Investigation into the Development of
Inclusive Dates: 12/31/00 - 04/30/01
Background - Several state Departments of Transportation (DOTs) have expressed an interest in deploying wireless communications infrastructures along the roadsides of their states to support traffic management and maintenance services. The common practice is to use a direct fiber-optic cable run to each wireless access point (AP) along the roadside. This research project investigates using 802.11b wireless Ethernet radios to form a wireless backbone for the interconnectivity between the wireless APs. This solution appeals to the state DOTs because of its reduced cost in comparison to running fiber-optic cabling to each communications AP along the roadside. The advent of a completely wireless solution also helps account for the need for communications in nonroutine locations, such as cameras monitoring a major accident.
Approach - A wireless backbone consists of wireless repeater nodes that carry the data from wireless APs to wired nodes connecting to the traffic message channel using fiber-optic or other high-speed wired solutions. A key element in this scenario is the capability to use wireless APs and wireless nodes that are operating in the same frequency band without interference with one another. The 802.11b radios used in this project partition the 2.4-gigahertz (GHz) ISM band into eleven separate channels. The dissection of the 2.4-GHz band helps to alleviate the interference issues that can arise between terminals operating in close proximity with one another. However, the amount of interference that does exist among adjacent and near channels can significantly raise the noise floor and thus drop the effective data throughput for these channels. The issue of radio interference between multiple radios increases when multiple wireless devices are trying to connect through the same AP. This scenario is likely to occur when a roaming device (e.g., ambulance, police car, or traffic control vehicle) tries to connect to a roadside AP that also serves as the communications link for a stationary intelligent transportation system device [e.g., camera, digital message sign (DMS), highway advisory radio].
A test network is prepared at the SwRI test track facility, where AP and backbone radios are mounted on towers along the track. Extensive throughput analyses were performed for varying scenarios. The performance tests included measuring system throughput along multiple legs of the backbone and between mobile end-device connections at various locations along the backbone. Different distances between the mobile end devices and the APs were also used in several of the scenarios.
Accomplishments - The first observation is that the wireless link provides roughly half the advertised data throughput. The advertised throughput for an 802.11b radio is a bit of a misnomer, really referring to the fact that the radio-frequency link can support 11 megabytes per second (Mbps) of raw radio frequency data, not 11 Mbps of Ethernet data. Even with a link of roughly 5 Mbps, the developed backbone design worked well with multiple mobile devices connecting to the various APs. Test results also conclude that multiple mobile end devices can share an AP as long as there is proper dispersion of the allotted 11 channels between the AP, backbone, and mobile radios. Special provisions must also be made for the access codes used between the radios to ensure appropriate security and to reduce cross-talk between the backbone and AP radios.