Cable modem
A cable modem is a device that transmits data to the end customer via cable television networks and is used for broadband Internet access via cable television connections .
A direct connection from modem to computer was only common in the early years of technology; instead, a router and usually also a network distributor in the form of an Ethernet switch and / or wireless access point are integrated in the cable modem . A separate driver software for the cable modem is not necessary, the configuration is done via a web interface. The counterpart to the cable modem at the cable network operator is called the Cable Modem Termination System (CMTS) . A CMTS serves up to 50,000 end customers, depending on the expansion stage.
history
The broadband -Wifi cable television networks, according to the means of telephone line realized ADSL process the broadband access technology most commonly used. At the end of 2006, there were around 57 million Internet access via cable modems in the 30 OECD countries (corresponding to a market share of almost 30% of the entire broadband market), slightly more than half of them in the USA. In Switzerland and Austria there were roughly 0.7 and 0.5 million Internet access via cable with 2.1 and 1.4 million broadband connections, respectively.
Cable television networks were v. a. In the 1980s it was only intended for analog broadcast distribution , so extensive and costly renovation work was necessary for bidirectional data transmission via a return channel . At the end of September 2007 there were around one million cable internet connections with 18 million DSL connections in Germany . Today only a few cable television networks are not modernized. In large residential complexes, however, it can still happen today that the operator of the system does not want a bidirectional expansion in order to avoid possible interference with television reception.
properties
The data received by the PC at the USB or Ethernet interface are converted in the cable modem into a transmission format compatible with the cable network . The frequency ranges of some cable television channels are used exclusively for data transmission. Adapted to the traffic load typical of Internet surfing on the Internet , the frequency ranges can be used in such a way that more transmission capacity is available in the direction of the participants than in the opposite direction. An optimal assignment of the frequencies is also important because the cable network has a tree topology . A cable from the headend branches many times on its way below the streets and serves up to several hundred cable television customers. All cable internet subscribers connected to a tree must share the frequency range reserved for data traffic. For modern cable networks, this tree topology and thus the bottleneck of sharing the total bandwidth only exists for the last mile (which, in contrast to the telecommunications network, is regulated), since the signal for all connected subnetworks in parallel via fiber optic cable networks from the cable headend to the last distribution station ( see also HFC ) can be transported.
Data encryption according to the Data Encryption Standard with a key length of 56 bits ( DOCSIS 1.0) or 128 bits (DOCSIS 1.1 / 2.0) is intended to guarantee the confidentiality of the data. DOCSIS 3.0 is now available from the major cable providers. It is thus possible to combine frequency bands or frequency channels and achieve well over 100 Mbit / s.
Depending on the version, cable modems can achieve a transmission speed of over 120 Mbit per second in both directions. The network operators determine the offered speeds themselves according to technical and economic aspects. They range from ISDN speeds up to 450 Mbit per second to the subscriber and usually significantly less in the opposite direction.
Usually the cable operator provides the modem to the end customer. Since the cable network operator of an apartment cannot be freely chosen by the tenant, a customer cannot freely choose his provider ("provider"). As long as no uniform standards for access had been established, it was also not possible to use a modem bought independently from the cable network operator in order to achieve Internet access via the cable network. Some cable modems with a built-in router can be switched to the so-called bridge mode, in which the built-in router is deactivated. Some network operators now allow the use of modems that you have bought yourself if the modem is certified for the DOCSIS version used. In Germany, this permission has been mandatory for all network operators since the router-free law came into force on August 1, 2016. In any case, your own modem must be registered with the network operator in order to allow access.
The cable modem can be connected to any cable television connection point in the home. Only in some old cable networks in the home may it be necessary to connect the cable modem directly to the access point (usually in the basement). This is the case when old cable amplifiers are still installed in the house distribution that do not work bidirectionally. These can be exchanged for new models in order to be able to operate the cable modem at all connection points in the house.
The internet data is extracted in the cable modem itself (see below). A separate frequency splitter ( DSL splitter ) is therefore not necessary.
As a telephone solution is Voice over Cable used a variant of IP telephony . For this purpose, an analog telephone is usually connected to the cable modem. In Germany, cable network operators have also been providing cable modems with an S 0 bus for connecting ISDN terminals and telephone systems since the end of 2007 .
Alternatively, an IP data stream can be used for telephony via the network connection according to the SIP standard.
A cable modem usually offers two connection options for the PC:
- The modem was connected to the PC via a USB port. This requires a special driver for the operating system, which is usually only available for Microsoft Windows . Apart from the limitation to Windows, there is also the disadvantage that other computers in the home network (LAN) only have internet access if the computer to which the modem is connected is switched on and configured to forward the network data. In addition, due to the limited maximum cable length of the USB, the modem must not be too far away from the computer. This procedure is mostly no longer used today.
- On the other hand, today's variant of connecting the PC to the modem via an Ethernet cable is completely independent of the operating system; this cable can also be considerably longer than with USB. If the cable modem does not have a built-in router , a router can be connected between the PC and modem, which enables independent Internet access for several computers in the LAN or WLAN .
