DOCSIS Modem Transmission

The DOCSIS modem maintains two-way communications with a CMTS that interfaces to the Internet. Data is transported into the modem via the forward or downstream spectrum and is conveyed from the modem to the CMTS upstream via the RF return path.

In DOCSIS 1.0 through 2.0, the downstream data travels on a selected channel between the frequencies of 88 to 864 MHz and the upstream data travels on an RF carrier between the frequencies of 5 to 42 MHz (Figure 1). The downstream data is modulated onto a QAM television (TV) channel with a bandwidth of 6 MHz, but the bandwidth of the upstream QAM carrier can range from 0.2 MHz to 6.4 MHz. Because the bandwidth of the downstream frequency spectrum is over 700 MHz, there are many more channels available for downstream DOCSIS data than there are for upstream data.

Figure 1: The DOCSIS frequency spectrum.

DOCSIS 3.0 introduced downstream and upstream channel bonding, which allowed up to eight downstream channels to be bonded together. Eventually, modem and CMTS manufacturers increased the number of downstream bonded channels to 32.

In the upstream, up to four 6.4 MHz wide carriers could be bonded, requiring up to 25.6 MHz (4 × 6.4 = 25.6) of upstream spectrum. To have enough bandwidth for upstream carriers from previous DOCSIS versions, DOCSIS 3.0 provided the option to increase the top frequency of the upstream to 85 MHz, and bumped the downstream range to 108 MHz to 1,002 MHz (Figure 2A).

DOCSIS 3.1 provides the option to expand the top of the upstream spectrum to 204 MHz while raising the low end of the downstream spectrum to 258 MHz, but increasing its high end to 1,218 MHz (Figure 2B). These spectrum changes enable the full capabilities of DOCSIS 3.0 and 3.1 to be realized. Because DOCSIS 3.0 and 3.1 are compatible to previous versions, it is unnecessary to modify the downstream and upstream spectrum ranges to use them.


Figure 2: DOCSIS 3.0 and 3.1 Spectrum Plans.