There’s a great deal of talk around the capability of Wi-Fi 7 (IEEE 802.11be) to revolutionize the wireless experience. It’s not hype. A key feature that delivers this transformative impact is multi-link operation (MLO). A mandatory and defining component of 802.11be, MLO enables a multi-link device (MLD) to simultaneously operate across multiple frequency bands, including 2.4 GHz, 5 GHz, and 6 GHz.
Access point (AP) and non-AP MLDs learn each other’s MLO parameters and capabilities through the multi-link information elements exchanged in frames like Beacons and Association Request/Response. In this blog, I’ll illustrate MLO’s impact on wireless connectivity and show you how it works in simultaneous transfer/receive (STR) mode.
How does multi-link operation (MLO) enhance wireless connectivity?
MLO introduces significant benefits for a variety of use cases. Key enhancements include:
- Simultaneous use of multiple bands. MLDs can transmit (Tx) and receive (Rx) data over more than one band at the same time. This is useful in environments with heavy congestion, as it avoids interference on any single band.
- Improved throughput. MLO leverages the combined capacity of multiple channels across different bands to enable higher aggregate throughput. This makes Wi-Fi 7 ideal for bandwidth-heavy applications like video streaming, virtual reality, and online gaming.
- Reduced latency. By offloading traffic across multiple channels. This is particularly useful in gaming, video conferencing, or other apps that require real-time communication.
- Better reliability and robustness. If one band (for example, 2.4 GHz) experiences congestion, then station (STA) MLDs can seamlessly switch to a less congested band (such as 6 GHz) without dropping the connection. This is extremely helpful in spaces with busy radio frequency (RF) traffic, such as stadiums, apartments, and offices.
Type of MLO operation modes
Wi-Fi 7 defines several single and multi-radio MLO modes, with stations able to support these modes based on their respective hardware capabilities. Various software thresholds—such as bandwidth requirements, band preferences, RF congestion, and QoS—will influence and guide a station’s choice of operating mode.
Among these modes, MLSR is required to be supported by all AP and non-AP MLDs. Support for EMLSR and STR modes is mandatory for AP MLDs, but optional for non-AP MLDs (stations). STR is currently incorporated by most vendors, making this mode an excellent starting place for dissection.
MLO’s STR mode in action
In STR operation, each link can be used to Tx or Rx concurrent physical layer protocol data units (PPDUs) without any synchronization. Figure 2 illustrates an example where an AP MLD and a non-AP MLD are operating over an STR link pair. Both devices contend for access to the wireless medium and engage in subsequent frame exchanges on those links.
After the AP MLD and the non-AP MLD complete a multi-link setup to successfully establish link 1 and link 2, and with the links enabled, AP 2 can receive data frames from STA 2 on link 2. Meanwhile, AP 1 contends for the wireless medium and, upon securing a transmit opportunity (TXOP), transmits data frames to STA 1 on link 1.
Next, let’s conduct a lab test using Cisco’s CW9178I AP running on Catalyst 9800 Wireless LAN controller (WLC) to demonstrate STR in action.
The access point under test (APUT) is configured to operate on 2.4 GHz (20 MHz) and 5 GHz (40 MHz) bandwidths with a WPA3-SAE WLAN. In the first step of the test, Wi-Fi 7/802.11be/MLO is enabled on both bands. We are using a Qualcomm 7800-based STR/MLMR-capable station, while the CW9178I AP serves as the sniffer—capable of capturing data across multiple bands and decoding Wi-Fi 7 frames.
Next, let’s associate the STAUT and check the capability details in both the WLC and Wireshark. During the association process, multiple elements are exchanged: the MLO information elements for the 5 GHz Association link, as well as the “Per-STA Profiles” information elements containing details about the non-association link (2 GHz).
The WLC identifies the STA as STR capable if the “Maximum Number of Simultaneous Links” value in the ML information element of the association request is non-zero. This indicates the number of radios the station is using for its association. See Figure 4 below for the corresponding Wireshark capture.
