A long-range serial telemetry radio moves 57 to 115 kilobits a second. On a pipe that narrow, every byte of header and every buffer Atlas cannot see decides whether a call fits at all. Where both ends of a radio pair run Atlas, [[link]] type = "serial" puts Atlas datagrams straight onto the radio’s serial port: no adapter, no pseudo-terminal, no pppd, no IP or UDP, no address plan per radio pair. Atlas writes only when the radio has room, reads the radio’s signal in-line, comes back on the first probe after a reset, and lets several radios share one channel. Everything above the frame — sessions, probes, scheduling, routing, classes — sees one more link.
The classic way to carry IP over a serial radio is a bridge process that owns the port and strips the radio’s own status frames, a pseudo-terminal that pppd attaches to, a PPP session with its own negotiation and liveness, an IP address per radio pair, and finally a UDP socket in the daemon. Each stage adds bytes, a buffer or a way to fail. The native link removes all of them: the daemon opens the serial device and does the four jobs that matter itself — separating the radio’s status frames, framing, pacing and liveness.
IPv4’s 20 bytes, UDP’s 8, PPP’s protocol field, frame check sequence and flag, and HDLC byte escaping — two byte values with asyncmap 0, thirty-four with pppd’s default. On a 104-byte voice datagram that is about 29 bytes, a fifth of the packet.
The adapter process, the pseudo-terminal, pppd and its setuid root, an IP address plan per radio pair, LCP negotiation after every reset, LCP echo running beside Atlas’s own probes, and a second telemetry path over a Unix socket.
The outer and inner Atlas headers, session sealing, sealed probes, the scheduler, shortest-path routing, transport classes, end-to-end encryption and the signed control plane. A serial link is one more member of the bond: it can carry everything alone, share traffic with a cellular modem, or stand by as the last link that still reaches. the data path ↗
Each Atlas datagram is followed by a CRC-16, encoded with Consistent Overhead Byte Stuffing and closed with a single zero byte. That is the whole frame. Below, one voice packet drawn to scale both ways: the Atlas datagram is the same 104 bytes in each row; everything around it is what the radio has to carry for it.
Consistent Overhead Byte Stuffing removes every zero from the frame, so a zero byte can only ever be a boundary. It costs one code byte per 254 bytes — 0.4 % in the worst case — where HDLC escaping, PPP’s and SLIP’s method, doubles a frame in the worst case and adds about 13 % to encrypted bytes under pppd’s default asyncmap. Because the cost is fixed, the airtime of every frame is known before it is written, which is what the pacer and the fragment ceiling compute with. Up to 254 bytes, a datagram costs exactly four bytes: the code byte, the CRC and the delimiter.
The receiver gathers non-zero bytes and decodes on the delimiter; there is no escape state to fall out of. A lost or corrupt byte damages one frame, and the next zero starts a clean one. When the port opens, everything up to the first delimiter is discarded and counted, so the tail of a frame that began before a reset never reaches the CRC. After a quiet line, and on every addressed frame, an extra leading zero closes whatever a reset radio, a replug or another node’s interleaved bytes left behind.
The frame check is CRC-16/X-25 — PPP’s FCS-16 — so the native link keeps exactly the error detection of the path it replaces: every 1-, 2- and 3-bit error and every burst up to 16 bits in any frame Atlas sends. Two more bytes for a 32-bit check would buy nothing here. The radio already checks a CRC per air packet, so what reaches the port is mostly whole air packets or missing ones; the damage the link catches is structural — a lost air packet joining the head of one frame to the tail of the next, bytes dropped on overflow, a baud mismatch. And every Atlas datagram is authenticated above the frame: a corrupt frame that slips past the CRC is refused there and never delivered.
pppd’s default asyncmap escapes the 32 control byte values as well as the flag and escape bytes. Encrypted payloads look random, and the clear headers in front of them are full of small values, so the escapes add up: 182.6 bytes per voice packet against 133.6 with asyncmap 0. Measured
Three rates matter on these radios. 57.6 and 115.2 kbit/s are the common serial line rates; with a start and a stop bit, every byte costs ten bits on the line, so they carry 5 760 and 11 520 bytes a second. 64 kbit/s is a common air rate, 8 000 bytes a second. The table gives the bytes Atlas hands the radio for each kind of packet and how long each occupies the line or the air; the radio adds its own air framing on top, the same for both chains.
