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РусскийUnveiling the DOCSIS 3.1 technology
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.
DOCSIS 3.1 ANALYZERS OVERVIEW
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 |
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
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
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
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.
DOCSIS 3.1 ANALYZERS OVERVIEWCodewords
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
New modulation schemes
DOCSIS 3.1 allows for a use of higher order QAM modulations to offer increased spectrum efficiency.
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.
DOCSIS 3.1 ANALYZERS OVERVIEWPROMAX is a leading manufacturer of test and measurement systems, broadcast and TV signal distribution equipment. Our product lines include measuring instruments for cable TV, satellite TV, broadcast, wireless and fibre optics networks, FTTH and GPON analyzers. DVB-T modulators, IP streamers or IP converters (ASI, DVB-T) are among the company’s latest developments.