Modulation Types

Both DOCSIS and Euro-DOCSIS dictate that digital information be modulated and demodulated between modem devices on the cable network using QAM, which is a type of phase-amplitude modulation. The phase and amplitude of the signal are modulated to encode 0s and 1s. QAM modulation encodes symbols on RF waveforms. The number of symbols per second is represented by a modulated signal that is proportional to the bandwidth of the transmission frequency. For example, an 8-MHz wide channel can carry more symbols per second than a 6-MHz wide channel. Each symbol that is modulated onto the waveform represents several bits and thus data. The number of bits that can be represented by a symbol is dependent on the type of modulation used. Modulation types used in DOCSIS networks can encode from 2 bits per symbol up to 8 bits per symbol. However, the number of bits that can be successfully encoded and decoded per symbol is extremely reliant on signal quality. In cable environments where noise and interference are prevalent, modulation formats that supply greater data throughput are more easily corrupted, resulting in either high bit error rates (BERs) or loss of the entire communication channel. Modulation types that encode fewer bits per symbol are more noise tolerant and are often used to mitigate the issues caused by noise and interference.

In the downstream direction on the cable plant the DOCSIS specification includes two types of QAM modulation: 64 QAM and 256 QAM. Upstream transmission occurs in frequency ranges with higher noise levels and interference and DOCSIS dictates the use of more robust modulation types—16 QAM and quadrature phase shift keying (QPSK). DOCSIS 2.0 provides provisions for clean upstream systems (often found where fiber nodes have small numbers of households passed) and allows for 32 and 64 QAM upstream.

Bandwidth for downstream transmission varies from region to region with the CATV standard used locally. For NTSC systems, transmission downstream is on a band 6-MHz wide. On other cable systems the band is 8-MHz wide. Overall throughput on the cable network is directly proportional to the number of symbols that can be transmitted per second. For DOCSIS networks, the frequency range for downstream signals is from 55 to 870 MHz.

Both DOCSIS and Euro-DOCSIS specifications detail upstream frequencies and bandwidth that is not fixed but instead can be one of several widths—200 kHz, 400 kHz, 800 kHz, 1.6 MHz, and 3.2 MHz. DOCSIS 2.0 allows for the use of an upstream channel width of 6.4 MHz, although this is only useful in networks with unusually small amounts of noise and interference. These frequencies can be located anywhere between 4 and 45 MHz but typically are above 20 MHz because of noise limitations. Euro-DOCSIS allows for a frequency range of 5 to 65 MHz. The range of upstream bandwidths allows cable operators to mitigate concerns of noise and interference by placing appropriately sized upstream DOCSIS bands between known noisy frequencies in the upstream spectrum. Multiple upstream frequencies can be used on a single fiber node or group of nodes to supply additional throughput. Certain cable modem and CMTS vendors offer features that allow automatic changes in modulation, bandwidth, and frequency based on the appearance of noise or interference on an active channel.

Determining Modulation Type

Although some CMTS vendors have configured a default downstream modulation of 256 QAM, most vendors have chosen the more common 64 QAM as a default modulation profile. This is because 256 QAM offers less tolerance for network operating and alignment errors and requires better carrier-to-noise ratio performance. Only exceptionally clean cable plants are of high enough quality to run 256 QAM in every home with appropriately low error rates. Often some form of plant repair or in-house wiring repair is required before 256 QAM is reliable in the home. Sometimes connectors and other passive devices inside the home must be replaced. Other solutions include amplitude correction or even complete coaxial cable replacement. 64 QAM is more frequently deployed than 256 QAM as a result of these challenges.

Likewise, most CMTS vendors default to the noise-tolerant QPSK in the upstream. Example 13-1 shows sample output from a Cisco uBR7246 detailing the modulation profile for a particular CMTS upstream channel. The output notes channel width, modulation profile (in this case 16 QAM) and symbol rate for the channel.

Example 13-1 Example Output of show controller Command on a Cable Interface uBR7246#show controller cable 5/0 upstream 0

Cable5/0 Upstream 0 is up

Frequency 19.984 MHz, Channel Width 1.600 MHz, 16-QAM Symbol Rate 1.280 Msps Spectrum Group 1 SNR 27.6040 dB

Nominal Input Power Level 2 dBmV, Tx Timing Offset 2293 Ranging Backoff Start 0, Ranging Backoff End 4 Ranging Insertion Interval automatic (50 ms) Tx Backoff Start 0, Tx Backoff End 4 part_id=0x3137, rev_id=0x03, rev2_id=0xFF nb_agc_thr=0x0000, nb_agc_nom=0x0000 Range Load Reg Size=0x2C Request Load Reg Size=0x07

Minislot Size in number of Timebase Ticks is = 8

Minislot Size in Symbols =64

Bandwidth Requests = 0x52B

Piggyback Requests = 0x1AA

Invalid BW Requests= 0x1AA

Minislots Requested= 0x2C1A

Minislots Granted = 0x2570

Minislot Size in Bytes = 32

UCD Count = 3250

Continue reading here: Throughput for Docsis and EuroDOCSIS

Was this article helpful?

0 0