A New York town finds benefits of spread-spectrum radio frequency

By James B. Jones
Special to GCN

Spread-spectrum radio frequency is a fast-growing alternative for providing data connectivity between locations.

SSRF has been around since the late 1920s, but it was during World War II that its potential was first fully realized.

For SSRF to work, a line of sight must be available between the antennas, which must be within the distance limits of the signal strength.

The geography in Tonawanda, N.Y., was particularly good for this technology to advance the town's WAN. For the past seven years, the town had used the local telephone company infrastructure to connect its buildings. There was an assortment of connections: 1.5-Mbps T1 lines, 56-Kbps leased lines, standard dial-up lines and Integrated Services Digital Network metered lines. Thirteen civic buildings throughout the town were brought online via the telephone lines. The town paid $65,000 per year for this connectivity.

The town's Technical Support Department and its consultant, Aurora Consulting Group of East Aurora, N.Y., began research into switching to an SSRF system in the spring of last year, and by fall presented its findings to town officials.

The argument for making a capital investment of about $115,000 for an SSRF system was simple: A greater-bandwidth WAN would be more reliable and less complicated, and would save at least $65,000 per year.

The team enumerated the benefits of the system:

' Resistance to jamming by outsiders

' Encoded signals that restrict the communications to legitimate users

' Resistance to interference by precipitation and other environmental factors

' Multiple levels of physical and electronic security

' Easy accessibility for multiple users

' Excellent signal resolution

' Operation on a segment of the radio spectrum that's exempt from federal licensing.

Another factor in support of the project was the 180-foot cellular phone and radio tower recently built at the town's centrally located police station.

The town then prepared a performance specification for a 3-Mbps wireless WAN, the fastest available at the time.

The project required line-of-sight analyses for connecting 14 buildings throughout the town. The WAN was to be a multipoint hub-and-spoke topology, with the tower at the police station as the hub. The sites' distances from the hub range from across the street to three miles.

The network had to be Ethernet 802.3-compliant. The equipment to be furnished and installed at each site included a wireless radio bridge, an antenna and cabling between the two.

The town's Internet, e-mail and financial system servers are in the Municipal Building, about 1.5 miles from the police tower. Most of the network traffic is bound for these servers. Internet access is gained and controlled at one point at the proxy server. The network design had to take this traffic pattern into consideration.

The town sought bids for the project in November of last year; by then, Ethernet bridges at 10 Mbps were coming onto the market. The faster devices were sought as an alternative to the original 3-Mbps system. In December, bids were received for the project, and the 11-Mbps wireless bridges offered by Aironet Wireless Communications Inc. through a local radio contractor, Transwave Communication Systems, were selected.

The $112,500 project was awarded to Transwave, which completed the installation in April.

Catch some rays

The contractor began by installing the main antenna near the top of the police station tower. The antenna, about the size of a broom handle, is omnidirectional. The bridge was installed on a small shelf near the existing hubs and was configured to act as the root device to which all other bridges were to be directed.

The antennas installed at the other sites were directional. In most cases, the contractor mounted a galvanized steel pipe, or mast, to a high point on the building. This formed the structure for the antenna. A 45-foot and a 55-foot tripod tower were installed at two sites to create the necessary line of sight, and existing tripods were used at two other sites.

The bridge operates from a standard power source. Antennas are connected to the bridge by coaxial cable, and the bridge is then attached to the LAN by Category 5 Ethernet cable.

The configuration of the bridge includes a unique site identifier along with the common radio signal identifier. The bridge is configured through a serial cable from a notebook PC, but configuration can also be confirmed and the system monitored via Telnet or through a standard Web browser. The initial configuration was done by the contractor at his shop and then modified on site.

Bridges operate by providing an origin and destination database for all network TCP packets. Town personnel and staff from Aurora Consulting completed the conversion of the network by disconnecting all of the telephone devices and connecting the new bridges to existing LANs at each building.

Having a new Ethernet connection among all town departments made it possible to deploy several network management strategies. One is the configuration of the file server at the Municipal Building to be the Dynamic Host Control Protocol server. This permits the automatic assignment of IP addresses to all clients on the network. This strategy was implemented at the same time as the conversion.

Another important management tool was virus protection for all servers. The virus software is continually updated from the Web, and these updates are then pushed to the other servers.

More users

In addition to the town's conversion, the local school district joined the project under a $30,000 shared-service grant sponsored by New York.

The school district connected three schools by using the town's contractor and by colocating an antenna on the tower at the police station. It signed on to the project because it had two telephone company leased lines that were about to expire.

Other plans for the network call for using Microsoft Systems Management Server for network management and system maintenance, and developing intranet enterprise resource applications development.

James B. Jones is a project engineer in the Tonawanda, N.Y., Technical Support Department.