Tuesday, 31 December 2013

DCF - Distributed Coordination Function

Distributed Coordination Function (DCF) - mandatory access method of 802.11 standard

Medium access method that utilizes multiple checks and balances to try to minimize collisions.
These checks and balances can also be thought of as several lines of defense.

HCF - Hybrid Coordination Function - specifies advanced QoS methods

Components of DCF:

  • Interframe Space (IFS)
  • Duration/ID Field
  • Carrier Sense
  • Random back-off timer

These above are checks and balances that work together at the same time to ensure that only one 802.11 radio is transmitting on the half-duplex medium.

Interframe Space (IFS):

IFS is a period of time that exists between transmissions of wireless frames
There are 6 types of IFS (from shortest to longest):

  • Reduced IFS (RIFS), highest priority
  • Short IFS (SIFS), second highest priority
  • PC IFS (PIFS), middle priority
  • DCF IFS (DIFS), lowest priority
  • Arbitration IFS (AIFS), used by QoS stations
  • Extended IFS (EIFS), used with retransmissions

Only ACK frames, data frames, and CTS frames may follow a SIFS.

ACK frame is the highest priority frame.

Two most common IFS are: SIFS and DIFS

RIFS < SIFS < PIFS < DIFS < AIFS < EIFS

Interframe spaces are all about what type of 802.11 traffic is allowed next.


Duration/ID field:

A value form 0 to 32,767. The value of the Duration/ID field indicates how long the RF medium will be busy before another station can contend for the medium.
It's a field in the MAC header of an 802.11 frame.

Carrier Sense:

The first step that an 802.11 CSMA/CA device needs to do to begin transmitting is to perform a carrier sense. This is a check to see whether the medium is busy.

Two ways of carrier sense:

  • Virtual Carrier Sense
  • Physical Carrier Sense

Virtual Carrier Sense:

Uses a timer mechanism known as the Network Allocation Vector (NAV).
The NAV timer maintains a prediction of future traffic on the medium based on Duration value seen in a previous frame transmission.

The listening station will use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0.

When an 802.11 radio is not transmitting, it is listening.

A station cannot contend for the medium until its NAV timer is 0, nor can a station transmit on the medium if the NAV timer is set to a non zero value.

Physical Carrier Sense:

Physical carrier sensing is performed constantly by all stations that are not transmitting or receiving. When a station performs a physical carrier sense, it is actually listening to the channel to see whether any other transmitters are taking up the channel.

It has two purposes:

  • Determine whether a frame is inbound for a station to receive. If the medium is busy, the radio will attempt to synchronize with the transmission.
  • Determine whether the medium is busy before transmitting. This is known as a Clear Channel Assessment (CCA). The CCA involves listening for 802.11 RF transmission at the Physical Layer.
             The medium must be clear before a station can transmit.

Both virtual and physical carrier senses are always happening at the same time.
Virtual carrier sense is a Layer 2 line of defense, while Physical carrier sense is a Layer 1 line of defense.

Random Back-off Timer:


The station selects a random back-off value. The value is chosen from range of 0 to the initial contention window value. This value is then multiplied by slot time (it differs among different spread spectrum techniques). This back-off timer is used before a station can transmit.

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