A WLAN designer must determine whether clients will require seamless roaming
from access point to access point, as depicted in Figure
.
As a
client roams across the wireless network, it must establish and maintain an
association with an Aironet access point.
The following steps are taken
to ensure seamless roaming:
- The client sends out a request for association and immediately receives a
response from all access points within its coverage area.
- The client decides which access point to associate with based on signal
quality, strength, the number of users associated, and the required number of
hops to reach the backbone.
- After an association is established, the client's Media Access Control
(MAC) address drops into the table of the selected access point. If the client
encounters difficulty, it will roam for another access point. If no other
access point is available, the client will lower its data transmission rate and
try to maintain connection.
- After the client roams to another access point, its MAC address drops into
the table of the new access point, which sends a broadcast message basically
stating that it received "MAC address X".
- The original access point forwards any data it had for the client to the
other access point, which responds by sending the same to the client.
The following two factors need to be considered when designing a WLAN
with seamless roaming capabilities for devices that are powered on while moving
from one point to another:
- Coverage must be sufficient for the entire path.
- A consistent IP address should be available throughout the entire path. The
IP subnet for each access point could be on different switches and separated by
Layer 3 devices. If so, consider using Layer 2 switching technologies such as
ATM-LANE, ISL, or 802.1q, to span the VLANs. This will help ensure that there
is a single broadcast domain for all access points. Figure
illustrates such a scenario.
Association process
When a client comes online, it will
broadcast a probe request. An AP that hears this will respond with information
about the AP such as RF hops to the backbone, load, and so on. If more than one
AP replies, then the client will decide which AP to associate with, based on
the information returned from the AP. APs broadcast ‘beacons’ at periodic
intervals. A beacon contains details similar to that in the probe response. The
client listens to all APs it can hear and builds an information table about the
APs. The association process is illustrated in Figure
.
Re-Association process
As the client is moving out of range of
its associated AP, the signal strength will start to drop off. At the same
time, the strength of another AP will begin to increase. The re-association
process that occurs is shown in Figure
. The same type
of handoff can occur if the load on one AP becomes too large as long as the
client can communicate with another AP.