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 speed of 100 kilobits per second (Kbps), with a throughput averaging 28.8Kbps. Planned system upgrades will increase throughput up to 40Kbps using existing radio modems. Metricom also plans to offer service with throughput up to 128Kbps This service will use spectrum in the 2.3GHz
range and will require a radio modem upgrade.
Some advantages and disadvantages of spread-spectrum systems are summarized below:
Spread Spectrum Systems
* High bandwidth
* Secure communications
* Low initial cost
* Easy for provider to expand coverage
* Limited availability for wide area
* Must be quasi-stationary to use
* Recurring monthly costs
By most estimates, more than 90
percent of traffic on the U.S. cellular telephone network is voice, but data transmissions are increasing rapidly. In 1995, there were approximately 1 million wireless-data users, with the market projected to grow to nearly 10 million users by the year 2000.
While its popularity and coverage has expanded since Advanced Mobile Phone Service (AMPS) was introduced in the 1960s, analog cellular radio is still the base technology used for cellular service today. 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.
CDPD supports TCP/IP and is most appropriate for short bursts of data,
such as e-mail, credit-card authorization or database queries. Currently, CDPD provides data rates of 19.2Kbps with throughput averaging 14.4Kbps. Next-generation systems will allow data rates
Nationwide, approximately 45 percent of CDPD agencies are in public safety. Many of these users are using the service as an adjunct to their existing private mobile-data systems. Typical applications include database inquiry, automated field reporting and unit-to-unit messaging. Although there are only limited examples of use in public safety, CDPD does provide the capability for electronic dispatch of units.
Major CDPD providers generally have roaming agreements to allow users to access the service when outside their home coverage area.
Some advantages and disadvantages of CDPD systems are summarized below:
Cellular Digital Packet Data
* Supports short, frequent messages well
* Inexpensive end-user equipment
* Available today
* Protocol provides easy access to the Internet
* Transparent roaming available
* Moderate data rates (19.2Kbps)
* Secure (data encryption provided by carrier)
* Service in major population areas
* System designed for data
* Not yet fully deployed
* Coverage not available in less-populated areas
* Priority access not available for government users
* Potentially significant ongoing costs
* Newer technology
* Does not support sustained data transfers well
Cellular switched-circuit data is today's most popular and widely available option for wireless data transfer. It creates a dedicated connection or circuit over the analog cellular network only for the duration of the call, in contrast to the dedicated connection provided by a packet-switched network. Transfer rates are up to 14.4Kbps. Transferring data with CSCD requires a laptop computer, data-capable cellular telephone, a connector cable and a cellular modem (typically a PC card).
As with voice service, charges are determined by the duration of calls, making CSCD cost-effective for larger data transmissions with file transfer, fax and e-mail applications. Cellular switched-circuit data is a good approach for session-based interactive transactions, such as logging onto a host
application or accessing a private intranet. CSCD networks are low security but can be improved through user-provided encryption applications. CSCD is compatible with most off-the-shelf modem software. Since this service is available wherever analog cellular service is available, there is a variety of service providers.
Some advantages and disadvantages of CSCD systems are summarized below:
Cellular Switched-Circuit Data (CSCD)
* Inexpensive and easy-to-use user devices
* Transparent roaming
* Service in major population areas (covers 90 percent to 95 percent of population base)
* Supports sustained data transfers well
* Voice and data capabilities
* Extensive applications software
* Good developer support
*Dial-up connection required for each data message
*Does not support short, frequent messages well
*Priority access not available to government users
*Potentially significant ongoing costs
*Reliability (transmissions can drop when moving between calls)
*Roaming can be expensive
*Security (data encryption is an add-on)
Personal Communications Systems
Personal communications systems (PCS) are the next generation of terrestrial-based commercial wireless communications, providing inexpensive voice and data services. PCS include a broad range of telecommunications services intended to provide subscribers with enhanced features and wireless access to the public switched network. "One person, one number" has become the familiar motto of PCS in recent years.
The Personal Communications Indu-stry Association predicts that there will be more than 167 million subscribers to PCS services by 2003. To accommodate this expected demand, the FCC has allocated both narrowband (901-902MHz, 930-931MHz, 940-941MHz) and broadband (1850-1990MHz) frequency spectra for PCS services. Blocks of spectra were auctioned by the FCC between 1995 and 1997.
PCS design is similar to cellular design, but PCS use all-digital technology. PCS systems use a large number of low-power transmission sites to support high levels of data throughput.
Examples of enhanced services available from PCS providers include voice mail; call hold, forwarding, waiting, and three-way calling; paging; text messaging; distinctive ringing; fraud control (through authentication and encryption); and better reception than analog cellular within the coverage area. Current data communication capability is provided via a dial-up connection, similar to switched-circuit cellular.
The jury is still out regarding the effectiveness of PCS for wide-area use. While existing PCS services rely on cellular-type architectures, combinations of PCS services with satellite and other technologies may provide a greater functionality in the future. However, since providers of PCS services have designed their systems using competing technologies, wide-area roaming may be difficult.
