CS485 Sylabus

Modes of operations

The 802.11 protocol can operate in 2 different modes:

Centralized mode:

The access to the transmission medium is controlled by the Access Point (AP)
The AP point coordinator sends CF-Poll frame to stations.
A polled station is permitted to transmit one frame.
In case the polled stations does not have any frames to send, then it must transmit null frame.
There are no collisions in the centrilized mode

This mode is implemented as the Point Coordinated Function (PCF) in 802.11

Most 802.11 devices does not have the PCF function built-in.... !!!

"PCF seems to be implemented only in very few hardware devices as it is not part of the Wi-Fi Alliance's interoperability standard." (from Wikipedia)

Distributed mode:

The access to the transmission medium is not controlled by a centralized agent
Each node must determine on its own whether it is safe to transmit a frame
In the distributed mode, stations will content for the channel
There will be collisions in the distributed mode

This mode is implemented as the Distributed Coordinated Function (DCF) in 802.11

All 802.11 devices have the DCF function built-in !!!

MAC level Acknowledgement

Unlike IEEE 802.3 (Ethernet), the wireless 802.11 scheme uses a MAC level Acknowledgement (just like Aloha)

MAC level ACK:

Interframe Spacing

All nodes must use the Distributed Coordination Function (DCF) to gain access to the transmission medium
First step in DCF: Carrier sensing before transmission
Notes:
Different type of frames uses different IFS durations !!!

Types of IFS:

SIFS = Short Interframe Spacing (has the shortest duration)
SIFS is used as sensing delay for transmitting highest priority ACK frames
PIFS = Point Coordination Function (PCF) Interframe Spacing (has the second shortest duration)
PIFS is used as sensing delay by a base station (= coordination point) that operate in a special transmission coordinator role
DIFS = Distributed Coordination Function (DCF) Interframe Spacing (has the "normal" duration)
DIFS is used as sensing delay for transmitting ordinary priority data frames
EIFS = Extended Interframe Spacing (has the longest duration)
The EIFS is derived from the SIFS and the DIFS and the length of time it takes to transmit an ACK frame at 1 Mbit/s by the following equation:
EIFS is used as sensing delay for transmitting ordinary priority data frames when it has recently received a corrupted frame
(More later)

Interframe Spacing and Priority of frames

The length of the interface space determines the priority of a frame.
Property:

Example:

Suppose node A just finished sending a data frame to node B
Node B wants to send an ACK frame back to node A
But in the mean time, node C wants to send a data frame to a node D:
Timing diagram of the events:

The use of the EIFS duration

The EIFS is a very long waiting duration in 802.11
It is not intended for regulating priority but to overcome a problem

When is EIFS used:

Pre-requisite knowledge:
The EIFS is used in DCF when a station wants to send a data frame after it has monitored an erroneous frame
I.e., the previous MAC frame was incomplete or has an incorrect CRC.

Example:

The reason behind its use:

The node has detected a corrupted frame
It could be because the node was nearly out of range of the sender
The intended receiver may have received the frame OK
The receiving station would send an ACK...
Now.... the EIFS will prevent a station from corrupting the ACK frame.

Example:

Sequence of events:

A sends a frame to B
C receives the frame with an error (it might be interference at C or it's slightly out of range)
B receives the frame correctly
B will send the ACK frame
C will not hear it
Under normal operation, C will transmit after delaying DIFS
C will sense an idle channel after DIFS and transmit...

Result: collision an important ACK frame at node A

Solution: