Wireless technology has exploded in the last several years, and it provides an expanding array of technology choices. Wireless technology refers to the transmission and receipt of information (voice, fax, data) using radio frequency (RF) energy. It can be point-to-point, analogous to telephone or leased-circuit connections, or broadcast, such as commercial television and radio.
However, not all commercial wireless technologies are for government users, and a few are only now becoming viable in this regard. For both voice and data, wireless networks are merely an extension of wired networks from the user perspective. Within the context of government users and applications, what follows are several of the most pervasive and applicable wireless technologies available today and a brief glimpse at some promising technologies for tomorrow.
Specialized Mobile Radio (SMR)
The Federal Communications Commission (FCC) established specialized mobile radio (SMR) services in the mid-1970s by allocating a portion of the 800MHz frequency band for private land mobile-radio systems. SMR networks are operated by commercial system providers. Types of services provided include voice radio networks (including dispatch service), mobile packet data networks, and telephone and paging services. Initially developed around interstate highways and population centers, some of these networks have extended their service to include outlying areas. The main differentiators for SMR networks are transmission speed, transmission protocols, coverage areas and cost.
SMRs specializing in data communications typically use a packet-switching protocol. Data is segmented and routed in discrete data envelopes called "packets," each with its own control information for routing, sequencing and error checking. Packet switching allows a communications channel to be shared by multiple users, each using the circuit only for the time required to transmit a single packet. Users are able to maintain a continuous connection to the network without permanently tying up a channel.
Some advantages and disadvantages of SMR systems are summarized below:
Specialized Mobile Radio
* Cellular-style roaming through out coverage area
* Easy access to public switched telephone network
*Supports short, frequent messages well (e.g., data inquiries, text mes-sages)
*Services designed specifically for data
*Coverage lacking in less-populated areas
*Priority access to regular telephone networks not available for government users
*Potentially significant ongoing costs for usage fees
*Does not support sustained data transfers well (e.g., long reports, images)
Today, over 80 percent of the customers who subscribe to SMR services are in the construction, service or transportation industries. However, over the last 10 years, SMR network providers have increased their marketing efforts to public agencies.
Spread spectrum is a modulation technique that takes an input signal, mixes it with frequency modulated (FM) noise and "spreads" the signal over a broad frequency range. The signal then hops from frequency to frequency at defined intervals, resulting in the spread signal having greater bandwidth than the original
message. Spread-spectrum receivers
have unique user codes to recognize, acquire and "de-spread" a spread signal, thus returning the signal to the original message.
Popularly available spread-spectrum data networks use a mesh topology of shoebox-size radio transceivers (microcell radios), which are mounted to streetlights or utility poles. These microcells are strategically placed every quarter- to half-mile in a checkerboard pattern. Each microcell radio employs multiple-frequency-hopping channels and uses a randomly selected hopping sequence. Frequency hopping allows for a very secure network. These types of networks use digital-packet-switched protocols similar to that employed by SMRs.
Microcells transmit messages to wired access points (WAPs). WAPs convert the data packets into a format for transmission to a wired Internet protocol network backbone. Each WAP and the microcells that report to it can support thousands of subscribers.
The major spread-spectrum data provider is Metricom. Its system transmits data at a raw