How a Cable Modem Works

Web Diagnostic Interface Reference


SNMP Management


USRobotics Regulatory Information



A part of a communications network that is responsible for handling the majority of the traffic using the highest-speed, and often longest, paths in the network.

The amount of data that can be transmitted over a given time period. Bandwidth is usually described in terms of analog signals in units of Hertz (Hz), which describes the maximum number of cycles per second, or in terms of digital signals in units of bits per second.

Baseline Privacy Interface (BPI)
This is used to protect users and their data from harm. DOCSIS has defined BPI as the required standard. BPI uses a public/private key exchange system to encrypt data that is transmitted between the cable modem and the CMTS.

This is the communication channel. The bandwidth corresponds to the difference between the lowest and highest frequency signal which can be carried by the channel.

Cable Modem Termination System (CMTS) All of the cable modems attached to a cable TV company coaxial cable line communicate with a CMTS at the local cable TV company office. Cable modems can receive and send signals only to the CMTS, but they cannot send signals directly to other cable modems on the line.

Community String
Is required to access any device via SNMP. A community string is equivalent to a password.

Customer Premises Equipment (CPE)
Includes any piece of equipment in a communication system that resides within the home or office. Examples include modems, television set-top boxes, telephones, and televisions.

Data link encryption
Data encryption standard. A cryptographic encryption algorithm that is part of many standards.

Dedicated connection
A communication link that operates constantly.

Digital signal
A signal that takes on only two values, off or on, typically represented by "0" or "1." Digital signals require less power but (typically) more bandwidth than analog, and copies of digital signals can be made exactly like the original.

Domain Name System (DNS)
A protocol used for assigning text addresses (such as for specific computers and computer accounts on the Internet.

Data Over Service Interface Specification (DOCSIS)
Data Over Service Interface Specification.

Data being sent from the Internet to the computer.

Dynamic Host Configuration Protocol (DHCP)
This is a protocol for automatic TCP/IP configurations providing static and dynamic address allocation and management.

The number of oscillations in an alternating current that occur within one second, measured in Hertz (Hz).

1,000,000,000 bytes, or 1,000 megabytes.

Graphical User Interface (GUI)
An interface that has pictures as well as words on the screen to assist the user.

HFC Plant
Hybrid Fiber Coax Plant. A type of network that includes coaxial cables to distribute signals to a group of individual locations (typically 500 or more), and a fiber optic backbone to connect these groups.

Documents or other information with embedded links that enable a reader to access tangential information at specific points in the text.

Hypertext Markup Language (HTML)
The computer language used to create hypertext documents, allowing connections from one document or Internet page to numerous others. HTML is the primary language used to create pages on the World Wide Web.

Hypertext Transfer Protocol (HTTP)
The first part of an address (URL) of a site on the Internet, signifying a document written in Hypertext Markup Language (HTML).

Initial Maintenance Opportunity
Occurs on the first transmission a cable modem makes. It references the MAP to locate the transmit opportunity.

International Organization of Standardization (ISO)
Develops, coordinates, and circulates international standards that facilitate world trade.

International Telecommunication Union (ITU)
A United Nations organization that coordinates use of the electromagnetic spectrum and creation of technical standards for telecommunication and radio communication equipment.

International Telecommunication Union/Telecommunication Standardization Sector (ITU-T)
The branch of the ITU that is responsible for telecommunication standardization.

Internet Engineering Task Force (IETF)
The standards organization that standardizes most Internet communication protocols, including Internet protocol (IP) and hypertext transfer protocol (HTTP).

Internet Protocol (IP)
The standard signaling method used for all communication over the Internet.

Internet Service Provider (ISP)
An organization offering and providing Internet access to the public using computer servers connected directly to the Internet.

A network serving a single organization or site that is modeled after the Internet, allowing users access to almost any information available on the network. Unlike the Internet, intranets are typically limited to one organization or one site, with little or no access to outside users.

One thousand bits.

One thousand bytes.

Light Emitting Diodes (LEDs)
A type of semiconductor diode that emits visible or infrared light when current passes through it. Visible LEDs are used as indicator lights; for example, the light that shows a computer or printer is turned on. Infrared LEDs are used in remote-control devices.

Local Area Network (LAN)
A network connecting a number of computers to each other or to a central server so that the computers can share programs and files.

Media Access Control layer.

