The term ‘Cable Modem’ is quite new and refers to a modem that operates over the ordinary cable TV network cables. Basically you just connect the Cable Modem to the TV outlet for your cable TV, and the cable TV operator connects a Cable Modem Termination System (CMTS) in his end (the Head-End). Actually the term "Cable Modem" is a bit misleading, as a Cable Modem works more like a Local Area Network (LAN) interface than as a modem. Cable modems allows consumers access to the Internet at higher speeds and at a fraction of the time it takes traditional telephone modems.
This is true for two reasons:
1) Broadband networks make the connection up to a hundred times faster 2) The service is "always on," meaning customers get the information they want, when they want it. Unlike telephone modems, cable modems allow consumers to keep their telephone lines open for voice conversations.
1.1 IMPORTANT TERMS
A short list of some of the technical terms and acronyms that you may come across in trying to understand the cable modem world. CATV: Community Antenna Television or Cable TV system. Can be all coaxial or HFC (Hybrid Fiber Coax) based. Cable modem (CM): Client device for providing data over a cable TV network. Read all about it here. Channel: A specific frequency and bandwidth combination. Used in this context about TV channels for television services and downstream data for cable modems. CMTS: Cable Modem Termination System. Central device for connecting the cable TV network to a data network like the internet. Normally placed in the headend of the cable TV system. CPE: Customer Premises Equipment. Used to describe the PC and/or other equipment, that the customer may want to connect to the cable modem. DHCP: Dynamic Host Configuration Protocol. This protocol provides a mechanism for allocating IP addresses dynamically so that addresses can be reused. Often used for managing the IP addresses of all the cable modems in a cable plant and the PC's connected to the cable modems. DOCSIS: Data Over Cable Service Interface Specification. The dominating cable modem standard. Defines technical specifications for both cable modem and CMTS. Downstream: The data flowing from the CMTS to the cable modem. Downstream frequency: The frequency used for transmitting data from the CMTS to the cable modem. Normally in the 42/65-850 MHz range depending on the actual cable plant capabilities. Headend: Central distribution point for a CATV system. Video signals are received here from satellites and maybe other sources, frequency converted to the appropriate channels, combined with locally originated signals, and rebroadcast onto the HFC plant. The headend is where the CMTS is normally located. HFC: Hybrid fiber-coaxial (cable network). Older CATV systems were provisioned using only coaxial cable. Modern systems use fiber transport from the headend to an optical node located in the neighborhood to reduce system noise. Coaxial cable runs from the node to the subscriber. The fiber plant is generally a star configuration with all optical node fibers terminating at a headend. The coaxial cable part of the system is generally a trunk-and branch configuration. MAC layer: Media Access Control sub layer in the network stack. Read more about that later in this presentation. MCNS: Multimedia Cable Network System Partners Ltd. The consortium behind the DOCSIS standard for cable modems. Minislot: Basic timeslot unit used for upstream data bursts in the DOCSIS standard. MSO: Multiple Service Operator. A cable TV service provider that also provides other services such as data and/or voice telephony. QAM: Quadrature Amplitude Modulation. A method of modulating digital signals using both amplitude and phase coding. Used for downstream and can be used for upstream. QPSK: Quadrature Phase-Shift Keying. A method of modulating digital signals using four phase states to code two digital bits per phase shift. Ranging: The process of automatically adjusting transmit levels and time offsets of individual modems, in order to make sure the bursts coming from different modems line up in the right timeslots and are received at the same power level at the CMTS. SID (Service ID): Used in the DOCSIS standard to defines a particular mapping between a cable modem (CM) and the CMTS. The SID is used for the purpose of upstream bandwidth allocation and class-of-service management. Subscriber Unit (SU): An alternate term for cable modem.
Upstream: The data flowing from the CM to the CMTS.
Upstream frequency: The frequency used to transmit data from the CM to the CMTS. Normally in the 5-42 MHz range for US systems and 5-65 MHz for European systems.
1.2 CATV NETWORK
A CATV network is designed and used for cable TV distribution. With an upgrade of the system, it is normally possible to allow signals to flow in both directions. Higher frequencies flow toward the subscriber and the lower frequencies go in the other direction. This is done by upgrades to the amplifiers in the cable distribution network etc. Most CATV networks are Hybrid Fiber-Coax (HFC) networks. The signals run in fiber-optical cables from the Head-End center to locations near the subscriber. At that point the signal is converted to coaxial cables, that run to the subscriber premises. One CMTS will normally drive about 1-2000 simultaneous Cable Modem users on a single TV channel. If more Cable Modems are required, the number of TV channels are increased by adding more channels to the CMTS.
TYPES OF MODEM
A number of different Cable Modem configurations are possible. The following 3 configurations are the main products that we see now. * External Cable Modem
* Internal Cable Modem
* Interactive Set-Top box
2.1 EXTERNAL CABLE MODEM
The external Cable Modem is a small external box that connect to your computer normally through an ordinary Ethernet connection. The downside is that you need to add a (inexpensive) Ethernet card to your computer before you can connect the Cable Modem. A plus is that you can connect more computers to the Ethernet. The ailable Cable Modems w...