3.6.6. ISDN, ADSL, VDSL, Cable Internet

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Modem technology for connecting to the Internet is still in use, but more and more come to the fore where the technology is based on the transmission of digital signals. This has become possible only with the development of postal exchanges, which today can be directly processed by a digital signal. Two of the most interesting ways to transfer data for small business or home connectivity of ISDN (Integrated Services Digital Network) and the ADSL (Asymmetric Digital Subscriber Line). Use another Cable television - cable modem. Common to these methods of communication that the connection is realized by the application or user is not constant, as in modem communications. So, with the classic postal traffic is increasingly offered the possibility of connecting to the Internet from a provider of services that can enable (Inernet Service Provider - ISP), which is not necessarily related to the company that offers the PTT service, old regular public telephone traffic (PSTN - Public Switched Telephone Network / POTS - Plain old telephone service). It will particularly consider the relationship between the possibilities of transmitting (TX) and receiving (RX) of various data.


ISDN is a modern addition to the existing, analog fixed telephone network. Allows voice communications (telephone conversation) and non-voice communication (data transfer - fax, Internet). Voice communication is based on the use of 'digital' phones to analog voice signal is converted to digital or will be used for the conversion of a special device that will be able to connect to either of two ways to transfer data (analog and digital), to perform the required conversion to a digital signal and digital communication the circuits in the PTT central. The basic configuration of the device carries the acronym NT1 (Network Terminator Type 1). Essentially converts two-wire communication with the postal exchanges in the four to the user. It may even be part of a device (router, for example) or a standalone device.

Way communication takes place over a group of data channels bandwidth 64 kbps (B-channel) and a signal-control channel (D-channel) bandwidth 16 kbps Transmission channel group realized time division multiple PBX (Private Branch eXchange), and at intervals miss the signals of each channel in a sequence and cyclically, a little first and second and third, etc., and again. According to the number of channels for information there are two different conceptions of ISDN:

 ISDN concept of communication  iStripper
Figure* 3.6.23 Concept of ISDN data transfer. ( + / - )  

The previous Figure are shown ways to connect to ISDN service provider (ISP - Internet Service Provider), using the symbols of the company Cisco devices. It is easy to calculate the proper power of each of the presented concepts. Home users is sufficient BRI conception that connects to the phone via NT1 NT2 (Network Termination Type 2) and PC via TA (Terminal Adapter) for NT2. As mentioned wiring between NT1, NT2, TA and ISDN modem in the computer is four to although the computer uses 8 core cable with an RJ45 connector. (pins 1,2,7,8 are not used). All these devices are usually for home use together in a single chassis. CSU / DSU (Channel Service Unit / Data Service Unit) is the equipment that provides the user with the service provider to connect to the Internet and this concept is used in firms with higher requirements by turnover or number of telephones for employees. BRI is a peculiarity of the concept which allows the use of speech and computer simultaneously, each in its channel or only computers on both channels simultaneously, and what is this 'flipping' automatically. If your computer uses both B-channels will automatically transfer switch in one B-channel to be freed as soon as the second detects the external voice call. In addition to the connection with the invitation to the user's need at the modem and a regular phone call.

Bad qualities are almost gone, except for one: the modern multimedia facilities can not meet the higher bandwidth of 128 kbps (2 B channels) and today imposes ADSL technology as a solution to this limited bandwidth BRI concept.


ADSL uses broadband Internet access. The standard is upgraded by the time and hence the various versions. It belongs to the group of DSL technologies that allow users to access the Internet or sustained by consumption by establishing connections at data rates of 128 kbps - 1 Mbps upstream and 512 kbps - 7 Mbps incoming traffic. Due to the different speed of the outgoing and incoming traffic has the prefix - asymmetrical (A). To communicate using the existing analog telephone wires but speaking (300 Hz-4000 Hz) is used as an analog channel and a frequency of 25 kHz-2200 kHz for additional digital channels - groups 'subchannels'. Subchannels in the range of 25 kHz-140 kHz are used for transmitting data, and the subchannels in the range of 140 kHz-1100 kHz (ADSL 2), respectively range of 140 kHz-2200 kHz (ADSL 2+), are used for receiving data. Lower limit can be 0 Hz instead 4000 Hz if not used previous older telephone communications POTS (Plain old telephone service), therefore all modes of transport are unified. The latest revision of this standard provides for a range of 0-280 kHz channels for transmitting and 280 kHz-2200 kHz for the receiveing of which one of the 'subchannels' is used for 'voice telephony'. This distribution channel is not exactly as at figure 3.6.24b, which tend to use two basic incompatible standards (CAP and DMT). In Figure 6.3.25b (chart ' A ') are shown ADSL 2 and ADSL 2+ frequency ranges compared to other systems. Receiving and transmitting part containing 'subchannels', which allows for different bandwidth depending on how much is 'subchannels' benefits. About traffic and coding, special care 'chips' in ADSL modem and a theoretical maximum throughput capacity is about 24 Mbps. Higher frequency range requires better infrastructure and the telephone before installing equipment needed to measure regarding possible work. ADSL works at a maximum distance of approximately 5 km to the PTT central. POTS voice channel is separated from other ADSL channels by a special filter (splitter). Voice channel is routed to the phone handset, and the other to the computer via a modem / router that takes care of the distribution of incoming and outgoing ADSL traffic. Except as provided in the ADSL frequency range, for incoming and outgoing traffic can be made available to the user more 'subchannels' and thus deliver different bandwidth which is analogous to differently charged by the service provider. Concept of connecting to the Internet is shown in the following figure.

