permits. Roaming is enabled by complete coverage with wireless cells. The benefits of Cisco Aironet WLAN products include the following: Wireless clients associate to another access point if necessary. This process is called roaming between the wireless cells. The wireless client initiates the roaming if one of the following conditions is detected: Roaming without service interruption requires the identical configuration of SSIDs, VLANs, and IP subnets on all access points. Roaming is initiated by the client. The client searches for another access point with the same SSID and sends a reauthentication request to the new access point. A short roaming time is important for delay-sensitive applications, such as voice and video. Roaming maintains network connectivity while moving from one access point to another. Roaming between access points that reside on a single IP subnet (or VLAN) is considered Layer 2 (data link layer) roaming. Roaming between access points that reside in different IP subnets is considered Layer 3 (network layer) roaming. Roaming at Layer 2 is managed by the access points using a combination of multicast packets that inform the switches in the network that the device has moved. The protocol between the access points is called Inter Access Point Protocol (IAPP). First generation Layer 3 roaming was provided by Mobile IP, a technology which works with fixed IP addresses within a geographically distributed IP subnet. It relies on routers acting as so-called home agents and foreign agents, to tunnel traffic on behalf of a mobile device. Modern WLAN implementations allow for seamless Layer 3 roaming. Layer 3 roaming using Mobile IP has been replaced by the implementation of lightweight access points in combination with WLAN controllers.
Content 6.2 Describing Wireless LAN Topologies 6.2.4 Wireless VLAN Support Switches use VLANs to separate traffic. Access points can extend VLANs to the wireless LAN by mapping VLANs to SSIDs. The wireless VLANs share the same wireless cell and channel. The result is a virtualization of the access points. The access point appears as multiple different access points. The VLAN deployment example in Figure shows how VLANs may be used to segregate user groups and provide unique access policies. The Cisco Aironet Access Points support only the 802.1Q trunking protocol standard. Cisco switches and routers support both the ISL and 802.1Q protocols. WLANs can fit nicely into the larger network because VLANs have been enabled on the access points. This approach allows WLAN users to roam from access point to access point, maintaining connectivity to the proper VLAN. In Figure , the notebook user is able to maintain access to the proper VLAN (VLAN102) and communicate with the router while roaming from access point to access point. Roaming without service interruption requires the identical configuration of SSID, VLANs, and IP subnets on all access points. Switches do not allow different VLANs to talk to one another, so a router is needed. The VLAN number of the switch and the access point has to match. You can configure Cisco Aironet Access Points with 8 to 16 different VLANs (depending on implementation) for system design flexibility. For client cards that require broadcast SSID support, the access point has to be configured for SSID broadcast per VLAN. IP phone networks can be extended with wireless IP phones. The new 802.11e standard specifies QoS both upstream and downstream for WLAN networks. QoS is driven by the following:
Content 6.2 Describing Wireless LAN Topologies 6.2.5 Wireless Mesh Networking A mesh networking infrastructure is decentralized and inexpensive because each node needs to transmit only as far as the next node. Nodes act as repeaters to transmit data from nearby nodes to peers that are too far away to reach. This approach results in a network that can span a large distance, especially over rough or difficult terrain. Mesh networks are also extremely reliable because each node is connected to several other nodes. If one node drops out of the network because of hardware failure or any other reason, its neighbors find another route. Extra capacity can be installed by adding more nodes. Mesh networks allow many possible paths from a given node to other nodes. Paths through the mesh network can change in response to traffic loads, radio conditions, or traffic prioritization. Wireless mesh networks differ from other wireless networks in that only a subset of the nodes needs to be connected to the wired network. The network can cover more distance by using nodes that are not connected to the wired network. Unlicensed bandwidth and wireless routing allow microcells to interconnect over wireless backhaul links. Mesh applications may be used to provide wireless coverage throughout a campus, manufacturing environment, or city. Deploying mesh access points allows the network to extend beyond the typical boundaries that would require each access point to be wired to the LAN. The Cisco Adaptive Wireless Path (AWP) protocol allows each device to find a way back to the wired rooftop access point and thus to the network. Access points are authenticated as they join the network, allowing the controller to send configuration parameters. Each access point runs the Cisco AWP protocol. AWP is a new protocol that was designed specifically for the wireless environment. It allows access points to communicate with each other to determine the best path back to the wired network. After the optimal path is established, AWP continues to run in the background to establish alternative routes back to the roof-top access point (RAP) if the topology changes or conditions cause the link strength to diminish. The RAP connects the mesh network to the wired network. Cisco AWP considers factors such as interference and characteristics of the radio so that the mesh can be self-configuring and