| Packet | Atlas datagram | Native · pppd | 57.6 kbit/s line | 64 kbit/s air | 115.2 kbit/s line |
|---|---|---|---|---|---|
| Probe or probe reply | 44 B | 48 B · ≈76 B | 8.3 ms · 13.2 ms | 6.0 ms · 9.5 ms | 4.2 ms · 6.6 ms |
| Voice, 20 ms frame | 104 B | 108 B · ≈137 B | 18.8 ms · 23.8 ms | 13.5 ms · 17.1 ms | 9.4 ms · 11.9 ms |
| Voice, 20 ms + previous frame | 168 B | 172 B · ≈201 B | 29.9 ms · 34.9 ms | 21.5 ms · 25.1 ms | 14.9 ms · 17.4 ms |
| Voice, 40 ms frame | 136 B | 140 B · ≈169 B | 24.3 ms · 29.3 ms | 17.5 ms · 21.1 ms | 12.2 ms · 14.7 ms |
| Voice, 40 ms + previous frame | 232 B | 236 B · ≈266 B | 41.0 ms · 46.2 ms | 29.5 ms · 33.2 ms | 20.5 ms · 23.1 ms |
| Telemetry, a 40-byte MAVLink message | 112 B | 116 B · ≈145 B | 20.1 ms · 25.2 ms | 14.5 ms · 18.1 ms | 10.1 ms · 12.6 ms |
| Video fragment, 256-byte piece | 300 B | 305 B · ≈334 B | 53.0 ms · 58.0 ms | 38.1 ms · 41.8 ms | 26.5 ms · 29.0 ms |
An Atlas datagram is the inner IP packet plus 44 bytes: 8 outer header, 20 inner header, 16 AEAD tag; a sealed probe is 44 bytes in all. Voice is a 32-byte codec frame in a 60-byte IP packet (92 bytes at 40 ms); the previous frame adds a copy of the last packet and a 4-byte trailer. The pppd column is IPv4 and UDP, PPP’s protocol field, FCS-16, one flag and the escaping of the datagram’s bytes under asyncmap 0; native counts the worst-case COBS code bytes.
Per direction. A half-duplex radio shares its air rate between both directions, so a two-way call needs the voice line twice on air; a push-to-talk net needs it once.
| Stream | Native · pppd | 57.6 kbit/s line | 64 kbit/s air | 115.2 kbit/s line |
|---|---|---|---|---|
| Voice, 50 frames/s | 43.2 · 54.8 kbit/s | 94 % · 119 % | 68 % · 86 % | 47 % · 59 % |
| Voice, 50 frames/s + previous | 68.8 · 80.4 kbit/s | 149 % · 174 % | 108 % · 126 % | 75 % · 87 % |
| Voice, 25 frames/s | 28.0 · 33.8 kbit/s | 61 % · 73 % | 44 % · 53 % | 30 % · 37 % |
| Voice, 25 frames/s + previous | 47.2 · 53.2 kbit/s | 102 % · 115 % | 74 % · 83 % | 51 % · 58 % |
| Telemetry, 10 messages/s | 9.3 · 11.6 kbit/s | 20 % · 25 % | 14 % · 18 % | 10 % · 13 % |
| Probes, one round a second | 0.8 · 1.2 kbit/s | 1.7 % · 2.6 % | 1.2 % · 1.9 % | 0.8 % · 1.3 % |
| Radio heartbeat, every 250 ms | 0.5 kbit/s · the same | 1.2 % | 0.9 % | 0.6 % |
Kilobits are the bytes Atlas hands the radio, times eight. Shares above 100 % do not fit. The heartbeat is the 17-byte MAVLink message that makes the radio report its status; either chain writes it.
Three link plans, worked from the budgets above. Voice goes in the Voice class, flight commands in Control, the vehicle’s position in Position — the high band, sent first and never held — while general telemetry and video share the low band, which waits in Atlas’s own queue.
