Data Over Cable Service Interface Specification (DOCSIS)

  • 125_systemy_standardu_docsis.jpgDeveloped by CableLabs and many other contributing companies such as Broadcom, Cisco, Conextant, ARRIS, BigBand Networks, Correlant, Harmonic, Intel, Motorola, Netgear, terayon, and Texas instruments. DOCSIS defines the communications and operation support interface requirements for a data over cable system. It allows high-speed transfer of data through a Cable TV (CATV) system. It is employed by many cable television operators to provide Internet access over their existing hybrid fiber coaxial (HFC) infrastructure.

  • DOCSIS was first deployed as version 1.0 in March 1997, then later on upgraded to 1.1 which added quality of service (QoS) in April 1999. As cable internet became more popular in demand and offering faster speeds than DSL DOCSIS was revised to enhance upstream transmission speeds. DOCSIS 2.0 was later released in December 2001 this time offering a significantly increase transmission speeds in both downstream and upstream also introducing support for Internet Protocol Version 6 (IPv6). In 2006 DOCSIS 3.0 first came to light when CableLabs outlined the method of combining the downstream and upstream channels into one offering the highest level of speeds possible yet.


Physical layer

  • Channel Width: With DOCSIS 1.0/1.1 the established upstream channel between 200 kHz and 3.2MHz, however it is backward compatible to the earlier, narrower channel widths.
  • Modulation: DOCSIS 1.0/1.1/2.0 individualizes that 64-level and 256-level QAM or Quadrature amplitude modulation can be used for modulation of downstream data and QPSK (Quadrature phase-shift keying) or 16-level QAM be used for upstream modulation. DOCSIS 2.0 specifies 32-QAM, 64-QAM and 128-QAM is ready for use for upstream usage.

MAC layer

  • DOCSIS applies a mixture of deterministic access methods, accuately TDMA for DOCSIS 1.0/1.1 and both TDMA and S-CDMA for DOCSIS 2.0, with a confined use of contention for bandwidth requests. In contrast to the pure contention-based MAC CSMA/CD employed in earlier Ethernet systems, DOCSIS systems expereinece few collisions. For DOCSIS 1.1 and higher the MAC layer also accommodate extensive Quality of Service features that help to accurately support applications, for example Voice over IP, that have specific traffic requirements, such as low latency.


  • All of these features connected enable a total upstream throughput of 30.72 Mbit/s per channel (although in DOCSIS 1.0/1.1 the upsteam speed is only limited to 10.24 Mbit/s). However, all the versions of the DOCSIS standard support a downstream throughput of up to 42.88 Mbits/s per channel with 256-QAM.
  • In DOCSIS 3.0 the features management over IPv6 and channel bonding, which enables multiple downstream and upstream channels to be used together at the same time by a one subscriber.
  • In DOCSIS 2.0 + IPv6 standard also assist IPv6, which may on the cable modem side only require a firmware upgrade.


  • sb5101.jpgDOCSIS- Date over Cable Service Interface Specification- A cable modem standard from the CableLabs research consortium, which provides equipment certification for interoperability. It supports IP traffic over digital cable TV channels, and most cable modems are DOCSIS compliant.
  • There are three types of modem's that either are available or will be available for 3.0.
  • Motorola SB6120(only available from some ISP's,
  • Cisco DPC3000(Available in the Spring of 2009), and
  • Linksy's WCM300(availability unsure)
  • DOCSIS uses a Typical Cable Modem Termination System- it is a device which hosts downstream and upstream ports. Use a bi-directional communication, at least two physical ports are needed downstream and upstream, because of the noise in the return path. Until DOCSIS 2.0 upstream ports couldn't' transfer data as fast as downstream ports, but the main reason why upstream ports outnumber downstream ports is line noise.
  • Before a cable company can even run DOCSIS, it must upgrade its HFC network to support a return path for upstream traffice, without it, the old DOCSIS 1.0 standard still allows use of data over able system. by implementing the return path using the plain old telephone services. Chances are high that DOCSIS can run it.
  • If the customer's PC and associated peripherals are termed Customer Premise Equipment. It's conneected to the cable modem, which in turn connected through the HFC network to the CMT's. The CMT will then route traffice between the cable network and the internet. Cable operators have full control of the cable modem's configuration.


Here are all the versions of DOCSIS and their downstream and upstream speeds.

Euro Downstream
42.88 Mbit/s
55.62 Mbits/s
10.24 Mbit/s
42.88 Mbit/s
55.62 Mbit/s
30.72 Mbit/s
3.0 (4 channel bonding)
+171.52 (+152) Mbit/s
+222.48 (+200) Mbit/s
+122.88 (+108) Mbit/s
3.0 (8 channel bonding)
+343.04 (+304) Mbit/s
+444.96 (+400) Mbit/s
+122.88 (+108) Mbit/s


DOCSIS has several layers of protection from unauthorized access to the network, however, these defenses to not extend past the network. Once data has entered the Internet backbone, the following security measures no longer apply. There have been several different versions of security over the various versions of DOCSIS, including Baseline Privacy Interface, Baseline Privacy Interface Plus, and now what is simply known as SEC (for "security"). SEC is the most recent upgrade which is AES-compliant for 128-bit encryption.

external image moz-screenshot.png
image property of Cable Labs

The main method of security for DOCSIS is encryption of information over the network between the CM (cable modem) and the CMTS (cable modem termination system). On this level of security, DOCSIS uses an encapsulation protocol for protecting the packet data. It uses the Cipher Block Chaining mode of protection which defines the frame format for carrying encrypted data, a set of cryptographic algorithms which are designed to protect the data, and rules for the application of included algorithms. CBC can use multiple encryption algorithms for an added level of data security, because one can never be too safe.

The CMTS continually enforces encryption of the data flow over the network, and additionally uses authenticated client/server key management protocol. In this protocol, the CMTS (which acts as the server) controls the distribution of secure keying material to specific (and authorized) client CMs. The key management protocol uses X.509 digital certificates to occasionally refresh and reauthorize the key. These certificates include a serial number, a cryptographic public key, the ethernet MAC address and the manufacturers' identification. The network operator is in charge of determining the appropriate length of time between refreshes.

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