Transmission method
The DOCSIS standard ( Data Over Cable Service Interface Specification ) developed in the USA has established itself as the global standard for the modulation method and other interface properties . In Europe there were competing proposals ( DVB-RCCL , DAVIC), but they could not establish themselves on the market. The special features of the European cable networks such as frequency plan and higher channel bandwidth of eight instead of six megahertz are taken into account in an appendix to the DOCSIS standard ( EuroDocsis). There are different versions of the standard (1.0, 1.1, 2.0, 3.0 and 3.1). While the expansion from DOCSIS 1.0 to 1.1 consisted only of software adjustments, DOCSIS 2.0 contains improved procedures for error correction and multiple access (S- CDMA and A- TDMA ). This increases the usable data rate again, especially for the return channel. The latest DOCSIS standard 3.1 from October 2013 supports data rates of up to 10 Gbit / s in the receiving direction ("downstream") and 1 Gbit / s in the sending direction ("upstream").
The data for both transmission directions are modulated onto different frequency bands in order to enable bidirectional transmission. Highly integrated circuits with digital signal processors are used for the corresponding digital signal processing.
Because of the tree structure of the cable television network, different modulation methods are used in the transmit and receive directions. In the receiving direction, channels above 450 MHz are used. With the complex quadrature amplitude modulation (QAM), the digital signals are modulated onto the carrier frequencies .
Structure and functionality
The main functional blocks of a cable modem are shown in the picture:
- The tuner sets the frequencies to be used for the outgoing and return channels. The diplexer forwards the receive frequencies to the demodulator and inserts the signals coming from the modulator into the cable network. It thus fulfills a function that was fulfilled in classic analog telephone technology by the hybrid circuit .
- The demodulator generates a baseband signal from the broadband signal, which is decoded to correct errors in the component for the media access control . The data is processed in the CPU so that it can be sent to the PC via Ethernet or USB interface.
- The modulator takes over the data stream coming from the MAC module and converts it into the broadband signal to be transmitted.
- The MAC component ( Media Access Controller ) has a central function. In addition to encoding and decoding the data, it controls access to the return channel for data to be sent. He shares these tasks with the
- CPU, which also takes over device control and data exchange with the PC.
Radio connection boxes and multimedia connection boxes
The cable modem is connected to a cable connection of the cable television via a suitable antenna socket. In addition to the two IEC sockets for radio and television sets, these connection sockets can have a third connection option for a cable modem. With these junction boxes, also known as multimedia junction boxes, the interference of interference signals in the cable network via the radio and television connection socket into the transmission data stream, which usually takes place in the frequency range of 5–65 MHz, can be significantly reduced with filters.
Cable modem malfunctions
The transmission and reception level of the cable modems is standardized. Frequent causes of intermittent interference are fluctuating, weak or too high levels in the cable network or interference due to inadequate shielding.
A cable modem, which is also active in the event of interference, enables the provider to measure the level of the return channel and the signal-to-noise ratio up to the connection of the respective customer.
Suitable TV sockets should be installed to ensure safe and trouble-free operation of a cable modem. The use of high-quality installation material and the waiver of additional antenna sockets , distributors or switches up to the cable modem can help reduce additional sources of interference.
If the topography of the cable television network requires the use of amplifiers, these must be bidirectional (return channel capable) for the modem connection.
By using an additional capacitive sheath current filter at the antenna input, interference from other devices due to so-called hum loops can be avoided.
With the introduction of DVB-T2 in Germany, the susceptibility to interference on downstream channels also increased due to the high transmission power of DVB-T2 transmitters and poor shielding, since the same frequencies are usually used. Even with the introduction of DOCSIS 3.1 standards, this resulted in the OFMD -Downstreamkanälen by mobile phones and transmitting stations in connection with the LTE -Mobilfunkstandard to irradiation because the same frequencies are used here as well. Irradiation leads to interruptions and instability during operation of the modem .
Requirement of the domestic infrastructure
In order to ensure Internet access via the cable television network, the distribution of the cable connection, which has hitherto mostly been used only for television and radio, in the customer's household must be suitably designed or, if necessary, replaced. It is particularly important that appropriately shielded coaxial cables and distributors (e.g. according to the so-called Class A standard) as well as amplifiers with return channel capability and the aforementioned multimedia sockets are used.
In addition (depending on the regional cable provider), the entire local cable television system must be integrated into the domestic equipotential bonding in order to compensate for possible differences in potential between the supply line coming from outside and within the house.
As a rule, this work may only be carried out by certified or specially trained technicians, as otherwise there is a risk of considerable interference in the cable television network.
Cable modem hardware manufacturer
literature
- Mark E. Laubach, David J. Farber, Stephen D. Dukes: Delivering Internet Connections over Cable. New York 2001, ISBN 0-471-38950-1
Individual evidence
- ↑ Distribution of broadband connections in the OECD countries
- ↑ Bridge mode with WLAN modem UPC. Retrieved October 29, 2017 .
