Cable Modems

Cable Modems share the cable TV line coming in your home. It provides an asymmetrical transfer rate with the upstream being significantly lower then the downstream. For example, upstream 2 Mbps and downstream 30 Mbps.

The downstream speed depends on number of users using the cable system in the area as the line is shared with other subscribers on cable line.

Cable Modem


There are three competing standards:

  • DVB/DAVIC European standard
  • MCNS United States standard
  • IEEE 802.14 Geneva standard is the only one that guarantees transfer rate
The premise equipment uses Dynamic IP addressing and is connected to the head end:

Cable ModemWiring

Security Issues

Originally there were security issues as you could see others through Microsoft's Network Neighborhood. Now that has been addresses and each cable modem connection is encrypted.


You can get very fast transfers through the high download speeds, faster then ADSL. The cost is very competitive and is usually bundled with your cable TV. The best part is that your home is prewired making it easy to install.

DOCSIS Protocol Stack

Data Over Cable Service Interface Specifications (DOCSIS) is the standard in North America for running data over the cable service.

DOCSIS and the OSI Model

Data Link Encryption uses the Baseline Privacy Interface (BPI) because of the shared medium. It uses a public/private key exchange to encrypt data between cable modem and Cable Modem Termination System (CMTS).

Transmission Convergence layer: MPEG-2 is used on the Transmission Convergence layer. All data, voice, audio and Internet is encoded in MPEG-2 frames. This is used for the downstream communications only.

Media Access Control layer (MAC) - uses Hybrid Fiber Coax (HFC) plant which is similar to Ethernet. It is used only for upstream communications. It is used for determining "when to transmit data" and "for how long" on the CMTS (Cable Modem Termination System).

PHY - Physical Layer uses QPSK or QAM-16 for uUpstream modulation and QAM 64 or QAM 256 for downstream modulation.

Cable Frequency Plan

The Hybrid Fiber Coax (HFC) plant operates on NTSC (National Television Standards Committee) with 6 MHz per channel in the frequency bandwidth of 48 to 870 MHz which gives approximately 137 channels.

DOCSIS Sequence

  1. Finds Channel - QAM Lock
    • Scans entire cable frequency for an available 6 MHz channel
    • Looks for an MPEG PID (Program Identifier) of 0x1ffe which indicates it's a DOCSIS data frame
    • QAM locks - modulation synchronizes

  2. Ranging
    • Downloads upstream parameters:
      • Time Reference (SYNC)
      • Upstream parameters (UCD)
      • When to transmit (MAP)
    • Then does the fine adjustments:
      • Power levels adjusted to within 0.25 dbmV
      • Frequency locked to within 10 Hz
      • Time synchronized to within 1 uSec

  3. DHCP and IP Configured
    • CMTS forwards request to a DHCP server
    • Basis on registered Ethernet MAC address

  4. DOCSIS Configuration file is downloaded and contains:
    • TFTP server address for configuration files
    • DOCSIS configuration file name
    • Time of Day (TOD) server
    • Network specific information
    • Network Access: Enabled or not
    • Class of Service:
      • Max upstream and downstream data rates
      • Priority
      • SNMP information

  5. Registration with CMTS starts with a verification of all previous steps the cable modem has progressed through. The CMTS parses this list, validates it, and then sends a registration response.

  6. Data Ready - cable modem is now ready for use.

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Copyright July 2013 Eugene Blanchard