Background

Cable modem terminology

The words a DOCSIS plant is described in, from the RF on the coax to the CMTS in the central office. Read this once and the tables in docsis-server make sense.

Downstream and upstream

We start off with a downstream and an upstream. The downstream runs from 54 MHz or 108 MHz or 258 MHz up to 860 MHz or 1002 MHz or 1218 MHz depending on the plant design. The upstream runs from 5 MHz up to 42 MHz or 85 MHz or 204 MHz depending on where the split is with the downstream. Those three pairings are the North American sub-split, mid-split and high-split. Any frequency below 10 MHz is usually too noisy for any useful work. Any frequency below 20 MHz is also fairly noisy but still usable with the right modulation and error correction.

020040060080010001218 MHzSplit at 42 / 54 MHzSplit at 85 / 108 MHzSplit at 204 / 258 MHz86010021218 MHzUpstreamDownstreamDownstream ceiling, depends on plant designBelow 10 MHz: too noisy · 10–20 MHz: noisy but usable
The three common splits, to scale. The upstream is a sliver next to the downstream, which is why upstream capacity is what a plant runs out of first.
CMTScentral office Downstream · 54–1218 MHz · one continuous signal, every modem receives it Upstream · 5–204 MHz · short bursts, one modem at a time per carrier Each house holds a cable modem on the same coaxial line.

Every modem hears the downstream. The upstream is scheduled.

Each cable modem on a downstream is receiving exactly the same RF signal. All of the IP packets have been encrypted and a single cable modem can only decrypt the packets for itself, but every cable modem is constantly decoding all of the RF signals. (This is a bit different with downstream OFDM, but we will just handwave that distinction away for now.) Whereas the downstream signals are constant, the upstream signals are very intermittent. An upstream can carry 1 or more carriers and is usually configured for at least 3. For each of those carriers only a single cable modem can be transmitting at any one time. Embedded in the downstream signal is a special timing map that tells each cable modem when it can transmit and on which carrier it can transmit. The central office (CMTS) sends that map hundreds of times a second.

Downstream54–1002 MHz · ref −10 dBmV · 10 dB/div
8 × 6 MHz SC-QAM carriers, always on OFDM block · thousands of subcarriers 542915287651002 MHz
Steady carriers on a flat noise floor. The picture looks the same from every modem on the line.
Upstream0–85 MHz · ref +40 dBmV · 10 dB/div
noise floor rises toward 0 MHz burst idle burst OFDMA · low modulation,high error correction OFDMA · high modulation,low error correction 01734516885 MHz
Carriers appear only while a modem has been granted a slot to transmit; in the OFDMA blocks, groups of subcarriers are granted to different modems at once. The block sitting in the noisy low band runs a lower modulation with heavier error correction; the clean upper block runs high modulation with little. Below 20 MHz the noise climbs steeply; below 10 MHz there is nothing usable.

The node

A downstream and upstream together is in our terminology known as a "node". All of the cable modems in that node share the same RF spectrum on the cable line. Because there is so much more bandwidth available on the downstream than on the upstream, the same downstream signal is often used by multiple upstreams. RF noise on the upstream is often the biggest problem on an RF cable plant, and segregating groups of homes to separate upstreams both doubles the upstream bandwidth to those homes and segregates the noise from the homes in one upstream from the homes on another upstream.

Node DS 1 · shared US 1 US 2 US 3 US 4 splitter noise stays on US 3 One downstream, four upstreams: each group of homes gets its own upstream capacity, and a noisy upstream does not degrade the others.

Remote PHY

On older cable modem systems, all of the RF modulation/demodulation happened in a central office. On newer systems, we use something called "Remote PHY" where we move the modulation/demodulation outside of the central office in the place of something we used to call the "optical node". The optical node was fed an RF signal over fiber optic cable from the central office and it converted that optical RF signal to a cable line RF signal. With the new RPHY node, the former optical node is connected to the central office using 10 Gbit or higher optical Ethernet. Using a digital signal instead of an RF signal improves the quality of the signals coming and going to the central office. Depending on the bandwidth being used, you can also daisy chain the RPHY nodes together so that a single fiber optic cable going back to the central office can support multiple RPHY nodes.

Each RPHY node can have 1 or more downstreams as well as multiple upstreams.

A Remote PHY node hanging on the strand with its lid open. The lid carries the RF amplifier board, silkscreened with its own signal path: four thirty-decibel test points at the corners, blocks marked US conditioning, DS conditioning and DPD conditioning, and the AC routing that passes plant power through. The base below holds the Remote PHY modules and the optical connection, joined to the lid by a ribbon of fine leads. Hard line runs out to both sides.
A Remote PHY node opened on the strand. The optical Ethernet and the modulation hardware are in the base; the lid is the amplifier that drives the coax.
RPHY node modulation and demodulation happen here 10G in 10G out from the CMTS to the next node DS 1 US 1 US 2 DS 2 US 3 US 4 One RPHY node, two downstreams, each shared by two upstreams: four nodes in our terminology.

The CMTS

At the central office we have a device called a "Cable Modem Termination System". This device used to be quite large and it would generate all of the RF signals. With the advent of RPHY, the CMTS is now just a computer. That computer takes the packets coming in from the internet, determines which cable modem they are for, checks to see if that cable modem is going over its bandwidth reservation, it then encrypts the packets and sends them to the RPHY node over optical gigabit Ethernet. The CMTS also calculates the upstream transmit map hundreds of times a second that gets sent out to the cable modems. The RPHY node receives RF signals on the upstream ports and converts those RF signals to digital packets. Those digital packets get sent back to the CMTS where they are decrypted using the encryption keys it negotiated with that cable modem. The decrypted IP packet is then sent to the internet.

Internet CMTS central office · a computer classify per modemcheck bandwidth reservationencrypt · decrypt · build MAPs 10G+ optical Ethernet encrypted packets + MAPs, hundreds per second daisy chain from node 1 own fiber to the CMTS RPHY node 1 RPHY node 2 RPHY node 3 1 · Packets arrive from the internet; the CMTS finds the modem, checks its reservation, encrypts. 2 · Sent digitally over fiber to the RPHY node, which modulates them onto the downstream RF along with the transmit MAP. 3 · Modems burst on the upstream in their MAP slots; the node demodulates to packets; the CMTS decrypts and forwards to the internet. downstream RF upstream RF optical Ethernet