 ADSL concept of communication  iStripper
Figure* 3.6.24 Concept ADSL data transmission. ( + / - )  

Install the splitter and the router / modem implements a service provider. Very often both of these devices in the same chassis, and an additional switch to connect multiple computers. Usually the modem / router has a built in switch that allows you to connect several computers and the use of a special switch is not necessary. One useful feature of the modem / router is the possibility of an electronic lock. Pressing the lock button, the device disconnects itself from the network and it is not possible to establish any communication by the service provider (ISP), and vice versa. Of course at the ISP must be a splitter to separate voice from digital traffic. When the specified voice traffic refers to an analog signal and the ability to use the 'old phone on the wheel. A possible solution is to transform traditional telephone signal into a digital signal and 'move' in one of the ADSL subchannels and distributed through a special exchange Internet traffic (Voice over IP - VoIP), which eliminates the need for a distributor. Of course, the ISP must have devices that allow specified.

Wiring between the ISP and the user is one pair (two-wire communication), which used to be the interface uses RJ11 connector, while between ISDN devices use the RJ45 connector in which the interface devices use two pairs (four-wire), and network devices after router using RJ45 connector and four pairs (eight wires) out of which interface is used two or all four pairs, depending on the type of communication. ISDN technology is generally in decline and will not be specifically described, and ways of connecting in the network RJ45 connector to cable is shown in section 3.6.4. RJ11 connector looks like RJ45 connector, but is smaller in size and uses a different (thinner) cable.

As ADSL asymmetric data transfer technique, in which the speed of downloading data from multiple higher speed transfer of data, it is not suitable for communication, where a significant one, and another form of data transmission, such as video conferencing. But can fully satisfy standard requirements of small distant village regarding voice communication and data transfer (double play - 2D services).

When the functions of a modem, router and switch in one unit, who also supports the wireless Wi‑Fi communications, it is termed an access device for BROADBAND Internet, respectively SOHO (Small Office, Home Office) ADSL or CABLE device. On the side of the ISP is DSLAM (Digital subscriber line access multiplexer) device that is able to serve several SOHO devices, as shown in figure 3.6.25a. Modern Internet services (services) require higher speeds than previously described devices. The solution is a combination of communication through fiber optic cables between the ISP and DSLAM devices, and of DSLAM equipment to the SOHO device communication is accomplished TT twisted pair, coaxial cable, optic cable or wireless - Hybrid networks. Typically solution for ADSL and VDSL is TT twisted pair, and for Cable TV solution is coaxial cable. The optical connection is used when needed very high speed Internet access, and wireless communication usually is specific portable devices like smart phone, tablet and even laptops to be used when Wi‑Fi is not available.

As the data transfer rate is greater, inasmuch DSLAM device must be closer to the user (Shannon's theorem), what is easy if the ISP in the vicinity of the user. Otherwise DSLAM devices are placed in special cabinets that serve several surrounding houses, or if it is a skyscraper placed in him. Due to the large number of users in a skyscraper possible need to set up more cabinets with DSLAM devices. Hybrid systems for data transfer is feature of the ADSL and CABLE TV systems.

VDSL / Cable Internet

Development ADSL labeling revisions of this standard. One of its upgrading is VDSL (Very high bit-rate Digital Subscribe Liner), which is the basic feature of the existing versions and expanding standards (Annexes standards) to the frequency range with an upper limit of 1100 KHz (1.1 MHz) is increased to 12 MHz way to expand the receiving area of the first 3.8 MHz and adds another area of the transmit and two receiving areas (Annex B) and a further increase in the upper limit at 30 MHz with the addition of another area of the transmitting and receiving (Annex C). In this way, both versions of its VDSL provides enhanced delivery. But there is one small 'quirks'. What is the frequency range that uses larger, so the distance to the communications system provider of smaller (Shannon's theorem). So that is below 1 km. Therefore, in the vicinity of interested group of users sets street cabinet with electronic access, which is further connected to the PTT central office to another technology. So, densely populated areas are at an advantage.

