A method for imprinting digital signals onto a carrier for transportation from one place, such as a cable headend, to another, such as a home. The business take-away from QAM is the message of throughput, or how many bits per second can be crammed into a single, 6 MHz channel, modulated with QAM. Early implementations used 64-QAM, which afforded about 27 Mbps per channel. Contemporary networks generally use 256-QAM modulation now, which yields 38 Mbps per 6 MHz channel. Even though the technique is called quadrature amplitude modulation, it also involves the use of “phase modulation. What happens, loosely, is this: Digital bits are grouped into symbols, which are then imprinted onto a carrier by manipulating two transmission characteristics: Amplitude (power) and phase (frequency shape). The advantage of this dual modulation is the ability to stuff twice as much information into the transmission channel, at the same time.

Although 64-QAM and 256-QAM are the workhorses of digital modulation, a slimmer variation — 16-QAM — is available for upstream, home to headend transmissions. The advantage of 16-QAM in the upstream is more carrying capacity inside the skinny, 5-42 MHz upstream (headend-to-home) signal path. However, using 16-QAM in the upstream requires a carrier-to-noise ratio that is difficult to support in that inherently noisy spectral zone.
From a services perspective, QAM is used in the downstream (headend to home) signal path for all digital offers: Video, broadband Internet, and voice (circuit switched or IP). (In some cases, digital signals from traditional broadcasters are excepted, and particularly if the broadcasters send signals using vestigial sideband modulation.)

There’s no easy way to describe this complicated form of modulation. The name itself is impressively nerdy, and probably not the best way to describe whats really happening. Quadrature, for example, means “at right angles.” There are four 90-degree angles in a 360-degree totality, so “quadrature” implies a fourth, or a quarter. The whole discussion of quadrature, and right angles, relates to the phase manipulation. Basically speaking, “phase” is the shape of the frequency, and is shifted by 90 degrees before being manipulated to carry digital information. Hence “quadrature.”

In simplest terms, the process of modulating with QAM involves grouping bits into symbols, then simultaneously imprinting the grouped bits onto the phase or the amplitude of the transmission carrier. With 64-QAM, 8 symbols are transmitted; in 256 QAM, 16 symbols.

Pronunciation: Most say it like a word; take your pick on the vowel sound. Some say kw-ahh-m (as in, rhymes with mom.) Others, like anyone at Broadcom (the biggest producer of QAM chips) say it so that it rhymes with Sam.

QAM is used in cable systems because its efficient, and fast, compared to other digital modulation techniques, like vestigial sideband (used in digital terrestrial TV broadcasts) and QPSK (quaternary phase shift key, used in satellite and upstream cable transmissions).

The technologies that make it possible for some packets to get preferential treatment over other packets. Services that run over broadband have different “nutritional” needs; QoS recognizes that not all packets are created equal. It enables broadband service providers to differentiate their broadband services, maybe by goosing available speeds, or by assuring that technologically “needy” services, such as voice or video, run smoothly. In short, QoS is what makes managed broadband networks work better than the public Internet.

Conversationally, its spoken either as its constituent letters, or as a word: “Kwoss.”

The quality additions can be applied in speed — you need more speed to view a streamed clip — or in transit timeliness. The latter matters especially to isochronous services, like phone. Without QoS, voice calls over the IP data path run the risk of sounding awkward — where one or both callers feels the need to follow each uttered phrase with “over,” or where callers start to sound like the teacher in Charlie Brown.
Technically, at least as it relates to the cable industry, QoS is rooted in the DOCSIS 1.1 cable modem specification. Essentially, 1.1 describes a way to “stripe” packets that flow to and from a cable modem. The striping is done with “service identifiers” (SIDs, pronounced “sihds”). There are 16 of them in DOCSIS 1.1. How they’re applied depends on the strategic roadmap of varying service providers, and what qualitative, competitive differences they want to offer.

Prior to DOCSIS 1.1, cable modem technology followed a best-effort practice: Available bandwidth was shared by the number of simultaneously-used modems connected, per 500-home HFC node. Nobody could take priority over each other for more bandwidth. To keep heavy users from being bandwidth hogs, cable providers capped everyone at maximum downstream/upstream rates, usually 1-2 Mbps/384 kbps. (This was before the “speed wars” between cable and DSL.)

Note that the qualitative and quantitative improvements that can be put into motion with QoS are not contained to broadband speeds. Consider, for instance, Customer Bob, who happens to buy a cell phone with built-in WiFi capabilities. Let’s say Bob isn’t particularly enthused with the cellular service he gets inside his house. However, Bob also happens to have a WiFi network, and a cable modem, in his house.

In the growing discussions between cable and wireless providers, one obvious use of cable’s QoS is to jointly solve Bob’s problem: When he’s in his house, he’s talking on the WiFi part of his cell phone. The cable modem, behind the WiFi antenna, hands his speak-bits off to his cellular carrier. When Bob climbs into his car and backs out of the driveway, the call reverts to cellular mode.

The technology behind making that scenario work, behind the scenes, is QoS. It “protects” Bob’s speak-bits, so that they get to where they’re going (the person he’s talking to) without incident.

Couldn’t the cellular carrier just bypass the cable operator, and do the WiFi connection to Bob without the knowledge of the broadband connection?

Yes. The notion of “bypass” started with companies like Vonage adding voice services, undetectably, over cable modem passageways. Yet it can be extended to practically anything the imagination can cook up. But, without QoS, “bypass” services will only be as sturdy as the weakest link in their delivery chain — which means they have an “end-and-end” connection, not an “end-to-end” link.

A subset of quadrature amplitude modulation, used to imprint digital signals onto a transmission carrier by manipulating the phase of the carrier. In terms of carrying capacity, QPSK is functionally equivalent to 4-QAM, and can transmit four symbols per second. This is equivalent to roughly 1.67 bits/Hz, or about 10 Mbps of effective throughput.

Essentially, with QPSK, the phase carrier is split into two: The original phase, and the manipulated phase. The manipulated phase is generally shifted by some increment of 90 degrees — a right angle — which is where the term “quadrature” comes in.

QPSK is generally used in circumstances which require as much speed as possible, in harsh conditions. Satellites, for example, use QPSK to blast digital signals down through space to Earth-based receivers. Cable providers generally want to use QPSK for the upstream (home to headend) signal path.