Umber Networks · Fi-Wi-L4S Whitepaper
The natural standards venue for scheduling a contended Fi-Wi network is the IETF, not IEEE 802.11: the mechanisms are L3/L4 machinery, above the air interface that 802.11 governs, and the credible path is running code plus an Internet-Draft, the QUIC and L4S pattern. Umber intends to bring the contended-last-hop scheduling problem to the IETF, beginning with running code, measurements, and a problem-statement discussion. Whether the work ultimately warrants a new working group will depend on community interest and implementation experience. The asks of transport engineers are specific: review the grant semantics and their congestion-control interaction, attack the clock-discipline exchange, run the code, and say on the record if the contended last hop is your problem too. Section 6 gives the details.
The client's 802.11 MAC arbitrates whenever its transmit queue is non-empty. Nothing above the MAC can suspend arbitration directly; the only state in which the MAC is guaranteed quiet is an empty FIFO. Every mechanism in this note is therefore an instrument for manufacturing the absence of payload at all instants outside a grant, so that the transition from FIFO-empty to FIFO-non-empty happens only when the Concentrator intends it.
The target is not zero arbitrations. An 802.11ax trigger frame does not eliminate arbitration either: the AP wins one EDCA contention, and the triggered responses follow within SIFS (the short interframe space, a fixed 16 µs turnaround in the 5 GHz OFDM PHYs, too short for any competing station to begin contending), consolidating N stations' arbitrations into one. Consolidation, not elimination, is the correct emulation target, and it gives us the figure of merit for everything below: arbitrations per delivered aggregate (equivalently, arbitrations per delivered megabyte at fixed offered load). A secondary consequence of consolidation is that when grants are non-overlapping, the surviving arbitrations occur on an idle medium, so each one costs only an AIFS deferral (the arbitration interframe space, a per-access-category wait of a few tens of microseconds) plus a random backoff, rather than risking a collision. Per-transmission overhead remains; contention losses largely do not.
Native 802.11ax trigger-based uplink, in which the AP's Trigger frame solicits a trigger-based (TB) PPDU response from the client, requires PHY and MAC support that the installed base does not have and will not have for years. Coverage is gated by silicon replacement cycles, and even among shipping 11ax clients, trigger support quality is uneven and UORA (UL OFDMA-based Random Access), the trigger mechanism through which a station announces demand the scheduler does not yet know about, is essentially unused.
Most of the trigger machinery is also more than this system needs. Its weight serves simultaneous multi-user uplink: resource unit (RU) allocation, per-STA power pre-correction toward a target RSSI, tight timing and frequency pre-synchronization, and commanded MCS (modulation and coding scheme) all exist so that several clients' TB PPDUs arrive at the AP aligned in time, frequency, and power. Fi-Wi does not require simultaneity within a channel. Its scheduling unit is the TXOP (transmit opportunity), a whole-channel time grant to one station, which drops the synchronization, power-control, and RU-signaling burden entirely and leaves rate adaptation with the client as part of its autonomous execution of an eligibility decision. The frequency-domain multiplexing that RUs provide within a BSS, Fi-Wi obtains across the radio-head array instead: clients scheduled concurrently on different RRHs and channels against the same master timeline. The residual cost of time-only grants, per-station preamble and interframe-space overhead under many small uplink flows, is the case the grant policy exists to compress.
Triggers also scope a single BSS with the AP as scheduler; cross-BSS coordination waits on 11be MAPC and 11bn coordinated scheduling, which at fleet scale is a 2030s proposition. Software coverage iterates in months. The mechanisms in this note run on silicon that has already shipped, and when trigger and MAPC primitives do arrive, they slot in as actuators under the same control plane rather than replacing it.
There is no protocol mechanism by which a node in the forwarding path installs state in an endpoint's kernel. SO_TXTIME and TCP_TX_DELAY are syscalls, not protocols: the resulting departure time lives in skb->tstamp, which has no wire representation and dies at the NIC. This is the Earliest Departure Time (EDT) model of pacing, in which each packet carries a "not before" timestamp decided in the sender's stack and enforced by the sender's own qdisc or NIC launch-time hardware; EDT is a host-stack construct with no existence in any IP or TCP header. The contrast with the MAC is exact: 802.11 gives the network an in-band control channel into the client's transmit machinery (EDCA parameter sets, TWT agreements, triggers), while IP was designed with essentially none. The path-visible knobs are the receive window, ACK arrival timing, one ECN bit, and the timing of our own downlink. That asymmetry, a rich control plane at L2 and a near-empty one at L3/L4, is the gap this program fills.
It follows that the FIFO empty-to-non-empty instant is writable from exactly three positions. From the path, we can only induce the transition by shaping the inputs endpoints already respond to. From code resident on the client, we can set it directly; that is a resident client library. From the MAC, triggers and TWT let client firmware honor a grant sent over the air. Section 4 states the interface the first two positions require; the mechanisms that realize them are developed in the engineering note. The MAC stratum comprises the AP-controlled levers, TWT, Quiet elements and NAV, EDCA parameter sets, and ADDBA window sizing.
