Wireless Technologies

There are two basic types of wireless services: fixed, which delivers services to a fixed location, such as a building, and mobile, which reaches a moving target. Fixed wireless can be further subdivided into terrestrial and satellite services, though one mobile satellite service is also discussed. In terms of the ability to transmit data reliably, fixed wireless is far ahead of mobile.

The figure shows the radio frequency (RF) spectrum and the areas that are used for transmitting data.

Fixed Terrestrial Wireless
Some of these technologies have been around for awhile and others are just emerging. Both microwave and infrared/laser frequencies have been used in point-to-point topologies for several years. These technologies require a line-of-sight between the two endpoints of the wireless link and, depending on the specific service, may need extremely precise alignment, as well.

Earlier fixed wireless technologies exploited the lower end of the radio frequency spectrum for data communications because, when the power is boosted enough, these lower-frequency signals can travel a long distance and can even penetrate buildings, as does television.

Following are descriptions of some of the many emerging and existing fixed wireless technologies:

Local Multipoint Distribution Service (LMDS): LMDS is a new fixed wireless technology that shows considerable promise. LMDS uses microwave (actually, milimiter-wave) signals to send voice, video, and data at 1 Gbps or more over frequency bands ranging from 27.5 GHz to 31 GHz. Transmission is within small cells roughly three miles in diameter. Hewlett-Packard has predicted throughputs as fast as 1.5 Gbps downstream, with upstream rates as high as 200 Mbps.

LMDS can provide multi-services because a single device can send and receive multiple signals simultaneously, each with its own carrier frequency. LMDS can be engineered to provide 99.999 percent availability. And, analysts expect LMDS services to cost 50 to 75 percent less than traditional leased-line services.

The FCC first auctioned LMDS licenses in March 1998. The FCC excluded ILECs and cable television companies from participating in order to promote entrepreneurial opportunities. The FCC was to reopen bidding on the remaining licenses in April 1999. 1999 is predicted to be the year when license holders detail their service plans and begin trials.

How it Works: LMDS uses the "very high frequency" Ka band, but over a much shorter distance than the early wireless offerings. (The Ka band is above the UHF band and below the far infrared region.)

Fortunately, because of the combination of the very high frequencies and the small cell sizes (cells are spaced three to six miles apart), LMDS signals can be transmitted and received with only a small, six-inch square antenna, but it must be in line-of-sight of a cell. The graphic illustrates this.

Repeaters or reflectors can be used to spread a strong signal into shadow areas, thereby increasing coverage. Adding overlapping cells can bring total coverage to almost 85 percent of all homes in an area.

Cellsize is also influenced by the amount of local rainfall. Because LMDS signals are microwaves, they are attenuated by water (called "rain fade"). Leaves, trees, and branches can also cause signal loss. Because it uses frequency modulation (FM) rather than the amplitude modulation (AM) used by most cable television providers, LMDS can generate signals with 10 times higher signal quality than cable television.

Multipoint Multichannel distribution Service (MMDS):MMDS networks operate in the 2.5 to 2.7 GHz range. Unlike LMDS, it is a fixed wireless technology that is available today.

Licenses were distributed about 15 years ago to be used to offer asymmetric wireless cable television services. That never happened and the FCC has now authorized two-way traffic in the MMDS range in a handful of cities, with approval expected this year for about 500 additional cities.

Winstar and Teligent: Two companies, Winstar and Teligent, are currently offering wireless services in the 24-GHz and 38-GHz bands today using proprietary technologies. Teligent also plans to offer services in the LMDS range. Both of these approaches use point-to-multipoint toplogies.

Wavespan: Wavespan of Mountain View, CA sells a wireless Ethernet bridge.

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Fixed Satellite Wireless
High-speed microwave using geosynchrous satellites has been available for quite some time. Now, however, there are many new choices appearing. Some of the newer satellite wireless services include the following:

DirectPC: This offering from the DirectTV satellite television providers uses a standard telephone line and modem for the uplink, a 12 Mbps connection between the DirectPC network operations center and the satellite, and a connection of up to 400 Kbps to deliver data from the satellite to the user. Television service can use the same dish.Two-way service will be offered beginning sometime in 2001.

