Cable modem

As a cable modem refers to a device that transmits data over cable television networks and for the realization of broadband Internet access via cable connections (cable internet) is employed.

The cable modem located at the customer between the cable TV connection and the router or computer. The connection from the modem to the computer is done either via Ethernet or via the USB port. There is also cable modems, which are combined with a wireless access point and set up a wireless connection to the computer. Such models are often referred to as a wireless cable modem gateway. To drive the cable modem via an Ethernet connection cable modem no specific drivers must be installed on the computer normally. Often, directly at the cable modem, a router or a wireless router for sharing mehrenen computers are connected. The counterpart to the cable modem with the cable company called Cable Modem Termination System ( CMTS ). A CMTS uses depending on the configuration of up to 50,000 subscribers.

Dissemination

The broadband Internet access over cable TV networks is the broadband access technology most commonly used by the realized phone line ADSL method. In the 30 OECD countries, there was the end of 2006 approximately 57 million Internet access via cable modem (corresponding to a market share of nearly 30 % of the total broadband market ), of which slightly more than half in the United States. In Switzerland and Austria each roughly 0.7 and 0.5 million Internet connections by means of cable connection at 2.1 respectively 1.4 million broadband connections.

The German cable television networks are developed for historical reasons, usually just for television distribution; In many places, the signals can not be transmitted bidirectionally, so that the signal transmission from the user to the headend, the TV channels are fed into the cable from which is not possible without modernization. Since these alterations are complex and costly, they are spread over several years; End of September 2007 there were in Germany about a million cable Internet access at 18 million DSL lines.

Properties

The data received from the PC to the USB or Ethernet interface be converted to a format compatible with the cable network transmission format in the cable modem. The frequency ranges of some cable television channels are used exclusively for data transfer. Adapted to the typical "surfing " the Internet traffic load, the frequency ranges can be used so that more transmission capacity in the direction to the participants than in the opposite direction is available. An optimal allocation of frequencies is also important because the cable network has a tree topology. A cable from the headend branches on its way beneath the streets often and serves up to several hundred cable television customers. All cable Internet subscribers connected to a tree have to share the space reserved for the traffic frequency range. For modern cable networks this tree topology and thus the bottleneck of dividing the total bandwidth only for the last mile (which is in contrast to the telecom network but regulated) exists because of the cable headend to the final distribution station, the signal for all connected subnetworks in parallel over fiber networks ( see also HFC) can be transported.

A data encryption using the Data Encryption Standard with a key length of 56 bits ( DOCSIS 1.0) or 128 bits ( DOCSIS 1.1/2.0 ) to ensure confidentiality of the data. There are now DOCSIS 3.0 in the major cable providers. Thus, it is possible to combine the frequency bands or frequency channels, and to achieve more than 100 Mbit / s.

Depending on the design cable modems can achieve a transmission speed of 120 Mbit per second in both directions. The speeds offered insert the network provider for technical and economic points of fixed itself. They range from ISDN speed up to 150 Mbit per second to the subscriber and usually significantly less in the opposite direction.

Usually the modem will be provided or sold by the cable operator to the end user. Since the cable operator an apartment by the tenant is not free to chose, a customer 's provider can not choose freely so. As long as there are no uniform standards for access had prevailed, it was also not possible to use an independently even bought from the cable modem network operators to implement Internet access via the cable network. Some network operators allow now the use of self- purchased the modem when the modem is certified for DOCSIS version used. In any case, your modem must also be registered with the network operator to allow access.

In the household, the cable modem can be connected to any cable television connection point. Only in some old inner domestic cable networks, it may be necessary (usually in the basement) to connect the cable modem directly to the access point. This is the case when in the house distribution amplifier old cable installed yet that does not work bidirectionally. You can exchange these for new models in order to operate the cable modem at all connection points in the house can.

The extraction of the Internet data is carried in the cable modem itself (see below). A separate frequency splitter, as used for DSL connections to the frequency separation of voice and data signals, that is not necessary.

As telephone solution Voice over Cable is used, a variant of IP telephony. To this end, an analog phone to the cable modem is usually connected. In Germany provide cable operators since the end of 2007, cable modems with S0 -Bus for connecting ISDN terminals and PBXs ready.

Alternatively can be used over the network port on the SIP standard for telephony, an IP data stream.

