DIFS (Distributed interframe space)

Introduction:

Distributed Interframe Space (DIFS) is a fundamental concept in the IEEE 802.11 standard for wireless local area networks (WLANs). It is a time interval that is used by stations to contend for the medium access in the absence of higher priority frames. This time interval is part of the Media Access Control (MAC) protocol and is used to avoid collisions between multiple stations trying to transmit at the same time.

Background:

IEEE 802.11 is a standard that defines the physical and media access control (MAC) layers of wireless local area networks (WLANs). The MAC layer is responsible for providing access to the wireless medium by multiple stations. The access method used by the MAC layer is known as the Distributed Coordination Function (DCF), which is a contention-based protocol. In other words, multiple stations compete with each other to access the wireless medium. To avoid collisions, the MAC layer uses a set of rules called Interframe Spaces (IFS). These IFS values define the time intervals that must elapse between two consecutive frames transmitted by a station.

What is DIFS?

DIFS is one of the four Interframe Spaces (IFS) defined by the IEEE 802.11 standard. The other three IFS values are Short Interframe Space (SIFS), Point Coordination Function Interframe Space (PCF-IFS), and Extended Interframe Space (EIFS). DIFS is the longest IFS value among these four IFS values. It is used by a station to contend for the medium access in the absence of higher priority frames.

The duration of DIFS is defined in terms of time slots. A time slot is the smallest unit of time that is used by the MAC protocol. The duration of a time slot is fixed and depends on the physical layer parameters of the WLAN. For example, in a WLAN that uses the 2.4 GHz band, the duration of a time slot is 20 μs.

The duration of DIFS is defined as follows:

DIFS = SIFS + (2 * SlotTime)

where SIFS is the duration of the Short Interframe Space and SlotTime is the duration of a time slot. In other words, DIFS is equal to the duration of SIFS plus two time slots. The value of SIFS is fixed and is usually set to 10 μs.

The role of DIFS in WLANs:

DIFS is used by a station to contend for the medium access in the absence of higher priority frames. When a station has data to transmit, it first listens to the wireless medium. If the medium is idle for a duration of DIFS, the station can start transmitting its data. If the medium is busy, the station waits until the medium becomes idle for a duration of DIFS before attempting to transmit its data.

DIFS is also used to avoid collisions between multiple stations that attempt to transmit at the same time. When multiple stations attempt to transmit at the same time, they all listen to the medium and wait for a duration of DIFS. If the medium is still idle after the DIFS duration, each station chooses a random backoff time and waits for that time before attempting to transmit its data. The backoff time is chosen randomly to avoid collisions between multiple stations. If two or more stations choose the same backoff time, a collision occurs, and the stations must wait for a random backoff time before attempting to transmit again.

DIFS also plays an important role in the interoperation between different WLANs. When a station roams from one WLAN to another, it must use the appropriate IFS values for the new WLAN. If the new WLAN has a different IFS value for DIFS, the station must wait for the appropriate duration of DIFS before attempting to transmit its data.

Conclusion

Overall, Distributed Interframe Space (DIFS) is an essential concept in the IEEE 802.11 standard for wireless local area networks. It defines a time interval that is used by stations to contend for the medium access in the absence of higher priority frames. This time interval is part of the Media Access Control (MAC) protocol and is used to avoid collisions between multiple stations trying to transmit at the same time.

DIFS is the longest Interframe Space (IFS) value among the four IFS values defined by the IEEE 802.11 standard. Its duration is defined in terms of time slots, and it is used by a station to avoid collisions and contend for the medium access in the absence of higher priority frames.