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DSL and Cable Modem Networks

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Mar 14, 2003.

Chapter Description

Before committing to a DSL or cable-modem, learn exactly what these technologies consist of and what kind of service they can provide for you.

Cable Access Technologies

Cable television (CATV) is a unidirectional medium carrying broadcast analog video channels to the most customers possible at the lowest possible cost to the CATV service provider. Since the introduction of CATV more than 50 years ago, little has changed beyond increasing the number of channels supported.

Fearing loss of market share when DSL was introduced (in the 1990s) and recognizing the need to offer advanced services to remain economically viable, key multiple system operators (MSOs) formed the Multimedia Cable Network System Partners, Ltd. (MCNS). The goal of the MCNS was to define a standard product and system capable of providing data and future services over the CATV infrastructure. MCNS partners included Comcast Cable Communications, Cox Communications, Tele-Communications Inc., Time Warner Cable, MediaOne, Rogers CableSystems, and Cable Television Laboratories (CableLabs).

The MCNS defined the Data Over Cable Service Interface Specification (DOCSIS) 1.0 standard, which was in turn accepted as the North American standard. These key MSOs defined upgrade and construction programs to provide two-way functionality to the end-user over the CATV infrastructure.

Cable Access Architecture

To deliver data services over a cable network, one television channel (50 to 750 MHz range) is allocated for downstream traffic to homes and another channel (5 to 42 MHz band) is used to carry upstream signals.

Figure 7-5 illustrates the architecture of a cable access network for both CATV and cable modem services.

Figure 7-5 Figure 7-5 Cable Access Architecture

The following list details the cable access network architecture:

  • Residential and business end-users are connected to fiber nodes by coaxial cables. Users attach to this cable through an Ethernet network interface card (NIC) installed in the PC, in turn connected to a cable modem, as illustrated in the Figure 7-6.

    Figure 7-6 Figure 7-6 Cable Modem Access

  • The fiber nodes house the cable modem termination system (CMTS) at the head-end, communicating with the cable modems at the end-user premise. This communication creates a LAN connection between the end-user and the cable modem service provider.

  • Most cable modems are external hardware devices connecting to a PC through a standard 10Base-T Ethernet card or Universal Serial Bus (USB) connection.

  • These fiber nodes are connected by fiber rings (such as SONET) to the distribution hubs, which are in turn connected by fiber rings to a regional cable head-end.

  • The cable head-end then forwards the traffic to the appropriate network—the PSTN for VoIP applications and the public Internet for all other IP traffic.

A single downstream 6 MHz television channel can carry up to 27 Mbps of downstream data throughput from the cable head-end; upstream channels can deliver 500 Kbps to 10 Mbps from home and business end-users. This upstream and downstream bandwidth is shared by other data subscribers connected to the same cable network segment, which is often 500 to 2000 homes on a modern network.

An individual cable modem subscriber can reach speeds from 500 Kbps to 1.5 Mbps or more, depending on the network architecture (for example, oversubscription ratio) and traffic load.


Although other users on the network segment affect cable modem speed, the CATV-signal does not affect this speed because each signal (CATV and cable modem) uses a different frequency on the line. This means that your cable modem connection will not be slower if you are watching TV.

When you are surfing the World Wide Web, your system's performance can be affected by Internet backbone congestion. The local access provider has no direct management control over this backbone congestion; it's the Internet.

DOCSIS Standards, Signaling Protocols, and Applications

Data Over Cable Service Interface Specification (DOCSIS) is a set of standards for transferring data by CATV and cable modems. The DOCSIS interface specifications enable multivendor interoperability for transporting Internet Protocol (IP) traffic. The DOCSIS layers are compared with the OSI Reference Model layers in Figure 7-7.

Figure 7-7Figure 7-7 OSI Layers and DOCSIS Layers

The following list details the correlation between the OSI Reference Model and the DOCSIS standard:

  • TCP/IP support:

    • IP services at the network layer (OSI Layer 3)

    • TCP/UDP services at the transport layer (OSI Layer 4)

  • Data-link layer:

    • Logical Link Control (LLC) sublayer conforming to Ethernet standards

    • Link security sublayer for basic privacy, authorization, and authentication

    • Media Access Control (MAC) sublayer supporting variable-length protocol data units (PDU)

  • Physical (PHY) layer comprised of the following:

    • Downstream convergence layer conforming to MPEG-2

    • Physical Media Dependent (PMD) sublayer for downstream and upstream data transmission; through Time Division Multiplexing (TDM).

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