In the old time, the bit rate was the same as the signaling rate, that is, the number of times the signal level changes per second. However, we can utilize more than two different values, four or eight being common. This means that each signal element may then represent two bits(four values) or three bits (eight values) of binary information. The bit rate is then two or three times the signaling(baud) rate. An example is a PSK modem in which four different phase changes (0, 90, 180, 270 degrees) are employed instead of just two. This enables each phase change to convey two bits. Higer bit rates are achieved using 8 and 16 phase changes. In practice, however, there is a limit to how many different phase changes can be used, as the reducing phase differences make the scheme progressively more prone to noise and phase impairments introduced during transmission. Hence to increase the bit rate further, it is more common to introduce amplitude as well as phase variations of each vector. This type of modulation is then known as quadrature amplitude modulation(QAM). An example is shown in the above picture. This has 16 levels per signal element- and hence 4-bit symbols- and is known as 16-QAM. The phase diagram is also known as a 16-point constellation.

To deliver data services over a cable network, one television channel (in the 50 - 750 MHz range) is typically allocated for downstream traffic to homes and another channel (in the 5 - 42 MHz band) is used to carry upstream signals. A single downstream 6 MHz cable channel may support up to 27 Mbps of downstream data throughput from the cable headend using 64 QAM transmission technology. Speeds can be boosted to 36 Mbps using 256 QAM. Upstream channels may deliver 500 Kbps to 10 Mbps from homes using 16QAM or QPSK (quadrature phase shift key) modulation techniques, depending on the amount of spectrum allocated for service.

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