WiFi

QoS Field

WiFi Quality of Service (QoS) is a group of technologies and rules that help manage how data travels over a wireless network, making sure that important apps and services work as well as they can. The main goal of WiFi QoS is to make sure that important network traffic, like voice calls over the internet (VoIP) and watching videos online, gets the fast connection and quick response it needs, even when the network is busy. This is really important for wireless networks, where the amount of data that can be sent is limited and can change a lot.

The term "QoS field in WiFi" talks about special settings and methods used in a certain part of the network system (Layer 2 of the OSI model) to organize and give priority to data traffic.

By adjusting these settings and methods, people who run networks and the devices that connect to them can sort and give priority to data traffic effectively. This makes sure that important apps get the resources they need to work well, leading to a better experience for everyone using the network

Followings are the description of Acronyms shown in the table :

TID (Traffic Identifier): This is a label used in WiFi networks to classify data packets according to the type of service they require. TID helps in prioritizing traffic by identifying the nature of the data being transmitted, such as voice, video, best effort, or background. This classification allows the network to apply the appropriate QoS policies to ensure that high-priority traffic like voice and video is delivered promptly and efficiently.
EOSP (End of Service Period): This indicates the end of a transmission opportunity (TXOP) period granted to a device for sending its data. In WiFi QoS, a TXOP is a time window during which a particular device has the right to transmit its data without interruption from other devices. EOSP signals to the network that the device has finished using its current TXOP, and the opportunity to transmit can be passed to another device. This mechanism helps in managing how devices share the wireless medium, especially in busy networks to ensure smooth data flow.
TXOP (Transmission Opportunity): This is a specific duration when a device is allowed to send multiple data frames sequentially without competing for access to the wireless medium again. TXOP is crucial for QoS in WiFi because it ensures that once a device starts transmitting, it can continue to send a series of packets based on its priority and QoS requirements. This is particularly important for applications that need continuous data transmission, like video streaming or VoIP, as it helps reduce latency and ensure a stable connection.
MSDU (MAC Service Data Unit): In the context of WiFi networking, an MSDU is the payload or the actual data that needs to be transmitted over the network. It is encapsulated within a MAC protocol data unit (MPDU) for transmission. The MSDU is essentially what you're trying to send over the network, such as a piece of an email, a segment of video, or a portion of a webpage. The process of handling MSDUs efficiently is vital for maintaining good network performance, especially when dealing with QoS to ensure that data packets are delivered in a timely and reliable manner.

< Example for (5) >

This is an example of the structure and content of a QoS Data frame in an IEEE 802.11 (Wi-Fi) network, highlighting how QoS is implemented at the MAC (Media Access Control) layer.

IEEE 802.11 QoS Data, Flags: .......TC

Type/Subtype: QoS Data (0x28)

Frame Control Field: 0x8801

.... ..00 = Version: 0

.... 10.. = Type: Data frame (2)

1000 .... = Subtype: 8

Flags: 0x01

.... ..01 = DS status: Frame from STA to DS via an AP (To DS: 1 From DS: 0) (0x01)

.... .0.. = More Fragments: This is the last fragment

.... 0... = Retry: Frame is not being retransmitted

...0 .... = PWR MGT: STA will stay up

..0. .... = More Data: No data buffered

.0.. .... = Protected flag: Data is not protected

0... .... = Order flag: Not strictly ordered

.000 0000 0011 1100 = Duration: 60 microseconds

Receiver address: Anritsu_07:91:0e (00:00:91:07:91:0e)

BSS Id: Anritsu_07:91:0e (00:00:91:07:91:0e)

Transmitter address: e0:cb:ee:f9:4a:de (e0:cb:ee:f9:4a:de)

Source address: e0:cb:ee:f9:4a:de (e0:cb:ee:f9:4a:de)

Destination address: Anritsu_07:91:0e (00:00:91:07:91:0e)

Fragment number: 0

Sequence number: 13

Frame check sequence: 0x1adb5852 [correct]

[Good: True]

[Bad: False]

Qos Control: 0x0000

.... .... .... 0000 = TID: 0

[.... .... .... .000 = Priority: Best Effort (Best Effort) (0)]

.... .... ...0 .... = QoS bit 4: Bits 8-15 of QoS Control field are TXOP Duration Requested

.... .... .00. .... = Ack Policy: Normal Ack (0x0000)

.... .... 0... .... = Payload Type: MSDU

0000 0000 .... .... = TXOP Duration Requested: 0 (no TXOP requested)

Let's break down the key parts:

Type/Subtype (0x28): Indicates this is a QoS Data frame, a type of frame used in Wi-Fi networks to transmit data with Quality of Service considerations.
Frame Control Field (0x8801): This field provides information about the frame:

