Control Frames

Control frames assist in the delivery of data frames. They administer access to the wireless medium (but not the medium itself) and provide MAC-layer reliability functions.

Common Frame Control Field

All control frames use the same Frame Control field, which is shown in Figure 4-12.

Figure 4-12. Frame Control field in control frames

Protocol version

The protocol version is shown as 0 in Figure 4-12 because that is currently the only version. Other versions may exist in the future.

Type

Control frames are assigned the Type identifier 01. By definition, all control frames use this identifier.

Subtype

This field indicates the subtype of the control frame that is being transmitted.

ToDS and FromDS bits

Control frames arbitrate access to the wireless medium and thus can only originate from wireless stations. The distribution system does not send or receive control frames, so these bits are always 0.

More Fragments bit

Control frames are not fragmented, so this bit is always 0.

Retry bit

Control frames are not queued for retransmission like management or data frames, so this bit is always 0.

Power Management bit

This bit is set to indicate the power management state of the sender after conclusion of the current frame exchange.

More Data bit

The More Data bit is used only in management and data frames, so this bit is set to 0 in control frames.

Protected Frame bit

Control frames may not be encrypted. Thus, for control frames, the Protected Frame bit is always 0.

Order bit

Control frames are used as components of atomic frame exchange operations and thus cannot be transmitted out of order. Therefore, this bit is set to 0.

Request to Send (RTS)

RTS frames are used to gain control of the medium for the transmission of "large" frames, in which "large" is defined by the RTS threshold in the network card driver. Access to the medium can be reserved only for unicast frames; broadcast and multicast frames are simply transmitted. The format of the RTS frame is shown in Figure 4-13. Like all control frames, the RTS frame is all header. No data is transmitted in the body, and the FCS immediately follows the header.

Figure 4-13. RTS frame

Four fields make up the MAC header of an RTS:

Frame Control

There is nothing special about the Frame Control field. The frame subtype is set to 1011 to indicate an RTS frame, but otherwise, it has all the same fields as other control frames. (The most significant bits in the 802.11 specification come at the end of fields, so bit 7 is the most significant bit in the subtype field.)

Duration

An RTS frame attempts to reserve the medium for an entire frame exchange, so the sender of an RTS frame calculates the time needed for the frame exchange sequence after the RTS frame ends. The entire exchange, which is depicted in Figure 4-14, requires three SIFS periods, the duration of one CTS, the final ACK, plus the time needed to transmit the frame or first fragment. (Fragmentation bursts use subsequent fragments to update the Duration field.) The number of microseconds required for the transmission is calculated and placed in the Duration field. If the result is fractional, it is rounded up to the next microsecond.

Figure 4-14. Duration field in RTS frame

Address 1: Receiver Address

The address of the station that is the intended recipient of the large frame.

Address 2: Transmitter Address

The address of the sender of the RTS frame.

Clear to Send (CTS)

The CTS frame, whose format is shown in Figure 4-15, has two purposes. Initially, CTS frames were used only to answer RTS frames, and were never generated without a preceding RTS. CTS frames were later adopted for use by the 802.11g protection mechanism to avoid interfering with older stations. The protection mechanism is described with the rest of 802.11g in Chapter 14.

Figure 4-15. CTS frame

Three fields make up the MAC header of a CTS frame:

Frame Control

The frame subtype is set to 1100 to indicate a CTS frame.

Duration

When used in response to an RTS, the sender of a CTS frame uses the duration from the RTS frame as the basis for its duration calculation. RTS frames reserve the medium for the entire RTS-CTS-frame-ACK exchange. By the time the CTS frame is transmitted, though, only the pending frame or fragment and its acknowledgment remain. The sender of a CTS frame subtracts the time required for the CTS frame and the short interframe space that preceded the CTS from the duration in the RTS frame, and places the result of that calculation in the Duration field. Figure 4-16 illustrates the relationship between the CTS duration and the RTS duration. Rules for CTS frames used in protection exchanges are described with the protection mechanism.

Address 1: Receiver Address

The receiver of a CTS frame is the transmitter of the previous RTS frame, so the MAC copies the transmitter address of the RTS frame into the receiver address of the CTS frame. CTS frames used in 802.11g protection are sent to the sending station, and are used only to set the NAV.

Figure 4-16. CTS duration

Acknowledgment (ACK)

ACK frames are used to send the positive acknowledgments required by the MAC and are used with any data transmission, including plain transmissions, frames preceded by an RTS/CTS handshake, and fragmented frames (see Figure 4-17). Quality-of-service enhancements relax the requirement for a single acknowledgment per Data frame. To assess the impact of acknowledgments on net throughput, see Chapter 25.

Figure 4-17. ACK frame

Three fields make up the MAC header of an ACK frame:

Frame Control

The frame subtype is set to 1101 to indicate an ACK frame.

Duration

The duration may be set in one of two ways, depending on the position of the ACK within the frame exchange. ACKs for complete data frames and final fragments in a fragment burst set the duration to 0. The data sender indicates the end of a data transmission by setting the More Fragments bit in the Frame Control header to 0. If the More Fragments bit is 0, the transmission is complete, and there is no need to extend control over the radio channel for additional transmissions. Thus, the duration is set to 0.

If the More Fragments bit is 1, a fragment burst is in progress. The Duration field is used like the Duration field in the CTS frame. The time required to transmit the ACK and its short interframe space is subtracted from the duration in the most recent fragment (Figure 4-18). The duration calculation in nonfinal ACK frames is similar to the CTS duration calculation. In fact, the 802.11 specification refers to the duration setting in the ACK frames as a virtual CTS.

Figure 4-18. Duration in non-final ACK frames

Address 1: Receiver Address

The receiver address is copied from the transmitter of the frame being acknowledged. Technically, it is copied from the Address 2 field of the frame being acknowledged. Acknowledgments are transmitted in response to directed data frames, management frames, and PS-Poll frames.

Power-Save Poll (PS-Poll)

When a mobile station wakes from a power-saving mode, it transmits a PS-Poll frame to the access point to retrieve any frames buffered while it was in power-saving mode. The format of the PS-Poll frame is shown in Figure 4-19. Further details on the operation of power saving modes appears in Chapter 8.

Figure 4-19. PS-Poll frame

Four fields make up the MAC header of a PS-Poll frame:

Frame Control

The frame subtype is set to 1010 to indicate a PS-Poll frame.

Association ID (AID)

Instead of a Duration field, the PS-Poll frame uses the third and fourth bytes in the MAC header for the association ID. This is a numeric value assigned by the access point to identify the association. Including this ID in the frame allows the access point to find any frames buffered for the now-awakened mobile station.

Address 1: BSSID

This field contains the BSSID of the BSS created by the access point that the sender is currently associated with.

Address 2: Transmitter Address

This is the address of the sender of the PS-Poll frame.

The PS-Poll frame does not include duration information to update the NAV. However, all stations receiving a PS-Poll frame update the NAV by the short interframe space plus the amount of time required to transmit an ACK. The automatic NAV update allows the access point to transmit an ACK with a small probability of collision with a mobile station.