Contention-Free Access Using the PCF

If contention-free delivery is required, the PCF may be used. The PCF is an optional part of the 802.11 specification; products are not required to implement it. However, the IEEE designed the PCF so stations that implement only the distributed coordination function (DCF) will interoperate with point coordinators.

Contention-free service is not provided full-time. Periods of contention-free service arbitrated by the point coordinator alternate with the standard DCF-based service. The relative size of the contention-free period can be configured. 802.11 describes the contention-free periods as providing "near isochronous" services because the contention-free periods will not always start at the expected time, as described in the "Contention-Free Period Duration" section later in this chapter.

Contention-free service uses a centralized access control method. Access to the medium is restricted by the point coordinator, a specialized function implemented in access points. Associated stations can transmit data only when they are allowed to do so by the point coordinator. In some ways, contention-free access under the PCF resembles token-based networking protocols, with the point coordinator's polling taking the place of a token. Fundamentals of the 802.11 model remain in place, however. Although access is under the control of a central entity, all transmissions must be acknowledged.

PCF Operation

Figure 9-1 shows a transfer using the PCF. When the 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 case is controlled by the PCF, while access to the medium in the latter case is controlled by the DCF and the rules from Chapter 8. The contention period must be long enough for the transfer of at least one maximum-size frame and its associated acknowledgment. Alternating periods of contention-free service and contention-based service repeat at regular intervals, which are called the contention-free repetition interval.

Figure 9-1. Using the PCF

Reserving the medium during the contention-free period

At the beginning of the contention-free period, the access point transmits a Beacon frame. One component of the beacon announcement is the maximum duration of the contention-free period, CFPMaxDuration. All stations receiving the Beacon set the NAV to the maximum duration to lock out DCF-based access to the wireless medium.

As an additional safeguard to prevent interference, all contention-free transmissions are separated only by the short interframe space and the PCF interframe space. Both are shorter than the DCF interframe space, so no DCF-based stations can gain access to the medium using the DCF.

The polling list

After the access point has gained control of the wireless medium, it polls any associated stations on a polling list for data transmissions. During the contention-free period, stations may transmit only if the access point solicits the transmission with a polling frame. Contention-free polling frames are often abbreviated CF-Poll. Each CF-Poll is a license to transmit one frame. Multiple frames can be transmitted only if the access point sends multiple poll requests.

The polling list is the list of privileged stations solicited for frames during the contention-free period. Stations get on the polling list when they associate with the access point. The Association Request includes a field that indicates whether the station is capable of responding to polls during the contention-free period.

Transmissions from the Access Point

Generally, all transmissions during the contention-free period are separated by only the short interframe space. To ensure that the point coordinator retains control of the medium, it may send to the next station on its polling list if no response is received after an elapsed PCF interframe space. (Such a situation is illustrated in Figure 9-1.) The access point polls the second station on its list but receives no response. After waiting one PCF interframe space, the access point moves to the third station on the list. By using the PCF interframe space, the access point ensures that it retains access to the medium.

The access point may use several different types of frames during the contention-free period. During this period, the point coordinator has four major tasks. In addition to the "normal" tasks of sending buffered frames and acknowledging frames from the stations, the point coordinator can poll stations on the polling list to enable them to send frames; it may also need to transmit management frames.

Time in the contention-free period is precious, so acknowledgments, polling, and data transfer may be combined to improve efficiency. When any subset of these functions are combined into a single frame, the result is a bit strange. A single frame could, for example, acknowledge the receipt of the previous frame, poll a different station for buffered data, and send its own data to the station on the polling list.

Several different frame types can be used in the contention-free period:

Data

The standard vanilla Data frame is used when the access point is sending a frame to a station and does not need to acknowledge a previous transmission. The standard Data frame does not poll the recipient and thus does not allow the recipient to transmit any data in return. The data only frame used in the contention-free period is identical to the Data frame used in contention-based periods.

CF-Ack

This frame is used by stations to acknowledge the receipt of a frame when no data needs to be transmitted. Contention-free acknowledgments are longer than the standard control frame acknowledgment, so this frame may not be used in actual implementations.

CF-Poll

CF-Poll frames are sent by the access point to a mobile station to give the mobile station the right to transmit a single buffered frame. It is used when the access point does not have any data for the mobile station. When a frame for the mobile station is available, the access point uses the Data+CF-Poll frame type.

Data+CF-Ack

This frame combines data transmission with an acknowledgment. Data is directed to the frame recipient; the acknowledgment is for the previous frame transmitted and usually is not for the recipient of the data.

Data+CF-Poll

This frame is used by access points to transmit data to a mobile station and request one pending frame from the mobile station. The Data+CF-Poll can only be sent by the access point during the contention-free period.

CF-ACK+CF-Poll

This frame acknowledges the last frame from one of the access point's clients and requests a buffered frame from the next station on the polling list. It is directed to the next station on the polling list, although the acknowledgment may be intended for any mobile station associated with the access point.

Data+CF-ACK+CF-Poll

This frame brings together the data transmission, polling feature, and acknowledgment into one frame for maximum efficiency.

CF-End

This frame ends the contention-free period and returns control of the medium to the contention-based mechanisms of the DCF.

CF-End+CF-Ack

This is the same as the CF-End frame but also acknowledges the previously transmitted Data frame.

Any Management

No restriction is placed by the standard on which management frames can be transmitted during the contention-free period. If the rules applying to a particular frame type allow its transmission, the access point may transmit it.

Contention-Free Period Duration

The minimum length of the contention period is the time required to transmit and acknowledge one maximum-size frame. It is possible for contention-based service to overrun the end of the contention period, however. When contention-based service runs past the expected beginning of the contention-free period, the contention-free period is foreshortened, as in Figure 9-2.

Figure 9-2. Data+CF-Ack and Data+CF-Poll usage

When the contention-free period is foreshortened, the existing frame exchange is allowed to complete before the beacon announcing the start of contention-free operation is transmitted. The contention-free period is shortened by the amount of the delay. Contention-free service ends no later than the maximum duration from the expected beginning point, which is referred to as the Target Beacon Transmission Time (TBTT).

The point coordinator may also terminate the contention-free period prior to its maximum duration by transmitting a CF-End frame. It can base this decision on the size of the polling list, the traffic load, or any other factor that the access point considers important. Products will sometimes selectively use the PCF for a particular frame exchange by starting contention-free service, performing the desired frame transmission, and then ending it.