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QAM, or Quadrature Amplitude Modulation, is the modulation scheme used in cable plants. Basically, QAM is how the modem encodes digital information to be sent over the RF interface of the modem.

64QAM and 256QAM are two very common modulation schemes used in the downstream channels for cable modem or digital cable plants. 16QAM and QPSK are two modulation schemes commonly used for the upstream channels in a cable plant.

In digital applications, the modulating signal is generally quantised in both its in-phase and 90 components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation. Wikipedia explanation

The dots mentioned in the cited description are also called decision points. When the modem plots these points, each will fall into the boundary of a decision area, which corresponds to a sequence of 0s and 1s. This is called the QAM constellation.

QAM Constellation

The number before QAM (for example 16QAM) refers directly to the number of decision points in the constellation. Meaning, 256QAM has 192 more decision points than 64QAM. The number before QAM is also always a power of two. (EG. 2^6=64QAM)

There are several trade offs to this:
•The more Decision points, the more throughput per channel.
•The more Decision points, the more susceptible the channel is to noise (as the decision boundaries get smaller to accommodate more decision points, it becomes harder to distinguish which boundary the point was intended to lie in)

More Decision points=More Bandwidth=Higher susceptibility to noise or poor signal (the same applies inversely as well)


So what does this mean to me?
Ideally, the customer shouldn't notice any difference between a 64QAM or a 256QAM modulated downstream channel.

Despite the fact that 256QAM means more possible throughput per downstream channel, that doesn't necessarily equate to the customer seeing faster speeds. At best, it'll enable the cable company to either:

•Deliver the same number of customers incrementally faster speeds.
•Deliver a larger number of customers the same speeds as before.

Modulation Schemes and Speed table

*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.

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by Nerdtalker See Profile edited by Johkal See Profile
last modified: 2008-11-01 15:00:10