A radio left at 57 600 baud carries 5 760 bytes a second, whatever its air rate. Voice in 40 ms frames, ten 40-byte telemetry messages a second, one probe round and four radio heartbeats a second need 4 824 bytes a second natively — 84 % of the line — and about 5 900 through pppd, more than the line holds. The same call and the same telemetry fit natively and do not fit through PPP. With 20 ms frames the voice alone takes 94 % of the line natively and 119 % through PPP: on this radio, choose 40 ms frames or raise the line rate.
At 115 200 baud the line outruns the air, and the radio’s buffer is where queues form. Fifty voice frames a second take 43.2 kbit/s of the 64 natively. Offered alongside about 110 kbit/s of 1 400-byte video, paced voice arrives one-way at p95 100.6 ms with full-duplex air and 131.4 ms with half-duplex air, 97 to 100 % of frames delivered, and the video still carries about 17 kbit/s. Without pacing the same voice waits about 1.95 s and 6 % of it arrives, because the radio’s buffer fills with video and overflows. Measured
The 100 ms is airtime, not queueing. A voice frame waits at most behind the low-band frame just released plus one fragment’s airtime of queue; while a call is on the link, Atlas cuts video into fragments of about 20 ms of airtime, and a 256-byte fragment is 38 ms on 64 kbit/s air. Holding the queue tighter would starve the video entirely.
Carrying each voice frame’s predecessor in the next packet means one lost packet costs no audio. With 20 ms frames that is 68.8 kbit/s natively — more than 64 kbit/s air holds, so on that radio use 40 ms frames, at 47.2 kbit/s with redundancy, or a faster air rate. At 115.2 kbit/s the 20 ms stream with redundancy takes 75 % of the line. Through PPP the same plans cost 80.4 and 53.2 kbit/s. transport classes ↗
A 40-byte telemetry message costs 116 bytes natively and about 145 through PPP: ten a second are 9.3 or 11.6 kbit/s. Position reports and commands ride the high band with voice and go out first; bulk telemetry and logs wait in the low band and fill what is left. Store-carry-forward and deadlines work on a serial link as on any other: what cannot cross in time is held or dropped in Atlas’s queue, never in the radio’s.
A serial radio keeps a transmit buffer of a few kilobytes and sends it in turn, oldest first. Whatever reaches that buffer is committed: a voice frame behind two kilobytes of video waits a quarter of a second on 64 kbit/s air, and a buffer that overflows drops bytes from the middle of frames. The radio reports how full the buffer is, and the native link writes only while the bytes below Atlas stay under a small target — so the queue that decides the order is Atlas’s own, where a voice frame overtakes video.
Frames handed to the writer thread and not yet in the kernel are counted exactly. The serial line — the kernel’s tty queue, the USB adapter and the UART — drains at a tenth of the baud rate in bytes; the kernel part is read with TIOCOUTQ and the rest is integrated from what was written. The radio’s transmit buffer drains at the air rate: each status report replaces the estimate with the radio’s own figure, carried forward by what was delivered and sent since, and two reports from a busy buffer measure the air rate the radio actually gets, never above the declared one.
The low band is held while the bytes below exceed the larger of the class target and a floor that lets the next frame cross the line while the radio sends the last. While the high band has been used in the last second, the floor is capped at one fragment’s airtime, 20 ms, so a voice frame waits behind at most that and the frame just released. With no high band in use, the floor is at least 100 ms of airtime, so a half-duplex radio fills its transmit windows instead of sending one short packet per turn.
The sum of the three queues is handed to the low-band pacer as a queue reading, so a serial link is paced by the same code, counters and dashboard chip as an Ethernet or cellular link with a readable device queue — even though there is no queueing discipline under it. It is paced by default, with or without transport classes, because nothing else stands between Atlas and the radio. The high band — Control, Voice, Position — is never held. queues below the tunnel ↗
Without hardware flow control, the default on these radios, a radio sent more than its buffer holds drops the excess and corrupts the frames it cut — exactly what pacing prevents. With flow_control = "rtscts" the radio also stops the line when its buffer is full and the kernel holds the rest, which the model reads. A radio that never reports, a generic byte pipe, is paced on the line and the declared rate alone.
