Backward compatibility in WiFi

Wi-Fi's backward compatibility with previous generations is needed as it allows people to upgrade their devices gradually without concern about their older devices becoming obsolete. Depending on the type of business, the WiFi infrastructure upgrade in enterprise can be every four to five years. Enterprise customers usually upgrade the WLAN and uplink infrastructure but may not always upgrade the connected wireless clients.

We can consider backward compatibility in WiFi as a mixed blessing. On one hand, backward compatibility ensures that older devices can still connect to newer WiFi networks, protecting investments in existing equipment and providing continuous service to users with older hardware. On the other hand, supporting older devices can impact the performance and efficiency of the network. Legacy devices often use old protocols that are less efficient and slower compared to modern standards. These older devices can reduce the overall network speed and capacity, as the network must accommodate their slower data rates and less efficient communication methods. Additionally, maintaining compatibility with outdated technology can complicate network management and increase security risks, as older devices may not support the latest security protocols. Thus, while backward compatibility ensures inclusivity and cost savings, it can also impose significant performance and security drawbacks

How is backward compatibility maintained in WiFi?

As WiFi professionals, it’s important to know how backward compatibility is maintained in WiFi. To understand that, we should consider the WiFi signal transmission over the RF medium. The WiFi technology works on PHY (Physical) and MAC (Data Link Layer) layer of OSI. The PHY layer consists of the PLCP (Physical Layer Convergence Protocol) sublayer and the PMD (Physical Medium Dependent) sublayer. The PLCP sublayer prepares the data from the MAC layer by adding a PHY header to it, resulting in a data unit called the PPDU (PLCP Protocol Data Unit). The PMD sublayer then transmits these PPDUs over the RF medium.

On the physical layer the information is converted to 0 and 1 and is transmitted and received using transceivers. The information getting transmitted uses a specific modulation scheme and we can differentiate the signal based on the modulation scheme we observe. To understand what different modulation scheme each WiFi technology uses we need to know the PPDU format of the technology.

With each new WiFi technology, new PPDU formats are introduced. To better understand how backward compatibility is maintained, let's distinguish between legacy, HT, and VHT PPDU formats and see how these transmissions can be differentiated at the receiver.

802.11a/g PPDU:

If we consider the above format of the 802.11a/g PPDU, we have the following information:

STF: Short training Field, this is used for initial timing sync and frequency estimate

LTF: Long Training Field, used for fine timing and frequency sync, and channel response estimation.

Signal: Metadata which contains info about the upcoming data: MCS, Len, CRC. Encoded similar to a data symbol but always sent using BPSK

Data: This is the payload gets sent using 52 subcarriers.

802.11 n HT PPDU Format:

802.11n gives us the HT (High Throughput) preamble. In HT PPDU, we have the Legacy information (so it’s called L-LTF and L-STF and L-Sig) followed by the HT-SIG, HT-STF and HT-LTF. The HT-SIG consists of two different symbols: HT-SIG1 and HT-SIG2 and both use different form of BPSK (Binary Phase Shift keying) called QBPSK (Quadrature- BPSK).

802.11 AC VHT PPDU Format:

The VHT (Very High Throughput) PPDU also has legacy information followed by VHT-SIG-A, VHT-STF, VHT-LTF and VHT-SIG-B.

To tell if a transmission is Legacy, HT, or VHT, we need to check the SIG field of the PPDUs. Here's a simple way to understand this process:

1. Legacy Preamble: Every transmission starts with a legacy preamble. After this, we need to identify what comes next: HT or VHT preamble.

2. Checking Data Rate:

- If we see a data rate greater than 6 megabits per second in the legacy preamble, it's a legacy frame (802.11a or 802.11g).

- If the rate is exactly 6 megabits per second, it could be either legacy, HT, or VHT.

3. Modulation Types:

- QBPSK after Legacy Preamble: If we see QBPSK right after the legacy preamble, it's a high throughput (HT) frame.

- BPSK after Legacy Preamble: If we see BPSK (Binary Phase Shift Keying), it could still be legacy. But if we see another symbol --> QBPSK after BPSK: This indicates a very high throughput (VHT) frame.

4. Further Checks:

- If the transmission uses a modulation scheme other than BPSK or QPSK after the legacy preamble, such as QAM (Quadrature Amplitude Modulation), it's likely a legacy frame in its data portion.

In summary, by looking at the data rate and the type of modulation immediately following the legacy preamble, we can determine if the frame is legacy, HT, or VHT. For instance, QPSK after the legacy preamble means HT, BPSK followed by QPSK means VHT, and anything greater than 6 Mbps or other modulation types generally point to a legacy frame.

Legacy = BPSK (SIG -> BPSK)

HT = BPSK + QBPSK (L-SIG -> BPSK + HT-SIG1, HT-SIG2 -> QBPSK)

VHT = BPSK + BPSK + QBPSK (L-SIG -> BPSK + VHT-SIG-A1 -> BPSK + VHT-SIG-A2 -> QBPSK)

No backward compatibility in WiFi 6E

While Wi-Fi has always supported older devices, Wi-Fi 6E does not do so intentionally to fully utilize the benefits of the 6 GHz spectrum. This decision helps achieve the best performance, security, and future readiness for new wireless devices. By focusing on new features and keeping older devices on the 2.4 GHz and 5 GHz bands, Wi-Fi 6E offers a faster and more efficient wireless experience.

Will we ever need to support older devices on the 6 GHz band? Probably, but that’s a discussion for the future, maybe in couple of decades when Wi-Fi 8, Wi-Fi 9, and Wi-Fi 10 are common. For now, we can enjoy the clean and efficient 6 GHz band for Wi-Fi.