|Saturday, 14 October 2006|
Modems are used to transfer information between a computer, and a communication medium, such as a twisted pair telephone line for communication with a second device at a telephone company central office (TCCO). A modem may be integrated internally into a computer or may be external in a separate enclosure and connected to a computer by a cable. To connect the modem to the telephone line, a user typically plugs one end of a cable into the modem and the other end into a telephone wall jack. Telephone wall jacks are commonly provided in homes, businesses, hotels and other buildings to provide access to the public telephone network. Personal computer users currently have two distinct options to access the internet, dial-up modems and high-speed data connections, such as digital subscriber loop (DSL) and cable-modem services. In the past, nearly all modems used in homes and small offices operated by dial-up over telephone lines, and were therefore constrained by telephone circuitry. Digital subscriber line signal architectures, generally denoted as DSL, provide simultaneous voice and high-speed data services over a signal copper wire pair. DSL allows data transmission at speeds much faster than the best available analog modems. There exist several variations of DSL systems that use copper wire cabling to move data between the site and the serving central office. Data, voice and video are separated at the serving central office. Voice is delivered to the public switched telephone network while data is delivered to the host destination over high speed service access links. Digital subscriber line is a technology which allows for simultaneous voice arid data traffic to coexist over a communication channel comprising a standard telephone transmission line. As an example, ADSL or Asymmetric Digital Subscriber Line services generally use existing unshield twisted pair (UTP) copper wires from a telephone company's central office to the subscriber's premise, utilize electronic equipment in the form of ADSL modems at both the central office and the subscriber's premise, send high-speed digital signals up and down those copper wires, and send more information one way than the other. ADSL systems can provide very high data speeds, such as on the order of several megabits per second, over a standard twisted pair. Unlike the traditional data modems used for analog communication with a TCCO via a twisted pair, ADSL requires modems both at the subscriber end and at the TCCO end.
A cable modem is a device that hooks up to the Internet through a cable system provided by a cable supplier. With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. The growth of the cable television (CATV) industry has provided an infrastructure of high frequency coaxial (HFC) cables connected to the homes of subscribers. This availability and the high bandwidth offered by HFC cables make the cable infrastructure a desirable medium for data networks. In the cable television network, broadcasting signals are transmitted from a CATV station to each subscriber via a trunk line and a branch line. The trunk line may be a multistage tree type or a multistage star type. Since the trunk line and the branch lines are used to transmit the wide-band broadcasting signals, coaxial cables and optical fiber cables are chiefly used for the trunk line and the branch lines. With cable television, a TV analog signal received at the head end of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 60 to 700 MHz. Accordingly, the CATV system has a superior wide-band transmission ability. Since the CATV system has a relatively large cable transmission capacity, it is proposed that the CATV system should provide service for connecting to the Internet. In this case, a cable modem as a cable-line end is connected to the CATV system, and a plurality of personal computers and the like are connected to this cable modem via a local area network (LAN) using an Ethernet etc. Cable modems offer greatly improved bandwidth capable of delivering services hundreds, or even thousands, of times faster than conventional modems. Cable modems can achieve data-transfer rates of up to 40 Mbits/s by connecting directly to coaxial lines as opposed to dial-in modems, that use twisted-pair copper telephone lines.
A cable modem network consists of a number of cable modems in subscriber homes, a cable modem headend system, and a coaxial or hybrid-fiber/coax communication link between the headend and the subscriber modems. In a typical cable modem configuration, the cable modem is external to and separately addressable from the end user's computer system. The end user's computer is then connected to the cable modem using, for example, a network card. Communication over a cable network using a cable modem is typically performed using a standard such as the well-known data over cable service interface specification (DOCSIS) 1.0 standard, which establishes uniform data transmission standards. Cable modems are being deployed today that allow high-speed Internet access in the home over a cable network, often referred to as a hybrid fiber copper (HFC) cable network. Cable modem includes a cable modem integrated circuit (IC) which may comprise a single IC or a plurality of IC's. The cable modem IC performs the modulation and demodulation for the cable modem and the operations necessary to interface with the PC. Cable modem also includes support circuitry such as memory elements and other components, not shown. Cable modem includes a tuner coupled to cable modem IC through a power amplifier that is referred to as a programmable gain amplifier (PGA). Tuner interfaces the cable modem with the CMTS of the cable network and handles the upstream and downstream signal paths or data streams. Cable modems fall into two broad categories, parallel tone modems and serial tone modems. Parallel tone modems operate by dividing the band into many sub-bands and then sending data at a low rate through each sub-band. Serial tone modems send one symbol at a time at a rate such that the signal occupies the entire bandwidth. There may be several modems operating at different frequencies at the same time, but each modem operates serially and independently of the others. Serial tone modems are more robust than the parallel tone modems, but once the ingress becomes large enough to interfer with the signal, all data transmission is degraded or lost. The most common type of modulation used in cable modems is quadrature amplitude modulation (QAM).
Cable modem systems include bi-directional communications between a cable modem termination system (CMTS) and a plurality of cable modems. During normal operation, the CMTS sends sequences of MPEG frames to each cable modem. The MPEG frames include media access control (MAC) packets. One important type of MAC packet is the SYNC packet. SYNC packets permit cable modems to synchronize their local clock (the CM clock) with the CMTS clock. The cable modem termination system receives data packets from the data network and transmits them downstream via the cable television network to a cable modem attached to the customer premise equipment. The customer premise equipment sends responses data packets to the cable modem, which sends response data packets upstream via the public switched telephone network to the telephony remote access concentrator, which sends the response data packets back to the appropriate host on the data network. Cable modems may be bi-directional or telephony return cable modems. Bi-directional modems connect to HFC networks with the capability of sending and receiving data over the coaxial cable. Because CATV has been implemented as a, uni-directional communication system, bi-directional cable modems are not widely available. Telephony return cable modems use the uni-directional communication as a downstream (data sent to the subscriber) connection, but add a telephone (PSTN) connection as an upstream connection. Because the cable modem does not know beforehand the loss between it and a cable modem termination system (CMTS), the cable modem initially transmits at a minimum specified level and then increases the level throughout the ranging process. The ranging process is performed so that the cable modem does not transmit at levels that can overdrive station equipment and cause impairments to other channels. After transmitting the ranging message, the cable modem awaits an acknowledgement from the CMTS.
Unlike traditional dial modems, cable modems transmit and receive digital signals at radio frequencies. Digital data on upstream and downstream channels of the cable network is carried over radio frequency (RF) carrier signals. Cable modems convert digital data to a modulated RF signal for upstream transmission and convert a downstream RF signal to digital form. The conversion is done at a subscriber's home. The CMTS converts downstream digital data to a modulated RF signal, which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. In general, a cable modem device is equipped on its back with a radio frequency (RF) coaxial connector for connection to a CATV trunk line, an Ethernet connector or jack for connection to a computer, a universal serial bus (USB) connector or jack, a reset switch to return the modem to its initial state, and a power supply connector to receive electric power. Digital modems are increasingly being used by consumers as cable modem network service providers obtain additional subscribers. Cable modem devices designed for the mass-market are designed to be affordable to as many consumers as possible. The use of cable modems has increased significantly over the last few years due to the demand for larger bandwidth and faster connection to the Internet. Cable modems allow for much faster information exchange than is available over plain old telephone service. An individual cable modem end user may experience information exchange speeds of from 0.5 Mbps to 1.0 Mbps or more depending on the cable network architecture and traffic load. Cable modem is thus suitable for high speed file transfer, video teleconferencing and pay-per-view television. Further, cable modems may simultaneously provide high speed Internet access, digital television and digital telephony.