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Cisco Vs. Meru: The Vendors Speak: Page 3 of 9

Nowhere is this more acute and nowhere is there a greater imperative to play by the rules than in the wireless LAN industry, which uses the IEEE 802.11 standard as its rules of the road. WLANs are half-duplex networks operating in a shared, unlicensed radio medium, with a limited number of available channels. Furthermore, the physics of RF (radio frequency) propagation mean that overlapping WLANs compete with each other for channel use.

The 802.11 standard has been carefully engineered to let devices operate fairly within these critical design parameters. Ignoring the 802.11 industry standard to improve performance is not innovation; it is irresponsible because it can have detrimental effects on other devices and networks, violating the spirit of fairness inherent in the 802.11 standard.

The hundreds of members of the 802.11 working group have toiled for more than 15 years to create a standard that uses a well-understood mechanism for fairly sharing the limited resource of the unlicensed radio band. The standard efficiently provides equal access to all 802.11 devices. The standard also has evolved over this time to address the evolving requirements of the users of 802.11, including the addition of a new standard for providing priority access and quality of service (802.11e) extensions early in 2005.Meru

The basis of the 802.11 standard access mechanism is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to the CSMA/CD (Collision Detection) mechanism used by Ethernet. The carrier sense part relies on hearing the transmissions of other 802.11 devices and refraining from transmitting while those other transmissions are going on. The collision avoidance portion relies on virtual carrier sense, where every 802.11 frame carries a value that indicates the amount of time that frame and its acknowledgement will need in order to complete. This duration value is used by every receiver to indicate that the radio channel is busy, even if the radio can't hear any activity. The standard requires that this duration value accurately indicate this time and specifies exactly how to do this.

As long as everyone adheres to the use of the duration as it is described in the standard, everyone shares the benefits of having equal access to the radio channel. But abuse of this duration value provides an easy way to tip the access mechanism heavily in favor of the abuser, locking out all other 802.11 devices from the channel for any amount of time the abuser chooses. If this behavior were discovered on an unauthorized device in your network, it would be considered a denial-of-service attack.