Multi-Link Operation (MLO) Modes
802.11be defines four ways a Multi-Link Device (MLD) can use two or more links at once. Pick a mode below and watch the same traffic scenario play out on 5 GHz and 6 GHz. The differences between MLSR, EMLSR, STR, and NSTR become visible the moment the links have to share time.
— Shankar K. · Source: IEEE 802.11be-2024 Section 35.3, 9.4.2.312 · Wireshark: wlan.eht.multi_link
A Wi-Fi 7 device with MLO can use 2 bands (say 5 GHz + 6 GHz) at the same time against the same AP. That device is called a Multi-Link Device. How it uses those bands — one at a time, in parallel, or with constraints — is the mode. The four modes below trade throughput for cost, complexity, and power. Scroll to the bottom for the Basic / Intermediate / Expert guide if the acronyms are new.
wlan.eht.multi_link expands the tree; the EML Capabilities subfield is at wlan.eht.multi_link.common_info.eml_capabilities.
A new Wi-Fi 7 flagship phone on a Wi-Fi 7 mesh router. Speed test next to the router: 2 Gbps. Walk into the next room: 800 Mbps. Living room: 400 Mbps. Reviewers called this "game-changing." The user is unimpressed.
The 2 Gbps headline assumes the device is running MLO in Simultaneous Transmit/Receive (STR) mode — two radios powered at the same time, on 5 GHz and 6 GHz, aggregating traffic. STR requires both radios at close to maximum power continuously, which is battery-hostile. Most client devices, including most flagship phones, actually run enhanced Multi-Link Single Radio (eMLSR), which rapidly switches between bands instead of running them simultaneously. eMLSR keeps the authentication context across bands so handoff is seamless, but peak throughput is bounded by one radio at a time.
Even in eMLSR mode, MLO delivers a real user-visible benefit: when the 5 GHz link degrades mid-session, the device can switch to 6 GHz in milliseconds without re-authenticating or renegotiating a handshake. The user experiences "things just keep working" instead of "Zoom call paused for 3 seconds." The real-world win isn't peak throughput, it's resilience.
Headline Wi-Fi 7 throughput numbers in reviews often assume STR. Before buying a device specifically for MLO aggregate throughput, check the datasheet for "STR capable" or equivalent language. If it says MLO but not STR, it's eMLSR, and the peak-throughput math is closer to Wi-Fi 6E than the marketing suggests.
Multi-Link Operation (MLO) lets a Wi-Fi 7 device use more than one radio channel at the same time against the same access point. Before Wi-Fi 7, a laptop on 5 GHz couldn't also use 6 GHz for the same session — the two bands operated independently, with separate associations and no coordination.
A link is one band + channel combination. For example, "5 GHz Channel 36 at 80 MHz" is one link. A device that supports MLO has two or more of these links active at once.
A Multi-Link Device (MLD) is the logical device that owns those links. To applications and the network layer it looks like one device with one MAC address, even though underneath it may have two or three physical radios. That single identity is what makes MLO clean: no roaming between bands, no double association, no separate IP per band.
How the AP knows which mode a client supports: the STA's Association Request carries a Multi-Link Element. Inside, the EML Capabilities subfield advertises EMLSR and/or EMLMR support. The MLD Capabilities subfield carries the STR capability bit and the maximum simultaneous links. The AP inspects these on day one and provisions the exchange accordingly.
TID-to-link mapping: after association, the AP and STA negotiate which TIDs are served by which links via the Multi-Link Element TID-to-Link Mapping subelement. Voice (TID 6–7) typically maps to the low-latency link; best-effort (TID 0–4) to the high-throughput link. The UMAC at the sender distributes each outgoing frame to the correct LMAC before any EDCA backoff. This is why MLO latency reduction is structural, not probabilistic.
EML Padding Delay. Minimum MAC padding duration the AP adds to the Initial Control Frame (ICF) directed at an EMLSR STA. Gives the STA's PHY time to collapse NSS from the listen configuration (1 SS per link) to the full transmit configuration on the target link. Negotiated during association via the EML Capabilities subfield.
EML Transition Delay. The interval between ICF reception and the first PPDU exchange. Covers the STA's radio retuning and chain reconfiguration. Upper-bounded by the EML Transition Timeout.
Medium Sync Delay Information. An NSTR timer that bounds how long an MLD may hold a TXOP on one link while the other link is in a synchronization-pending state. Prevents one link from starving the other. Present in the Common Info field of the Basic MLE when relevant.
IDC power leakage. In-device coexistence between co-located 5 GHz and 6 GHz radios typically achieves 40-60 dB of isolation. STR requires closer to 80 dB of isolation to transmit one link while receiving another without desensitization. NSTR accepts the lower isolation budget and mandates same-direction operation to avoid the cross-direction saturation case.
UMAC/LMAC split. IEEE 802.11be Section 35.3 formally defines the two-layer MAC for MLO. Each affiliated STA (one per link) consists of a PHY layer and a Lower MAC (LMAC) that handles channel access independently. A single Upper MAC (UMAC) sits above all affiliated STAs, provides the LLC with a single MAC Service Access Point, and owns the MLD MAC (U-MAC) address. The UMAC implements TID-to-link mapping and distributes outgoing frames to the appropriate LMAC before any EDCA backoff occurs on that link.
SR-Tx (Selective Transmit) on UL. In MLO STR mode, the client UMAC selects the transmit link per frame on the uplink — not per session. The selection criterion is the cleanest available band at that instant. ACK frames return on whichever link the AP designates as cleanest, which may be different from the UL path. The TCP window stays open because ACKs continue arriving even when one link is momentarily congested. This is the mechanism behind the WBA Phase 2 UL improvement numbers: +116% UL throughput under CCI is SR-Tx working correctly.
ML-Rx (Multi-Link Receive). The EMLSR client keeps its receiver open on all active links simultaneously at reduced spatial streams (1 SS per link in listen mode). When an Initial Control Frame (ICF / MU-RTS) arrives from the AP on any link, the STA collapses all streams to the target link within the EML Padding Delay window. This is the mechanism that makes EMLSR make-before-break: the receiver never goes dark between links, so there is no gap in connectivity from the TCP layer's perspective.
Per-STA Profile subelement. Length-prefixed inner body that carries per-link association state. "Complete Profile" flag means the inner (Re)AssocReq/Resp body is fully included; "incomplete" means it's compressed against the association link's parameters and only deltas are carried. Link ID in the subelement maps to the 4-bit Link ID in the Common Info.
Wireshark. The MLE tree expands at wlan.eht.multi_link. Per-STA Profile subelements show up as nested trees, each with its own Link ID and inner assoc frame body. On a real capture, the first place to look for mode negotiation evidence is the AssocReq's MLE inside the STA's first AssocReq to the AP MLD.
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