802.11 Medium Access Mechanisms

802.11-based WLANs use a similar mechanism known as carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is a listen before talk (LBT) mechanism. The transmitting station senses the medium for a carrier signal and waits until the carrier channel is available before transmitting.

Wired Ethernet is able to sense a collision on the medium. Two stations transmitting at the same time increase the signal level on the wire, indicating to the transmitting stations that a collision has occurred. 802.11 wireless stations do not have this capability. The 802.11 access mechanism must make every effort to avoid collisions altogether.

CSMA/CA

CSMA/CA is more ordered than CSMA/CD. To use the same telephone conference call analogy, you make some changes to the scenario:

• Before a participant speaks, she must indicate how long she plans to speak. This indication gives any potential speakers an idea of how long to wait before they have an opportunity to speak.
• Participants cannot speak until the announced duration of a previous speaker has elapsed.
• Participants are unaware whether their voices are heard while they are speaking, unless they receive confirmation of their speeches when they are done.
• If two participants happen to start speaking at the same time, they are unaware they are speaking over each other. The speakers determine they are speaking over each other because they do not receive confirmation that their voices were heard.
• The participants wait a random amount of time and attempt to speak again, should they not receive confirmation of their speeches.

The 802.11 implementation of CSMA/CA is manifested in the distributed coordination function (DCF). To describe how CSMA/CD works, it is important to describe some key 802.11 CSMA/CA components first:

• Carrier sense
• DCF
• Acknowledgment frames
• Request to Send/Clear to Send (RTS/CTS) medium reservation

In addition, two other mechanisms pertain to 802.11 medium access but are not directly tied to CSMA/CA:

• Frame fragmentation
• Point coordination function (PCF)

Carrier Sense

A station that wants to transmit on the wireless medium must sense whether the medium is in use. If the medium is in use, the station must defer frame transmission until the medium is not in use. The station determines the state of the medium using two methods:

• Check the Layer 1 physical layer (PHY) to see whether a carrier is present.
• Use the virtual carrier-sense function, the network allocation vector (NAV).

The station can check the PHY and detect that the medium is available. But in some instances, the medium might still be reserved by another station via the NAV. The NAV is a timer that is updated by data frames transmitted on the medium. For example, in an infrastructure BSS, suppose Martha is sending a frame to George (see Figure 2-4). Because the wireless medium is a broadcast-based shared medium, Vivian also receives the frame. The 802.11 frames contain a duration field. This duration value is large enough to cover the transmission of the frame and the expected acknowledgment. Vivian updates her NAV with the duration value and does not attempt transmission until the NAV has decremented to 0.


Note that stations only update the NAV when the duration field value received is greater than what is currently stored in their NAV. Using the same example, if Vivian has a NAV of 10 milliseconds, she does not update her NAV if she receives a frame with a duration of 5 milliseconds. She updates her NAV if she receives a frame with a duration of 20 milliseconds.

DCF

The IEEE-mandated access mechanism for 802.11 networks is DCF, a medium access mechanism based on the CSMA/CA access method. To describe DCF operation, we first define some concepts. Figure 2-5 shows a time line for the scenario in Figure 2-4.

In DCF operation, a station wanting to transmit a frame must wait a specific amount of time after the medium becomes available. This time value is known as the DCF interframe space (DIFS). Once the DIFS interval elapses, the medium becomes available for station access
contention.

In Figure 2-5, Vivian and George might want to transmit frames when Martha's transmission is complete. Both stations should have the same NAV values, and both will physically sense when the medium is idle. There is a high probability that both stations will attempt to transmit when the medium becomes idle, causing a collision. To avoid this situation, DCF uses a random backoff timer.

The random backoff algorithm randomly selects a value from 0 to the contention window (CW) value. The default CW values vary by vendor and are value-stored in the station NIC. The range of values for random backoff start at 0 slot times and increment up to the maximum value, which is a moving ceiling starting at CWmin and stopping at a maximum value known as CWmax. For the sake of this example, assume that the CWmin value begins at 7 and CWmax value is 255. Figure 2-6 illustrates the CWmin and CWmax values for binary random backoff.

A station randomly selects a value between 0 and the current value of the CW. The random value is the number of 802.11 slot times the station must wait during the medium idle CW before it may transmit. A slot time is a time value derived from the PHY based on RF characteristics of the BSS .

Getting back to the example, Vivian is ready to transmit. Her NAV timer has decremented to 0, and the PHY also indicates the medium is idle. Vivian selects a random backoff time between 0 and CW (in this case, CW is 7) and waits the selected number of slot times before transmitting. Figure 2-7 illustrates this process, with a random backoff value of four slot times.


The Acknowledgment Frame

A station receiving a frame acknowledges error-free receipt of the frame by sending an acknowledgment frame back to the sending station. Knowing that the receiving station has to
access the medium and transmit the acknowledgment frame, you would assume that it is possible for the acknowledgment frame to be delayed because of medium contention. The transmission of an acknowledgment frame is a special case. Acknowledgment frames are allowed to skip the random backoff process and wait a short interval after the frame has been received to transmit the acknowledgment. The short interval the receiving station waits is known as the short interframe space (SIFS) . The SIFS interval is shorter than a DIFS interval by two slot times. It guarantees the receiving station the best possible chance of transmitting on the medium before another station does.

Referring to Vivian's transmission to George, Vivian deferred her transmission attempt for four slot times. The medium was still available, so she transmitted her frame to George, as depicted in Figure 2-9. The AP receives the frame and waits a SIFS interval before sending an acknowledgment frame.


The Hidden Node Problem and RTS/CTS

Vivian might be unable to access the medium because of another station that is within range of the AP yet out of range of her station. Figure 2-10 illustrates this situation. Vivian and George are in range of each other and in range of the AP. Yet neither of them is in range of Tony. Tony is in range of the AP and attempts to transmit on the medium as well. The situation is known as the hidden node problem because Tony is hidden to Vivian and George.

802.11 Frame Fragmentation

Frame fragmentation is a MAC layer function that is designed to increase the reliability of frame transmission across the wireless medium. The premise behind fragmentation is that a frame is broken up into smaller fragments, and each fragment is transmitted individually, as depicted in Figure 2-13. The assumption is that there is a higher probability of successfully transmitting a smaller frame fragment across the hostile wireless medium. Each frame fragment is individually acknowledged; therefore, if any fragment of the frame encounters any errors or a collision, only the fragment needs to be retransmitted, not the entire frame, increasing the effective throughput of the medium.