Starband: A two-way data service that is available now from Starband, owned by Gilat Satellite Networks. Television service from The Dish can be received using the same dish.

The figure illustrates how a one-way satellite service works.

Spaceway: Hughes plans to offer faster speeds than land-based networks and at a cost 20-30% less. The initial U.S. network will use two geosynchronous satellites operating in the Ka-band with an in-orbit spare. Once this is operational, Hughes will add low-Earth orbit (LEO) satellites to expand the system's reach worldwide. The low orbits avoid the long signal delay normally experience with geosyncronous satellites (up to 5 seconds round-trip).

Astrolink: By mid-2002, Lockheed Martin plans to use four (later nine) geosynchronous satellites operating in the Ka-band to deliver data rates from 416 Kbps (65-cm dish) to 10.4 Mbps (1.8-m dish). Up to 100 gateways will connect Astrolink to terrestrial networks worldwide.

Teledesic: Craig McCaw plans to use LEO satellites to create a global, broadband "Internet-in-the-sky" service. It will use small, low-power terminals and antennas which will mount flat on a rooftop and provide two-way connections with up to 64Mbps on the downlink and up to 2 Mbps on the uplink. Higher-speed terminals will offer upwards of 64 Mbps each way. The service will operate in a portion of the Ka-band, using 288 satellites, divided into 12 planes of 24 satellites each. For efficient spectrum use, frequence will be allocated dynamically and reused many times within each satellite footprint.

Cyberstar: Loral Space and Communications Ltd. will use the Ku-band to initially provide IP multicast services at speeds up to 29 Mbps. Within two years, Loral expects to deliver two-way communication services to businesses and consumers.

SkyBridge: Alcatel will offer Ku-band service using 40 LEO satellites (expanding to 80). The service will provide global connectivity with downstream speeds in multiples of 20 Mbps and upstream speeds in multiple of 2 Mbps. SkyBridge will use an innovative re-use scheme to avoid interference problems with geostationary satellite systems and terrestrial services also using the Ku-band. Traffic management and routing will be done in the ground stations, so satellites will not need direct links between them.

 

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Mobile Wireless
Though there is a lot happening in this area, not many good solutions, in terms of bandwidth, reliability, and response time, have yet appeared. Mobil computing has always faced a big obstacle: As wireless users move around, their connections fade in and out: some applications can shrug this off, while others may become unstable or even damage data.

Another problem to contend with is latency, which ranges from .5 second to more than 5 seconds, round trip. This delay is such that many end-users, after staring at their screens for what seems a long time, might assume the application or machine has hung and either terminate the application or reboot their PC.

Wireless middleware is starting to appear which reduces both the amount of information that travels wireless and the number of messages that must be exchanged. In some cases, it can queue messages when end-users are outside a covereage area. Of course, wireless middleware also adds cost and complexity to the solution. These problems can also be at least partially alleviated by proper configuration.

Growth in mobile wireless is likely to be primarily in metropolitan areas, since vendors delivering services aren't likely to build antennas in areas where there aren't enough customers to make the upfront expenditures pay off. Already there are new "packages" being offered in metropolitan areas that include all long distance, roaming, voicemail, paging, and other features, all for one fixed monthly price, often with no activation or cancellation charges either. One area that has seen successful application of mobile wireless technology is in the situation where drivers are dispatched from a central office to various locations within an area. Data transmissions tend to be small and session durations short.

Some of the current and emerging mobile wireless technologies include the following:

Analog Cellular: No other wireless coverage can match the analog cellular network. With the right PC Card cellular modem, cable, and cellphone, any end-user can dial an ISP and communicate at 9.6 Kbps. Compatibility between modems and cellphones, however, has been a problem and carriers have been inconsistent in deployment of gateways between the cellular and landline networks.

