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Understanding how cable delivers high speed data: DOCSIS 3.1 – 4.0

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Cable operators are moving to high speeds

  • Spectrum
  • Speeds from 300Mbs to 940Mbs
  • Xfinity
  • Speeds from 75Mbs to 6Gbs
  • Mediacom
  • Speeds from 100Mbs to 1Gbe
  • COX
  • Speeds from 25Mbs to 940Mbs
  • WOW
  • Speeds from 100Mbs to 1.2Gbs

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Cable modem’s WAN port is moving to 2.5Gbe

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Some cable modems are moving to 10Gbe WAN ports!

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How to use coax and RF transmission to deliver data to the home/business?

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RG-6 Frequency range is 3Ghz @ 75 ohms

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CableLabs

A research and development company specializing in RF technologies

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CableLabs has developed

  • 10G/25G/50G-PON Passive Optical Networks
  • FDX Point-to-Point Coherent Optics “a single existing fiber can be used to carry upwards of 50 Tbps in each direction”
  • DOCSIS® 3.1 and 4.0 Technology
  • Wi-Fi Easy Mesh: how multiple access points (AP) in homes and small offices work together to form a unified network
  • LoRa Server: components for Low Power Wide Area Network critical for IoT
  • IWiNS: Intelligent Wireless Network Steering: enables seamless transitions between overlapping wireless networks (LTE, Wi-Fi, CBRS, etc)

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DOCSIS vs LTE vs 5G vs 802.11b/g/n/ac/ax

  • DOCSIS borrowed many RF technologies using by cellular and by the WiFi Alliance
  • OFDM (Orthogonal Frequency-Division Multiplexing)
  • QAM (Quadrature Amplitude Modulation)
  • Channel bonding
  • FEC (Forward Error Correction)

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Complex RF signals

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European cable TV conforms to PAL/DVB-C standards of 8 MHz RF channel bandwidth (called downstream channels)

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North American cable TV conforms to NTSC/ATSC standards which specify 6 MHz per channel

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DOCSIS is RF to Ethernet

  • Cable operators had a well-developed method of using Radio Frequency (RF) to deliver TV and video content
  • Cable Labs designed a Layer 1 and Layer 2 RF technology so cable systems could begin to deliver ethernet to homes and businesses.

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Using coax for the “last mile” connection

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Gave cable operators the bandwidth they needed for video and data.

RG-6 Frequency range is 3Ghz @ 75 ohms

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Coax is a lower cost transmission line than fiber to the home

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Cable operators already had 40% of all households in the US connected as of 2023. In 2019 it was 47%!

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RF as a Layer 1 is complex

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DOCSIS

  • DOCSIS is a layer 1 physical and layer 2 MAC

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We will focus on data

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DOCSIS is a RF standard

  • Designed by a company called “Cable Labs”
  • DOCSIS is a layer 1 and layer 2 RF technology

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DOCSIS 3.1

  • Shares many of the RF technologies of cellular LTE/5G and wireless 802.11 b/g/ac/ax
    • OFDM (Orthogonal Frequency-Division Multiplexing)
    • QAM (Quadrature Amplitude Modulation)
    • Channel bonding
    • FEC (Forward Error Correction)

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Delivering High Speed Data

is complicated

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1. CMTS 2. Hybrid Fiber-Coax Network 3.Cable modem

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Modern cable infrastructure: Remote-PHY��Allows full duplex and 10Gbe

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Architecture of Hybrid-Fiber and C-RAN

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PON moving into cable system

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radio frequency over glass (RFoG) is a deep-fiber network design

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More downstream spectrum

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More upstream bandwidth

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Improved operational expenses

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We will focus on three components of cable

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CMTS

  • Responsible for:
  • Initialize and registering your modem
  • Setting download and upload speeds
  • Ensuring security
  • Allowing customer service to see your modem

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CMTS (cable modem termination system)

  • CMTS can service between 4K to 150K subscriber modems
  • Inputs are Ethernet
  • Outputs are coax / fiber
  • IP traffic only
  • Provide QoS for VoIP
  • Acts as a router or switch

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CMTS monitoring software

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CCAP (Converged Cable Access Platform)

  • a single platform for offering traditional video and IP-based broadband
  • Includes a CMTS

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The basics of cable

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CCAP can see video/data problems in the cable plant

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What is between my modem and CMTS?

