Upgrading and Repairing Networks

The Data Over Cable Service Interface Specification (DOCSIS)

Newer cable systems, however, based on the DOCSIS (Data Over Cable Service Interface Specification) standards allow for bidirectional, asymmetrical communications. Version 1.1 and 2.0 of DOCSIS include the necessary authentication and encryption features that make cable modem access as secure as DSL. In general, if you can see other computers in your Network Neighborhood, your cable provider is using an older system and you might want to take steps to protect yourself, such as using a firewall appliance.

DOCSIS specifications can be found at the URL www.cablemodem.com/. This is the Web site of an organization responsible for establishing specifications and testing hardware to ensure that it meets the standards for cable televisionCable Television Laboratories, Inc. (CableLabs). This testing ensures that a cable company can buy hardware from different manufacturers and be assured that the system that is installed will work. For example, the router may come from one company, and the cable modems used at customer sites may come from another manufacturer.

DOCSIS version 2.0 is the most recent standard. This version has been adopted by the International Telecommunications UnionTelecommunication Sector (ITU-T) as a worldwide standard.

DOCSIS 2.0 specifications (developed by both CableLabs and Cable Television Laboratories, Inc.) cover many aspects of using a cable system for Internet access:

• Radio Frequency Interface Specification: This document describes the physical media specification, including items from signal processing to a scrambler for the upload channel. This specification also defines parameters, such as the protocol used for downloading as well as other specs to Media Access Control mechanisms, and quality of service. This specifications document is more than 500 pages long and can make quite interesting reading for electrical engineers, like the following specifications.

• Operations System Support Interface Specification: This specification document covers networking subjects such as the Simple Network Management Protocol (see Chapter 49, "Network Testing and Analysis Tools"). This specification also provides the documentation for the Management Interface Base used by SNMP to collect data about the network. You will also find here such things as how subscriber billing should work and fault management. This specification is relevant to most users as the one that defines the interface between the cable system and the user's cable modem.

• Baseline Privacy Plus Interface Specification: Encryption and key management are covered in this document, including the use of X.509 certificates.

• Cable Modem to Customer Premises Equipment Interface Specification: This document covers the "last few feet" to the user's cable modem. Here you'll find information about the Ethernet interface, as well as other information about the end-user equipment and how it interfaces with the cable network.

• Cable Modem Termination System Network Side Interface Specification: Here you will find out the specifications concerning connections from the cable company's service and high-speed connections such as Asynchronous Transmission Mode (ATM, covered in Chapter 15, "Dedicated Connections").

• Acceptance Test Plan: This document defines the tests that cable modem technology must meet to be certified by CableLabs.

This list of specifications can be read in more detail by developers by visiting the DOCSIS Web site (www.cablemodem.com). For most end users it will not matter at all if you get the service you expect. For network and application developers, reading the specs is the only way you'll be informed of the technologies being used by most cable Internet providers today.