Version: 0 indicates the Wi-Fi version.
Type: Data frame (2) specifies that this is a data frame.
Subtype: 8 identifies it specifically as a QoS Data frame.
Flags:

DS status: Frame from STA to DS via an AP (To DS: 1 From DS: 0) indicates the direction of the frame, from the station (STA) to the distribution system (DS), through an access point (AP).
More Fragments: This is the last fragment means no more fragments follow this frame.
Retry: Frame is not being retransmitted indicates this is the first transmission of the frame.
PWR MGT: STA will stay up suggests the sending device will remain active and not go into power-saving mode.
More Data: No data buffered means the sender does not have more data to send at this moment.
Protected flag: Data is not protected signifies the frame is not encrypted.
Order flag: Not strictly ordered indicates the frames are not required to be processed in strict order.

Duration: 60 microseconds specifies the time the channel is reserved for the frame's transmission.
Addresses: Includes Receiver address, BSS Id (Basic Service Set Identifier), Transmitter address, Source address, and Destination address, identifying the involved devices and network.
Fragment number: Identifies the fragment number of the frame, with 0 indicating it's the first or only fragment.
Sequence number: 13 is a unique identifier for the frame to keep track of the order of frames.
Frame check sequence: A form of error checking to ensure the integrity of the frame's data.
QoS Control (0x0000): Provides specific QoS parameters:

TID: 0 (Traffic Identifier) and Priority: Best Effort indicate the type of traffic is "Best Effort," which is the default level of service without any special priority.
Ack Policy: Normal Ack means the receiver should acknowledge receipt of this frame using standard acknowledgment procedures.
Payload Type: MSDU specifies that the payload is a MAC Service Data Unit, the actual data being transmitted.
TXOP Duration Requested: 0 indicates no specific transmission opportunity was requested for this frame.

NOTE : In this example, duration fields says 60 micro seconds but TXOP Duration Requested set to 0, which looks conflicting. How would you comment on this ?

The apparent discrepancy between the "Duration" field being set to 60 microseconds and the "TXOP Duration Requested" being set to 0 in the QoS Control field can be understood by distinguishing the purposes of these two fields in the context of an IEEE 802.11 QoS Data frame. Therefore, there is no conflict between these two fields. The Duration field ensures the successful transmission and acknowledgment of the current frame, while the TXOP Duration Requested indicates that no additional extended transmission time is being requested. This setup is typical for scenarios where the device is either sending data opportunistically, without the need for extended access, or where network conditions or policies do not favor or permit extended TXOP allocations.

Duration Field: This field, set to 60 microseconds, specifies the time period that the medium is reserved for the frame's transmission, including the time required for the acknowledgment (ACK) to be received after the frame is transmitted. It's a mechanism used in Wi-Fi networks to prevent other devices from attempting to access the medium immediately after this frame, ensuring that the current transmission is completed successfully and acknowledged without interference. The value is not directly related to the QoS TXOP; rather, it's a standard field in all 802.11 frames that helps manage airtime fairness and efficiency.
TXOP Duration Requested (in the QoS Control field): This part of the QoS Control field specifies the duration that the transmitting station requests to send multiple frames in a single TXOP. A value of 0 means that the frame is not requesting any specific TXOP duration beyond the immediate opportunity to transmit this single frame. In other words, the station is not asking for any additional reserved time on the medium to send more frames back-to-back without competing again for medium access.

The key difference lies in their purpose and context of use:

The Duration field is used for immediate medium reservation for the current frame and its ACK, applicable to all frames.
The TXOP Duration Requested specifies a desire for extended transmission rights beyond the immediate frame, relevant only in QoS contexts where multiple frames might be sent in a rapid sequence (burst)

NOTE :Does 'Duration' corresponds to NAV ?

Yes, the "Duration" field in an IEEE 802.11 frame corresponds to the Network Allocation Vector (NAV). The NAV is a virtual carrier sensing mechanism used in Wi-Fi networks to inform all other stations in the network about the medium's status, specifically how long the medium will be busy or reserved. This helps in managing access to the wireless medium and in reducing collisions by preventing stations from attempting to transmit while the medium is occupied.

When a Wi-Fi device transmits a frame, it includes in the Duration field the amount of time that the medium will be required for the transmission of the frame and for the subsequent acknowledgment (ACK) frame, or for the entire frame exchange sequence if multiple frames are involved. Other devices that receive this frame read the Duration field and set their NAV timers accordingly. While the NAV timer is running, these devices refrain from initiating their transmissions, effectively giving the transmitting device exclusive access to the medium for the specified duration.

The Duration field, therefore, plays a critical role in coordinating access to the wireless medium, ensuring that transmissions are orderly and minimizing the likelihood of collisions. By setting the NAV through the Duration field, a device communicates its need for the medium, allowing for more efficient use of the network resources and better overall network performance.