  • Optical nodes
  • Coax trunk lines
  • Fiber
  • Amplifiers (Repeaters)
  • Splitters
  • Taps

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Distances

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Coax Trunk lines

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  • A coax trunk line can carry from 200 -2K household IP traffic
  • 75 ohms

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Optical Ground Wire

  • OPGW: steel cables surround fiber strands for telephone pole transmission lines
  • Key to long haul cable delivery
  • Protects from lightning
  • Most cables include power delivery for repeaters/amplifiers and fiber nodes

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Fiber optic nodes

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Optical nodes: Fiber to coax

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Cable – data to Hospitals/Schools

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Cable-data to remote areas

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TAPs

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A coax trunk line is TAP-ed and coax splitter breaks up service for each room inside a hotel

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Cable amplifiers (repeaters)/splitters

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Safety: FVD “foreign voltage detector”

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  • Never touch an amplifier or TAP without checking the outside case with a FVD.

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  • These devices are AC powered and can have AC voltage leakage and you will get a nasty shock

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  • A FVD will alert you to this danger, touch the metal case with the FVD and hold the button down to determine is the case is safe to touch.

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Splitters: VoIP – Data - video

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Signals are impacted by HFC

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CPE

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Customer Premise Equipment

  • Most customers do not own their cable modem
  • CMTS controls this device
    • upgrades the firmware via a TFTP server
    • Reboots the modem remotely
    • Allows customer service to see SNR values and errors in the RF signal
    • Controls modem output RF power
    • Requires modem to “handshake” every 30 seconds

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Gateways or cable modems

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  • Most customers have CPE gateways
    • Cable modems
    • Router
    • Firewall
    • Wireless AP

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  • Businesses want just a cable modem
    • Ethernet RJ-45 jack
    • IP address

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Cable modems: DOCSIS 3.1

  • Maximum download 10Gbe
  • Typically, ethernet out: 2.5Gbe
  • IP address
  • You pay a rental fee/month

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Gateway

  • Cable modem
  • VoIP
  • Router
  • Firewall
  • Wireless AP

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Booting a modem:

  • Internal firmware
  • Begins boosting its RF amplifier to find a CMTS device
    • This is a single downstream channel
    • CMTS broadcasts enough information to identify downstream channels and upstream channels
    • Begins “ranging”
    • CMTS request a digital certificate from the modem

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Modem gets an IP address:

  • CTMS now sees the modem in the system
  • A DHCP server provides an IP address
  • A TFTP server sends a binary file to configure the modem
  • A “Time of Day” server delivers an accurate time to the modem
  • The CTMS activates the MAC layer resources for the modem

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All modems time-share

  • Access to the CMTS is time-shared by all the subscribers in a service group
    • FDMA (Frequency Division Multiple Access)
      • bandwidth is divided into various frequency bands
    • TDMA (Time Division Multiple Access)
      • time slot given to each modem

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Upstream/Downstream channels

  • Your modem’s download speed is dependent on how many downstream channels have been assigned by CTMS

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Channel width vs speed

Download channels can be from 24Mhz wide to 192Mhz wide

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DOCSIS is a complex RF system

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Your main cable-data problems

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80km max distance

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Ingress noise

  • As external RF signals leak into the return path from various sources, that noise accumulates and funnels upstream to the CMTS.
  • 80% of this problem comes from the customers home or drop
  • Tightening a connector can solve the problem
  • A good cable tech can identify which modems in an area are the problem using a with PreEqualization Analyzer

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CMTS can attenuate bands to remove ingress

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Impulse Noise

  • noise coming into the cable plant impacting upstream data
  • VoIP or Zoom calls are impacted with dropouts.

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  • Some sources of impulse noise
  • Poor insulators on electrical lines (arcing generates powerful RFI and EMI)
  • Welding machines

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Cable modem BSoD?

