contrast to Ethernet which uses carrier-sense multiple access with collision detection (CSMA/CD). The 802.11 specification includes provisions designed to minimize collisions because mobile units have to contend with the hidden node problem where two mobile units may both be in range of a common access point, but out of range of each other. Bridge A bridge can be used to connect networks, typically of different types. A wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to a wireless network. The bridge acts as the connection point to the wireless LAN. Wireless distribution system A wireless distribution system (WDS) enables the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the need for a wired backbone to link them, as is traditionally required. The notable advantage of a WDS over some other solutions is that it preserves the MAC addresses of client packets across links between access points. An access point can be either a main, relay, or remote base station. A main base station is typically connected to the wired Ethernet. A relay base station relays data between remote base stations, wireless clients or other relay stations to either a main or another relay base station. A remote base station accepts connections from wireless clients and passes them to relay or main stations. Because data is forwarded wirelessly, consuming wireless bandwidth, throughput in this method is halved for wireless clients not connected to a main base station. Connections between base stations are done at layer-2 and do not involve or require layer-3 IP addresses. WDS capability may also be referred to as repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). All base stations in a WDS must be configured to use the same radio channel, and share WEP keys or WPA keys if they are used. They can be configured to different service set identifiers. WDS also requires that every base station be configured to forward to others in the system as mentioned above. Roaming There are two definitions for wireless LAN roaming: Internal roaming: The mobile station (MS) moves from one access point (AP) to another AP within a home network if the signal strength is too weak. An authentication server performs the re-authentication of MS via 802.1x (e.g. with