Black Box LW0050A, Руководство пользователя

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PRO 11 SERIES WIRELESS ETHERNET
The 802.11 standard defines an Exponential Backoff Algorithm that must be
executed in the following cases:
• When the station senses the medium before the first transmission of a packet,
and the medium is busy
• After each retransmission, and
• After a successful transmission
The only case when this mechanism is not used is when the station decides to
transmit a new packet and the medium has been free for more than DIFS.
The following figure shows a schematic of the access mechanism:
Figure E-4. Access Mechanism.
E.4 How Does a Station Join an Existing Cell (BSS)?
When a station wants to access an existing BSS (either after power-up, after sleep
mode, or just entering the BSS area), the station needs to get synchronization
information from the Access Point (or from the other stations when in ad-hoc
mode, which will be discussed later).
The station can get this information by one of two means:
1. Passive Scanning: In this case the station just waits to receive a Beacon Frame
from the Access Point (the beacon frame is a frame sent out periodically by the
Access Point containing synchronization information), or
2. Active Scanning: In this case the station tries to locate an Access Point by
transmitting Probe Request Frames, and waits for Probe Response from the
Access Point.
Immediate access when medium
is free
≥
DIFS
Busy Medium
Defer Access
Slot Time
Select Slot and Decrement Backoff
as Long as Medium Is Idle
DIFS
Contention Window
Backoff Window Next Frame
PIFS
SIFS
DIFS