As standard, VDSL means distribution data using a digital signal using TT twisted pairs at already existing telephone infrastructure, provided that from the ISP to the DSLAM device uses an optical cable and from DSLAM to the SOHO device TT twisted pair, according to Figure 3.6.25a. Distribution of signals within the SOHO network is UTP cable with data rate of 100 Mbps to 1000 Mbps, depending on the device used (Figure 3.6.26a). Depending on the available speed for receiving of video content used STB device that performs decoding of TV signals (chart ' B ' at Figure 3.6.25b). Basically it is about distribution of TCP / IP packets - IPTV. If not using POTS for voice communication, but the voice signal is digitized and sent through the existing sub-channel, splitter is not required.

If you offer special services which should be two-way communication to the ISP for special services (S), as well as a 'video library', the higher frequency range is required as shown in the chart ' C ' in Figure 2.6.25c. VDSL without particular problems supporting 'triple play - 3D' service; video, voice communication and Internet.

 VDSL and Cable communication concept    VDSL and SOHO
Figure* 3.6.25 HFC / ADSL, VDSL, DOCSIS. + / - ) Figure* 3.6.26 VDSL / DVB concept.

Although for a time in the 80' and 90', television receivers and VCRs were equipped to receive the analog signal at VHF and UHF frequency band channels, analog cable-ready tuners are rarely used now. The conversion to digital broadcasting has put all signals - broadcast and cable - into digital form, rendering analog cable television service obsolete. But digital system requiring the set-top box (STB) for decryption of TV signal. Analog television sets are still present (Figure 3.6.26b), and regardin to DOCSIS 3.0 standard can receive analog signal (Figure 3.6.25b, chart ' I ' i ' II '). Modern cable systems are large, with a single network and headend often serving an entire metropolitan area. Most systems use Hybrid fiber-coaxial (HFC) distribution. After 2004 the traditional cable television providers and traditional telecommunication companies increasingly compete in providing video, voice and data services to users. The combination of television, telephone and Internet access is commonly called 'triple play - 3D' service, regardless of whether CATV or telcos offer it. Scheme of 3D cable service shown in Figure 3.6.26b.

The chart shows a large disproportion between outgoing and incoming traffic, a special scope ' S ' is used for a specific two-way services, such 'video library' and the like. Cable operator is not so much a confined with the standards and each of them purpose scope 'S' can be defined as desired. This includes verbal communication by telephone. Given the above, can achieve high data download speed but not upload speed.

After 2013, and updated several times since. The DOCSIS 3.1 suite of specifications (chart ' III ' at Figure 3.6.25b) support capacities of at least 10 Gbit/s downstream and 1 Gbit/s upstream using 4096 QAM. The new specs do away with 6 MHz and 8 MHz wide channel spacing and instead use smaller (20 kHz to 50 kHz wide) orthogonal frequency-division multiplexing (OFDM) subcarriers; these can be bonded inside a block spectrum that could end up being about 200 MHz wide. Analog signal is out.

Special categories in relation to the listed are SATELLITE and low voltage - electrical power BPL (Broadband over Powerlines) systems. BPL is an emerging technology that is available in very limited areas. Sattelite system is advantageous where above solutions are not possible, and BPL is an emerging technology that is available in very limited areas.


For the user's interesting and collection of these technologies. ISDN is paid according to time spent and ADSL to its turnover, or constant price regardless of the traffic continuously. But regardless of the IP address of the user dynamically changes say every 12 hours, unless you buy a fixed IP address. Similarly ADSL Cable in doing it but it works on frequencies that are larger, it is possible to achieve a throughput over 30 Mbps. In both cases, usually brings a bandwidth ranging from 256 kbps-10'000 kbps Large bandwidth within the home (ISP's network) does not mean that you will realize a large bandwidth 'out there', especially if the user some important overseas facilities. So, before concluding with the ISP's useful to ask which characteristics can be achieved with some of their offerings. The user should evaluate whether his short fast connection and can be obtained and the cost of turnover, or permanent unlimited connection but slower with a fixed cost (flat rate).

Indicate one important difference. ADSL and VDSL technology to the user sending pure digital signal, and Cable TV uses modulation of high-frequency signal to a digital signal, like DVB. So, SOHO devices are not compatible. Did not compatible frequency ranges for different geographical areas (Europe, America, Japan ...), so need to watch what is being purchased during tourist trips. The charts in Figure 3.6.25b is closest to the European standards.


Citing of this page:
Radic, Drago. " IT - Informatics Alphabet " Split-Croatia.
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