Sign in Subscribe

AIFSN (Arbitration Interframe Space Number)

Last updated on 

The AIFSN, or Arbitration Interframe Space Number, is a parameter used in the operation of wireless communication networks, specifically in the context of the IEEE 802.11 standard for wireless LANs. The AIFSN is a value used to determine the duration of the interframe space (IFS) between frames that are contending for transmission on a wireless network. In this article, we will explain what the AIFSN is, why it is important, and how it works.

Wireless networks use a shared medium for communication, meaning that multiple devices can attempt to transmit data at the same time. To avoid collisions between frames, the IEEE 802.11 standard specifies a set of rules for medium access control (MAC) that determine when and how devices are allowed to transmit data.

One of these rules is the use of interframe spacing (IFS) between frames. The IFS is a duration of time that must elapse between the transmission of one frame and the next. The IFS serves to prevent collisions between frames by allowing time for the wireless medium to clear before the next frame is transmitted.

There are several types of IFS defined in the IEEE 802.11 standard, each with a different duration. These include the shortest IFS, called the Short Interframe Space (SIFS), which is used between frames that are part of the same transmission, and the Distributed Interframe Space (DIFS), which is the longest IFS and is used between frames from different devices.

The AIFSN is a value that is used to determine the duration of the DIFS, which is the IFS used between frames from different devices. The AIFSN is a value between 0 and 7, and is set by the network administrator. The AIFSN is used in conjunction with the contention window (CW) to determine the duration of the DIFS.

The CW is another parameter used in the IEEE 802.11 standard, which determines the duration of a random backoff period that devices must wait before attempting to transmit data. The CW is based on the number of previous failed transmission attempts, and is increased after each failed attempt, in order to reduce the probability of collisions between frames.

The duration of the DIFS is determined as follows:

DIFS = SIFS + (AIFSN x SlotTime) + (2 x DIFS)

Where:

  • SIFS is the duration of the Short Interframe Space, which is a fixed duration defined in the IEEE 802.11 standard.
  • AIFSN is the Arbitration Interframe Space Number, which is set by the network administrator.
  • SlotTime is the duration of a time slot, which is a fixed duration defined in the IEEE 802.11 standard.
  • DIFS is the duration of the Distributed Interframe Space, which is the IFS used between frames from different devices.

As can be seen from the above equation, the duration of the DIFS increases as the AIFSN value increases. This means that frames from different devices will have to wait longer before attempting to transmit data if the AIFSN value is set to a higher value.

The purpose of the AIFSN is to provide a mechanism for prioritizing access to the wireless medium for certain types of frames. Frames that require higher priority, such as time-sensitive data, can be given a lower AIFSN value, which will reduce the duration of the DIFS and allow these frames to be transmitted more quickly.

For example, voice data is typically time-sensitive, and requires low latency in order to maintain a high-quality user experience. To ensure that voice data is given priority access to the wireless medium, the AIFSN value for voice frames can be set to a lower value than for other types of frames, such as data frames. This will result in a shorter DIFS duration for voice frames, which will reduce the delay before these frames can be transmitted.

The AIFSN value can also be used to balance the load on the wireless network. Frames that require less priority, such as background data transfers, can be given a higher AIFSN value, which will increase the duration of the DIFS and reduce the probability of collisions with higher priority frames. This can help to ensure that time-sensitive frames are not delayed due to collisions with lower priority frames.

It is worth noting that the AIFSN value is not a guarantee of priority access to the wireless medium. Even frames with a lower AIFSN value must still contend with other frames for access to the medium, and collisions can still occur. However, by using the AIFSN value in conjunction with other parameters, such as the CW, it is possible to increase the probability of successful transmission for frames that require higher priority access to the medium.

In summary, the AIFSN is a parameter used in wireless networks to determine the duration of the interframe space between frames that are contending for transmission. The AIFSN is used in conjunction with other parameters, such as the CW and the IFS, to determine the priority and access to the wireless medium for different types of frames. By adjusting the AIFSN value, network administrators can prioritize access to the medium for time-sensitive frames, balance the load on the network, and reduce collisions between frames.