DCF and HCF coordination functions
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DCF and HCF coordination functions

Overview of DCF and HCF

DCF (Distributed Coordiantaion Function) and HCF (Hybrid Coordination Function) are QoS methods for gaining access to the wireless medium.
802.11-2012 Standard defines:

• DCF (default / mandatory)
• PCF (optional)
• HCF

QoS in the original 802.11 standard

The original 802.11 standard defined two methods in which an 802.11 radio card may gain control of the half-duplex medium:

• Distributed Coordination Function (DCF)
  • DCF is the default method and mandatory method for 802.11 access
  • DCF is a contention based method determining who gets to transmit on the wireless medium next
  • Utilizes multiple checks and balances to try to minimize collisions
  • DCF medium contention mechanisms discussed earlier allow for an 802.11 radio to transmit a single frame.
  • After transmitting a frame, the 802.11 station must contend for the
  • medium again before transmitting another frame.
• Point Coordination Function (PCF)
  • PCF was never adopted by WLAN vendors
  • In PCF, the access point briefly takes control of the medium and polls the clients
  • PCF medium contention mechanisms discussed earlier allow for an 802.11 radio to transmit a single frame.
  • After transmitting a frame, the 802.11 station must contend for the
  • medium again before transmitting another frame.

802.11e (QoS for Wireless)

802.11e (QoS for Wireless) is the new standard that defines enhanced medium access methods; Hybrid Coordination Function (HCF).
HCF combines capabilities from both DCF and PCF and adds enhancements to them to create two channel-access methods:
Enhanced Distributed Channel Access (EDCA) and HCF Controlled Channel Access (HCCA):

• Enhanced Distributed Channel Access (EDCA)
  • EDCA is an extension to DCF
  • The EDCA medium access method will provide for the“prioritization of frames” based on upper-layer protocols i.e. Application traffic, such as voice or video
• Hybrid Coordination Function Controlled Channel Access (HCCA)
  • HCCA has never been adopted by WLAN vendors
  • HCCA is an is an extension of PCF
  • HCCA gives the access point the ability to provide for “prioritization of stations.” In other words, certain client stations will be given a chance to transmit before others

Specifics of DCF and HCF

DCF (Distributed Coordination Function)

DCF has four components as 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, these four checks, all function at the same time:

• Interframe space
  • IFS is a period of time that exists between transmissions of wireless frames.
  • 6 types of interframe spaces (listed shortest to longest):
    • Reduced interframe space (RIFS), highest priority
    • Short interframe space (SIFS), second highest priority
    • PCF interframe space (PIFS), middle priority
    • DCF interframe space (DIFS), lowest priority
    • Arbitration interframe space (AIFS), used by QoS stations
    • Extended interframe space (EIFS), used after receipt of corrupted frames
  • The length of time of each IFS varies depending on the transmission speed of the network
  • Interframe spaces are one line of defense used by CSMA/CA to ensure that only certain types of 802.11 frames are transmitted following certain interframe spaces
    For example, only ACK frames, block ACK frames, data frames, and clear-to-send (CTS) frames may follow a SIFS.
  • Two most common IFS are the SIFS and the DIFS
  • Interframe spacing also acts as a backup mechanism to virtual carrier sense
    

• Duration/ID field
  • One of the fields in the MAC header of an 802.11 frame is the Duration/ID field.
  • The value of the Duration/ID field indicates how long the RF medium wil be busy before another station can contend for the medium.
  • When a client transmits a unicast frame, the Duration/ID field contains a value from 0 to 32,767
  • The Duration/ID value represents the time, in microseconds, that is required to transmit an active frame exchange process so that other radios do not interrupt the process.
  • A client that is transmitting the data frame calculates how long it will take to receive an ACK frame and includes that length of time in the Duration/ID field in the MAC header of the transmitted unicast data frame.
  • The value of the Duration/ID field in the MAC header of the ACK frame that follows is 0 (zero).
  • In the rare case of a PS-Poll frame, the Duration/ID is used as an ID value of a client station using legacy power management.
    

• 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 types of Carrier sense:
  • Virtual Carrier Sense
    • Virtual carrier sense is a layer 2 carrier sense mechanism.
    • 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 information seen in a previous frame transmission.
    • A listening radio hears a frame transmission from another station
      • It looks at the header of the frame and determines whether the Duration/ID field contains a Duration value or an ID value.
      • If the field contains a Duration value, the listening station will set its NAV timer to this value.
      • The listening station will then use the NAV as a countdown timer, knowing that the RF medium should be busy until the countdown reaches 0.
        

• Physical Carrier Sense
  • Physical carrier sense is a layer 1 line of defense.
  • It is possible that a station did not hear the other radio transmitting and was therefore unable to read the Duration/ID field and set its NAV timer.
  • 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.
  • Physical carrier sense has two purposes:
    • To determine whether a frame transmission is inbound for a station to receive. If the medium is busy, the radio will attempt to synchronize with the transmission.
    • To determine whether the medium is busy before transmitting. This is known as the clear channel assessment (CCA). The CCA involves listening for RF transmissions at the Physical layer. The medium must be clear before a station can transmit.
      

• Random backoff timer
  • An 802.11 station may contend for the medium during a window of time known as the backoff timer
  • The station selects a random backoff value using a pseudorandom backoff algorithm.
  • The station chooses a random number from a range called a contention window (CW) value.
  • After the random number is chosen, the number is multiplied by the slot time value.
  • Slot time sizes are dependent on the physical layer specification (PHY) in use (DSSS, OFDM, etc.).
  • The random backoff timer is the final timer used by a station before it transmits.
  • When the backoff time is equal to 0, the client can reassess the channel and, if it is clear, begin transmitting.
  • If no medium activity occurs during a particular slot time, then the backoff timer is decremented by a slot time.
  • If the physical or virtual carrier sense mechanisms sense a busy medium, the backoff timer decrement is suspended, and the backoff timer value is maintained.
  • When the medium is idle for a duration of a DIFS, AIFS, or EIFS period, the backoff process resumes and continues the countdown from where it left off.
  • When the backoff timer reaches 0, transmission commences.
  • Unsuccessful transmissions cause the CW size to increase exponentially up to a maximum value as shown below:

Figure 8.6 Contention window length

The following example is a simple review of the process:

• An OFDM station selects a random number from a contention window of 0–15. For this example, the number chosen is 4.
• The station multiplies the random number of 4 by a slot time of 9μs.
• The random backoff timer has a value of 36μs (4 slots).
• For every slot time during which there is no medium activity, the backoff time is decremented by a slot time.
• The station decrements the backoff timer until the timer is zero.
• The station transmits if the medium is clear.

The random backoff timer is another line of defense and helps minimize the likelihood of two stations trying to communicate at the same time, although it does not fully prevent this from occurring. If a station does not receive an ACK, it starts the carrier sense process over again.

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.