their own dedicated equipment. However, all radio frequency local-area networks (LANs) follow roughly the same architecture.

The most common approach to radio frequency implementation uses wireless terminals (either handheld or vehicle-mounted) that include a radio transmitter/receiver, keyboard, LCD display, and usually a barcode scanner. The terminals communicate with the master radio transceiver, a.k.a. base station or wireless gateway, which receives and routes messages from the individual terminals to the radio frequency network controller, and also routes messages such as instructions and exceptions from the host system to the terminals.

The base station receives data from the terminals by means of polling or contention protocols. With polling, each radio frequency terminal is polled or queried in a specific sequence. In a contention system, each terminal transmits on its own accord; if the channel is busy, the terminal retransmits after a randomly set delay. The polling method is preferred for systems with few terminals and approximately uniform transaction rates. Systems with many terminals and high transaction rates achieve more consistent response rates with the contention method.

The radio frequency network controller (which may be a PC) acts as the interface between the radio network and the host computer and/or data collection program. The controller/PC may interface via wire with an autonomous host, or the PC may itself be the host with the data collection program/database resident on it.

Because of the possibility of electromagnetic and other radio signal interference within a facility, the placement of the antenna and base station(s) as well as other system components is critical to system performance; a professional site survey is highly recommended. Depending upon the size and layout of a facility, repeaters and/or multiple base stations may be required to assure complete radio coverage.

Hardware
Wireless data communications by radio frequency usually requires four key pieces of hardware:
· A host computer - the system component that runs the application program and maintains the database.
· A network controller - hardware that acts as a gateway for communications between the host computer and other components of the RF System. Depending on the host, a network controller is not always needed.
· Base station - a radio transceiver unit that passes communications between the network controller and the Radio Frequency Data Communication terminals.
· Radio Frequency Data Communication terminals - hand-held or vehicle-mounted units that send and receive messages by radio frequency. Data entry prompt and/or responses and instructions from the host are displayed on the screen of the unit.

Host Connectivity
Terminal emulation is the most common means of host connectivity, where emulation software, running on both the terminals and the network controller, makes the radio frequency terminals appear to be standard terminals acceptable to the host, resulting in a quick and easy host connection.

However, the latest generation of radio frequency devices uses a client/server approach, where each individual terminal uses a network driver, giving access to the network so it can communicate directly with the host or any other application. The client/server architecture provides extremely fast response rates, because the data collection program doesn't have to run on the host as it does with terminal emulation (with client/server RFDC, the host is only used for look-ups and updates, considerably reducing transaction traffic).

Area Networks - LAN and WAN
Where a radio frequency local area network (LAN) may cover a 50,000 to 1,000,000 square-foot facility, a radio frequency wide area network (WAN) can cover entire geographical regions. With radio frequency WANs, network operations are managed by vendors (ARDIS and RAM Mobile Data are two operators offering nationally networked RFDC support), and users rent air time for monthly fees. Terminals require special WAN radio modems, and data is transmitted via satellite networks with coverage in nearly all of North America.

Transmission Options
Historically, commercial radio frequency applications have relied on narrow band transmission, where signals are broadcast over a narrow portion of the radio spectrum at specific frequencies which are licensed to users by the Federal Communications Commission (FCC). Most licensed narrow band systems in the U.S. operate at 25 KHz bandwidth, in the 450 MHz to 470 MHz frequency range at a speed of up to 19.2K bit per second (bps). Narrow band has been and continues to be the transmission method of choice for large, single-site industrial applications because of its consistent system performance and coverage combined with protection from interference offered by FCC licensing.

Within recent years another transmission option, spread spectrum, has experienced rapid growth within commercial RFDC-based applications. Spread spectrum transmission offers about one-quarter the coverage (compared with narrow band transmission) per base station, but on the other hand, it offers faster baud rates of 2 Mbps. Spread spectrum systems also offer more flexibility for small, multi-site, and global applications, because they don't require FCC licensing.

Spread spectrum radio technology was originally developed during World War II for covert radio transmissions. It provides immunity to interference by broadcasting redundant data across a broad range of frequencies within the unlicensed 902 MHz to 928 MHz, 2.4 GHz to 2.48 GHz, or 5.725 GHz to 5.850 GHz bands; or by continually changing frequencies within those bands.
Spread spectrum transmission development in the AIDC industry began with the 900 MHz band, using a technique called direct-sequence modulation to spread the signal over a range of channels. Each data bit is transmitted over at least 10 different channels for redundancy, and the spread spectrum receiver decodes the binary data and reconstructs it into the original bit stream. Transmitting over an extended bandwidth results in quicker data throughput, but the tradeoff is diminished range.

Many spread spectrum networks, particularly those operating in the internationally sanctioned 2.4 GHz range, use frequency hopping rather than direct-sequence techniques to spread the signal over the radio spectrum. With frequency hopping, the radio transmits short bursts of data, skipping from frequency to frequency. Frequency hopping can overcome interference better than direct sequencing, and it also offers greater range (though still not the degree of range offered by narrow band transmission). This is because direct sequencing uses available power to spread the signal very thinly over multiple channels, resulting in a wider signal with less peak power. In contrast, the short signal bursts transmitted in frequency hopping have higher peak power, and therefore greater range.

Standards/Regulation
When it comes to protection from interference, users of FCC-licensed narrow band systems have a regulatory edge and an avenue of redress if radio interference does occur. Spread spectrum technology is based upon interference avoidance techniques, but if some outside transmission does interfere, users can only switch to another frequency setting. These problems are more likely to occur within the heavily-trafficked 900 MHz band than within the 2.4 GHz band. Interoperability among system components is the long-sought goal of the emerging 2.4 GHz marketplace. Since 1990, the 802.11 committee of the Institute of Electrical and Electronics Engineers (IEEE) which includes representatives from the electronics, communications, and semiconductor industries, has been working on a standard for interoperability among compliant manufacturers of both direct-sequence and frequency hopping spread spectrum RFDC systems. In July 1997 the 802.11 committee ratified the standard and it is anticipated that standardization of 2.4 GHz radio technology should give users far greater flexibility in hardware choices.

Reprinted with permission from AIM, Inc.
www.aimglobal.org

Back to Index