802.11 Protocol Architecture
mechanism does a good job of avoiding collisions; however, stations
on networks with high utilization will experience substantial delays
while waiting to transmit frames. In 802.11 and 802.11b networks, the
values for aCWmin and aCWmax are 31 (first transmission attempt)
and 1023 (5th and all subsequent retries until the retry limit is reached).¹
In 802.11g networks, the values for aCWmin and aCWmax are 15 (first
transmission attempt) and 1023 (6th and all subsequent retries until the
retry limit is reached).² This gives 802.11g a statistical transmission
advantage in 802.11b/g mixed mode networks.
Extended Rate Physical (ERP)
ERP is a term used to specify PHYs able to both transmit and receive
certain OFDM physical modulations defined by Clause 19, also known
as 802.11g. In the case of ERP networks (i.e., where 802.11g access
points are present), ERP-OFDM stations (also called ERP stations) may
need to implement protection mechanisms (when mandated by the
access point), which enable NonERP stations (e.g., 802.11b –
commonly referred to as Clause 15 and 18 stations) to understand when
ERP-stations are transmitting. If all of the stations in the area are ERP
then protection mechanisms are unnecessary. If one or more of the
stations in this scenario are NonERP types, a condition which is made
known to the BSS by the access point using the beacon frame, then all
ERP stations must use a protection mechanism that announces the
pending use of the medium using a modulation that the NonERP client
stations can understand.
1
IEEE 802.11 - 1999 (R2003) – Sections 9.2.4 & 15.3.3
2
IEEE 802.11g - 2003 – Section 19.8.4
