Title - An Introduction to the DOCSIS Protocols


In the United States, an organization that supports the common development interests of the cable operators was created and it is known as CableLabs® (www.cablelabs.org) . They have defined several standards for the Cable TV industry that the operators then use to purchase their equipment. One such standard is for cable modems and it is known as DOCSIS (Data Over Cable Service Interface Specification).


The major components in a typical Cable TV system are shown in Figure 1.


Figure 1. A typical Cable TV system


As shown, the Headend or sometimes called the Master Head End (MHE), contains the major equipment for the network. It assembles the program material from the many different video sources. It also has the main link funneling data from the cable modem to the Internet.


The Headend serves all customers in a large geographical area and may typically be 100,000 subscribers. The headend sends the assembled data and video to branch locations around the geographic area to a facility known as a Distribution Hub. A Distribution Hub contains the modulators to put a video channel on a particular TV channel. (e.g. NBC goes on TV channel 12). It also contains the receivers for data that is sent from the homes.


The headend uses lasers to feed optical fibers that spread out across the area. Each fiber will feed approximately 2,000 homes. When the fiber reaches a neighborhood, it is converted from an optical signal to an electrical signal and split into four different paths feeding 500 homes. This combination of optical (fiber) and electrical (coaxial) is called Hybrid Fiber Coax or HFC for short.


The data signal is carried just like any other video signal through the system. Unlike the video signal, the data system also requires a channel that goes back to the head end since Internet users are interactive. The way the system carries both the downstream (head end to subscriber) and upstream (subscriber to head end) signals is by using separate frequencies.

The different versions of DOCSIS

There are currently three generations of DOCSIS. DOCSIS 1.0 is defined primarily as an Internet access service for consumers. It allows the single coaxial cable to be shared by a variety of subscribers in the neighborhood. DOCSIS 1.1 adds the ability to delivery data very accurately allowing the system to support telephone services. In a system, it is possible for there to be more than one DOCSIS channel on the system. DOCSIS 1.1 devices can work in DOCSIS 1.0 systems but they loose the ability to support accurate delivery of data.


DOCSIS 2.0 devices support DOCSIS 1.1 features and add the ability to use higher upstream data rates. The technical aspects of these three generations will be described in greater detail in the next few seminars. As a point of reference, most cable systems deliver data at 27 Mb/s (megabits per second ) on the downstream path. This 27 Mb/s can be done separately for each fiber link serving 2,000 homes if so desired. Should more data be required, more than one channel of DOCSIS can be used.


For upstream data transmissions, the channel data rates are in the range of 320 Kb/s (Kilobits per second) to 27 Mb/s). Each user's modem is set by the operator to some maximum speed thus allowing the operator to charge more for greater performance.


The suite of DOCSIS specifications

DOCSIS covers several different aspects of the system to provide the data delivery solution and there are different specifications for each piece of the system. The DOCSIS reference model is shown in Figure 2.


Figure 2. The DOCSIS reference model


DOCSIS covers many aspects of the Cable network. Included in the architecture are:

  • The link between the PC and the Cable Modem (CM)

  • The link between the CM and the Cable Modem Termination System (CMTS)

  • Network Management

  • Link Privacy

  • Generic information about the network beyond the CMTS

In order for a manufacturer to claim that they meet the DOCSIS specification, CableLabs® puts the equipment through a compliance test to ensure it meets the requirements. Most US cable operators will only buy cable modem equipment that is DOCSIS certified.


The most important specification is the CMRFI (Cable Modem Radio Frequency Interface) specification that defines the messages that flow over the coax and into the cable modem. The description of those messages is in DOCSIS Protocols seminar. A description of the electrical modulation is contained in the DOCSIS Physical Layer Technology seminar.


The cable modems you buy in the stores are compliant to this specification. The cable modem connect with your computer in one of the several ways specified in the CMCI (Cable Modem to Computer Interface) specification. These modem may be internal to the computer, use the Ethernet Interface, Universal Serial Bus (USB), or the Home Phone Network Architecture (HPNA).


The DOCSIS system also includes security features (DOCSIS). This security ensure that only valid users are able to use the system. Security also includes that the data is encrypted so that a user is assured of privacy of their data.


Management of the system is provided by the Data Over Cable System Operation Support System Interface (DOCS-OSSI). This allows the operator to detect problems in the network, and determine the status, capability and provisioning of the parameters for the Cable Modem.

Cable Modem Terminating System (CMTS)

The Cable Modem Terminating System (or CMTS), which is located in the distribution hub, provides the interface to the HFC network. The CMTS provides the control functions to arbitrate and schedule transmissions from the many cable modems.


There are many upstream channels for each downstream channel. In the DOCSIS protocol the CMTS controls all aspects of managing the bandwidth in both directions on the link. This control allows for assigning different levels of priority and bandwidth to each individual cable modem (CM). Because of the tight control available, upstream and downstream transmission can be accurately scheduled thus enabling the desired Quality of Service (QoS) for each type of connection (i.e. voice, video, and data packets).


The upstream channel is divided into units of 6.25 ms. These units are called mini-slots, which are 8 bytes long for QPSK or 16 bytes long for 16-QAM. Blocks of mini-slots are assembled together to transmit variable length packets. Cable Modems (CM) send requests for the amount of mini-slots that are needed to send the data packets. The CMTS schedules the proper number of slots and tells the CM when to transmit the data.


More Information

The DOCSIS specifications are available at: www.cablemodem.com/specifications.html .


Additional DOCSIS seminars:

The DOCSIS Protocol - A description of the messages between the CMTS and the CM. This includes the mechanism to share the coax, ranging and registration.

The DOCSIS Physical Layer - The downstream/upstream modulation and data rates.

Quality of Service (DOCSIS 1.1) - The changes to DOCSIS 1.0 to implement QoS.


In Summary:

  • DOCSIS was created by CableLabs and they certify the equipment.

  • DOCSIS is composed of many different specifications for each major system interface.

  • There are three generations of DOCSIS protocols and these are backward compatible.

  • The DOCSIS protocol allows complete control of data being transmitted by the CM thus allowing telephone and other QoS data streams to be supported.


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