LAN implementation
There are some short haul microwave systems used for corporate LANs that do require an FCC license because they operate in a more crowded portion of the spectrum. This is the case with the 18-GHz frequency band, which is used by Motorola's Altair product line, for example. The FCC allocates licenses for the 18-GHz region of the spectrum for five channels, each consisting of two 10-MHz bands in the 18- to 19-GHz band. The two-channels-per-license arrangement permits full-duplex communications using one channel in each direction.
While a license may be construed as a liability, it offers the guarantee of interference-free communication. Unlike unlicensed spectrum, such as that often used by spread-spectrum technologies, licensed spectrum gives the license-holder a legal right to an interference-free data communications channel. Owners of wireless LANs operating on unlicensed spectrum are continuously at risk of unauthorized interference destroying their data communications capabilities, and do not always have legal recourse.
Since Motorola has obtained licenses for 18-GHz operation in all U.S. metropolitan areas with populations above 30,000, Altair microcells in these areas are fairly well protected from unauthorized interference. Motorola customers do not have to deal directly with the government or wait for approval to operate Altair equipment. Upon purchasing the equipment, they fill out a simple registration form, and immediately place their equipment into operation. Motorola's Frequency Management Center dedicates and coordinates frequencies to specific customer locations. In the event a customer moves the Altair system to another location, Motorola provides an 800 number to report that fact in compliance with FCC requirements. Motorola handles frequency coordination for the customer at the new location.
Corporations are making greater use of wireless technologies for extending the reach of LANs where a wired infrastructure is absent or costly. Wireless bridges and routers can extend data communication between buildings in a campus environment or between buildings in a metropolitan area.
Wireless bridges. Short-haul microwave bridges equipped with directional antennas provide an economical alternative to leased lines or underground cabling. Because they operate over very short distances—less than a mile—and are less crowded, they are less stringently regulated and have the additional advantage of not requiring an FCC license.
The range of the bridge is determined by the type of directional antenna. A 4-element antenna, for example, provides a wireless connection of up to 1 mile. A 10-element antenna provides a wireless connection of up to 3 miles.
Directional antennas require a clear line of sight. To ensure accurate alignment of the directional antennas at each end, menu-driven diagnostic software is used. Once the antennas are aligned, and the system ID and channel are selected with the aid of configuration software, the system is operational. Front-panel LEDs provide a visual indication of link status and traffic activity. The bridge unit has a diagnostic port, allowing performance monitoring and troubleshooting through a locally attached terminal or remote computer connected via modem.
Because it is fully compatible with the IEEE 802.3 Ethernet standard, microwave bridges support all Ethernet functionality and applications without the need for any special software or network configuration changes. For Ethernet connections, the interface between the microwave equipment and the network is virtually identical to that between the LAN and any cable medium, where retiming devices and transceivers at each end of the cable combine to extend the Ethernet cable segments. Typically, microwave bridges support all Ethernet media types via AUI connectors for thick Ethernet (10Base5), 10Base2 connectors for thin Ethernet, and twisted-pair connectors for 10BaseT Ethernet. These connections allow microwave bridges to function as an access point to wired LANs.
Like conventional Ethernet bridges, microwave bridges perform packet forwarding and filtering to reduce the amount of traffic over the wireless segment. The microwave bridges contain Ethernet address filter tables, which help to reduce the level of traffic through the system by passing only the Ethernet packets bound for an interbuilding or intrabuilding destination over the wireless link. Since the bridges are "self-learning," the filter tables are automatically filled with Ethernet addresses as the bridge learns which devices reside on its side of the link. That way, Ethernet packets that are not destined for a remote address remain local. The table is dynamically updated to account for equipment that is either added or deleted from the network. The size of the filter table can be 1000 entries or more, depending on vendor.
With additional hardware, microwave bridges have the added advantage of pulling double-duty as a backup to local T1/E1 facilities. When a facility degrades to a pre-established error-rate threshold or is knocked out of service entirely, the traffic can be switched over to a wireless link to avoid loss of data. When line quality improves or the facility is restored to service, the traffic is switched from the wireless link to the wireline link.
Wireless routers. Wireless remote access routers scale wireless connect geographically disbursed LANs by creating a wireless WAN over which network traffic is routed at distances of 30 miles or more using spread-spectrum technology.
Unlike wireless bridges which simply connect LAN segments into a single logical network, wireless routers function at the network layer with IP/IPX routing, permitting the network designer to build large, high performance, manageable networks. Wireless routers are capable of supporting star, mesh, and point-to-point topologies that are implemented with efficient MAC protocols. These topologies can even be combined in an internetwork.
A polled protocol (star topology) provides efficient shared access to the channel even under heavy loading (Figure 1). For small-scale networks, a CSMA/CA protocol supports a mesh topology (Figure 2). Clusters of nodes can be connected using a point-to-point protocol when building large-scale internetworks (Figure 3). The single-hop node-to-node range is up to 30 miles, depending on such factors as terrain and antennas, with a multiple hop range extending on the order of a hundred miles.
Figure 1: In the star topology, remote stations interconnect with the central base station, and with other remote stations, through the base station. Only one location needs line-of-sight to the remotes. Networks and workstations at each location tie into a common internetwork. The maximum range between the central base station and remote stations is approximately 15 miles.
Figure 2: In the mesh topology, each site must be line-of-sight to every other. A CSMA/CA protocol ensures efficient sharing of the radio channel. The range with omni-antennas is up to 3.5 miles.
Figure 3: The point-to-point topology is useful where there are only two sites. It can also function as a repeater link between clusters of sites. The range using directional antennas is up to 30 miles.
End-to-end SNMP supports management of the radios and the wireless WAN along with the remainder of the enterprise network using industry standard SNMP tools. The result is a manageable network with reach extending to metropolitan, suburban, rural, remote, and isolated areas.
Applications of wireless routers include remote site LAN connectivity and network service dissemination. Organizations with remote offices such as banks, health care networks, government agencies, schools, and other service organizations can connect their computing resources. Industrial and manufacturing companies can reliably and cost-effectively connect factories, warehouses, and research facilities. Network service providers can distribute Internet, VSAT, and other network services to their customers.
A WAN built with wireless routers exploits the tariff-free wireless "infrastructure." A wireless WAN offers a substitute for a wired infrastructure with its associated costly service fees. In areas where a wired infrastructure is absent or underdeveloped, the use of wireless routers newly enables internetworking. Performance is comparable to commonly used wired WAN connections, approaching T1 speeds with a 1.3 Mbps data rate.
A user-supplied PC hosts the wireless router device through a connection to the PC's parallel printer port. Software includes an installation utility, network drivers, management, and IP/IPX routing. No radio license is required in the United States and many other countries, simplifying the network development process compared to licensed microwave.
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