CS485 Sylabus

### Prioritizing transmissions in (wireless) IEEE 802.11

• Assigning priority in a distributed manner

• Scenario:

• Multiple nodes are waiting for a transmission (node A) to finish:

The (multiple) nodes will transmit when the transmission is complete

• Problem description:

 The multiple nodes are not aware of each other We must make sure that node B transmits first We must do so without using any messages communicated between the nodes

• Solution:

• When a node want to transmit a frame:

1. The node must first wait ("defer") a predefined amount of time before the node can transmit its frame

2. If the channel is idle after waiting the predefined time:

• The node will transmit the frame (immediately)

Schematically:

Otherwise:

 The node must wait until the current transmission is over Then go back to step 1: defer the predefined wait time again before transitting !!!

• Key to prioritizing the ACK transmission:

• Different types of frames will use different waiting (defer) time !!!

In fact:

 Higher priority frames will use a shorter waiting time !!!

Example:

• Interframe Spacing

• Interframe Spacing

 Interframe Spacing (IFS) = the period of time that a (transmitting) node must wait (and listen) before the node can start transmitting a frame

• Partial flow chart of the 802.11 Medium Access Protocol:

Explanation:

• A (transmitting) node must listen (monitor) the transmission medium for IFS amount of time

 The IFS for different types of frames are different !!!

• If there are no transmissions for IFS amount of time, the node will transmit its frame

• If the node detect a transmission during the IFS wait time:

 The node will back off and try again later

• The various types of IFS in 802.11

• Types of IFS:

• SIFS = Short Interframe Spacing (has the shortest duration)

 SIFS = 28 μsec

Usage:

 SIFS is used as sensing delay for transmitting ACK frames !!!

• PIFS = Point Coordination Function (PCF) Interframe Spacing (has the second shortest duration)

 PIFS = SIFS + 1 Slot Time = 78 μsec

Usage:

 PIFS is used as sensing delay by a base station (= coordination point) that operates in a special coordination mode

• DIFS = Distributed Coordination Function (DCF) Interframe Spacing (has the "normal" duration)

 PIFS = SIFS + 2 × Slot Time = 128 μsec

Usage:

 DIFS is used as sensing delay for transmitting data frames

• EIFS = Extended Interframe Spacing (has the longest duration)

 EIFS = Ack frame duration + SIFS + DIFS

Usage:

 EIFS is used by a transmitting node that received a corrupted data frame

Explanation:

• Every node in IEEE 802.11 will continuously receive and CRC check all frames !!!

• If a received frame contains an (CRC) error then:

 A node will defer EIFS duration instead of DIFS before transmitting a data frame.

Schematically:

• Reason:

• The corrupt frame may be a data frame for another node (far away -- that's why it was corrupt)

• That (other) node may have received the data frame correctly !

• By waiting EIFS, the node will let the other node transmit the ACK frame without collision !!!