With MAVLink framing on, a SiK-family radio answers each heartbeat from its host with a status frame — RSSI and noise at both ends, and how full its transmit buffer is — injected into the same byte stream the data uses. The native link writes that heartbeat, four times a second by default, and pulls the radio’s answer out of the stream itself, with no adapter process.
The radio inserts its status frame between air packets, so it can land in the middle of an Atlas frame, and it contains zero bytes that would read as delimiters. The extractor therefore runs on the raw stream first and takes only bytes that match the exact MAVLink framing, length, message id and checksum of the two radio messages; anything else passes through untouched. On encrypted traffic a false match needs about forty random bits to line up, and a damaged match costs only the one frame it sat in. The peer’s heartbeats, which the radio forwards over the air, are stripped the same way.
Local and remote RSSI, noise and buffer fill go straight into the link monitor through the same call the telemetry socket uses, so the link score, RSSI-trend prediction and the per-link signal carried in each link-state advert all work as they do with an adapter. A radio whose signal is diving moves traffic off the link before its probes start to miss. The heartbeat costs 17 bytes four times a second: 0.9 % of 64 kbit/s. prediction ↗
PPP is point to point by definition. SiK-family radios with multipoint firmware can broadcast on one channel, so every radio hears every other — and with a node address in each frame, one radio reaches several peers. Setting address on a link switches it to addressed frames: three bytes in front of the datagram, a marker, the destination and the source.
Each node takes an address from 1 to 254 on the channel. peer_addresses tells a node where a peer sits before it has heard from it, which the side that starts a session needs; any other peer is learned from its first authenticated frame, the way endpoint roaming learns a UDP address. The marker byte can never open an Atlas datagram, so a receiver tells addressed and point-to-point frames apart without a negotiated mode.
Every peer on the channel is an ordinary neighbour to the routing layer: link-state adverts cross the channel, shortest paths run over it, and a node that also holds a cellular or Wi-Fi link can carry traffic between the radio net and the rest of the fleet. One radio replaces a stack of point-to-point pairs, each with its own PPP session and address plan.
Atlas probes are the liveness check on a serial link, as on every link: dead after the configured number of misses, alive again on the next completed round trip. There is no LCP and no second state machine. Sessions belong to peers, not links, so a link that returns carries traffic at once — there is nothing to renegotiate. Traffic resumed 53 ms after a radio reset and 194 ms after a radio was unplugged and plugged back in. Measured
| Failure | What the link does | What the operator sees |
|---|---|---|
| Radio reset | Probes miss, the link goes dead; the first frame after the radio is back is delivered. | Alive false, then true — traffic back 53 ms after the radio. |
| Unplug and replug | The reader sees the hang-up, closes the device and reopens it every 500 ms; a /dev/serial/by-id/ path finds the same radio again. | Open false with the last error, then a reopen count and a journal event — back 194 ms after the device reappears. |
| Baud mismatch | Bytes arrive as noise and no frame survives the CRC. | CRC, framing and resync counters climb while received frames stay flat; UART frame errors on a real port. |
| Garbage, partial frames, bit errors | The damaged frame is dropped; the next delimiter resynchronises. | The CRC, framing and resync counters. |
| Radio buffer overflow | Pacing keeps the buffer short; what would have been dropped is estimated. | Estimated overflow bytes and the radio’s buffer free, in percent. |
A forgotten pppd on the port | The link opens the device exclusively, so a later opener is refused instead of splitting the byte stream. | Open false, last error “device or resource busy”, until the other process lets go. |
Every figure appears per link in the stats snapshot, and on the dashboard’s link card under a serial chip with the line rate. The dashboard reads them like any other link.