Bandwidth allocation map. Contains information that indicates when a cable modem can transmit and for how long.

Megabits per second.

One million bits.

1,000,000 bytes, or 1,000 kilobytes.

Management Information Base (MIB)
A database containing ongoing information and statistics on each device in a network, used to keep track of each device's performance and make sure all are functioning properly. MIBs are especially used with SNMP.

Modem (MOdulator-DEModulator)
A device that converts digital data into analog signals and vice-versa for transmission over a telephone line.

Moving Pictures Experts Group (MPEG)
A committee formed by the ISO to set standards for digital compression of full-motion video. Also stands for the digital compression standard created by this committee.

An international standard for the digital compression of VHS-quality, full-motion video.

An international standard for the digital compression of broadcast-quality, full-motion video.

An international standard for the digital compression of broadcast-quality audio.

Multiple System Operator (MSO)
Also known as the cable company, this is the organization that provides Internet service.

Packet Rate
The number of packets divided by the seconds.

Periodic Maintenance
The CMTS will send a ranging response to the cable modem with instructions to adjust the timing, frequency, and power level of the cable modem transmission. This exchange repeats until the CMTS and Cable modem have properly adjusted the timing to within 1 microsecond, the frequency to within 10 Hz, and the power to within .25 dB.

Physical Layer.

Program Identifier (PID)
The identifiers chosen by programmers as entity names contain valuable information.

Quadrature Amplitude Modulation (QAM)
A technique of modulation in which there are 16 possible four-bit patterns, determined by the combination of phase and amplitude.

QAM Lock
A cable modem will scan the entire frequency spectrum looking for a 6 MHz channel where the MPEG-2 frames are stamped with a PID of 0x1ffe. Once the cable modem finds a 6 MHz channel that carries this type of MPEG-2 frame, it achieves what is called QAM lock.

Radio Frequency (RF)
Electromagnetic carrier waves upon which audio, video, or data signals can be superimposed for transmission.

The registration process is simply described as a verification of all previous steps which the cable modem has progressed. A registration request is sent to the CMTS with a list of operating parameters that the cable modem has established. The CMTS parses this list, validates it, and then sends a registration response.

The central switching device in a packet-switched computer network that directs and controls the flow of data through the network.

The DHCP server will, in the most basic of systems, register the cable modem by looking at its unique Ethernet MAC address (different from the MAC layer in the DOCSIS protocol model) and assign to it an IP address from a pool of IP addresses. This pool of IP addresses and related configuration is called a scope.

Small Computer System Interface (SCSI)
Pronounced "skuzzy" - A type of interface between computers and peripherals that allows faster communication than most other interface standards, often used to connect PCs to external disk drives.

Simple Network Management Protocol (SNMP)
The Internet standard protocol for network management software. Using SNMP, programs called "agents" monitor various devices on the network (hubs, routers, bridges, etc.). Another program collects the data from the agents. The database created by the monitoring operations is called a management information base (MIB). This data is used to check if all devices on the network are operating properly.

Time synchronization message used to provide a common time reference for all cable modems connected to the CMTS.

Transmission Control Protocol/Internet protocol - A method of packet-switched data transmission used on the Internet. The protocol specifies the manner in which a signal is divided into parts, as well as the manner in which "address" information is added to each packet to ensure that it reaches its destination and can be reassembled into the original message.

Trivial File Transfer Protocol (TFTP)
A simpler version of FTP that uses UDP (User Datagram Protocol) rather than TCP (Transmission Control Protocol) for data transport. It is easier to program than FTP but lacks directory and authentication services. It is often used to allow diskless workstations to boot over the network.

Upstream channel descriptor - this is a description of the upstream parameters including modulation, symbol rate, channel width, and frequency.

Uniform Resource Locator (URL)
A text-based address used to identify specific resources on the Internet, such as Web pages. URLs are arranged in a hierarchical form that specifies the name of the server on which a resource is located (such as and the name of the file on that server (

Universal Serial Bus (USB)
A computer interface with a maximum bandwidth of 1.5 Megabytes per second used for connecting computer peripherals such as printers, keyboards, and scanners.

Video on Demand (VOD)
A pay-per-view television service in which a viewer can order a program from a menu and have it delivered instantly to the television set, typically with the ability to pause, rewind, etc.

Interactive, audiovisual communication among three or more people at two or more sites.