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UGS (Unsolicited Grant Service)

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Unsolicited Grant Service (UGS):

Unsolicited Grant Service (UGS) is a Quality of Service (QoS) mechanism used in Wi-Fi networks that aims to provide consistent and predictable throughput for real-time traffic, such as voice and video streaming applications. UGS is part of the Enhanced Distributed Channel Access (EDCA) protocol, which is an extension of the Distributed Coordination Function (DCF) used in the IEEE 802.11 wireless LAN (Wi-Fi) standard.

Background:

Wi-Fi networks use a contention-based access method, where multiple devices (stations) compete for access to the shared wireless medium. This contention can lead to unpredictable delays and varying throughput, which is not suitable for real-time applications that require constant and low-latency data transmission. To address this issue, different QoS mechanisms, such as UGS, have been introduced to prioritize and optimize the transmission of specific types of traffic.

UGS Operation:

UGS is designed primarily for time-sensitive traffic, like voice and video, that requires a continuous and predictable data rate to maintain a smooth and jitter-free user experience. UGS works by reserving a specific time slot for the real-time traffic, guaranteeing access to the wireless medium during that reserved time period. This time reservation is typically done through the Wi-Fi's EDCA mechanism, which includes four access categories with different priorities for traffic differentiation: AC_VO (Voice), AC_VI (Video), AC_BE (Best Effort), and AC_BK (Background).

UGS Traffic Parameters:

UGS traffic is characterized by specific parameters that define the QoS requirements of the real-time applications. These parameters include:

  1. Transmission Interval (TI): The time interval between two consecutive data frames for real-time traffic. It defines the periodicity of the traffic.
  2. Maximum Service Interval (MSI): The maximum tolerable time between the transmission of two consecutive frames for real-time traffic. If the next frame is not received within this interval, the real-time application may experience disruptions.
  3. Nominal MSDU Size (NMSDU): The size of the data payload in a frame for real-time traffic.

UGS Operation in EDCA:

Within the EDCA protocol, UGS operates with the highest priority in the AC_VO access category. When a station has real-time traffic to send, it follows the following steps to access the wireless medium:

  1. Access Category Selection: The station selects the AC_VO access category to transmit its UGS traffic.
  2. Arbitration Inter-Frame Space (AIFS): The station waits for a specific time called AIFS before attempting to transmit in the AC_VO access category. The AIFS time is shorter for higher-priority access categories.
  3. Contention Window (CW): If the wireless medium is idle during the AIFS period, the station initiates contention by randomly selecting a backoff value within a contention window (CW). The CW size is dynamic and varies based on the previous transmission outcomes.
  4. Transmitting Unsolicited Data: Once the backoff timer reaches zero and the medium is idle, the station transmits its UGS data without waiting for any specific acknowledgment (unsolicited grant).
  5. Periodic Transmission: UGS traffic follows a periodic transmission pattern, adhering to the defined Transmission Interval (TI) and Maximum Service Interval (MSI).

Usage and Applications:

UGS is particularly beneficial for real-time applications like Voice over IP (VoIP) calls and video streaming, where a consistent and predictable data transmission is essential to maintain call quality and video playback without interruptions or jitter.

Conclusion:

Unsolicited Grant Service (UGS) is a QoS mechanism used in Wi-Fi networks to provide guaranteed and predictable throughput for time-sensitive traffic, such as voice and video applications. By reserving specific time slots for real-time traffic, UGS ensures that critical data gets transmitted without contention, leading to a smoother user experience for real-time applications in Wi-Fi networks.