Most traffic uses the DCF, which provides a standard Ethernet-like contention-based service. The DCF allows multiple independent stations to interact without central control, and thus may be used in either IBSS networks or in infrastructure networks.
Before attempting to transmit, each station checks whether the medium is idle. If the medium is not idle, stations defer to each other and employ an orderly exponential backoff algorithm to avoid collisions.
In distilling the 802.11 MAC rules, there is a basic set of rules that are always used, and additional rules may be applied depending on the circumstances. Two basic rules apply to all transmissions using the DCF:
Additional rules may apply in certain situations. Many of these rules depend on the particular situation "on the wire" and are specific to the results of previous transmissions.
Error Recovery with the DCF
Error detection and correction is up to the station that begins an atomic frame exchange. When an error is detected, the station with data must resend the frame. Errors must be detected by the sending station. In some cases, the sender can infer frame loss by the lack of a positive acknowledgment from the receiver. Retry counters are incremented when frames are retransmitted.
Each frame or fragment has a single retry counter associated with it. Stations have two retry counters: the short retry count and the long retry count. Frames that are shorter than the RTS threshold are considered to be short; frames longer than the threshold are long. Depending on the length of the frame, it is associated with either a short or long retry counter. Frame retry counts begin at 0 and are incremented when a frame transmission fails.
The short retry count is reset to 0 when:
The long retry count is reset to 0 when:
In addition to the associated retry count, fragments are given a maximum "lifetime" by the MAC. When the first fragment is transmitted, the lifetime counter is started. When the lifetime limit is reached, the frame is discarded and no attempt is made to transmit any remaining fragments. Naturally, higher-layer protocols may detect any loss and retransmit the data; when a higher-level protocol such as TCP retransmits data, though, it would be a new frame to the 802.11 MAC and all the retry counters will be restarted.
Using the retry counters
Like most other network protocols, 802.11 provides reliability through retransmission. Data transmission happens within the confines of an atomic sequence, and the entire sequence must complete for a transmission to be successful. When a station transmits a frame, it must receive an acknowledgment from the receiver or it will consider the transmission to have failed. Failed transmissions increment the retry counter associated with the frame (or fragment). If the retry limit is reached, the frame is discarded, and its loss is reported to higher-layer protocols.
One of the reasons for having short frames and long frames is to allow network administrators to customize the robustness of the network for different frame lengths. Large frames require more buffer space, so one potential application of having two separate retry limits is to decrease the long retry limit to decrease the amount of buffer space required.
Backoff with the DCF
After frame transmission has completed and the DIFS has elapsed, stations may attempt to transmit congestion-based data. A period called the contention window or backoff window follows the DIFS. This window is divided into slots. Slot length is medium-dependent; higher-speed physical layers use shorter slot times. Stations pick a random slot and wait for that slot before attempting to access the medium; all slots are equally likely selections. When several stations are attempting to transmit, the station that picks the first slot (the station with the lowest random number) wins. According to the standard, all slot numbers should be equally likely; see the sidebar on Spectralink Voice Priority later in this chapter for one notable exception.
As in Ethernet, the backoff time is selected from a larger range each time a transmission fails. Figure 3-7 illustrates the growth of the contention window as the number of transmissions increases, using the numbers from the 802.11b direct-sequence spread-spectrum (DSSS) physical layer. Other physical layers use different sizes, but the principle is identical. Contention window sizes are always 1 less than a power of 2 (e.g., 31, 63, 127, 255). Each time the retry counter increases, the contention window moves to the next greatest power of two. The size of the contention window is limited by the physical layer. For example, the DS physical layer limits the contention window to 1,023 transmission slots.
When the contention window reaches its maximum size, it remains there until it can be reset. Allowing long contention windows when several competing stations are attempting to gain access to the medium keeps the MAC algorithms stable even under maximum load. The contention window is reset to its minimum size when frames are transmitted successfully, or the associated retry counter is reached, and the frame is discarded.
Figure 3-7. DSSS contention window size
Introduction to Wireless Networking
Overview of 802.11 Networks
11 MAC Fundamentals
11 Framing in Detail
Wired Equivalent Privacy (WEP)
User Authentication with 802.1X
11i: Robust Security Networks, TKIP, and CCMP
Contention-Free Service with the PCF
Physical Layer Overview
The Frequency-Hopping (FH) PHY
The Direct Sequence PHYs: DSSS and HR/DSSS (802.11b)
11a and 802.11j: 5-GHz OFDM PHY
11g: The Extended-Rate PHY (ERP)
A Peek Ahead at 802.11n: MIMO-OFDM
Using 802.11 on Windows
11 on the Macintosh
Using 802.11 on Linux
Using 802.11 Access Points
Logical Wireless Network Architecture
Site Planning and Project Management
11 Network Analysis
11 Performance Tuning
Conclusions and Predictions