Private Licensed Links (Microwave)
Microwave links are the traditional workhorse of fixed-wireless systems and have been around long before the term wireless broadband was coined. These connections are point-to-point and require licenses. Frequencies available range from 1.7 GHz to 40 GHz, with most of the lower frequencies being used by carriers for backhaul networks, such as T3 connections at 45 Mbps. Many of these are multihop systems and commonly operate at 2, 4 and 6 GHz. A 155-Mbps connection (OC-3) represents the high end for microwave communication today, but there is no inherent upper limit.

If you aren't a telephone company with a backhaul network or a PCS carrier connecting thousands of base stations, and instead are looking to bridge a LAN between two points, the FCC has allocated a frequency band specifically for private use: the 21.2-GHz-to-23.6-GHz band. The license-application process is streamlined, affordable products are available and systems are relatively easy to deploy. For about $30,000 you can purchase a fully installed 10-Mbps connection with a five-mile (eight-km) range. Note that Ethernet type of bridging is less expensive than deploying T1 links because it can operate in a half-duplex asynchronous manner. Of course, if you need to carry voice or video, you may need a T1 or other form of synchronous connection.

Microwave links are very reliable and using licensed frequencies virtually eliminates any potential of interference. Unlike spread-spectrum connections, they offer considerable headroom for increasing throughput if your requirements expand in the future.

Private Unlicensed Links (Spread Spectrum)
An alternative to a microwave link is to use spread-spectrum bridging products. Many wireless LAN vendors offer such products because they incorporate much of the required technology within their access points. These wireless bridges, mostly operating in the 2.4-GHz band, offer rates of 1, 2, 3, 4 and 10 or 11 Mbps and distances up to 10 or 25 miles (16 to 40 km) depending on the type of antenna used. For longer distances you may not be able to achieve as high a throughput. Some products also operate in the 5.8-GHz band.

Government regulatory agencies, including the FCC, mandate the use of a spread-spectrum radio technique that minimizes interference by making radio signals appear like background noise to unintended receivers. Spread spectrum can employ frequency hopping or direct sequence. For more details on spread spectrum, see the Wireless LAN chapter. (http://www.networkcomputing.com/netdesign/wlan1.html)

These bridges offer the same types of features offered by wireline bridges: interconnection with Ethernet or token-ring networks; Spanning Tree Protocol support; remote configuration via telnet, FTP, SNMP, HTML; automatic configuration using BOOTP or DHCP; and SNMP compliance supporting standard MIBs.

For additional information about these types of bridges, refer to Network Computing's article Bridging the Miles With 10-Mbps Spread Sprectrum (http://www.networkcomputing.com/820/820r3.html)

38-GHz Carrier Service
Between 38.6 and 40.0 GHz, the FCC has made 14 pairs of 50-MHz-wide channels available for carriers to offer wireless last-mile communications. This band is also referred to as the 39-Ghz band.ı The primary license holders of this spectrum at this time in the United States are Advanced Radio Telecom, Teleport and Winstar. Forthcoming auctions for this spectrum will inevitably produce new wireless-broadband competitors as well. These carriers manage the entire wireless link themselves, using the wireless connection to extend the reach of their fiber networks. Placing wireless hubs centrally in higher-density population areas the carriers then make wireless connections to other buildings with which they have line of sight. This involves securing roof rights and installing antennas, radios and interface equipment. The carriers usually target buildings that do not have fiber available. The carrier can then market high-speed connections to tenants of the building. The protocols and interfaces are standard communications interfaces such as T1, E1, frame relay, Ethernet and ATM. To the customer, the service is indistinguishable from a wireline service. All he or she sees is a jack in the wall.

Different carriers emphasize different services. One approach is to offer basic telephony services to end users, bypassing the local exchange carrier. Another approach is to offer Internet access and associated services, such as Web hosting and mailboxes. Another approach is to sell connectivity to existing LECs, CLECs, IXCs, long distance service providers and ISPs, essentially being a carrier's carrier. Although service is available on a limited basis in dozens of cities in the United States, this market area is so new that no dominant business model has emerged.

LMDS (Local Multipoint Distribution Service)
LMDS, another type of wireless-broadband system, has recently received tremendous press attention, and for good reason. The FCC recently auctioned a larger block of spectrum than ever before in history: 1.3 GHz. The specific bands include 27.5 to 28.35 GHz, 29.1 to 29.25 GHz and 31 to 31.3 GHz. One band, called the Aı band, is 1,150 MHz wide, and the other, the Bı band, is 150 MHz wide. The smaller B band is available to any company, but restrictions on the A band have prevented incumbent LECs and TV operators from obtaining the spectrum. Although actual LMDS service is extremely limited at this time, it is projected to become a multibillion-dollar industry within five years, with nearly every large telecom and networking vendor having some involvement today.

