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6. High speed connections, high flow, ADSL, ATM. |
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6.1. Introduction - 6.2. Technologies xDSL (ADSL, SDSL, VDSL...) - 6.3. Rented lines - 6.4. Satellite Connection - 6.5. The TV cable - 6.6. ATM
This chapter of the hardware networks and communications course treats connections with high flow for the connection of Internet sites and connection high speed between user and Internet: connection xDSL, ATM, rented lines, cable television, satellite connection... All these solutions require a special subscription in your supplier of access.
The xDSL gather all that makes it possible to make pass from the floods of data at high speed on simple twisted telephone lines. There are various alternatives:
The essential differences between these technologies are businesses of:
Technologies xDSL are divided into two great families, those using a symmetrical transmission and that using an asymmetrical connection.
A solution xDSL symmetrical have the same speed of transfer in download (Internet towards user) that in upload (user towards Internet), contrary to the asymmetrical connections (ADSL for example). This is of primary importance for the lodging of a site within the company. The symmetrical solutions are especially used to replace the too expensive rented lines.
The first technique resulting from technology DSL was born at the beginning of the years 1990, it is the HDSL. This technique high flow divided the numerical trunk of the network, T1 in the United States, and E1 in Europe, on several pairs of wire (2 instead of 24 for T1 and 3 instead of 32 for E1). This was carried out thanks to the evolution of the theory of the signal making it possible to increase the number of bits per transmitted symbol.
With this technique, it is possible to reach a flow of 2Mbps on three twisted pairs and 1,5Mbps on two pairs. This while having a length of local loop of 4,5km and without additional addition of repeaters.
HDSL is currently in strong progression. The first networks of access HDSL were deployed by the American local operators. The principal argument of the HDSL is of order economic. The HDSL is particularly well adapted for:
In short, the HDSL allows:
In Europe, the operators just start to massively deploy these technologies and the prices are long in lowering fault of competition. The great innovation should come from HDSL2. This technology, derived from the HDSL, offers the same performances but on only one twisted pair. It is currently tested in the United States with 1,5Mbps. The current problem of this technology is a still imperfect standardization.
The precursor of technology HDSL2 is the SDSL. Like HDSL, SDSL supports the symmetrical transmissions on T1 and E1, however, it differs from HDSL by three important points:
Just like the HDSL, this symmetrical solution are reserved for the replacement of rented lines T1 and E1. The use of the line with a phone call is impossible.
It is possible to couple 2 lines to reach 4 Mb/s. The distance from the distributor is in theory of 1,5 km, but of the tests go up up to 2 km
Last symmetrical solution SHDSL (Individual-pair
High-speed DSL) goes back to it (2002) gathers technologies HDSL and HDSL2 and
SDSL. The rates of transfer (in payload) are identical in the two directions and
can vary:
- 192 Kb/s with 2,3 Mb/s in mode two wire (a pair).
- 384 Kb/s to 4.6 Mb/s in mode four wire (two pairs).
This solution uses all the band-width of the telephone line. It is not thus possible any more to use the telephone line at the same time. The use of filters is thus not necessary. These lines also allow the passage of "digitized" telephone signals of normal type or ISDN via specific apparatuses (PABX for example).
The other specificity of a connection SHDSL comes from the flow. The line is configured for a fixed flow (until 2,3 Mb/s). If the modem cannot reach this speed, there is no connection. This must allow a fixed flow. Nevertheless, some manufacturers authorizes a car detection speed by the modem interior speeds. The maximum distance is of 5 km on a simple pair of copper.
By various tests, one realized that it was possible more quickly to transmit the data since the exchange of the public network towards the user. As the concentration of the cables is more important when one approaches the exchange. The latter thus generate more cross talk near the switch. The signals coming from the user, more attenuated, are more sensitive to the noise caused by these electromagnetic disturbances. It is thus preferable to transmit low frequency (or on a less broad frequency band) the data resulting from the user.
