WLAN DCF vs PCF: Understanding the Difference in Medium Access

WLAN stands for Wireless Local Area Network. It’s essentially a small network composed of Access Points (APs) and Stations (STAs). Access points are similar to base stations, while stations act as clients, workstations, PCs, or laptops. Both DCF and PCF are 802.11 medium access types.

• DCF stands for Distributed Coordination Function.
• PCF stands for Point Coordination Function.

This page will outline the key differences between DCF and PCF medium access types as defined in the IEEE 802.11 WLAN standard. These are the mechanisms for implementing CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) in Wireless LANs.

Understanding Carrier Sense

Before diving into the differences between DCF and PCF, let’s understand what “carrier sense” means. Carrier sense refers to a station’s ability to detect whether the wireless medium is currently occupied by another station or if it’s free for transmission.

There are two primary mechanisms for sensing the medium:

• Physical Layer Check (Layer-1): Examining the physical layer to detect the presence of a carrier signal.
• Virtual Carrier Sense (NAV): Using the NAV (Network Allocation Vector).

It’s best practice to utilize both mechanisms to accurately determine the idle state of the medium. The NAV is essentially a timer maintained by each station, updated based on the duration information found in 802.11 frames. Every 802.11 frame includes a duration field. This field is usually set to a value large enough to accommodate the transmission of the WLAN frame and the subsequent reception of the ACK (Acknowledgement) frame.

As illustrated in Figure 1:

• Suppose Annie is transmitting a frame to Gracy.
• Joseph, in this scenario, updates his NAV to match the duration specified in Annie’s MAC frame.
• Joseph ensures he doesn’t transmit anything until his NAV timer reaches zero.

It’s important to note that stations can update their NAV if the frame duration they overhear is longer than their currently set NAV value. However, if the overheard frame duration is shorter than the existing NAV setting, the NAV remains unchanged.

DCF - Distributed Coordination Function

DCF forms the foundation of the standard CSMA/CA access mechanism. Similar to Ethernet, it first verifies that the radio link is clear before initiating a transmission. To minimize collisions, stations employ a random backoff procedure after each frame transmission. The station that wins the backoff seizes the channel. In specific situations, DCF might utilize the CTS/RTS (Clear To Send/Request To Send) clearing technique to further decrease the chances of collisions.

DCF is the most common MAC access type used in WLAN traffic. This is because it’s similar to standard Ethernet, offering a contention-based service.

Key features of DCF:

• Allows multiple STAs to communicate without centralized control.
• Can be used in both IBSS (Independent Basic Service Set) and infrastructure network types.
• Each station checks the medium before transmitting, transmitting only when the medium is idle.
• If the medium is busy, stations defer transmission and use a simple exponential back-off algorithm to avoid collisions.

There are two main rules applied to transmissions based on the DCF concept:

• Rule 1: A transmission can be initiated immediately if the medium has been idle for a period greater than the DIFS (Distributed Interframe Space) period. Carrier sensing is performed using both the PHY layer and the NAV.
  • The medium is considered free for at least the duration of DIFS if the previous frame was received without errors.
  • If errors occurred during the previous frame reception, the medium must be free for a duration equal to EIFS (Extended Interframe Space).
• Rule 2: If the medium is occupied, the STA must wait until the channel becomes free or idle. This waiting period in 802.11 is termed “access deferral”.

When access is deferred:

• The STA waits for the medium to be free for the duration of DIFS.
• The STA then prepares for exponential backoff.

WLAN PCF - Point Coordination Function

Point Coordinators are utilized to ensure contention-free access to the medium. Point coordinators reside within access points, restricting PCF usage to infrastructure networks. To gain priority over standard contention-based services, PCF allows stations to transmit frames after a shorter interval.

The PCF isn’t widely implemented. However, the IEEE designed the PCF to ensure that stations that only implement the distributed coordination function (DCF) can still interoperate with point coordinators.

Contention-free service isn’t provided continuously. Instead, periods of contention-free service (arbitrated by the point coordinator) alternate with standard DCF-based service. When PCF is used, time on the medium is divided into the Contention-Free Period (CFP) and the Contention Period. Access to the medium in the former is controlled by the PCF, while access in the latter is controlled by the DCF.