Some advantages and disadvantages of PCS are summarized below:
Personal Communications Systems (PCS)
* Telephone interconnect/easy access to Public Switched-Telephone Network (PSTN), or "regular" telephony
* Support for high-volume data applications
* Increased competition, lower prices
* Difficult to eavesdrop
* Advanced digital features
* Low-weight, multipurpose, low-cost devices
* System design allows for reduced power consumption, longer battery life
*Low power requiring numerous sites for coverage (limited initial coverage)
*Priority access not available for government users
*Competing technologies inhibit roaming
*Potentially high recurring costs
Satellites function as radio repeaters in the sky. Radio signals are beamed to the satellite from an earth station via an uplink. At the satellite, the signal is filtered, converted and retransmitted via a downlink to ground-station or mobile receivers. Satellites can receive and retransmit thousands of signals simultaneously, from simple digital data to the most complex television programming. Satellite systems provide effective and ubiquitous mobile communications for users requiring a large coverage area (e.g., transportation, military, exploration, and maintenance). Recently, satellite companies have begun to show a higher level of interest in the public-sector market.
Two main types of satellite systems offer communications services applicable to government users: geosynchronous earth orbit (GEO) and low earth orbit (LEO) satellites. Satellite system providers use both circuit-switched and packet-switched technologies.
GEOs orbit the earth at an altitude of approximately 22,300 miles, traveling at the same angular speed as the earth rotates on its own axis. Thus, GEOs appear to remain "stationary" relative to a reference point on the earth. A single GEO can "see" approximately 40 percent of the Earth's surface. Three such satellites, spaced at equal intervals, can provide global coverage.
Due to a GEO satellite's distance from Earth, reception of the repeated signal can be delayed as much as 12 to 25 milliseconds for each outbound and inbound transmission. Data throughput rates range from 4.8Kbps to 9.6Kbps. In addition, these large distances cause GEO transmissions to require more power than closer terrestrial or LEO communications. This requirement has made it difficult to produce convenient hand-held radios that are able to access GEO satellites. GEO service vendors have historically focused on video, data broadcasting and long-haul transportation industries.
LEO satellites do not remain stationary above the Earth. They orbit 300 to 300-900 miles above the Earth's surface at speeds of 16,500 miles per hour. A LEO system is made up of satellites all traveling at the same speed and the same altitude. Satellites are positioned relative to each other such that each covers a portion of the Earth's surface. When the satellites travel around the world, their coverage area moves with them. As one satellite starts to leave a certain geographic area, it "hands off" communications to the next satellite as it enters the area, maintaining continuous coverage. A network control system interconnects the LEO satellites and links individual satellites. Since LEOs are closer to the Earth's surface, less power is required to send a message to them. User devices can be smaller and less sophisticated than those designed for use with GEO systems.
Significant efforts are currently under way to develop new LEO systems, with the earliest service anticipated for later this year. LEO vendors include:
* Globalstar -- A joint effort between Loral and Qualcomm, offering narrowband, dual-mode telephones with paging, low-speed data and position-location services.
* ECCO -- Designed by Constellation Communications, ECCO will offer
narrowband, dual-mode telephones with paging, low-speed data and fax services.
* Iridium -- This LEO service provider is from Motorola. Iridium will offer
narrowband, dual-mode telephones with paging, low-speed data, and fax
*Teledesic -- Co-founded by Microsoft's Bill Gates and Craig McCaw of McCaw Communications. It will offer broadband multimedia, videoconferencing and Internet services.
The tables below summarize some of the advantages and disadvantages of geosynchronous and low earth orbit satellites:
Geosynchronous Earth Orbit (GEO) Satellites
*Access to PSTN
*Many advanced digital features
*Accessibility from remote areas
*Ability to support voice and data
*Reduced coverage in "urban canyons"/line-of-sight
*Limited range of user equipment
*Low data-transmission rates
*High user equipment costs
*High, recurring costs
*Significant propagation delay
*Single point of failure
*Unproven for public safety and local government use
Low Earth Orbit (LEO) Satellites
*Advanced digital features
*Little propagation delay
*Access to PSTN
*Ability to support voice and data
*Entire system must be in place before operable
*Requires enormous infrastructure
*Limited range of user equipment
*Unproven; features and capabilities unclear
*Potentially high recurring and equipment costs
Evaluating wireless technology can be complicated and time-consuming. However, focusing on users' functional needs can enable more effective comparison of alternatives. Some specific areas to consider for wireless technology include:
* Application Integration -- Does the technology allow a smooth interface with current and planned applications? Would it facilitate software modifications if changes in system protocol or operational requirements occur?
* Performance -- Software and hardware components of the network must be responsive to user needs. Sensitivity to loading requirements, peak user demand and the ability to transfer information in the time frame required are critical parameters.
* Availability/Reliability -- On an annual basis, what is the percentage of time that the network is available for processing user requests? How does the availability change during emergencies or other periods of peak usage?
* Security -- Careful consideration must be given to the ability of system managers to control access to and use of the network on a user-by-user basis
* User Interface/Device -- The types of devices available for use on the network, their functionality -- including features, indications, ergonomic capabilities,
vendor sources, etc. -- should be compared to end-user requirements.
* Coverage -- The percentage of the service area over which the network can be used, usually defined by geographic areas with associated reliabilities for accessing the system. How does the coverage area compare to user-defined operating areas?
Gregory Walker is a senior consultant with The Warner Group, a Woodland Hills, Calif.-based management consulting firm specializing in the public sector. He has significant experience evaluating wireless voice and data systems and can be reached at (818) 710-8855.
November Table of Contents