The services planned for LMDS are quite similar to those of the 38/39-GHz band, namely Internet access, telephony, CLEC services, and resale via LECs, CLECs, long distance service providers and ISPs. The proposed architectures is point-to-multipoint with centralized hubs communicating to fixed-antennas and radios on neighboring buildings. Effective range is about 3 miles (5 km). Downstream radio channels will typically be 20 Mhz or 40 MHz wide and upstream channels 10 MHz, resulting in about 20 to 50 Mbps of downstream bandwidth and 10 Mbps of upstream bandwidth. The carriers will be able to reuse frequencies efficiently in a cellular fashion. Using a TDMA (Time-Division Multiple Access) approach, multiple customers will be able to share the same radio channel. Like 38/39-GHz service, carriers will support standard networking and telephony.

Satellite Systems
Why include satellites in a discussion of fixed-wireless systems? Satellites are anything but fixed, but the ground stations are. Satellites were first used for intercontinental telecommunications before undersea fiber was available and for communication in remote areas, including to remote islands and ocean-going ships. By the end of 1997, more than 180 communications satellites were deployed, and today satellite systems represent a sizeable industry. A variety of new LEO systems are in development and deployment, and they will result in an estimated 1,700 additional satellites by 2005. These new satellite networks will vastly increase the types of services available for both mobile and fixed use, but even now there are many instances when a satellite connection is the best option.

From a broadband perspective, companies, such as Comsat (www.comsat.com) in the United States, already offer flexible service ranging from 56 Kbps to 155 Mbps with E1/T1, frame relay and ATM interfaces operating via a geosynchronous Intelsat satellite. 2-Mbps service, for example, requires a 1.8-meter dish antenna that is relatively easy to install. Connection reliability matches landline quality though there is a somewhat greater delay of about a quarter of a second for the signal to travel to and from the satellite. This delay can affect protocols, such as TCP at data rates exceeding 1 Mbps. See When To Use Satellites, below, for scenarios in which a satellite connection makes the most sense. Also see Network Computing's article on broadband satellites at http://www.networkcomputing.com/905/905f2.html.

And the Rest
If all of the systems described above were not enough, there are a variety of other systems, both private and carrier-oriented, that are offering or about to offer broadband service. We quickly survey these, first private systems and then carrier solutions.

Laser
Laser technology can be used for reliable point-to-point communications. Like all the other systems discussed in this chapter, they require line of sight. For just a little more than $10,000 you can purchase a link offering 10 Mbps and a range of 1 km. Much higher data rates, e.g. 45 Mbps, are also available. Lasers offer the advantage of not requiring licensing anywhere in the world. They also can be extremely secure because any interception of the beam will block the transmission, which can instantly be detected. No wonder lasers are sometimes used by financial institutions. However, because of the small lens size and tightly focused beam, lasers are sensitive to moisture on the lens, though this can be solved by heaters. They also are subject to vibration, and the signal can easily be momentarily blocked by birds. Finally, direct sunlight also can affect signal reception and must be factored into the deployment.

UNII (Unlicensed National Information Infrastructure) Band
The FCC recently allocated 300 MHz of spectrum for unlicensed use from 5.15 GHz to 5.35 GHz and from 5.725 GHz to 5.825 GHz, a swath of spectrum far larger than all the other unlicensed bands combined. Though no products are yet available, intended uses include wireless LANs as well as communitywide networks thanks to the higher power of 4 watts EIRP (effective isotropic radiated power) allowed in the 5.725 to 5.825 portion of the band. Expect both private-link UNII products as well as carrier-based UNII services in the future.

MMDS (Multichannel Multipoint Distribution System)
This is a carrier service, initially intended for broadcast of television, and is commonly referred to as wireless cable. In the United States, this service is available at 2.5 GHz. MMDS service is usually analog and one way (transmit only), with a range of about 30 miles (50 km), and has been deployed in the United States, the Middle East, Latin America, Eastern Europe and Asia Pacific.

Carriers have not been widely successful with television programming and some have obtained waivers from the FCC to be able to offer two-way service for Internet. Others offer Internet service by using a hybrid approach with a PSTN (public switched telephone network) connection for the return path. Meanwhile the FCC is in the process of relaxing rules to allow two-way use of the band, which will facilitate data services. Two-way service reduces the effective range of MMDS to about 6 miles (10 km).

High-Altitude Long Endurance Systems
So far we have seen radio transmitters and receivers on buildings, towers and on satellites. Why not put them on specially designed aircraft that can fly in a circle above a coverage area for a long period of time? That is exactly what some companies are planning, including Angel Technologies Corp. (http://www.angelcorp.com). Such systems will offer broadband services with comparable protocols and interfaces as 38 GHz and LMDS carriers.