The Catalyst 9800 WLC provides a clear display of the STA’s 802.11be capabilities, including MLD links with Slot IDs and bands, MLO mode support (STR/eMLSR), and Tx/Rx RF and data statistics for each band. Equivalent CLI commands are also available, though not covered in this blog.
Now that the STA has associated on the 5G band with an STR link to both the 5G and 2G bands, let’s initiate traffic for one minute to verify STR operation. Using the IxChariot server, we will begin full-bandwidth Downlink UDP traffic. Initially, traffic will flow only on the 5G band, as it is the only active association link. However, the STA will soon assess the need for a secondary link to achieve higher bandwidth. It will then send a QoS Null data frame over the secondary (2G) link. The AP acknowledges this request and enables simultaneous data transmission across both bands.
Figure 6 shows the sequence starting with data on channel 36, followed by a QoS Null data frame on channel 6, and concluding with simultaneous data transmission on both channel 6 and channel 36.
The Catalyst 9800 WLC offers a comprehensive view of the client’s performance on each MLO link, with monitors providing detailed Tx/Rx data along with RF statistics.
Following the one-minute traffic run, the average throughput measured is 747 Mbps, as shown in Figure 8.
To provide a comparison, the test was repeated under the same conditions, but with 802.11be/MLO disabled, running in 802.11ax mode instead. The average throughput was 506 Mbps.
The table below summarizes the throughput comparison between clients. The impact is indeed transformative: Wi-Fi 7 with STR MLO significantly outperforms Wi-Fi 6, delivering a 47% throughput increase, along with more efficient spectrum utilization.
The CW9178I, CW9176I, and CW9176D1 APs, along with 9800 series wireless controllers, will fully support Wi-Fi 7 capabilities and features from IOS XE 17.15.2 onward.
Learn more about Cisco Wireless Wi-Fi 7 APs
and Cisco Catalyst 9800 Wireless Controllers.
Wi-Fi 7’s Multi-Link Operation (MLO) is a game-changer in wireless technology, revolutionizing speed, reliability, and efficiency. The deep dive into packet-level performance truly highlights how MLO optimizes data flow across multiple links. At Jazz Cyber Shield, we’re excited to see how this innovation enhances enterprise networking and cybersecurity. Looking forward to more insights on real-world implementations!
Figure 1 shows two rows as MLSR. I think the second should be EMLSR.
That’s correct! We updated it.
How many antennas does Qualcomm have that support STR? How many spatial streams does it send data with for each band when data transmission starts in STR mode?Also, how does the client decide on which bands to perform STR? Does it perform STR using 5G and 6G bands?
Figure 3 – Topology of STR Test
Switch model C9800 has to be C9300 I believe 🙂
Comprehensive write-up for STR, excited to explore other MLO modes.
Excellent technical breakdown of Wi-Fi 7’s MLO! The real-world testing data and 47% throughput improvement over Wi-Fi 6 are impressive. The diagrams and Wireshark captures make complex concepts easy to understand. Great work!
Great blog Shreyas! Do you have a list of clients that support 5/6 MLMR?
Hi Dliner, the results may vary depending on the HW/SF implementation, but you can test with any client that features the Qualcomm 7800 with High Band Simultaneous (HBS).
Great test, thank you. In the client stats GUI output, why does the AP hear client’s 2.4Ghz signal with -23dBm but hears the same client’s 5Ghz signal with -48dBm? Why is there so much difference? Maybe different EIRP on Qualcomm side but 25dB difference is HUGE!
Great blog! I have a question: Can we know which band is currently active in eMLSR mode from the beacon info? Thank you.
Hi Carl,
We have a CLI on the AP that displays the eMLSR mode and the corresponding link in real-time.
Additionally, eMLSR is triggered by encrypted Action frame which can we monitored in OTA.
Great overview of Wi-Fi 7’s MLO! I enjoyed the detailed explanation of STR mode, specifically how simultaneous multi-band transmission boosts throughput and reduces latency, making the real-world benefits over Wi-Fi 6 quite obvious. Nice to see actual data supporting these performance gains, and I look forward to seeing more analysis of eMLSR and other MLO modes to further our understanding of wireless networking.