Architectural Considerations


The primary problem that affects all these different scenarios is how measurement information is extracted and made available to the nodes that need it. Some of these methods require the collection of data from multiple nodes in the network, whereas some of the information is made available through MAC or routing layer messages.

Infrastructure Networks

In an infrastructure network, access points make a good choice for ALEs. The LIS can be provisioned with the location of each access point and can use range measurements from multiple access points effectively. Figure 7.10 shows this network architecture.

image from book
Figure 7.10: A LIS can use infrastructure (access points) to acquire location measurements.

To achieve the most basic level of location determination, the ALE in the access point only needs to report the MAC addresses of the mobile stations that it is currently serving. This information can be used to provide a service that is adequate for most purposes. Additional information can be retrieved from the ALE to gain better accuracy and precision in the final location estimate.

Ad Hoc Networks

In ad hoc networks, ALEs can be employed in a number of ways, depending on the particular network configuration. In order to guarantee that all the necessary information was available, all nodes in the network could be configured with ALEs. However, it is neither likely nor practical to expect that the network provider is able to control all the nodes in the network.

A more practical configuration uses select nodes within the network to relay location measurements. This function can be provided by fixed gateways that also provide a connection from the WLAN to the Internet or larger network. These nodes are usually under the control of the network provider and can therefore be configured with ALEs.

In addition to configuring ALEs within the access network, the target node is an excellent source of location measurements in an ad hoc network. Chapter 9 discusses a number of ways that the LIS can extract location measurements from a device.

ALE Overlays

A possible alternative to using the inherent measurement capability of mobile nodes and access points in the WLAN is to add independent ALEs to the network. These ALEs are independent nodes with passive radio receivers that monitor the radio environment to acquire location measurements. These nodes are typically fixed, so that they have a known location, and they report signal strength and timing measurements. The ALEs report these measurements to the LIS, which combines the measurements to determine the location of devices.

This overlay technology follows the concepts employed for cellular networks. Each ALE is analogous to the GSM Location Measurement Unit (LMU) that is used for network-based Uplink Time Difference of Arrival (U-TDOA). However, for many indoor environments the GPS used in GSM LMUs would not be applicable to WLAN ALEs. GSM LMUs use GPS to provide time synchronization and sometimes location; WLAN ALEs would be able to use the inherent network synchronization and would be manually located.

Dedicated location measurement devices can also provide measurements without some of the inherent limitations of access points. Access points are often limited in their ability to measure signals outside of the channel that they operate in. In order to monitor signals in other channels, the access point has to temporarily switch to the new channel, which can interrupt service. ALEs that monitor radio signals over the network coverage area provide a means of receiving location measurements. These ALEs can provide signal strength and timing information without disrupting the network service.

The Wireless Network Controller as an ALE

For networks that employ a wireless network controller, the network controller can act as an ALE to provide measurements to the LIS. A number of wireless network controllers determine location as a part of their operation, in which case they can report location to the LIS. Having the network controller report location has advantages in terms of simplicity, but it limits the opportunity for hybrid location determination methods where measurements from the wireless network are combined with other measurements from other sources, such as devices themselves taking pseudorange measurements.



IP Location
IP Location
ISBN: 0072263776
EAN: 2147483647
Year: 2004
Pages: 129

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