- 802.11 network uses
slotted transmissions:
- Slotted transmissions
(in general) will
reduce the
likelihood of
collisions
- Since nodes in the 802.11 network cannot detect collisions, there is no "wasting" of slots when collision is detected....
- Slotted transmissions
(in general) will
reduce the
likelihood of
collisions
- Duration of
a slot in
802.11 is:
- σ = 20 μsec
(A message can be longer than one slot).
- The 802.11 MAC protocol can operate in
2 different modes:
- Centralized mode
without
contention
- Distributed mode with contention
- Centralized mode
without
contention
- The Point Coordinated Function (PCF):
- Point Coordinated Function = the centralized (operation) mode of the 802.11 protocol
-
Operational overview:
- in Point Coordinated Function,
the access (= permission) to the
transmission medium
is controlled by the
Access Point (AP)/base station:
- Medium access
coordination:
- The AP point coordinator
transmits
CF-Poll frame
to stations (= wireless clients, laptops).
- A node (e.g., laptop) that
received a CF-Poll frame
must transmit
(exactly) one frame.
- If a node does not have any frames to transmit, then it must transmit a null frame.
- The AP point coordinator
transmits
CF-Poll frame
to stations (= wireless clients, laptops).
- in Point Coordinated Function,
the access (= permission) to the
transmission medium
is controlled by the
Access Point (AP)/base station:
- Property:
- There are no collisions in the PCF mode --- because transmissions are regulated by the access point !!!
Note:
- Most 802.11 devices do not support the Point Coordinated Function operational mode !!!
- The Distributed Coordinated Function
(DCF):
- Distributed Coordinated Function =
the distributed (operational) mode
of the 802.11 protocol
- The DCF is the most commonly used operational mode
- Distributed Coordinated Function =
the distributed (operational) mode
of the 802.11 protocol
- Operational overview:
- Each node
determine/decide on its own
whether it is
safe
to transmit a
frame
- In the distributed mode,
nodes/stations will
content (= "fight")
for the channel
- There will be collisions in the distributed mode
Note:
- All 802.11 devices support the DCF function
- Each node
determine/decide on its own
whether it is
safe
to transmit a
frame