# Contention and Latency in 802.11 Networks [TOC] ![](https://i.imgur.com/gDTbNHf.png) - 802.11 latency can be very low without congestion - Basic sequence: EDCA+DATA+ACK (~100’s µs for 100 bytes) ![](https://i.imgur.com/qIjsaO0.png) # 802.11ax Trigger-based Communications What is Trigger-based Communcations - mechanism to let AP schedules UL transmission from one or more STAs. - Trigger Frame is sent by AP to tell which UE can transmit during curent Transmission Opportunity How? - AP Gaining access to the channel - AP sends a TF packet - TF Packet received by HE STAs - STAs start their TB UL Transmission simulatenously enabled by using multiple antenna technologies or different OFDMA subcarriers - Once all of the transmission are finished, the AP acknowledges all the packets with a MACK Why? Single User transmissions from 9 STAs will take ~ 1.3 ms. With 802.11ax trigger-based Multi User UL transmissions, the same amount of data will take approximately 758 µs. ![](https://i.imgur.com/mcQIw92.png) -> Less Congestion -> More UE capacity -> Less Latency --> Also used for TF-RA ![](https://i.imgur.com/XEc1bXT.png) # BSS Coloring Intro ![](https://i.imgur.com/uoByIdz.png) CSMA/CA consumes a lot of the available bandwidth. This problem is referred to as contention overhead. Unnecessary medium contention overhead that occurs when too many APs and clients hear each other on the same channel is called an overlapping basic service set (OBSS), also commonly referred to as co-channel interference (CCI Every device with the same channel would deffer acces to each other. All these deferrals create medium contention overhead and consume valuable airtime because you have two basic service sets on the same channel that can hear each other, in the manner of OBSS. ![](https://i.imgur.com/rV7RDcB.png) Wi-Fi clients are the primary cause of OBSS interference. Due to the mobile nature of Wi-Fi client devices, OBSS interference isn’t static: It changes as client devices move. All this congestion and medium contention overhead means that efficiency at the MAC sublayer drops. (on a/b/g networks efficeiancy only 40-50%, on n/ac 60-70%) To increase capacity in dense environments, frequency reuse between basic service sets needs to be increased. What? to address medium contention overhead due to OBSS by assigning a different “color” — a number between 1 and 63 that is added to the PHY header [HE PLCP Protocol Data Unit (PPDU)] of the 802.11ax frame — to each BSS in an environment How? Mark the preamble of Wifi Frame with value in the range of 1 to 63 color remain static until AP consider to change it, new color may be chosen by the affected APs. If the color bit is the same, then the frame is considered an intra-BSS transmission and the listening radio will defer. If the color bit is different, then the frame is considered an inter-BSS transmission from an OBSS and the listening radio treats the medium as busy only for the time it took to determine the color bit was different. Why? BSS coloring can thus potentially decrease the channel contention problem increase capacity allow more simultaneous connection ## Two- NAV What? Utilizing two NAV, intra-BSS NAV and inter-BSS NAV for intra and inter-BSS frames How? a given transmitter can decrease its backoff counter only if both NAV timers are set to zero. Otherwise, it must remain idle for at least the duration of the ongoing transmission(s), which had previously activated the virtual carrier sensing. ![](https://i.imgur.com/P5FBvAX.png) Why? the intra-BSS NAV allows protecting STAs from intra-BSS transmissions, thus reducing the effect of certain anomalies such as the hidden-terminal problem. On the other hand, as a novelty, the inter-BSS NAV allows mitigating OBSS interference, which contributes to increasing the number of parallel transmissions. # Spatial Reuse Groups ## SRG-based Channel Access Rules # References Wilhemi, Francesc et al. "Spatial Reuse in IEEE 802.11ax WLANs". https://arxiv.org/pdf/1907.04141.pdf Qiao Qu, et. al. "Survey and Performance Evaluation of the Upcoming Next Generation WLAN Standard - IEEE 802.11ax". https://arxiv.org/ftp/arxiv/papers/1806/1806.05908.pdf Coleman, David and Lawrence C. Miller. "802.11ax for Dummies". John Wiley & Sons, Inc. Cavalcanti, Dave et al. "IEEE 802.11-18/1160r0 Controlling Latency in 802.11". IEEE. Chittabrata, Ghosh et al. "IEEE 802.11-15/0875r1 Random Access with Trigger Frames using OFDMA". IEEE. Chu, Liwem et al. "IEEE 802.11-15/0615r3 UL OFDMA Bandwidth". IEEE. WLANPedia. "Spatial Reuse & BSS Color“. https://www.wlanpedia.org/tech/11ax/11ax-spatial-reuse-bss-color/ Mathworks. “Spatial Reuse with BSS Coloring in 802.11ax Residential Scenario”. https://www.mathworks.com/help/wlan/ug/Spatial-Reuse-with-BSS-Coloring-in-802.11ax-Residential-Scenario.html Wilhelmi, Francesc, Sergio Barrachina Munoz, Cristina Cano, Ioannis Selinis, and Boris Bellalta. "Spatial Reuse in IEEE 802.11ax WLANs." ArXiv:1907.04141 [Cs], November 29, 2019. Wilhelmi, Francesc, Sergio Barrachina-Munoz, and Boris Bellalta. "On the Performance of the Spatial Reuse Operation in IEEE 802.11ax WLANs." In 2019 IEEE Conference on Standards for Communications and Networking (CSCN) , 1-6, 2019.