The idea is the use of an asymmetrical system, by imposing a lower flow of the subscriber towards the exchange. The systems using this technique were named ADSL. There are some at least in two alternatives: the RADSL and the VDSL
These asymmetrical solutions are null and void for the lodging of Internet site important, the speed of transfer server Internet towards Internet (towards the user) is definitely lower at the speed of user transfer towards server. On the other hand, this can completely function for the lodging of a small site of amateur or SME with the proviso of using an address TCP fixes or of using a redirection software of address TCP.
The most important characteristic of the ADSL is its capacity to offer fast numerical services on the existing coppered telephone network, in superposition and without interference with the traditional analogical telephone service. A circuit ADSL connects an exchange of the public network to modem ADSL of the user, thus creating three channels of information:
To create multiple channels, modem ADSL divide the bandwidth available of a
telephone line according to one of the two types: multiplexing with division of
frequency (FDM) and the cancellation of echo.
With one or the other of these techniques, transmissions ADSL leave the free
area around the 4kHz in order to let pass the telephone calls (POTS). For that,
in more of modem ADSL, it is necessary to install a separator of line (splitter
which filters the telephone signals sees - digital signal), as explained in filter
adsl installation
The ADSL allows, for a length of maximum loop of 5,6km, to provide flows of:
Speeds ADSL standards in Belgium are 4 Mb/s in download (1 Mb/s if the user is in extreme cases of distances). You can nevertheless increase the flow by changing subscription in your supplier of access (definitely more expensive). On the other hand, the subscriptions in France provide speeds of 256 Kb/s, 512 kb/s... according to the subscription.
These flows also depend on a certain number of factors including/understanding, the length of the loop, its section and the interferences. The attenuation of line increases with its length, the frequency of the signal emitted as well as the narrowness of the cable.
These speeds of transfer transform the existing
telephone public network (limited to the voice, the text and graphics low
resolution) into a powerful system able to support the multi-media one,
including the video real time.
Indeed, new wiring broad band will take decades to reach all the subscribers,
without speaking about a hypothetical profitability. While transmitting films,
programs of television, data of local area networks, and especially introducing
the Internet into the houses, ADSL makes the markets viable and profitable for
the companies of telephone and the suppliers of applications.
In December 1998, an important stage was reached by the UIT (International Telecommunication Union) with regard to the standardization of systems DSL. The standard more awaited was the ADSL-Lite, which hiding place a version reduced of the ADSL. It is intended for the fast accesses to Internet and functions with flows lower than those of its elder (which are however largely higher than those of maximum the V.92 modems in 55.600 kb/s). It is less complex to put in?uvre because it does not require a filter (splitter).
With RADSL (Spleen Adaptive DSL), the speed
transmission is fixed in an automatic and dynamic way, according to the quality
of the line of communication.
As a long time as it was a question of transfer of video data, he was out of
question of varying the flow. In this precise case, it is necessary to make a
synchronous treatment. However, since the failure of the VDT (Video Dial Tone),
which has undergoes competition of the cabled TV and by satellite, other
applications appeared:
These applications have two advantages, synchronization is not obligatory any more, and asymmetrical architecture becomes obvious (in measurement or one transmits more information in the direction serveur/client than in the other). The RADSL thus adapts its speed to the local conditions.
RADSL would allow constant flows (ascending of 128kbps with 1Mbps and descendant of 600kbps with 7Mbps), for a maximum length of local loop of 5,4km (like the ADSL). RADSL is in the course of standardization by the ANSI. The organization considers technologies QAM, CAPE and DMT like modulations RADSL.
VDSL is fastest of technologies xDSL. It is able to support, on a simple twisted pair, flows:
On the other hand, the maximum length of the loop is only of 1,5km. This distance is very low but it can be increased by using optical fibre, of the supplier to a special optical terminal near to the user. From this terminal this last can be connected in VDSL (see figure below).
With regard to the modulation, the two channels of data are separated from the bands used for telephony on the one hand, and from those used for the ISDN on the other hand. This would allow to the suppliers services to superimpose VDSL with the already existing services. For the hour it is considered that the two channels (ascending and descendant) are also separate in frequency.
The downward data could be transmitted to each final equipment (passive termination of network) or to a pivot which distributes the data to the final equipment (active termination of network).
