Detailed PCF Framing

Several frame types are used exclusively within the contention-free period. They combine, in various states, data transmission, acknowledgment, and polling. This section describes when various frames are used and how the different functions interact during frame exchanges. Contention-free frames combine several functions into a single frame.

Data+CF-Ack

The Data+CF-Ack frame combines two different functions for transmission efficiency. Data is transmitted in the frame payload, and the frame implicitly acknowledges the receipt of data received one short interframe space previously. Generally, the data and the acknowledgment are intended for two separate stations. In Figure 9-3, the contention-free acknowledgment is coupled with the data for transmission to the access point in the previous frame, but the data may be intended for any station on the 802.11 network.

Figure 9-3. Data+CF-Ack usage

This frame is used only in infrastructure networks because it is transmitted during the contention-free period. It may be transmitted by either the access point or a mobile station. During the contention-free period, however, the access point is responsible for polling, and it is unlikely that it would transmit this frame subtype because it does not include a poll.

Data+CF-Poll

The Data+CF-Poll frame is used by access points in infrastructure networks during the contention-free period. When the access point does not need to acknowledge any outstanding frames, it sends a Data+CF-Poll to transmit data to the recipient and allows the recipient to send one buffered frame in response. The data in the frame body must be intended for the recipient of the poll; the two operations cannot be "split" across two different receivers. In Figure 9-3, the access point uses a Data+CF-Poll frame to send one frame to the mobile station and to solicit the response.

Data+CF-Ack+CF-Poll

The Data+CF-Ack+CF-Poll frame is used by access points in infrastructure networks during the contention-free period. When the access point has data to transmit, must acknowledge a frame, and needs to poll a station on the polling list, all the functions can be combined into one frame. Figure 9-4 illustrates the usage of Data+CF-Ack+CF-Poll. As with Data+CF-Ack, the components of the Data+CF-Ack+CF-Poll frame are generally intended for different stations. The data transmission and polling must be intended for the same station, but the acknowledgment is for the previous transmission.

Figure 9-4. Usage of Data+CF-Ack+CF-Poll

The figure begins with mobile station 1 (MS1) transmitting a Data+CF-Ack frame. The Data must go to the access point, but the CF-Ack is used to acknowledge the previous Data frame transmitted by the access point. (That frame is not shown in the figure.) Moving down the polling list, the access point then polls mobile station 2 (MS2). However, the access point must acknowledge the data from MS1, which it does by transmitting a frame with a CF-Ack component. When the access point also has data to transmit, all three features can be combined into one omnibus frame. The Data and CF-Poll components are intended for the recipient of the frame, but the CF-Ack is intended for the transmitter of the previous frame. MS1 must listen to the access point frames to note the acknowledgment.

CF-Ack (no data)

When only an acknowledgment is required, a header-only frame with just the CF-Ack function can be transmitted. In Figure 9-4, if MS2 had no data to transmit, it would have responded with a CF-Ack frame.

CF-Poll (no data)

CF-Poll can also be transmitted by itself. Naturally, only access points perform this function, so the CF-Poll frame is transmitted only by access points in infrastructure networks during the contention-free period.

"Naked" CF-Polls are transmitted when the access point has no buffered data for the recipient and does not need to acknowledge the receipt of previous frames. One common situation in which no acknowledgment is necessary is when the access point transmits a CF-Poll and the polled station has no data and does not respond. If the access point has no data for the next station on the polling list, it transmits a CF-Poll, as in Figure 9-5.

Figure 9-5. CF-Poll framing usage

In Figure 9-5, the access point attempts to transmit data to MS1 but does not receive a response. After the PCF interframe space has elapsed, the access point can proceed down the polling list to MS2. No frame from MS1 needs to be acknowledged, and if the access point has no data for MS2, it can use a CF-Poll to allow MS2 to send data.

CF-Ack+CF-Poll (no data)

The final subtype of Data frame is the CF-Ack+CF-Poll, which is also transmitted by access points. Like all CF-Poll frames, it is used only during the contention-free period and only by access points. It incorporates the acknowledgment function and the polling function into a frame with no data. Figure 9-6 illustrates its usage.

Figure 9-6. CF-Ack+CF-Poll usage

The scenario is a slight variation on the previous setting. Instead of a timeout waiting for MS1 to respond, MS1 returns a frame. When the access point takes control of the medium, it uses a CF-Ack+CF-Poll to acknowledge receipt of the frame from MS1 and notifies MS2 that it is allowed to send a frame.

Contention-Free End (CF-End)

When the contention-free period ends, the access point transmits a CF-End frame to release stations from the PCF access rules and begin contention-based service. The format of the CF-End frame is shown in Figure 9-7. Four fields make up the MAC header of the CF-End frame:

Frame Control

The frame subtype is set to 1110 to indicate a CF-End frame.

Duration

CF-End announces the end of the contention-free period and thus does not need to extend the virtual carrier sense. Duration is set to 0. Stations that receive the CF-End frame cut the virtual carrier sense short to resume contention-based access.

Address 1: Receiver Address

CF-End is relevant to the operation of all mobile stations, so the receiver address is the broadcast address.

Address 2: BSSID

CF-End is announced by the access point to all the stations associated with its BSS, so the second address field is the BSSID. In infrastructure networks, the BSSID is the address of the wireless interface in the access point, so the BSSID is also the transmitter address.

Figure 9-7. CF-End frame

CF-End+CF-Ack

When the contention-free period ends, the access point transmits a CF-End frame to release stations from the PCF access rules and then begins contention-based service using the DCF. If the access point must also acknowledge receipt of data, it may simultaneously end the contention-free period and acknowledge the previous frame by using the CF-End+CF-Ack frame, which combines both functions. The format of the CF-End+CF-Ack frame is shown in Figure 9-8. Four fields make up the MAC header of the CF-End+CF-Ack frame:

Frame Control

The frame subtype is set to 1111 to indicate a CF-End+CF-Ack frame.

Duration

CF-End+CF-Ack announces the end of the contention-free period and thus does not need to extend the virtual carrier sense. Duration is set to 0.

Address 1: Receiver Address

CF-End+CF-Ack is relevant to the operation of all mobile stations, so the receiver address is the broadcast address.

Address 2: BSSID

CF-End+CF-Ack is announced by the access point to all the stations associated with its BSS, so the second address field is the BSSID. In infrastructure networks, the BSSID is the address of the wireless interface in the access point, so the BSSID is also the transmitter address.

Figure 9-8. CF-End+CF-Ack frame

CF Parameter Set

Access points that support contention-free operation may include the CF Parameter Set information element, which is shown in Figure 9-9. CF Parameter Set elements are included in Beacon frames to keep all mobile stations apprised of contention-free operations. They are also included in Probe Response frames to allow stations to learn about contention-free options supported by a BSS. Four fields make up the CF Parameter Set information element:

CFP Count

This field, which is one byte in length, tells how many DTIM frames will be transmitted before the start of the next contention-free period. Zero indicates that the current frame is the start of contention-free service.

CFP Period

This one-byte field indicates the number of DTIM intervals between the start of contention-free periods.

CFP MaxDuration

This value is the maximum duration of the contention-free period as measured in time units (TUs). Mobile stations use this value to set the NAV to busy for the entire contention-free period.

CFP DurRemaining

This value is the number of TUs remaining in the current contention-free period. Mobile stations use it to update the NAV throughout the contention-free period. When DCF-based contention-free service is provided, it is set to 0.

Figure 9-9. CF Parameter Set information element