802.11 Medium Access Methods

By Jim Geier

November 26, 2002

Medium access is a key aspect of the 802.11 standard. Learn how 802.11 implements medium access, and at the same time gain a better understanding of how to deploy a more effective WLAN.

The 802.11 standard ensures that all stations, both radio-based network interface cards (NICs) and access points, implement access methods for sharing the air medium. When installing wireless LANs (WLAN), most people don't give much thought to these mechanisms. A solid understanding of 802.11's medium access methods, however, will enable you to deal more effectively with issues such as radio frequency interference, denial of services attacks and throughput issues.

Distributed Coordination Function (DCF)

The 802.11 standard makes it mandatory that all stations implement the DCF, a form of carrier sense multiple access with collision avoidance (CSMA/CA). CSMA is a contention-based protocol making certain that all stations first sense the medium before transmitting. The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions.

If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame. The collision avoidance aspect of the protocol pertains to the use of acknowledgements that a receiving station send to the sending station to verify error-free reception. Think of this process of accessing the medium as a meeting where everyone is polite and each person only speaks when no one else is talking. In addition, everyone who understands what the person is saying nods their head in agreement.

The DCF protocol is somewhat more complex than this, though. For example, an 802.11 station utilizes information it gains from other frames that stations are sending over the wireless network. In the control field of each frame, there is a duration field that a sending station places a value in, to indicate how long the station will require the medium. As part of making a decision on whether to transmit a frame, a station must see that the time associated with the duration value of the last frame sent has expired, as well as sense that no physical transmission is taking place. The duration field enables stations to reserve the medium for subsequent frames of some specific 802.11-defined frame exchanges (e.g., RTS/CTS).

Because of its nature, DCF supports the transmission of asynchronous signals. A distinguishing factor of asynchronous signaling is that there are no timing requirements between data carrying frames. For example, the DCF protocol doesn't make any attempt to deliver a series of data frames within any timeframe or at any instant in time. As a result, there is a random amount of delay between each data frame transmission. This form of synchronization is effective for network applications, such as e-mail, Web browsing and VPN access to corporate applications.

DCF Protocol Issues

The DCF protocol is the heart of many WLAN troubles. RF interference is probably the biggest problem. If a source of RF interference (e.g., cordless phone or other WLAN) is present, the DCF can block stations from transmitting for as long as the interfering signal is present. The stations sense enough energy on the medium and wait patiently, in most cases for just a few seconds or minutes. Of course this causes the throughput of the network to drop significantly. That's why you should perform an RF site survey in the facility before installing a WLAN.

Similar to the impact of typical RF interference, someone could implement a denial of service attack, which is a deliberate action to instill RF interference at a level high enough to block a majority of the stations from transmitting. Again, all of the stations will not transmit because they respectfully follow the DCF protocol.

Instead of lasting for only a few seconds, however, a denial of service attack could be planned in a way to corrupt the network for hours or days until the jamming source is found. This type of attack will generally cause the network to be useless (i.e., throughput equal to zero). In order to reduce this impact, maximize the use of directional antennas to minimize the reception of RF signals from outside the facility where someone could conceal themselves with a high-powered jamming device.

Point Coordination Function (PCF)

As an optional access method, the 802.11 standard defines the PCF, which enables the transmission of time-sensitive information. With PCF, a point coordinator within the access point controls which stations can transmit during any give period of time. Within a time period called the contention free period, the point coordinator will step through all stations operating in PCF mode and poll them one at a time. For example, the point coordinator may first poll station A, and during a specific period of time station A can transmit data frames (and no other station can send anything). The point coordinator will then poll the next station and continue down the polling list, while letting each station to have a chance to send data.

Thus, PCF is a contention-free protocol and enables stations to transmit data frames synchronously, with regular time delays between data frame transmissions. This makes it possible to more effectively support information flows, such as video and control mechanisms, having stiffer synchronization requirements.

Timing mechanisms within the 802.11 protocol ensure that stations on the WLAN alternate between the use of DCF and PCF. As a result, the WLAN can support both asynchronous and synchronous information flows. For a period of time, stations will fend for themselves by using CSMA. For the following time period, the stations will wait for a poll from the point coordinator before sending data frames.

Don't run to your access point, though, and expect to find a switch that enables PCF. The only access point that I know of on the market supporting PCF is AOpen's WarpLink AOI-706. The big name vendors, such as Cisco, Proxim, and Symbol, don't include PCF mode.

Some chipsets have PCF functionality embedded somewhere in the firmware, but access point vendors seem to be reluctant to activate it, even though PCF has been part of the 802.11 standard since its inception in 1997. The problem is that the 802.11 standard is fairly vague in defining portions of the PCF protocol. As a result, you'd probably need to use the same vendor for the access points and radio cards to make it work properly. The Wi-Fi Alliance does not include PCF functionality in their interoperability standard.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, WLANs (SAMs, 2001) and offers computer-based training (CBT) courses on WLANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.



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