The DOCSIS Physical Layer

Data between the Cable Modem Terminating System (CMTS) in the headend and the Cable Modem (CM) travels along the fiber and coaxial cable. In a manner similar to a radio, many different "channels" are simultaneously transmitted. It is then up to the receiver to select the appropriate channel to receive the data vs. the TV channels.

It is possible to send and receive data simultaneously and this is done by using low frequencies to sent the information from the CM to the CMTS and higher frequencies to send the data from the CMTS to the CM. This is shown graphically in Figure 1.

Figure 1. Different frequency bands are used for sending data upstream vs. downstream.

The downstream channel uses a modulation technique called Quadrature Amplitude Modulation (QAM). QAM uses both both shnges in amplitude and phase to transmit the signal. 64-QAM uses 64 combinations of amplitude and phase while 256-QAM uses 256 combinations of amplitude and phase.

Table 1. The DOCSIS Downstream Channel

In the US, each TV channel is 6 MHz wide and so the DOCSIS channel replaces one of the cable operators video channels. In Europe, a TV channel is 8 MHz wide and European version of DOCSIS, Euro-DOCSIS, is optimized for that environment.

The characteristics for the upstream channel are shown in Table 2 below. QPSK is very similar to QAM but it uses only 4 combinations of amplitude and phase to modulate the signal. The reason this is done is that QPSK is more reliable but slower than 64-QAM. Noise from around the home and due to radio stations, CB radios and electric mixers makes it more difficult to hear the data signal.

Table 2. The DOCSIS Upstream Channel (DOCSIS 1.0 & 1.1)

The data rates for the upstream data channels are listed in Table 3. It is the Cable Operator that determines the data rate the system will use and the frequencies that are assigned.

Table 3. Upstream Data Rates (DOCSIS 1.0 & 1.1)

The primary difference between DOCSIS 1.x and DOCSIS 2.0 is the addition of higher upstream data rates. DOCSIS 2.0 uses A-TDMA (advanced frequency agile time division multiple access) and S-CDMA (synchronous code division multiple access) for the upstream modulation. This results in the speeds listed in Table 4.

Table 4. Upstream Data Rates (DOCSIS 2.0)

Downstream Processing Steps

Another processing step, the convolutional interleaver mixes up data from several packets and this results in noise bursts effecting fewer bits in any one packet. Scrambling the order of the bits and adding and adding trellis coding rounds out the additional processing of the data before being applied to the QAM modulator. The sequence of these processing steps is show below in Figure 2.

Figure 2. Order of the downstream. processing steps

The purpose of scrambling of the bits is not for reason of security but to keep too many 1s or 0s from occurring together. This would cause a DC voltage to occur and decrease the reliability of the transmission.

Figure 3. Bit Scrambling

More Information

The DOCSIS specifications are available at: www.cablemodem.com/specifications.html .

Additional DOCSIS seminars:

Overview - An overview of the components of a cable TV system.

The DOCSIS Protocol - A description of the messages between the CMTS and the CM. This includes the mechanism to share the coax, ranging and registration.

Quality of Service (DOCSIS 1.1) - The changes to DOCSIS 1.0 to implement QoS.

In Summary:

• Upstream and downstream transmissions share the same cable but use different frequencies.

• The system is scalable since there may be several downstream and upstream DOCSIS channels operating simultaneously.

• There are many different options for the upstream bandwidth.

• Upstream transmission is more challenging because of the presence of noise around the home getting into the cable system.