A cable modem is a type of modem that provides access to a data signal sent over the cable television infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of unused bandwidth on a cable television network. They are commonly found in Australia, New Zealand, Canada, Europe, the United Kingdom, Costa Rica, and the United States. In USA alone there were 22.5 million cable modem users during the first quarter of 2005, up from 17.4 million in the first quarter of 2004
Modem (from modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.
The most familiar example is a voiceband modem that turns the digital 1s and 0s of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Serial, or Network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by Baud, the number of times the modem changes its signal state per second.
Baud is NOT the modem's speed. The baud rate varies, depending on the modulation technique used. Original Bell 103 modems used a modulation technique that saw a change in state 300 times per second. They transmitted 1 bit for every baud, and so a 300-bps modem was also a 300-baud modem. However, casual computerists confused the two. A 300-bps modem is the only modem whose bit rate matches the baud rate. A 2400-bps modem changes state 600 times per second, but due to the fact that it transmits 4 bits for each baud, 2400 bits are transmitted by 600 baud, or changes in states.
Faster modems are used by Internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second. One kilobit per second (kbit/s or kb/s or kbps) as used in this article means 1000 bits per second and not 1024 bits per second. For example, a 56k modem can transfer data at up to 56,000 bits per second over the phone line.
The most familiar example is a voiceband modem that turns the digital 1s and 0s of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those 1s and 0s back into a form used by a USB, Serial, or Network connection. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps". They can also be classified by Baud, the number of times the modem changes its signal state per second.
Baud is NOT the modem's speed. The baud rate varies, depending on the modulation technique used. Original Bell 103 modems used a modulation technique that saw a change in state 300 times per second. They transmitted 1 bit for every baud, and so a 300-bps modem was also a 300-baud modem. However, casual computerists confused the two. A 300-bps modem is the only modem whose bit rate matches the baud rate. A 2400-bps modem changes state 600 times per second, but due to the fact that it transmits 4 bits for each baud, 2400 bits are transmitted by 600 baud, or changes in states.
Faster modems are used by Internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optical fibers. Most intercontinental data links now use optical modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second. One kilobit per second (kbit/s or kb/s or kbps) as used in this article means 1000 bits per second and not 1024 bits per second. For example, a 56k modem can transfer data at up to 56,000 bits per second over the phone line.
Cable modems in the OSI model or TCP/IP model
In network topology, a cable modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The cable modem bridges Ethernet frames between a customer LAN and the coax cable network.
With respect to the OSI model, a cable modem is a data link layer forwarder, rather than simply a modem.
A cable modem does support functionalities at other layers. In physical layer , the cable modem supports the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. It is to this cable-specific PHY that the name cable modem refers. In the network layer , the cable modem is a IP host in that it has its own IP address used by the network operator to manage and troubleshoot the device. In the transport layer (or layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the application layer , the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.
Some cable modem devices may incorporate a router along with the cable modem functionality, to provide the LAN with its own IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit. So, the cable modem function will have its own IP address and MAC address as will the router.
With respect to the OSI model, a cable modem is a data link layer forwarder, rather than simply a modem.
A cable modem does support functionalities at other layers. In physical layer , the cable modem supports the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. It is to this cable-specific PHY that the name cable modem refers. In the network layer , the cable modem is a IP host in that it has its own IP address used by the network operator to manage and troubleshoot the device. In the transport layer (or layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the application layer , the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.
Some cable modem devices may incorporate a router along with the cable modem functionality, to provide the LAN with its own IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit. So, the cable modem function will have its own IP address and MAC address as will the router.
Cable modems and VoIP
With the advent of Voice over IP telephony, cable modems can also be used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). Because most telephone companies do not offer naked DSL (DSL service without a POTS line), VoIP use is higher amongst cable modem users.
A cable modem subscriber can make use of VoIP telephony by subscribing to a third party service (e.g. Vonage or Skype). As an alternative, many cable operators offer a VoIP service based on PacketCable. PacketCable allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment, known as an Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA is basically a cable modem and a VoIP adapter (known as a Multimedia Terminal Adapter) bundled into a single device. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed Quality of Service across their entire path so that call quality can be assured.
A cable modem subscriber can make use of VoIP telephony by subscribing to a third party service (e.g. Vonage or Skype). As an alternative, many cable operators offer a VoIP service based on PacketCable. PacketCable allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment, known as an Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA is basically a cable modem and a VoIP adapter (known as a Multimedia Terminal Adapter) bundled into a single device. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed Quality of Service across their entire path so that call quality can be assured.
Hybrid Networks
Hybrid Networks developed, demonstrated and patented the first high speed, asymmetrical cable modem systems in 1990. A key Hybrid Networks insight was that highly asymmetrical communications would be sufficient to satisfy consumers connected remotely to an otherwise completely symmetric high speed data communications network. This was important because it was very expensive to provide high speed in the upstream direction, while the CATV systems already had substantial broadband capacity in the downstream direction. Also key was that it saw that the upstream and downstream communications could be on the same or different communications media using different protocols working in each direction to establish a closed loop communications system. The speeds and protocols used in each direction would be very different. The earliest systems used the public switched telephone network (PSTN) for the return path since very few cable systems were bi-directional. Later systems used cable for the upstream as well as the downstream path.
There was extreme skepticism to this approach initially. In fact, many technical people doubted that it could work at all. Hybrid's system architecture is the way most cable modem systems operate today.
LANcity
There was extreme skepticism to this approach initially. In fact, many technical people doubted that it could work at all. Hybrid's system architecture is the way most cable modem systems operate today.
LANcity
LANcity was an early pioneer in cable modems, developing a proprietary system that saw fairly wide deployment in the US. LANcity was sold to Bay Networks which was then acquired by Nortel, which eventually spun the cable modem business off as ARRIS. ARRIS continues to make cable modems and CMTS equipment compliant with the DOCSIS standard.
CDLP
CDLP
CDLP was a proprietary system that was made by Motorola. CDLP CPE was capable of both PSTN (telephone network) and RF (cable network) return paths. The PSTN return path cable modem service was considered 'one way cable' and had many of the same drawbacks as satellite Internet service, and as a result it quickly gave way to two way cable. Cable modems that used the RF cable network for the return path were considered 'two way cable', and were better able to compete with DSL which was bidirectional. The standard is more or less defunct now with new providers using, and existing providers having changed over to, the DOCSIS standard. The Motorola CDLP Proprietary CyberSURFR is an example of a modem that was built to the CDLP standard, capable of a peak 10 Mbit/s downstream and 1.532 Mbit/s upstream. (CDLP supported a maximum downstream bandwidth of 30 Mbit/s which could be reached by using several modems.)
The Australian ISP BigPond employed this system when it started cable modem trials in 1996. For a number of years cable Internet access was only available to Sydney, Melbourne and Brisbane via CDLP. This network ran parallel to the newer DOCSIS system for a number of years. In 2004 the CDLP network was switched off and now is exclusively DOCSIS.
The Australian ISP BigPond employed this system when it started cable modem trials in 1996. For a number of years cable Internet access was only available to Sydney, Melbourne and Brisbane via CDLP. This network ran parallel to the newer DOCSIS system for a number of years. In 2004 the CDLP network was switched off and now is exclusively DOCSIS.
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