Unveiling the DOCSIS 3.1 technology

May 21, 2019.

DOCSIS 3.1 brings a good number of new technologies to the table which put together become the essence of this new powerful CATV standard. Here you will find these innovative developments as well as their related jargon explained below.


PROMAX DOCSIS 3.1 analyzers
PROMAX DOCSIS 3.1 analyzers
    DOCSIS 3.0 DOCSIS 3.1
Frequency bands Upstream Up to 85 MHz Up to 204 MHz
Downstream Up to 1 GHz Up to 1218 MHz
(1.8 GHz optional)
Channel bandwidth Upstream 6.4 MHz (channel bonding) From 6.4 to 96 MHz
Downstream 6 MHz (channel bonding) From 24 to 192 MHz
RF carriers Up/Downstream SC-QAM OFDM
Carriers per channel Upstream 1 1900 (2K) or 3800 (4K)
Downstream 1 3800 (4K) or 7600 (8K)
Carrier spacing Upstream N/A 50 kHz (2K) or 25 kHz (4K)
Downstream N/A 50 kHz (4K) or 25 kHz (8K)
Non-user-content carriers Upstream N/A N/A
Downstream N/A Exclusion carriers, PLC, CP, SP
Modulation profiles Upstream QPSK, 8/.../128-QAM 16/.../4096-QAM
(16384-QAM optional)
Downstream 64/256 QAM 16/.../4096-QAM
(16384-QAM optional)
Packet framing Upstream DOCSIS DOCSIS
Downstream MPEG-TS DOCSIS + Codewords
FEC - Reed-Solomon LDPC + BCH
Main technical differences between DOCSIS 3.0 and DOCSIS 3.1 standards

DOCSIS 3.1 band extension

DOCSIS 3.1 extends the Upstream band up to 192 Mhz and the Downstream band up to 1.2 Ghz (optionally up to 1.8 Ghz) with the objective of increasing network capacity.

DOCSIS 3.1 band extension
DOCSIS 3.1 band extension

OFDM modulation

Both Upstream and Downstream DOCSIS 3.1 channels use OFDM RF transmission technique as opposed to SC-QAM (Single Carrier QAM) channels. These means that each channel is actually made up of thousands (2K, 4K or 8K) of narrow band subcarriers, so narrow and close to each other that together look as a unified carrier to the naked eye. This is the same transmission technique used in standards such as DVB-T/T2 or C2.

  • Number of subcarriers: 1900 (2K mode for Upstream), 3800 (4K mode for Upstream or Downstream), 7600 (8K mode for Downstream).
  • Subcarrier spacing in 6 Mhz channels: 25 kHz (4K mode for Upstream, 8K mode for Downstream) or 50 kHz (2K mode for Upstream, 4K mode for Downstream).
OFDM modulation
OFDM modulation

Exclusion bands: Ensuring compatibility with legacy DOCSIS

The exclusion bands are ranges of subcarriers that are deactivated within a channel for 2 main reasons: To ensure backwards compatibility with legacy DOCSIS channels that may be transmitted in that frequency range, or to avoid using that band because it significantly suffers from strong interferences or noise.

DOCSIS 3.1 exclusion bands
DOCSIS 3.1 exclusion bands

TDMA (Time Division Multiple Access)

Previous DOCSIS standards were already making use of TDMA in order to assign RF Upstream channels to a number of cablemodems, each using the channel during a specific timeslot (hence the term “time division”). DOCSIS 3.1 still makes use of TDMA in Upstream but assigning a number of subcarriers to each cablemodem during a timeslot.

DOCSIS 3.1 makes use of special non-user-content subcarriers:

  • PLC (Physical Layer Link Channel) carriers: for signaling, communication control and network management information.
  • CP (Continual Pilot) carriers: for synchronization.
  • SP (Scattered Pilot) carriers: for channel estimation.

Modulation profiles

A profile describes the set of parameters (modulation order, FEC, preamble, G.I., pilot pattern, etc) used for transmission between the CM and CMTS. Different modulation profiles may be used in US.



Several DOCSIS frames (packets) are sequentially embedded into codewords (larger frames). Codewords are associated with a common profile, so during the transmission of a specific codeword the same set of modulation parameters are used. Each codeword is mapped to a number of physical subcarriers for its transmission.

The NCP (Next Codeword Pointer) are special codewords that indicate at which subcarrier each standard CW starts. They are essential for the receiver to retrieve the data.

DOCSIS 3.1 codewords
DOCSIS 3.1 codewords

New modulation schemes

DOCSIS 3.1 allows for a use of higher order QAM modulations to offer increased spectrum efficiency.

DOCSIS 3.1 modulation schema
DOCSIS 3.1 4096 QAM modulation schema

Advanced FEC (Forward Error Correction)

DOCSIS 3.1 implements two new FEC (Forward Error Correction) algorithms so as to better detect and clear errors occurred during transmission.

  • LDPC (Low Density Parity Check): Iterative method that stops as soon as all errors are corrected or no more errors can be corrected.
  • BCH (Bose-Chaudhuri-Hocquenghem): This algorithm will get a second chance to clear any errors not corrected by the LDPC algorithm.

It is common to use as a quality measurement the number of correctable codewords errors and uncorrectable codewords errors on DOCSIS 3.1 channels, instead of the pre and post BER measurements used in previous standards.


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