DOCSIS History and Architecture

Initially, home Internet access was via a telephone modem and data was transported over telephone lines. Over the years, competition from direct broadcast satellite (DBS) providers motivated the cable television (TV) industry to diversify and explore business opportunities in areas other than TV content distribution. This competitive threat led to the development of the Data Over Cable Service Interface Specification (DOCSIS®) standard for cable modems as an alternative to dial-up telephone Internet access.
The first cable modem systems proved that cable TV providers could also be Internet providers. In addition, these first cable modem systems demonstrated distinct advantages over dial-up Internet access via telephone modems. Internet access was faster, more efficient, and had higher data rate throughputs. However, these first modem systems used proprietary technologies that limited their availability and distribution. Operators had already experienced these limitations with addressable set-top boxes (STB). 

Learning from this experience, cable system operators recognized that, in order to compete, cable modem technology had to be based on open standards similar to how telephone modems could operate with any conventional telephone connection. As a result, DOCSIS was created and the industry transformed itself from a cable TV provider to a broadband provider.

In order to service DOCSIS modems, the student must recognize the architecture and elements in the DOCSIS system and the significance of certified hardware. Finally, comparing DOCSIS to the Open Systems Interconnection (OSI) model exposes the student to communications concepts that will be introduced as the student’s career progresses.

The Emergence of Data Over Cable Systems

In 1996, the cable industry began efforts to develop cable modem standards that would provide vendor interoperability. CableLabs, the research and development consortium funded by cable operators, and a group of multiple system operators (MSO) formed a coalition known as Multimedia Cable Network System Partners Ltd. (MCNS). 

The coalition began researching specifications for cable modems and related equipment to provide high-speed data (HSD) service over hybrid fiber/coax (HFC) cable networks. MCNS joined forces with other companies, cable operators, equipment manufacturers, and technical groups to develop the Data Over Cable Service Interface Specification (DOCSIS) standard. By 1998, DOCSIS was officially approved as an international standard by the International Telecommunication Union (ITU). Shortly afterwards, enhancements were made to DOCSIS to improve quality of service (QoS), thus resulting in DOCSIS version 1.1, followed by subsequent versions.

DOCSIS can be credited with enabling the cable TV industry to make the transformation from a provider of only one product to a provider of multiple products. In the early years, the cable industry solely provided TV content; now they provide data and telephone service as well as TV content over an HFC network. As a result, sometime around the release of DOCSIS 2.0, cable operators adopted the moniker “broadband” in their service description as a reflection of the expanded availability of products and services.

The success of DOCSIS pushed telephone companies to develop and increase the availability of broadband access over their infrastructure using digital subscriber line (DSL) technology. Other competitors, including some telephone companies, use fiber-to-the-home (FTTH) technologies instead of DSL to provide competitive broadband service. In response to this competition, DOCSIS 3.0 and then DOCSIS 3.1 were introduced. These new versions are capable of providing broadband services at data rates that are comparable to those offered using FTTH technologies.

The Architecture of a DOCSIS System

The DOCSIS architecture employs a client/server model that is comprised of two main elements, a cable modem termination system (CMTS) and a cable modem (Figure 1). In a client/server relationship, the CMTS is the server, typically located in the headend or similar location, and the cable modem is the client that is placed inside the customer premises.  The CMTS communicates with a number of connected clients throughout the HFC network. Cable modems receive continuous downstream data from the CMTS and transmit data back upstream to the CMTS in time slots allocated and controlled by the CMTS via the HFC network.

DOCSIS Elements

The CMTS downstream and upstream radio frequency (RF) interfaces communicate with the cable modems via the HFC network (Figure 2). The CMTS interfaces with the operations support system (OSS) acting as a “proxy” or messenger that transfers messages and files between the modems and the OSS. Comprised of several servers, the OSS provides back office provisioning, modem configuration files, a billing system interface, and access control data to the DOCSIS modems. 

To protect customer privacy, the downstream and upstream data is encrypted at the CMTS using advanced security algorithms from a Security and Access Control server. The CMTS network interface connects to a switch which provides a connection to the Internet via a network of fiber-optic cable and routers that connect to Internet servers all over the world. The fiber-optic network may be owned by the cable operator or owned by a contracted Internet service provider (ISP).

The switch also provides a connection to a local content server, and the public switched telephone network (PSTN) for telephone service.

Figure 2: Main connections to the CMTS

As DOCSIS evolved, CableLabs introduced the Converged Cable Access Platform (CCAP) specification, which combines CMTS operations with those used for the delivery of digital television (DTV) content onto a single platform. Now an application within the CCAP specifications, CMTS references to either the CMTS hardware or CMTS application within CCAP, are interchangeable, unless stated otherwise.


In addition to being an equipment standard, what other recommendations are included in the Data Over Cable Service Interface Specification (DOCSIS)?
In addition to being an equipment standard, DOCSIS includes recommended parameters for hybrid fiber/coax (HFC) network performance. If the network meets or exceeds these recommendations, operators can expect reliable data transmission.

Which layers of the seven-layer Open Systems Interconnection (OSI) reference model are present in the DOCSIS implementation?
Within the DOCSIS implementation of the OSI model, layers one through four are specific to cable networks and are present only between the cable modem and the CMTS.

What are the main components of the DOCSIS architecture?
The main components of the DOCSIS architecture are the cable modem and the cable modem termination system (CMTS).

What criteria must a cable modem meet to be certified as a DOCSIS modem?
To be certified as a DOCSIS modem, verification of compliance with DOCSIS standards and the ability to interoperate with other certified DOCSIS modems must be met.

When choosing the installation location for a DOCSIS modem, what should you consider to ensure continuous and reliable operation?
To ensure continuous and reliable operation, the DOCSIS modem should have adequate air-flow across it and be plugged into a non-switched AC power outlet.

Why should an already installed cable service drop be inspected as part of any installation and service call?
Installation standards change, so what might have been a good installation at the time could be out of compliance today. Therefore, the entire drop system must be checked to ensure safety, technical, and company compliance during every installation and service call.

What is typically connected to the DOCSIS modem so that multiple computers can connect to the Internet through a single DOCSIS modem?
A router is connected to the DOCSIS modem so that multiple computers can connect to the Internet through a single DOCSIS modem.

What is the difference between a cable modem and a wireless gateway
Instead of connecting to only one computer device as the cable modem does, the gateway connects to several devices via wired-Ethernet connections or Wi-Fi.

What simplifies the installation if the customer wants the DOCSIS modem installed in a closet or utility room?
Installing the DOCSIS modem in a closet or utility room is relatively simple if there is an electrical outlet to power the router and DOCSIS modem, if a coaxial cable can be routed to the DOCSIS modem, and if the twisted-pair network cable is tagged and has its connectors installed using standard wiring configurations.