A roaming environment contains multiple wireless access points, such as the AirPort Base Station, that connect to the same wired network. The wireless client device, such as an iBook, chooses an access point from those within range based on criteria that include signal strength. The end result should be transparent to the user of the wireless device, since the wireless access points provide the same access to the wired network.
When a wireless client connects to an access point, the density setting is communicated to the client. This tells the client how "picky" to be about which access point it chooses. The High setting for Base Station Density means that the client is more prone to disconnect from a low-strength signal as a stronger signal becomes available. Essentially, the client is more selective. This means a mobile client switches sooner to a new access point that it is approaching. Were the Base Station Density set to low, it would maintain the connection to the weaker, outgoing signal for a longer period of time.
The Base Station Density setting also affects how sensitive the client's receiver is. When set to High, the receiver ignores more radio "noise," or interference. When set to Low, the client is more susceptible to effects from radio noise.
Notes
1. The density setting affects which networks a client will see when scanning. When connected to a base station that specifies High density, you do not see as many distant networks as you would when connected to a base station that specifies Low density.
2. Wireless Internet access requires an Internet service provider (fees may apply) and AirPort (or AirPort-compatible) wireless Ethernet card and base station. Some ISPs are not compatible with AirPort. For more information, see technical document 106590: "AirPort: Requirements for Wireless Internet Access".