Cabletron Systems 3Com Manual De Usuario

C

HAPTER

 1: O

VERVIEW

 

OF

 D

IGITAL

 W

IRELESS

 N

ETWORKING

and the addition of the AP to the AP table with a proximity classification. An 
unsuccessful AP packet transmission generates another wireless client probe on 
the same channel. If the wireless client fails to receive a probe response within the 
time limits, it repeats the probe process on the next channel in the sequence. This 
process continues through all channels in the range.

A wireless client can roam within the coverage area by switching APs. Roaming 
is transparent and virtually instantaneous in high-level applications. Roaming 
occurs when:

■

An unassociated wireless client attempts to associate or reassociate with an 
available AP.

■

The supported rate changes or the wireless client finds a better transmit rate 
with another AP.

■

The signal quality of a potential AP exceeds that of the current AP.

■

The ratio of good-transmitted packets to attempted-transmitted packets falls 
below a threshold.

■

The wireless client detects an imbalance in the number of wireless clients 
associated with available APs and roams to a less loaded AP.

A wireless client selects the best available AP and adjusts itself to the AP 
direct-sequence channel to begin association. Once associated, the AP begins 
forwarding any frames it receives addressed to the wireless client. Each frame 
contains fields for the current direct-sequence channel. The wireless client uses 
these fields to resynchronize to the AP.

The AirConnect Roaming feature enables a wireless client on the Internet to move 
from one AP to another within a single subnet. The scanning and associating 
process continues for active wireless clients. This allows the wireless clients to find 
new APs and discard out-of-range or deactivated APs. By testing the airwaves, the 
wireless clients can choose the best network connection available.

The AirConnect extended roaming (roaming across routers) feature enables a 
wireless client on the Internet to move from one subnet to another while keeping 
its IP address unchanged. extended roaming enables a wireless client to 
communicate with other hosts using only its home IP address after changing its 
point-of-attachment to the Internet/Intranet.

The Internet Protocol identifies the wireless client point of attachment to a 
network through its IP address. The AP routes packets according to the location 
information contained in the IP header. If the wireless client roams across routers 
to another subnet with extended roaming disabled, the following situations occur:

■

The wireless client changes its point of attachment without changing its IP 
address, causing forthcoming packets to become undeliverable.

■

The wireless client changes its IP address when it moves to a new network, 
causing it to lose connection.