IFS (Interframe Spacing)
Interframe spacing is the time delay that is deliberately added to avoid collision of frames during transmission. Firstly, we will introduce a term called slot time (in microsecs) that is a fixed time interval based on each modulation or PHY used.
| Modulation | Slot Time(micro sec) |
| DSSS | 20 |
| OFDM | 9 |
| HR/DSSS | 20 |
| ERP | 20 – 2.4 GHz long 9 – 2.4 GHz short |
| HT | 20 – 2.4 GHz long 9 – 2.4 GHz short, 5 GHz |
| VHT | 9 |
SIFS (Short IFS)
This delay is intended to be used 1. In RTS-CTS mechanism after the RTS receipt before CTS transmission 2. After the CTS frame receipt before sending DATA frame 3. After every DATA frame receipt before ACK is sent 4. In PCF mode for every frame exchange; except first exchange are error conditions 6. After the receipt of every fragment in a fragment burst
| Modulation | Time (micro sec) |
| FHSS | 28 |
| DSSS | 10 |
| OFDM | 16 |
| HR/DSSS | 10 |
| ERP | 10 |
PIFS (PCF IFS)
This interval is longer than SIFS and is used in PCF mode used by AP that wishes to switch from DCF to PCF mode.
| Modulation | Time = SIFS + Slot time (micro sec) |
| DSSS | 30 |
| OFDM | 25 |
| HR/DSSS | 30 |
| ERP | 30 – 2.4 GHz long 19 – 2.4 GHz short |
| HT | 30 – 2.4 GHz long 25 – 2.4 GHz short, 5 GHz |
| VHT | 25 |
DIFS (Distributed IFS)
This time interval is used in CSMA-CA process after the dot11 station performs physical and virtual carrier sensing methods to determine free medium and before the back off algorithm starts.
| Modulation | Time= SIFS + 2*Slot time (micro sec) |
| DSSS | 50 |
| OFDM | 34 |
| HR/DSSS | 50 |
| ERP | 50 – 2.4 GHz long 28 – 2.4 GHz short |
| HT | 50 – 2.4 GHz long 28 – 2.4 GHz short, 5 GHz |
| VHT | 28 |
AIFS (Arbitration IFS)
This delay is used in QoS stations for all management, few control frames – PS-POLL, RTS, CTS, BlockAckReq, BlockAck and all data frames.
EIFS (Extended IFS)
The time interval used to transmit the next frame after receiving a corrupted or incomplete frame is the EIFS.
RIFS (Reduced IFS)
This is the shortest IFS that is only used in 802.11n network in greenfield mode or network without any legacy devices. It is about 2 micro sec. This is also used in 802.11ad.
RIFS < SIFS < PIFS < DIFS < AIFS < EIFS
BackOff Timer
This timer is used when random backoff algorithm is used in CSMA-CA technique to select a random time period using the contention window. The random integer is multiplied by the slot time for each PHY used.
GI (Guard Interval)
The time period used to separate symbols to avoid inter symbol interference. This is 800ns generally. However, we can reduce this time period to 400 ns in 802.11n , 802.11ac and is called SGI (Short GI).
Beacon Interval
The time interval in which beacons are sent by the access points is a beacon interval. This value is 102.4 ms or 100 TU (time units) by default and can be configured . Though beacons are expected to be sent at every 100 TU, it might not happen always. These frames can be sent at TBTT (Target Beacon Transmission time), which means either at 100 TU or a little after that. This is only because beacons also have to contend for the medium.
Reference:
CWAP Official Study Guide
Home 