Beacon transmission in an IBSS network is slightly different. In an IBSS network – each station connected to the IBSS network competes to transmit the beacon at beacon interval time. At the start of each beacon period – there is a beacon generation window consisting of a number of slots.
If the window is 0 to ‘W’ slots (a window of [0,w] slot with fixed slot duration), each IBSS station, picks a different random back-off of slot times before attempting to send a beacon. The Station that wins the medium first will transmit the beacon and the rest of the IBSS stations will reset their slot count and compete to send a beacon at the next beacon transmission time.
A pictorial representation of IBSS Beacon transmission is provided below
FIG COURTESY: 802.11 Standard
Each IBSS station will perform the following action near an impending beacon transmission time
- Each station suspends any pending non-beacon transmission.
- Each station configures a random delay (D1) distributed over a uniformly calculated window of size = aSlotTime * aCWmin
- Each station waits for the period of random delay and if a beacon is not received by it till its random delay timer runs out, the station sends a beacon
- Each station in the IBSS network re-adjusts the TSF timer to the new time sent out in the last beacon transmission
- The Awake period in the above diagram is the time the IBSS station wakes up completely to transmit a beacon or any other frame
The Beacon frame format is shown in the next article