A cable modem usually offers two connection options for the PC:

• The modem is connected via a USB connection to the PC. This requires a special driver for the operating system, which is usually provided exclusively for Microsoft Windows. In addition to the limitation on Windows continue the disadvantage that other computers on the home network (LAN) only have internet access if the computer to which the modem is plugged in, turned on and configured for the forwarding of network data. And thus also the telephone connection - - In addition, the modem must due to the limited maximum cable length of USB, not be too far away from the computer.
• On the other hand completely independent of operating systems is the variant that you connect your PC via an Ethernet cable to the modem; This cable can also be significantly longer than USB. Does the cable modem does not have a built-in router, a router can be connected between PC and modem, whereby the independent Internet access to multiple computers in the LAN or WLAN is enabled.

Transmission method

As a global standard for the modulation method and other interface properties, the developed in the USA DOCSIS (Data Over Cable Service Interface Specification ) has prevailed. In Europe, there were competing proposals (DVB- RCCL, DAVIC ), but could not establish themselves in the market. The peculiarities of the European cable networks such as frequency plan and higher channel bandwidth of eight instead of six megahertz are in an annex to the DOCSIS standard addresses ( EuroDOCSIS ). There are different versions of the standard ( 1.0, 1.1 and 2.0). While the extension of DOCSIS 1.0 was to 1.1 only software adjustments, DOCSIS 2.0 includes improved methods for error correction and multiple access ( S- CDMA and A- TDMA). So that the effective data rate is further increased, particularly for the return channel.

The data for both directions of transmission are modulated in different frequency bands in order to allow bi-directional transmission. For the corresponding digital signal processing of high integrated circuits are used with digital signal processors.

Because of the tree structure of the cable television network different modulation methods are used in transmit and receive direction. In the receive direction channels are used above 450 MHz. With the complex quadrature amplitude modulation (QAM) digital signals are modulated on the carrier frequencies.

Design and operation

The major functional blocks of a cable modem are shown in the image:

• The tuner adjusts the for return channel frequencies to be used. The diplexer directs the receiving frequencies to the demodulator and adds the signals coming from the modulator into the cable network a. It therefore performs a function that was in the classic analog telephony satisfied by the hybrid circuit.
• The demodulator generates from the analog signal to a digital data stream is decoded in the error correcting Media Access Control part. The data are processed in the CPU so that it can be routed via Ethernet or USB interface to the PC.
• The modulator takes the next block from the MAC data stream and converts it into the analogue signal to be transmitted.
• The MAC block ( Media Access Controller) has a central function. In addition to the encoding and decoding of the data to control access to the upstream channel for data to be transmitted. He shares these tasks with the
• CPU also handles the instrument control and data exchange with the PC.

TV outlets and Multimedia Outlets

The connection of a cable modem via a suitable antenna socket to a cable box of cable television. These junction boxes in addition to the two IEC sockets for radio, TV sets have a third connection for a cable modem. These are also referred to as Multimedia Outlets Outlets pickup of sounds from noise in the cable network can be significantly reduced over the radio and TV socket in the upstream, which is usually the frequency range of 5-65 MHz, with filters.

By using a capacitive sheath current filter at the antenna input interference from other devices can be avoided by so-called ground loops.

Interference with cable modems

The transmit and receive level of the cable modems has been standardized. Common causes of intermittent faults are fluctuating, low or too high a level in the cable network or caused by insufficient shielding.

An active cable modem in case of malfunctions, it allows the provider to measure the level of the return channel and the signal -to-noise ratio.

Must be installed for safe and trouble- free operation of a cable modem for appropriate TV aerial sockets. The use of high-quality installation materials and the need for additional distributor or switch to cable modem can contribute to the reduction of additional sources of interference.

If amplifiers are required, these bidirectional ( back channel ) need to be.

Requirement of domestic infrastructure

To ensure Internet access via the cable television network, the existing customers in the household distribution of the hitherto most used purely for TV and radio cable connector must be suitably carried out or be exchanged if necessary. It is particularly important that a shielded coaxial cable and distribution (eg, after the so-called Class -A standard ) and return channel Amplifiers and above Multimedia doses are used.

In addition, should the entire local cable television system be integrated into the domestic potential equalization to compensate for any potential differences between the coming of the outside supply line and inside the house (depending on regional cable provider).

This work may only be performed by certified and specially trained technicians usually, otherwise the risk of serious disturbances may occur in the cable television network.

The use of a cable modem is not using the specified frequencies for radio and television reception in the cable network. Therefore, no contract for the simultaneous cable TV reception is technically necessary for the use of a cable modem for the internet.