For the ascending data, the multiplexing is more difficult. In a passive configuration, each final equipment must divide a common cable. A system of collision detection could be used, however, two other solutions can be considered.
A first solution would consist so that the optical terminal sends screens to all the final equipment. These screens would authorize only one equipment to be communicated and for a certain period (TDMA Time Division Multiplexing Access). This equipment is recognized, thanks to the screen, and would transmit for this period. However, this method is heavy insofar as it implies to insert a certain latency between two authorizations and where it requires many bytes for its only protocol of operation (what reduces the productive flow).
The second method would consist to divide the ascending channel into various frequency bands and to associate each band final equipment (FDMA Frequency Division Multiplexing Access). This method has the advantage of being freed from any protocol of dialogue. However, it would limit to a value fixes the flow available of each final equipment.
In conclusion, we saw that the increase in the band-width of the VDSL makes it possible to the supplier of access to offer services of television high definition and numerical video of quality, multi-media Internet and services LAN with the consumers.
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Technologies DSL |
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| Technology | Definition | Mode of transmission | Flow Internet - > PC (Download) | Flow PC - > Internet (Upload) | Maximum distance | A number of pairs |
| HDSL |
High dated spleen DSL | Symmetrical |
1.544 Mbps |
1.544 Mbps 2.048 Mbps |
3.6 km | 2 or 3 following desired flow |
| HDSL 2 | High dated spleen DSL 2 | Symmetrical | 1.544 Mbps | 1.544 Mbps | 3.6 km | 1 |
| SDSL |
Individual line DSL | Symmetrical | 768 Kbps | 768 Kbps | 3.6 km | 1 |
| SHDSL | Individual-pair High-Speed DSL | Symmetrical | - 192 Kb/s with 2,3 Mb/s (a
pair), - 384 Kb/s to 4.6 Mb/s (two pairs) |
- 192 Kb/s with 2,3 Mb/s (a
pair), - 384 Kb/s to 4.6 Mb/s (two pairs) |
5 km | 1 or 2 following desired flow |
| ADSL | Asymmetric DSL | Asymmetrical | 128 Kbps With 9 Mbps | 16-640 Kbps | 5.4 km | 1 |
| RADSL | Miss Adaptive DSL | Asymmetrical | 0.6- 7 Mbps | 128 kb/s-1 Mb/s | 5.4 km | 1 |
| VDSL | Very high dated DSL | Asymmetrical | 15-53 Mbps | 1.544-2.3 Mbps | 1.3 km | 1 |
Another method of connection to Internet or to a corporate network (interconnection) uses rented lines. They currently have the highest flow but the number of pairs is definitely higher (24 pairs for T1 and 32 pairs for E1 for example). This solution is inevitably definitely more expensive than the solutions standards. It is currently the surest connection since it directly connects 2 points without passing by intermediaries. Unfortunately, safety has a price. By compression, these lines make it possible to use as much the line in communication Internet, that in communication of the type ISDN or STN via PABX.
In the case of the suppliers of European accesses, the current types are E1 (2Mb/s, 50 km maximum), E2 (8Mb/s), E3 (34 Mb/s) and E4 (140 Mb/s)
In the case of the American suppliers, T1 (1,544 Mb/s), T2= 4X T1 (6,312 Mb/s) and T3= 7*T2 (44,736 Mb/s)
For Japan: T1 (1,544 Mbps), T2=4*T1 (6,312 Mbps), T3=5*T2 (32,064 Mbps) and T4= 3*T3 (97,728 Mbps).
Connection Internet by satellite has some advantages, in particular not to depend on installation terrestrial existing: cables of cable television, network telephone with close terminals DSL,..
The first connections by satellite used a hybrid system: reception by microwave link, emission by traditional modem STN. This not very advantageous solution makes it possible to use an antenna standard parabola.
The new parabolas allow the emission and the reception. Speed in upload (sending towards Internet) varies from 128 K with 1024k and 512kbps with 2 Mbps in download (Internet towards user) for the applications commercial. The theoretical limit of this connection borders the 155 Mbps (a record). This does not hold account of times of latent (nearly 700 milliseconds) between the emitted signal and the signal received from the other with dimensions of the connection. As the satellites are geostationary, the distance between the satellite and the ground is of 36.000 km (multiply by 2, that is to say 72.000 km, for the distance to be traversed). The travel time between the message sent and the beginning of sending of information is a little more important than in other connections high speed (it is the same problem with connections by satellite GSM). So the speed of ping is very low. This should nevertheless disturb only some critical applications or the players via Internet; to a lesser extent VoIP.