Both chains are supported. The native link is for radios with Atlas at both ends; pppd is for everything else. The difference is in what each one can see and what each one costs on a narrow pipe.
| pppd on both ends | Native serial link | |
|---|---|---|
| Works with | Anything that speaks IP: any application, dial-up modems, satellite terminals, tactical radios with PPP or IP ports | Atlas to Atlas |
| Bytes per small packet | Probe ≈76, voice ≈137 with asyncmap 0; more with the default asyncmap | Probe 48, voice 108 |
| Radio buffer and signal | Behind pppd and the kernel; the signal arrives over a separate adapter | Read in-line: writes paced on the radio’s buffer, RSSI and noise into routing |
| Recovery after a replug | PPP negotiation, seconds | The first frame after the port reopens |
| Topology | Point to point | Point to point, or several radios on one channel |
| Addressing | An IP address plan per radio pair | None point to point; one byte per node on a shared channel |
| Processes | Adapter, pseudo-terminal, pppd running setuid root | Two threads inside the daemon |
| Debugging | Standard tools: ping the far end, capture on ppp0 | Atlas’s own counters, snapshot, dashboard and journal |
| Standing | Thirty years of PPP; accreditors know it | COBS framing with PPP’s own CRC; every datagram authenticated above it |
The SiK adapter and pppd remain a supported way to put a serial radio in the bond. Atlas encrypts everything on either path, so security never depends on the choice.
A ground station, a gateway or a vendor box that expects IP over the radio. The native frame means something only to another Atlas node.
Dial-up and satellite modems, tactical radios with PPP or IP ports: they hand you an IP interface, and Atlas bonds it as an ordinary link.
When something outside the tunnel has to reach the far radio by address, the PPP link provides that address. Inside the tunnel, everything already has one.
Where an approved configuration specifies the standard path, keep it — and set asyncmap 0 on both ends: the default escapes 32 byte values and adds about 12 % to every encrypted packet.
Both ends of a serial link run the native framing or neither does. Moving a radio pair over is one change to its [[link]] on both nodes, with the adapter and pppd stopped there.
# Point to point: an RFD900-class pair, Atlas on both ends.
[[link]]
name = "rfd"
type = "serial"
device = "/dev/serial/by-id/usb-FTDI_FT231X_USB_UART_D30A1B2C-if00-port0"
baud = 57600 # must match the radio
radio = "rfd900" # rfd900 | sik | generic
air_rate = "64 kbps" # capacity = the lower of line and air
flow_control = "none" # or "rtscts"
probe_interval_ms = 1000 # radios are slow and half duplex
# Several nodes on one broadcasting channel: add an address.
# address = 1
# peer_addresses = { "10.0.100.2" = 2, "10.0.100.3" = 3 }Defaults: 57 600 baud; a heartbeat every 250 ms for rfd900 and sik, none for generic; a 2 048-byte radio buffer; pacing on. A serial link takes no bind_device; two serial links on one device, a reserved or duplicate address, or peer_addresses without an own address are refused by name when the configuration loads.
Stated in the same font size as everything above.
The frame carries Atlas datagrams and nothing else. A radio whose far end is not an Atlas node, or a device that offers only PPP or IP, stays on the adapter and pppd. Both nodes of a pair must run a build with the serial link.
On a shared channel the engine sends link-state floods and gossip as one unicast copy per peer, where one broadcast frame would reach all of them. When a radio’s transmit slot ends mid-frame, frames from two senders can interleave at a receiver: the split frame is lost and counted as a framing error, and the next one survives on its leading delimiter.
SiK-family radios talk in turns, and both directions share the air rate. The pacer measures the rate one direction actually gets from the radio’s reports, but it cannot create air: a two-way call needs its budget twice. Round trips include the other side’s turn, so a radio link wants a probe interval of 500 to 1 000 ms.
Atlas does not send AT commands. MAVLink framing, line rate, air rate, flow control and multipoint settings are set on the radio with its own tools. The radio’s buffer size is a link setting, radio_buffer_bytes, 2 048 bytes by default, and turns the reported fill into bytes.
On 64 kbit/s air a voice frame can still wait behind the low-band frame just released and one fragment’s airtime of queue, which is where the 100 ms comes from. Previous-frame redundancy with 20 ms frames needs more than 64 kbit/s of air.
Bring the radios, their line and air rates and what has to cross them. The answer is a link plan with the byte budget of every stream on it.