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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Multiple Cell Wireless LANsIf an organization requires greater range than a single cell, it can utilize a set of wireless local bridges (access points) and a wired network backbone to create a multiple-cell configuration (see fig. 2.9). This enables wireless users from different cells to communicate with each other, as well as lets wireless users access resources available on the wired network. Such a configuration can cover larger multi-floor buildings, campuses, and hospitals. In this environment, a portable PC with a cordless LAN adapter can also roam within the coverage area while maintaining a live connection to the corporate network. Typically, each access point utilizes a different hopping code or frequency.
The ideal wireless LAN configuration for your organization depends primarily on user requirements and geography. If you have a relatively small group that requires wireless interconnectivity within the immediate group, a single cell may do the job. If users are spread throughout the entire facility, however, then you might need a multiple-cell configuration. In either case, bridges may be necessary to support user access to resources located on the wired infrastructure. A critical function in a multiple-cell network is roaming, which allows wireless users to move from cell to cell seamlessly. Most wireless LAN companies who build wireless network bridges implement roaming. The roaming protocol works only at the MAC layer; therefore, it will not work over routers. This paragraph explains how WaveAROUND, which belongs to the WaveLAN product line, implements roaming1. All cells in a WaveLAN configuration are linked together in a wireless network group, called the Domain. Within this Domain, a mobile station will automatically switch between different cells to ensure continuous connectivity. The mobile station will monitor the communications quality with the WavePOINT of the current cell. If the communications quality drops below a preset value, the station will start searching for another cell. If found, the station will retrieve and adopt the WaveLAN Network ID of a new cell to ensure the network connection.
With WaveAROUND, multiple cells do not have to overlap. If a wireless user moves between two non-overlapping cells, as shown in figure 2.10, the WaveAROUND automatically reconnects the user to the new cell upon entering the new location. In situations where cells do overlap, WaveAROUND connects to a new WavePOINT before terminating the connection with the predecessor. This provides a constant connection to the network.
How does roaming work? Again, lets use WavePOINT as an example. The WavePOINT broadcasts beacon messages at regular intervals to support roaming mobile stations. Beacon messages contain the Domain ID, the WaveLAN Network ID of the WavePOINT, communications quality information, and cell search threshold values. The Domain ID identifies the WavePOINTs and mobile stations that belong to the same WaveLAN roaming network. A mobile station listening for beacons will only interpret beacon messages with the same Domain ID. The WaveLAN Network ID identifies a specific cell of the WaveLAN network. This is the Network ID assigned to each WavePOINT. It tells the mobile station which WaveLAN Network ID to use to communicate with the network on that location. You can encode the Network ID information by using a beacon key. Communications quality information helps the mobile station determine the quality of the link to the WavePOINT of a particular cell. The cell search thresholds are related to the level of communications quality. They activate the cell search mode when the communications quality drops, such as when the mobile station is moving to another location. In cell search mode, the mobile station will start searching for other WavePOINTs by listening for beacons. You can change the threshold values by selecting a different level of sensitivity, depending on your environment. A roaming station needs a number of beacons to determine the communications quality with a WavePOINT. Responsiveness parameters set beacon interval time and beacon time-out. The beacon interval time allows you to set the frequency of beacon transmissions. This frequency determines how fast a station can decide the actual communications quality. The beacon time-out sets the values of a timer mechanism. This timer activates the cell search mode when the mobile station does not receive a beacon within the specified time limit. Three user defined presets adjust the level of responsiveness:
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