Scheduling a hop the network does not own reduces to three functions at the interface between the Concentrator and the endpoint. Naming them separately is the useful move, because each has a different maturity, a different threat model, and a different answer to whether it belongs in a standard at all.
Grant. An eligibility decision: an identifier, a start time in a shared timebase, and an airtime budget, with a size hint for what the endpoint should release. A grant is not a command. The client MAC remains the autonomous executor, and the grant shapes the contention process rather than replacing it, which is what makes the model workable on silicon the network does not control.
Time. A shared timebase, without which a grant means nothing. Wi-Fi already distributes one: every associated station disciplines a hardware TSF counter from beacon timestamps inserted at PHY transmit time, a protocol-mandated synchronization service that reaches unmodified clients for free. The open question is the discipline exchange that lets a userspace peer track that timebase, and it inherits the delay-attack threat model that PTP deployments know well.
Telemetry. Per-packet observation of what the medium actually did: access category, backoff, retries, aggregate position, grant identity, and arrival timestamps in the shared timebase. Without it a scheduler cannot tell a decision that was wrong from one that met an uncooperative channel, and guard bands stay conservative because nothing measures them.
Three constraints are load-bearing from the first line of any specification. Grants and telemetry must be authenticated inside an encrypted session: a spoofed grant is a targeted denial of service, spoofed telemetry poisons the scheduler, and per-packet radio state is an RF fingerprint that must never travel in cleartext. Everything after a minimal invariant header should be encrypted for the anti-ossification reason QUIC established. And declared endpoint capability must be treated as a hint rather than a contract, with actual obedience classified from measurement, because a network scheduling hardware it does not own cannot take that hardware's word for anything.
Non-participating clients require no changes and remain standard 802.11 participants, receiving whatever shaping the path and the MAC stratum can induce. Their service under scheduling is a measured quantity rather than an assumption.
The figure of merit is arbitrations per delivered megabyte at fixed offered load. It follows directly from the control principle: the target is consolidation rather than elimination, so the metric has to count arbitrations against delivered work rather than count throughput alone.
The measurements that matter are comparative, on identical channel conditions, across an open-loop baseline and each successive degree of scheduling. Around that sit three questions the architecture cannot answer from first principles. Congestion control interacts with grant shape, and a window-limited ACK-clocked sender and a paced sender do not respond alike to the same grant, so the result may invert which congestion control looks better under scheduling. The delay between a grant and the first frame on air is a distribution rather than a constant, and guard bands should be adapted from it rather than set conservatively. And a residual floor of ungateable traffic, from ARP and ND through DHCP renewals, mDNS, DNS, keepalives and management frames, bounds the consolidation ratio from below no matter how good the scheduler is.
These are measurements on a conducted RF rig, not simulations, and they are the evidence any standardization argument would have to rest on. Umber is running them. The reference load generator is iperf 2, which already carries per-packet release-scheduling machinery in its UDP path, and the grant-obedient generator is being built in the open.
Cellular demonstrated that scheduled MACs beat contended MACs at density where the scheduler owns the medium, and C-RAN centralized the scheduler because vantage wins. Unlicensed spectrum withholds that ownership, which is what the empirical program tests. The Concentrator has the vantage, every queue, every flow's transport state, every radio head, the WAN side, and one master clock; the client MAC remains the microsecond executor of eligibility decisions planned at millisecond scale. Wi-Fi never got the control plane that makes this possible, and 802.11 cannot supply it, because the missing machinery sits above L2. The interface described here is that retrofit, and it is not Wi-Fi-specific in principle: it is an L3/L4 scheduling control plane for a contended last hop, with TSF as the timebase Wi-Fi happens to distribute for free.
The transport community has spent the last decade building the instruments this composes: earliest-departure-time pacing, L4S signaling, QUIC's userspace deployment model, and per-packet telemetry in the IOAM tradition. What remains is to aim them at the hop where the queues actually form.
The asks are specific. Review the grant semantics and their congestion-control interaction, since the question in Section 5 is genuinely open and may invert current assumptions. Scrutinize the clock-discipline exchange against PTP-class threat models before the security considerations of any draft harden. Run the code. And if the contended last hop is your problem too, say so on the record, because that is the evidence that decides whether this is one company's engineering problem or the community's.
The path runs through the standard gate, in the order the tradition prescribes: running code and measurements first, then a problem-statement draft, with the transport interactions belonging in TSVWG and CCWG and the per-packet telemetry in IPPM. Whether a working group follows is for that process to determine.
If you schedule packets for a living, the contended last hop is the queue you have not scheduled yet. Come schedule it with us.