CSMA/CA in IEEE 802.11

1

Physical carrier sense, andVirtual carrier sense using Network Allocation

Vector (NAV)NAV is updated based on overheard

RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission

Nodes stay silent when carrier sensed (physical/virtual)

Backoff intervals used to reduce collision probability

Backoff Interval

2

When transmitting a packet, choose a backoff interval in the range [0,cw]cw is contention window

Count down the backoff interval when medium is idleCount-down is suspended if medium becomes

busy

When backoff interval reaches 0, transmit RTS

DCF Example

3

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2

cw = 31

B2 = 10

Backoff Interval

4

The time spent counting down backoff intervals is a part of MAC overhead

Choosing a large cw leads to large backoff intervals and can result in larger overhead

Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

5

Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed

IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence

Binary Exponential Backoff in DCF

6

When a node fails to receive CTS in response to its RTS, it increases the contention windowcw is doubled (up to an upper bound)

When a node successfully completes a data transfer, it restores cw to Cwmin

cw follows a sawtooth curve

802.11 Handoff

Scanning Phase in Current 802.11Handoff ProcessAP discovery by scanning

Explicitly scan all channelsPassive scan

The client switches to a candidate channel and listens for periodic beacon packets from APs.

Active scan The client actively broadcasts a probe packet on

each channel to force APs to respond immediately.

New AP selectionCompare the signal strength

Problems in 802.11 HandoffApproximate latencies for current 802.11 handoff

Scanning: 350-500msAuthentication: < 10 ms(Re)association: < 10 ms (w/o IAPP)

The client only monitors the signal to the current AP and handoffs only after the service degrades below an acceptable threshold.

SolutionsContinuously monitoring the proximity of nearby

802.11 APs !Maintain AP neighboring graph