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802.11 Arbitration: I could have been a contender.

Part One: DCF

Wireless is a shared medium. The fundamental method for sharing this medium is DCF, Distributed Coordination Function. This is the method used when QoS is not present.

There are 3 major steps a STA must observe to gain access to the WM (wireless medium). They are Carrier Sense, IFS (Inter-Frame Spacing), and the Random Backoff Timer.

Now let's go through the details of using DCF for channel access.

When a STA needs to transmit it must contend for the medium. This applies to all wireless STA's including access points. The first step a STA performs is carrier sense. When performing physical carrier sense the STA does two things. The STA listens for raw RF energy above its energy detect threshold. It also listens for a PHY header.

During virtual carrier sense the STA listens for a MAC header. The Duration/ID field of the MAC header is what the STA uses to set its NAV (Network Allocation Vector). The STA is performing all of these task concurrently.

Once physical carrier sense determines the medium is idle and the STAs NAV timer has reached zero, the STA will observe a period of time know as the IFS. The IFS type and therefore the length of time observed depends on the type of frame that will be transmitted.

The final step in DCF arbitration is the random backoff timer. A STA must wait the duration of the random backoff timer before transmitting. The STA creates the random backoff timer by selecting an integer from the contention window, which I like to call the contention range. The STA then multiplies the integer selected from the CW by a slot time. Slot times are PHY dependent. They range in value from 9 to 20 microseconds.

Once the backoff timer reaches zero the station can transmit. This is done for every, single frame.

To review, the steps are:

  • Carrier Sense

  • Physical Carrier Sense

  • ED

  • PHY Header

  • Virtual Carrier Sense

  • NAV

  • IFS

  • Random Backoff Timer

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