  • The cable modem is on but there is no data flowing through it.
  • Reboot the modem and it works again.
  • A crashed modem happens when its internal operating system crashes.
  • are usually associated with dynamic changes in the modem
  • Happens often as new technology is rolled out in a cable plant
  • The single best recommendation you can do during this time is to keep OFDMA channels away from high noise areas, such as those below 30 MHz.

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Where not to place your OFDMA channel — at the lowest part of the noisy spectrum.

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Mismatch Impedance

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Staples

Nicked cables

Poor connectors

Old cable or connectors

Crimped cable

Bent cables

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correct signal level on your modem

  • Common DOCSIS 3.0/3.1 signal issues in home
  • Too many splitters in the house
  • Multiple Video Set top boxes for different rooms
  • Multiple TVs in the house directly connected to the ISP cable signal
  • Large house with multiple cable connection points
  • Downstream RF levels
  • -7 dBmV to +7 dBmV are "Recommended" values.
  • -8 dBmV to -10 dBmV / +8 dBmV to +10 dBmV are "Acceptable" values but out of spec from ISP. A technician should still fix this to get to recommended levels.

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Signal to Noise Ratio

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  • 256 QAM: 30 dB minimum. 33 dB or higher is recommended. Most ISPs run 256-QAM in downstream.
  • 64 QAM: 24 dB minimum. 27 dB or higher recommended. Some ISPs have 64-QAM in downstream but rare
  • 16 QAM: 18 dB minimum. 21 dB or higher recommended. It is very rare for ISP to use 16-QAM on newer DOCSIS 3.0 networks.
  • QPSK: 12 dB minimum. 15 dB or higher recommended. It is very rare for ISP to use QPSK as performance on this modulation is very low.

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Higher QAM = higher bit rate

  • How does your SNR compare to everyone else?
  • More QAM value = bits /hz
  • Higher QAM values require good SNR

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Cable modem rebooting or reinitializing

  • cable modem transmit power too low
  • too much attenuation between the modem and CMTS
  • Too much power into the modem
  • Overheating modem
  • Cables and connectors

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My DOCSIS 3.1 cable modem

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Cable infrastructure between the CMTS and CM

  • Coax or fiber optic cables
  • Amplifiers/repeaters
  • Splitters
  • Taps
  • Drop cables
  • NID Network interface device
  • Home cable inside your home/business
  • Cable modem/Gateway

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Cable

  • Telephone poles are the most common method connecting CMTS to your cable modem
  • Neighborhoods may have in-ground infrastructure

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Center-conductor carries the signal�coax cable creates a transmission line for RF��

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Cable modem Troubleshooting

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Cable Data problems

Slow speeds

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Connectors and damaged cables

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Quality connectors and cables

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Damaged cable changes the impedance

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  • Hard bends

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  • Stretching cable through conduct

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  • Kinking the cable

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  • Typically, higher frequency signals are more susceptible to attenuation as a result of cable damage.

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Key signal attributes for good high-speed data

  • RF power levels need to be at certain levels from CMTS to modem
  • SNR (Signal to Noise Ratio) is critical to speed of your modem

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RF power levels and SNR

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DOCSIS 3.1 Test Equipment

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DOCSIS 3.1 tester

  • Built-in DOCSIS 3.1 cablemodem
  • Datalogger
  • Test & Go
  • Scan
  • Hum
  • Web Browser
  • Tilt
  • IPTV, VOIP test
  • Snapshot
  • Input voltage Measurement
  • RF Power Measurement
  • Upstream Test Generator
  • Upstream Spectrum Analyzer
  • DOCSIS Bonding Group
  • Ranging Process
  • Registering Process
  • Channel Plan: Standard up to 10, customized up to 30
  • Optical Power Meter
  • Optical to RF Converter

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HD video content on cable

  • Each HD video stream uses 2MHz bandwidth and consumes about 8Mbs of network capacity
  • Viewing one HD video while recording another HD video on DVR set-top can consume about 26 Mbs of bandwidth
  • Four TV sets running content can demand 12Mhz bandwidth

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