Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 11 Mobile Access Router, Universal Bridge Client, and Cisco Unified Wireless
Cisco 3200 Series and Wireless Network Access
network, it can be supplemented by cellular services such as CDMA 1x RTT. By using this approach,
cellular services can be used to fill gaps in connections and to provide backup wireless connectivity. This
added backup interface requires Mobile IP to enable client roaming between the two separate networks.
cellular services can be used to fill gaps in connections and to provide backup wireless connectivity. This
added backup interface requires Mobile IP to enable client roaming between the two separate networks.
In IP networks, routing is based on stationary IP addresses, similarly to how a postal letter is delivered
to a fixed address on an envelope. A device on a network is reachable through IP routing by the IP
address to which it is assigned on the network. However, when networks are in motion, problems occur
when a device roams away from its home network and is no longer reachable using its existing IP route.
This causes the active sessions of the device to be terminated.
to a fixed address on an envelope. A device on a network is reachable through IP routing by the IP
address to which it is assigned on the network. However, when networks are in motion, problems occur
when a device roams away from its home network and is no longer reachable using its existing IP route.
This causes the active sessions of the device to be terminated.
Mobile IP offers a solution to these roaming problems by enabling users to keep the same IP address
while traveling to a different network (which may even be operated by a different wireless operator), thus
ensuring that a roaming client can continue communication without sessions or connection drops.
while traveling to a different network (which may even be operated by a different wireless operator), thus
ensuring that a roaming client can continue communication without sessions or connection drops.
Because the mobility functions of Mobile IP are performed at the network layer rather than the physical
or link layer, mobile devices such as the Cisco 3200 can span different types of wireless and wired
networks while maintaining connections and ongoing applications. Any application that requires that the
Session layer be maintained is a candidate for use on a Mobile IP-enabled network connection.
or link layer, mobile devices such as the Cisco 3200 can span different types of wireless and wired
networks while maintaining connections and ongoing applications. Any application that requires that the
Session layer be maintained is a candidate for use on a Mobile IP-enabled network connection.
For a comprehensive overview of Mobile IP networking, see
WMIC Roaming Algorithm
The following four basic triggers start the WMIC scanning for a better root bridge or access point:
1.
Loss of eight consecutive beacons
2.
Data rate shift
3.
Maximum data retry count is exceeded (the default value is 64 on the WMIC)
4.
A measured period of time of a drop in the signal strength threshold
Only #3 and #4 above are configurable via the packet retries command and mobile station period X
threshold Y (in dBm); the remainder are hard-coded.
threshold Y (in dBm); the remainder are hard-coded.
If a client starts scanning because of a loss of eight consecutive beacons, the following message is
displayed on the console: “Too many missed beacons”. The WMIC in this case is acting as a universal
bridge client much like any other wireless client in its behavior. An additional triggering mechanism,
“mobile station,” is not periodic but does have two variables; period and threshold. If a mobile station is
configured, the mobile station algorithm evaluates two variables (data rate shift and signal strength) and
responds as follows:
displayed on the console: “Too many missed beacons”. The WMIC in this case is acting as a universal
bridge client much like any other wireless client in its behavior. An additional triggering mechanism,
“mobile station,” is not periodic but does have two variables; period and threshold. If a mobile station is
configured, the mobile station algorithm evaluates two variables (data rate shift and signal strength) and
responds as follows:
•
If the driver does a long-term downshift in the transmit rate for packets to the parent, the WMIC
initiates a scan for a new parent (no more than once every configured period).
initiates a scan for a new parent (no more than once every configured period).
•
If the signal strength (threshold) drops below a configurable level, the WMIC scans for a new parent
(no more than once every configured period).
(no more than once every configured period).
The data-rate shift can be displayed with the following command.
debug dot11 dot11Radio 0 trace print rates
However, this does not show the actual “data rate shift” algorithm in action, but only the changes in data
rate. This determines the time period to scan depending on how much the data rate was decreased.
rate. This determines the time period to scan depending on how much the data rate was decreased.
The period should be set depending on the application; default is 20 seconds. This delay period prevents
the WMIC from constantly scanning for a better parent if, for example, the threshold is below the
configured value.
the WMIC from constantly scanning for a better parent if, for example, the threshold is below the
configured value.