Bellsouth Wireless Data & Ardis: As the two oldest wireless packet-data networks, these networks reach over 90 percent of the U.S. population. Though both use proprietary protocols, IP gateways are available. However, they use volume-based pricing, low effective throughput (4 Kbps for Bellsouth and 2.4 or 9.6 Kbps for Ardis), and trip latencies of three seconds or higher.

Cellular Digital Packet Data (CDPD): CDPD uses idle channels of existing, but CDPD-ugraded, cellular voice networks to transmit data at an effective throughput of about 10 Kbps. CDPD uses a full voice channel, but it can move your connection from one channel to another to avoid congesting voice communications. Each mobile end-stations has a fixed IP address. Service is available in most major cities.

Data-over-digital PCS Solutions: In 1999, 14.4 Kbps Internet access should become available for both Global System for Mobile communication (GSM) and Code-Division Multiple Access (CDMA) services. These services are still circuit switched, but are faster because there is no analog modem. Beginning late in 2000, CDMA, GSM, and Interim Standard 136 (IS-136) will all start offering packet data service at from 64 to 384 Kbps. Extensive roaming agreements still need to be worked out and availability may be limited to highly populated areas for quite some time.

Ricochet: Metricom offers Ricochet, a 28.8 Kbps wireless IP service in the San Francisco Bay area, Seattle, and Washington, D.C. Expansion to other cities and a 128 Kbps offering are both planned.

Universal Mobil Telecommunications System (UMTS): British Telecom with NEC and Nortel Networks began testing third-generation wireless communications services using UMTS, which promises data rates to 2 Mbps for stationary users, 384 Kbps for pedestrians, and 144 Kbps for users in cars or trains.

High Data Rate (HDR) Architecture: Qualcomm, with Cisco and U S West Wireless, is conducting trials of its IP-based HDR, which provides wireless Internet access at 1.8 Mbps or greater.

Wireless Application Protocol (WAP): A number of large wireless providers have banded together as the WAP forum. The group is developing standards that make Web content readily available to mobile wireless devices like smartphones.

IEEE 802.11: Released in 1997, this standard, currently supports data rates of 1 and 2 Mbps over three physical interfaces: direct-sequence spread spectrum (DSSS), frequency-hopping spread spectrum (FHSS), and infrared. Both DSSS and FHSS operate in the unlicensed 2.4 GHz band. 802.11 specifies a contention access method, CSMA/CA.

Direct-sequence provides slightly higher throughput and range, and better overall interoperability between current products. Frequency-hopping offers superior protection from interference and better overall performance when there are numerous users in a limited physical space.

802.11b uses on DSSS, but achieves up to 11 Mbps, a significant improvement over the original standard.. A 20-30 Mbps version which will operate in the 5 GHz range is also being worked on, but not expected to appear until after 2000.

Wireless LANs: Although it is unusual for sites to deploy strictly wireless LANs, it could become more popular as the technology improves. Wireless LANs offer limited mobility at best, with a maximum range measured in hundreds of feet. Nevertheless, it can be highly benficial where mobility is an important component of the business process. RadioLAN has a proprietary product that operates in the 5 GHz range and delivers 10 Mbps, but only up to about 100 feet.

Mobile IP: A companion standard to wireless LAN connectivity is RFC 2002, IP Mobility Support, which allows hosts with a fixed IP address to connect to any IP subnet and immediately be reachable from the Internet. Mobile IP is an extension to IP and is also being integrated into IPv6. There are also extensions to Internet Control Message Protocol (ICMP).

Iridium: Iridium launched its LEO service in 1998, offering handheld satellite communications from almost anywhere, but at very high rates. Iridium went bankrupt by 2000. The phones and the service was simply too expensive to win enough customers to keep it going.

Other: There are other specifications and proprietary implementations being developed both in the U.S. and Europe. There is even a standard for wireless home networks is being worked on.

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