March 2018


20 Years After The 1st DOCSIS, Cable Modems Are Capable Of The Same Speeds For Upload & Downloads

International R&D organisation - CableLabs has announced its specifications for Full Duplex Data Over Cable Service Interface Specification (DOCSIS), which will provide speeds of upto 10Gbps down and 10Gbps up.


A cable modem system consists of 2 main components:

 A cable modem at the customer premises, and

 A cable modem termination system at the CATV headend.


Cable TV distribution networks typically use optical fibre to carry digital signals from the Headend to nodes which convert optical signals to RF signals. The RF signals are distributed to the customer premises. This mix of both optical and RF, is referred to as a Hybrid Fibre-Coax (HFC) system.


A Cable Modem Termination System (CMTS) is the Headend for all Cable Modems operating in the network. The CMTS is usually located in the Cable TV headend premises.


Data sent by the CMTS is received at the premise by a Cable Modem. The Cable Modem also sends data back to the CTMS.


The earliest Cable Modems were engineered independently by companies. As a result, the the systems were incompatible.

Cable Labs established the DOCSIS - Data Over Cable Service Interface Specification, so that all equipment in the system are inter-compatible.

DOCSIS 1 was established in 1997. The latest DOCSIS 3.1 FDX (Full Duplex) was released in 2017 and commercial products are expected in 2018-19.


To deploy a Cable Modem system on a cable TV network, it is essential to enable the Cable TV network have a return path - i.e. to be able to carry signals in both directions - to and from the Cable Modem to the CMTS.


The data path from the CMTS to the customer's Cable modem is called Downstream.


The data path from the customer's Cable modem back to the CMTS is called Upstream.


Internet data traffic is often unsymmetrical, i.e. more data is required Downstream, for downloading, receiving web pages, receiving e-Mails etc.

Upstream data is required for Uploading files, sending out E-mails and sending requests for specific web pages.

As a result, until now, cable modem systems were engineered to provide much faster downstream than upstream.

Table 1 Summarises A Comparison of the various characteristics of DOCSIS Systems as they have evolved over the years.

(For more detailed information on DOCSIS 2 & 3, do read past technical articles carried in this magazine. You can read them for free, anytime in the "Tech Articles" section of our website. (


The main difference between DOCSIS 2 and DOCSIS 3 cable modems is that DOCSIS 3 provides higher speed mainly due to 'Channel Bonding" i.e. allocating 2 or more frequency bands for data downstream. For example, a DOCSIS 3 modem with 4 bonded channels will provide 4 times the bandwidth of a DOCSIS2 modem.

Earlier versions of DOCSIS also used a lower frequency band for Uploads (Return Path), and higher frequency bands for downloads. The downstream path was also allocated a much higher bandwidth. The Upstream path had limited bandwidth (5 MHz to 50 MHz or 100 MHz), and this lower frequencies were adversely affected by noise.

Higher download speeds are now achieved by using higher orders of QAM modulation, and increased bandwidth allotment.

To enable higher orders (more dense) QAM modulation, the data loss due to noise must be minimised. For this, improved error correction has been used.

DOCSIS 1 to 3.0 used FEC (forward error correction) to improve the BER (bit error rate). FEC is also used in when downlinking satellite TV signals.

For DOCSIS 3.1 adds Low Density Parity Check (LDPC) for 6 dB improvement (ie twice as good) error correction.

Before DOCSIS 3.1, the maximum modulation available was 64-QAM and 256-QAM for upstream and downstream respectively.

With LDPC in DOCSIS 3.1, can support up to 4096-QAM with future optional 8192-QAM and 16384- QAM in the downstream. Upstream now supports 1024-QAM and future optional support for 2048-QAM and 4096-QAM.


DOCSIS 3.1 also has increased the RF bandwidth for data transmissions. The upstream frequency range is from 5MHz to 204MHz, (earlier 5 MHz to 42MHz). The downstream starts at 258MHz extends to 1218MHz, and even 1,794MHz in future.

The digitisation of Cable TV signals, has made available spectrum that can now be used for data.


To further maximise data delivery, DOCSIS 3.1 enables the same CMTS to send different QAM signals to different cable modems. Cable modems located in a low noise environment will receive more dense QAM signals, resulting in higher speed data. Cable modems in the same distribution network that have high noise will receive lower QAM modulated signals.

This practically provides for 36% better efficiency.


DOCSIS 3.1 has introduced Orthogonal Frequency Division Multiplexing (OFDM). OFDM provides for 2 data streams at the same carrier frequency, but the Carriers are 90 degrees apart. there is also no wasted frequency spectrum kept aside for 'Guard Bands' between frequency bands.

OFDM may be new to DOCSIS, however it has been used for many applications including PLC, WiFi, and cell networks.


DOCSIS 3.1 also provides backward compatibility. The system will also work with DOCSIS 3.0 and earlier Cable modems, but at lower speeds. The reverse path has been restricted to 258MHz for backward compatibility.

DOCSIS 3.1 FDX Enables 10 Giga Bits In Both Directions


Announced February 2016, Full Duplex DOCSIS (FDX), an extension to DOCSIS 3.1, enables 10 gigabit speeds in both directions.

It uses the full Cable TV frequency spectrum (5 MHz to ~1.2 GHz) at the same time in both upstream and downstream directions. The FDX CMTS will simultaneously receive and transmit in the same FDX spectrum.

The FDX Cable Modems either receive or transmit in the same FDX spectrum, but simultaneously receive and transmit at different frequencies.

The FDX band is divided into (25 kHz or 50 kHz wide) sub-bands and the CMTS assigns which & how many sub-bands each Cable Modem uses for upstream or downstream operation. The allocated sub bands change continuously, depending of the data demand of each cable modem, at that instant. Cable Modems are distributed in sub-groups, and each sub group is allowed to transmit or receive at the allotted time, to avoid interference.

The CMTS at the headend uses a combination of interference cancellation and intelligent scheduling.

Cable Modems use echo cancellation to prevent upstream transmissions from interfering with adjacent downstream channels in the FDX band.


To practically make cable modems at consumer friendly prices, the DOCSIS 3.1 FDX technology will have to be made into an Integrated Circuit (IC). Work is in progress on this.

Arris President of Network, Cloud, and Services Dan Whalen said Arris will be trialling Full Duplex DOCSIS, with possible full-scale deployments across the globe in 2019. n