Does this solution have some additional defects since the price of the installation is very expensive (count at the beginning of 2004 since 1000? for the antenna of less than 1 meters diameter, terminal of transmission/reception), without counting the installation of the antenna by a specialized technician who will make point the antenna on a precise satellite. Moreover subscriptions remain also definitely higher than those of the ADSL. This solution is to be recommended only for the zones not served by technologies DSL.
This solution uses the network of cable television. The frequencies of transfer on these cables are spread out from 10 to 860 MHz but certain zones are blocked not to interfere with radio operator FM, military communications... On this broad spectrum, the tele chains numerical ones are gathered per packages of 8. Each one of these packages occupies a bandwidth of 8 MHz. On the other hand, the analogical chains of television cannot be gathered and also occupy 8 MHz.
Two zones of frequencies are reserved for Internet, for the side going up, the other for the downward side. Flow of rising, broad part the 30 MHz, reached 128 kbs/s. In the direction going down, the flow varies according to the commercial offers between 512 and 768 kb/s but can go up to 1,500 Mbps. Contrary to the ADSL, this bandwidth is divided between all the users of the same section, typically several buildings.
The ATM (Asynchronus Transfer Mode) was selected like standard towards the end of the Eighties. He is the direct heir to Frame Relay of which he differs by employment from packages from small size and fixes (called cell). Protocol ATM is thus directed connection.
The ATM transports a continuous flow of cells (screens) of fixed size comprising 5 bytes (byte) of heading and 48 bytes (byte) of data. The band-width is optimal. Connections ATM use a mechanism of priority of the data, called QOS (Quality Of Service). This means that the messages high priority are sent directly. When ATM finds white (not priority emission of data), it will send priority data to stop these white. This possibility makes it possible to transfer via connections ATM all kinds of information (given, voice and others). The transmission is supposed without error. This means that the message is never re-emitted. This requires a network (wiring) with weak losses.
In practice, the functions of routing of cell is directly established in hardware, contrary to the majority of routers IP. For connections ATM, the term used for the routers is Switch. ATM adds with technologies which preceded it the possibility of guaranteeing capacity and quality of the service. One can thus establish a connection ATM between 2 systems and specify for example which one wishes a guaranteed flow of 3 Mb/s, 100 ms a maximum time, a variation of time lower than 5 ms and a rate of loss lower than 10 10. Such guarantees are necessary to be able to transfer onto ATM the digital circuits (to 64 Kb/s, 2 Mb/s, 34 Mb/s... to 622 Mb/s currently). It is this quality of service which makes it essential compared to the Ethernet networks. Moreover, certain connections ATM can reach 10 Gb/s at almost any distance.
Connections ATM are independent of the type of support network, this is not a hardware technology. Technology ATM can thus be established on cable twisted, coaxial network or optical fibre with limitations for high speeds. It is not inevitably independent of technology IP, but complementary. Technology ATM do not have in its structure of the emission and TO addresses. It governs only the low layers of OSI models in charge of transport.
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Methods, safety measures of remote connections. |
Parameter setting router with firewall integrated and modem RJ45 in bridge |
Introduction to local network |
ADSL filters installation in the telephone catches |
1. Network hardware course - 2. Introduction to IT communication - 3. Base of transmission - 4. Ethernet - 5. Switch, router, ... Ethernet - 6. Internet connections - 7. Networks servers - 8. SCSI, SAS, RAID - 9. Backup Tape drives, NAS, ... - 10. Hardware Security - 11. Wireless and Wifi - 12. UPS, Inverter, Power Supply - 13 . Corporate network - 14. Alternative technologies - 15. Touch Screen, video projector 17. Exercise: hardware firewall
The "Hardware 1" course: PC and peripherals, the "Hardware 2" course: Network, servers and communication.
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